[10] T. Schneider, L. Wietschel, D. Schüppel, J. Riesner, K. Konrad, A. Thorenz, A. Tuma, D. Koch, Multicriteria optimization as enabler for Sustainable Ceramic Matrix Composites, Int J Appl Ceram Technol. 19 (2022) 3247-3254. https://doi.org/10.1111/IJAC.14174

[11] R. Rasheed, I. Anwar, F. Tahir, A. Rizwan, H. Javed, F. Sharif, Techno-economic and environmental sustainability analysis of filament-winding versus pultrusion based glass-fiber composite technologies, Environmental Science and Pollution Research. 30 (2023) 36276-36293. https://doi.org/10.1007/S11356-022-24817-5/FIGURES/10

[12] P. Mindermann, M.G. Pérez, J. Knippers, G.T. Gresser, Investigation of the Fabrication Suitability, Structural Performance, and Sustainability of Natural Fibers in Coreless Filament Winding, Volume 15, Issue 9. 15 (3260). https://doi.org/10.3390/ma15093260

[13] B. Iacopo, F. Archimede, G. Francesco, G. Luciano, M. Chiara, T. Giulio, Process and structural simulation for the development of a pressure vessel through filament winding technology, Materials Research Proceedings. 28 (2023) 347-356. https://doi.org/10.21741/9781644902479-38

[14] C. Laval, CADWIND 2006 - 20 years of filament winding experience, Reinforced Plastics. 50 (2006) 34-37. https://doi.org/10.1016/S0034-3617(06)70913-4

[15] A. Forcellese, M. Marconi, M. Simoncini, A. Vita, Life cycle impact assessment of different manufacturing technologies for automotive CFRP components, J Clean Prod. 271 (2020). https://doi.org/10.1016/j.jclepro.2020.122677

[16] L. Postacchini, M. Simoncini, A. Forcellese, M. Bevilacqua, F.E. Ciarapica, G. Andreassi, A.C. Russo, Environmental assessment of an automated impregnation process of carbon fiber tows, in: Procedia CIRP, 2020: pp. 445-450. https://doi.org/10.1016/j.procir.2020.05.077

[17] A. Benitez, C. Wulf, A. de Palmenaer, M. Lengersdorf, T. Röding, T. Grube, M. Robinius, D. Stolten, W. Kuckshinrichs, Ecological assessment of fuel cell electric vehicles with special focus on type IV carbon fiber hydrogen tank, J Clean Prod. 278 (2021). https://doi.org/10.1016/j.jclepro.2020.123277

[18] W. Kloepffer, Life cycle sustainability assessment of products (with Comments by Helias A. Udo de Haes, p. 95), International Journal of Life Cycle Assessment. 13 (2008) 89-95. https://doi.org/10.1065/LCA2008.02.376/METRICS

[19] R. Tong, Cost Analysis on L-shape Composite Componenet Manufacturing, in: 2012. https://api.semanticscholar.org/CorpusID:167899775
[10] T. Schneider, L. Wietschel, D. Schüppel, J. Riesner, K. Konrad, A. Thorenz, A. Tuma, D. Koch, Multicriteria optimization as enabler for Sustainable Ceramic Matrix Composites, Int J Appl Ceram Technol. 19 (2022) 3247-3254. https://doi.org/10.1111/IJAC.14174

[11] R. Rasheed, I. Anwar, F. Tahir, A. Rizwan, H. Javed, F. Sharif, Techno-economic and environmental sustainability analysis of filament-winding versus pultrusion based glass-fiber composite technologies, Environmental Science and Pollution Research. 30 (2023) 36276-36293. https://doi.org/10.1007/S11356-022-24817-5/FIGURES/10

[12] P. Mindermann, M.G. Pérez, J. Knippers, G.T. Gresser, Investigation of the Fabrication Suitability, Structural Performance, and Sustainability of Natural Fibers in Coreless Filament Winding, Volume 15, Issue 9. 15 (3260). https://doi.org/10.3390/ma15093260

[13] B. Iacopo, F. Archimede, G. Francesco, G. Luciano, M. Chiara, T. Giulio, Process and structural simulation for the development of a pressure vessel through filament winding technology, Materials Research Proceedings. 28 (2023) 347-356. https://doi.org/10.21741/9781644902479-38

[14] C. Laval, CADWIND 2006 - 20 years of filament winding experience, Reinforced Plastics. 50 (2006) 34-37. https://doi.org/10.1016/S0034-3617(06)70913-4

[15] A. Forcellese, M. Marconi, M. Simoncini, A. Vita, Life cycle impact assessment of different manufacturing technologies for automotive CFRP components, J Clean Prod. 271 (2020). https://doi.org/10.1016/j.jclepro.2020.122677

[16] L. Postacchini, M. Simoncini, A. Forcellese, M. Bevilacqua, F.E. Ciarapica, G. Andreassi, A.C. Russo, Environmental assessment of an automated impregnation process of carbon fiber tows, in: Procedia CIRP, 2020: pp. 445-450. https://doi.org/10.1016/j.procir.2020.05.077

[17] A. Benitez, C. Wulf, A. de Palmenaer, M. Lengersdorf, T. Röding, T. Grube, M. Robinius, D. Stolten, W. Kuckshinrichs, Ecological assessment of fuel cell electric vehicles with special focus on type IV carbon fiber hydrogen tank, J Clean Prod. 278 (2021). https://doi.org/10.1016/j.jclepro.2020.123277

[18] W. Kloepffer, Life cycle sustainability assessment of products (with Comments by Helias A. Udo de Haes, p. 95), International Journal of Life Cycle Assessment. 13 (2008) 89-95. https://doi.org/10.1065/LCA2008.02.376/METRICS

[19] R. Tong, Cost Analysis on L-shape Composite Componenet Manufacturing, in: 2012. https://api.semanticscholar.org/CorpusID:167899775
# Finite element analysis and life cycle assessment for CFRP laminates in marine applications

GENTILI Serena¹,ª, GRECO Luciano¹,ᵇ, FORCELLESE Archimede¹,ᶜ, MIGNANELLI Chiara¹,ᵈ*, PAPPADÀ Silvio²,ᵉ, SALOMI Andrea²,*f, VITA Alessio¹,g, ZANZARELLI Giuseppe²,ʰ

¹Università Politecnica delle Marche, Via Brecce Bianche 12, 60123 Ancona, Italy

²Cetma Composites, Via Enrico Fermi, 5/b, 72100 Brindisi, Italy

ªs.gentili@pm.univpm.it, ᵇl.greco@pm.univpm.it, ᶜa.forcellese@staff.univpm.it,

ᶜc.mignanelli@pm.univpm.it, ᵉsilvio.pappada@cetmacomposites.it,

ᵈandrea.salomi@cetmacomposites.it, ᵈalessio.vita@staff.univpm.it,

ʰgiuseppe.zanzarelli@cetmacomposites.it

**Keywords:** Finite Element Method, Life Cycle Assessment, Composite Material, Nautical Applications

**Abstract.** The present study focuses on structural analysis of a moth structure realized in carbon fiber reinforced polymers (CFRPs) prepreg laminates. A finite element method analysis was performed to investigate the flexural strength of different laminations and to identify the optimal layering. Moreover, a Life Cycle Assessment (LCA) analysis was conducted to evaluate the environmental impacts of the moth structure obtained using innovative processes and to compare them with those resulted by the same structure realized with manual composite processes. The results demonstrated the higher sustainability of the innovative process than the traditional one (335.75 eq. vs 228.22 kg CO₂ eq.). This research is part of the project NAUTILUS, in which Università Politecnica delle Marche and Cetma Composites are involved, that aims to develop, test and validate new processes for composite materials in nautical applications, with respect to environmental sustainability.

**Introduction**

In the last years, prepreg composite materials have emerged as revolutionary solutions in several sectors, for the production of aerospace, automotive, nautical and other high-performance applications, offering an excellent combination of strength, lightness, structural stability and versatility [1].

Furthermore, prepreg composites lead to several advantages as compared to hand-impregnation composites, such as the precise control of the resin content, uniformity and repeatability, high mechanical performances, reduced curing time, and smooth and glossy surface finish [2–5]. However, since the design of the composite laminated products is complex and time-consuming due to the large number of variables to consider, Finite Element Method (FEM) analysis can represent a suitable tool to optimize the composite layering. In particular, by using this method, simulations can be carried out to evaluate the mechanical performances of the composite structure and to define the best sequence of composite laminates based on the defined loading conditions [6,7]. Furthermore, a critical aspect in manufacturing is related to the environmental sustainability of composite materials. Prepreg composites are characterized by the production of a minimum amount of waste, optimizing the use of materials and thus contributing significantly to the environmental sustainability [8,9]. In the scientific literature, few papers on the design of composite nautical structures are available in scientific literature. Peterson investigated the potential application of additive manufacturing with Fiber Reinforced Plastic to the yacht design

<figure><img src="image_2682.png" /></figure>
# Finite element analysis and life cycle assessment for CFRP laminates in marine applications

GENTILI Serena¹,ª, GRECO Luciano¹,ᵇ, FORCELLESE Archimede¹,ᶜ,
MIGNANELLI Chiara¹,ᵈ*, PAPPADÀ Silvio²,ᵉ, SALOMI Andrea²,*f, VITA Alessio¹,g,
ZANZARELLI Giuseppe²,ʰ

¹Università Politecnica delle Marche, Via Brecce Bianche 12, 60123 Ancona, Italy

²Cetma Composites, Via Enrico Fermi, 5/b, 72100 Brindisi, Italy

ªs.gentili@pm.univpm.it, ᵇl.greco@pm.univpm.it, ᶜa.forcellese@staff.univpm.it,

ᶜc.mignanelli@pm.univpm.it, ᵈsilvio.pappada@cetmacomposites.it,

ᵇandrea.salomi@cetmacomposites.it, ᵉalessio.vita@staff.univpm.it,

ʰgiuseppe.zanzarelli@cetmacomposites.it

**Keywords:** Finite Element Method, Life Cycle Assessment, Composite Material, Nautical Applications

**Abstract.** The present study focuses on structural analysis of a moth structure realized in carbon fiber reinforced polymers (CFRPs) prepreg laminates. A finite element method analysis was performed to investigate the flexural strength of different laminations and to identify the optimal layering. Moreover, a Life Cycle Assessment (LCA) analysis was conducted to evaluate the environmental impacts of the moth structure obtained using innovative processes and to compare them with those resulted by the same structure realized with manual composite processes. The results demonstrated the higher sustainability of the innovative process than the traditional one (335.75 eq. vs 228.22 kg CO₂ eq.). This research is part of the project NAUTILUS, in which Università Politecnica delle Marche and Cetma Composites are involved, that aims to develop, test and validate new processes for composite materials in nautical applications, with respect to environmental sustainability.

**Introduction**

In the last years, prepreg composite materials have emerged as revolutionary solutions in several sectors, for the production of aerospace, automotive, nautical and other high-performance applications, offering an excellent combination of strength, lightness, structural stability and versatility [1].

Furthermore, prepreg composites lead to several advantages as compared to hand-impregnation composites, such as the precise control of the resin content, uniformity and repeatability, high mechanical performances, reduced curing time, and smooth and glossy surface finish [2–5]. However, since the design of the composite laminated products is complex and time-consuming due to the large number of variables to consider, Finite Element Method (FEM) analysis can represent a suitable tool to optimize the composite layering. In particular, by using this method, simulations can be carried out to evaluate the mechanical performances of the composite structure and to define the best sequence of composite laminates based on the defined loading conditions [6,7]. Furthermore, a critical aspect in manufacturing is related to the environmental sustainability of composite materials. Prepreg composites are characterized by the production of a minimum amount of waste, optimizing the use of materials and thus contributing significantly to the environmental sustainability [8,9]. In the scientific literature, few papers on the design of composite nautical structures are available in scientific literature. Peterson investigated the potential application of additive manufacturing with Fiber Reinforced Plastic to the yacht design

<figure><img src="image_2677.png" /></figure>
and manufacturing industry [10]. Bianchi et al. focused their attention on a simplified model for structural analysis of hydrofoil and moth masts realized with composite laminates [11]. Castegnaro et al. studied the bio-composite structures for the racing sailboats to cope with the environmental issues [12]. However, literature lacks environmental assessments of racing nautical structures and comparisons between the impacts of composite prepreg parts and ones produced via manual composite lamination.

Recently, the Italian Ministry of Economic Development funded a research project in which Università Politecnica delle Marche and Cetma Composites are involved, that concerns the development of innovative composites manufacturing processes for advanced racing nautical structures, characterized by lower environmental impacts than the traditional alternatives. In this context, this paper provides an environmental assessment of the use of carbon fibres (CF) prepreg for the production of a moth, that is a racing nautical structure. In addition, a comparison with an equivalent component produced using dry CF fabric and a manual lamination process was conducted. Given the interest in sailing competition and amateur applications, the moth structure was selected as representative case study to contribute to a sustainable development of high-end nautical component. The study aims to evaluate the difference between the use of dry fibres and resin, and prepregged fibers as composite material used to realize a moth structure. The goal is to identify which is the more sustainable solution. FEM analyses were employed to define the composite layering which guarantee the required mechanical performances and consequently the related weight. The latter represents an important input data for the environmental evaluation. The standardized methodology of Life Cycle Assessment (LCA) was used to identify which is the more sustainable solution.

## Material and methods

### Design phase

The moth structure is a racing sailing sailboat featured by hydrodynamic appendages called hydrofoils, which are submerged laminar surfaces with similar characteristics of aircraft wings. They are designed to generate vertical thrust as the boat speed increases, allowing the lift of the structure above the water level. This led to a minimization of the hydrodynamic resistance of the hull and the achievement of high sailing speeds and greater hydrodynamic efficiencies than traditional sailboats. For this reason, the CFRP results the most suitable material for this structure.

Firstly, the design phase needed the initial CAD model of the structure, which was provided by the Cetma Composites team, which also assessed the loads applied to it as well as the safety factor requirements of the structure. Then, several simulations based on the Finite Element Method (FEM) were carried out by using Siemens NX software. Figure 1 shows the CAD model of the moth structure investigated. An iterative process was performed to analyze the structural strength of the moth as a function of the structure thickness. This analysis allowed to identify the optimal prepreg layering in different areas of the moth, ensuring sufficient strength and stiffness while keeping a low weight of the structure.

### Life Cycle Assessment of the moth structure

The environmental impacts associated with the moth structure were evaluated through the LCA methodology. It is based on the UNI EN ISO 104040 – 14044 standards. The main phases of the LCA procedure are: i) goal and scope definition, ii) Life Cycle Inventory (LCI), iii) Life Cycle Impact Assessment (LCIA) and iv) Interpretation of results.
and manufacturing industry [10]. Bianchi et al. focused their attention on a simplified model for structural analysis of hydrofoil and moth masts realized with composite laminates [11]. Castegnaro et al. studied the bio-composite structures for the racing sailboats to cope with the environmental issues [12]. However, literature lacks environmental assessments of racing nautical structures and comparisons between the impacts of composite prepreg parts and ones produced via manual composite lamination.

Recently, the Italian Ministry of Economic Development funded a research project in which Università Politecnica delle Marche and Cetma Composites are involved, that concerns the development of innovative composites manufacturing processes for advanced racing nautical structures, characterized by lower environmental impacts than the traditional alternatives. In this context, this paper provides an environmental assessment of the use of carbon fibres (CF) prepreg for the production of a moth, that is a racing nautical structure. In addition, a comparison with an equivalent component produced using dry CF fabric and a manual lamination process was conducted. Given the interest in sailing competition and amateur applications, the moth structure was selected as representative case study to contribute to a sustainable development of high-end nautical component. The study aims to evaluate the difference between the use of dry fibres and resin, and prepregged fibers as composite material used to realize a moth structure. The goal is to identify which is the more sustainable solution. FEM analyses were employed to define the composite layering which guarantee the required mechanical performances and consequently the related weight. The latter represents an important input data for the environmental evaluation. The standardized methodology of Life Cycle Assessment (LCA) was used to identify which is the more sustainable solution.

## Material and methods

### Design phase

The moth structure is a racing sailing sailboat featured by hydrodynamic appendages called hydrofoils, which are submerged laminar surfaces with similar characteristics of aircraft wings. They are designed to generate vertical thrust as the boat speed increases, allowing the lift of the structure above the water level. This led to a minimization of the hydrodynamic resistance of the hull and the achievement of high sailing speeds and greater hydrodynamic efficiencies than traditional sailboats. For this reason, the CFRP results the most suitable material for this structure.

Firstly, the design phase needed the initial CAD model of the structure, which was provided by the Cetma Composites team, which also assessed the loads applied to it as well as the safety factor requirements of the structure. Then, several simulations based on the Finite Element Method (FEM) were carried out by using Siemens NX software. Figure 1 shows the CAD model of the moth structure investigated. An iterative process was performed to analyze the structural strength of the moth as a function of the structure thickness. This analysis allowed to identify the optimal prepreg layering in different areas of the moth, ensuring sufficient strength and stiffness while keeping a low weight of the structure.

### Life Cycle Assessment of the moth structure

The environmental impacts associated with the moth structure were evaluated through the LCA methodology. It is based on the UNI EN ISO 104040 – 14044 standards. The main phases of the LCA procedure are: i) goal and scope definition, ii) Life Cycle Inventory (LCI), iii) Life Cycle Impact Assessment (LCIA) and iv) Interpretation of results.
<figure><img src="image_2683.png" /><figcaption>Figure 1 CAD model of moth structure.</figcaption></figure>

**Goal and scope definition**

The LCA analysis aims at quantifying and comparing the environmental impacts associated with two different typologies of the moth structure production to identify which is the most sustainable solution. The functional unit (FU) is defined as “the production of a composite material moth structure of 6.71 kg”. The Life Cycle Assessment is based on a “from cradle to grave” approach, as it includes life cycle phases from the extraction of the raw materials to the disposal of the final product. More specifically, materials extraction, production of consumables, manufacturing and disposal are included within system boundaries. The service life is not included because the focus of the study is the environmental effects associated to the use of different materials.

Two different scenarios were considered in this analysis, which differ in terms of the typology of raw materials used. The description of the considered scenarios is reported below.

* Scenario 1: it includes the production of the moth structure made by CFRP composite manually impregnated. An epoxy resin is considered as the composite matrix. The process used to produce the structure is a manual lamination followed by a curing process at room temperature. The same materials and processes were considered to realize the mold for the moth structure. In addition, a milling process is considered to produce a polyurethane master used to realize the composite mold. Then, a landfill disposal is considered as end-of-life phase.

* Scenario 2: it includes the production of the moth structure made by a CFRP prepreg, with an epoxy resin matrix, as for the Scenario 1. A manual lamination followed by an oven curing process is considered to produce the structure. The same CFRP prepreg and the described process were considered to realize the composite mold. Moreover, as for the Scenario 1, the production of a polyurethane master to obtain the composite mold, through a milling process, was included. Then, a landfill disposal is considered as end-of-life phase.

Both scenarios include landfill disposal for the components as, at present, this is still the most common EoL alternative for thermosetting matrix composite materials. Figure 2 shows a schematic representation of the phases included within the system boundaries of the analysed scenarios.

**Life Cycle Inventory**

Different sources were considered for the collection of the Life Cycle Inventory data: primary data were measured directly, while secondary data were retrieved from scientific literature and Ecoinvent 3.1 commercial database [13]. A detailed description of the LCI phase and data retrieval is reported below. At first, inventory data related to the moth structure are listed, then data concerning the production of the moth mold and the master are given. The main inputs to realize the moth structure are the material used: carbon fibres and epoxy resin for both scenarios. Since the Scenario 1 does not consider the use of a prepreg, the quantity of each material used is higher than the one considered in the Scenario 2.
<figure><img src="image_2678.png" /><figcaption>Figure 1 CAD model of moth structure.</figcaption></figure>

## Goal and scope definition

The LCA analysis aims at quantifying and comparing the environmental impacts associated with two different typologies of the moth structure production to identify which is the most sustainable solution. The functional unit (FU) is defined as “the production of a composite material moth structure of 6.71 kg”. The Life Cycle Assessment is based on a “from cradle to grave” approach, as it includes life cycle phases from the extraction of the raw materials to the disposal of the final product. More specifically, materials extraction, production of consumables, manufacturing and disposal are included within system boundaries. The service life is not included because the focus of the study is the environmental effects associated to the use of different materials.

Two different scenarios were considered in this analysis, which differ in terms of the typology of raw materials used. The description of the considered scenarios is reported below.

* Scenario 1: it includes the production of the moth structure made by CFRP composite manually impregnated. An epoxy resin is considered as the composite matrix. The process used to produce the structure is a manual lamination followed by a curing process at room temperature. The same materials and processes were considered to realize the mold for the moth structure. In addition, a milling process is considered to produce a polyurethane master used to realize the composite mold. Then, a landfill disposal is considered as end-of-life phase.

* Scenario 2: it includes the production of the moth structure made by a CFRP prepreg, with an epoxy resin matrix, as for the Scenario 1. A manual lamination followed by an oven curing process is considered to produce the structure. The same CFRP prepreg and the described process were considered to realize the composite mold. Moreover, as for the Scenario 1, the production of a polyurethane master to obtain the composite mold, through a milling process, was included. Then, a landfill disposal is considered as end-of-life phase.

Both scenarios include landfill disposal for the components as, at present, this is still the most common EoL alternative for thermosetting matrix composite materials. Figure 2 shows a schematic representation of the phases included within the system boundaries of the analysed scenarios.

## Life Cycle Inventory

Different sources were considered for the collection of the Life Cycle Inventory data: primary data were measured directly, while secondary data were retrieved from scientific literature and Ecoinvent 3.1 commercial database [13]. A detailed description of the LCI phase and data retrieval is reported below. At first, inventory data related to the moth structure are listed, then data concerning the production of the moth mold and the master are given. The main inputs to realize the moth structure are the material used: carbon fibres and epoxy resin for both scenarios. Since the Scenario 1 does not consider the use of a prepreg, the quantity of each material used is higher than the one considered in the Scenario 2.
<figure><img src="image_2684.png" /><figcaption>Figure 2 System boundaries of analysed scenarios.</figcaption></figure>

In fact, prepreg offers a good control of resin content which cannot be guaranteed by impregnating the fibres in a resin bath. By considering a composite with 60wt% of fibres and 40wt% of matrix, 4.8 kg of epoxy resin and 7.2 kg of carbon fibres are considered in the Scenario 1, while 2.68 kg of epoxy resin and 4.03 kg of carbon fibres are analysed in the Scenario 2. Since the impregnation process included in the Scenario 1 is a manual process, no energy consumption is considered, while in the Scenario 2 an energy consumption of 40 MJ per kg of composite is included for the realization of prepreg [14]. To fabricate the moth structure a manual lamination process is used, so no consumptions are considered. An oven curing process is considered only in the Scenario 2, because the Scenario 1 includes a curing process at room temperature. The consumptions related to this process are the needed energy and the consumable materials as the polyamide (PA66) for the vacuum bag, the polyethylene (PET) for the breather, the polypropylene (PP) for the release film and a synthetic rubber for the sealant tape. Table 1 summarizes the moth inventory data.

<table><caption>Table 1 LCI input data of moth structure.</caption><thead><tr><th rowspan="2">Input</th><th rowspan="2"></th><th colspan="2">Quantity</th><th rowspan="2">Unit</th></tr><tr><th>Scenario 1</th><th>Scenario 2</th></tr></thead><tbody><tr><td rowspan="2">Materials</td><td>Epoxy resin</td><td>4.8</td><td>2.68</td><td>kg</td></tr><tr><td>Carbon fibres</td><td>7.2</td><td>4.03</td><td>kg</td></tr><tr><td>Production</td><td>Prepregging</td><td>-</td><td>40.0</td><td>MJ/kg</td></tr><tr><td rowspan="5">Curing process</td><td>Vacuum bag (PA66)</td><td>-</td><td>3.0</td><td>kg</td></tr><tr><td>Breather (PET)</td><td>-</td><td>1.2</td><td>kg</td></tr><tr><td>Realase film (PP)</td><td>-</td><td>0.4</td><td>kg</td></tr><tr><td>Sealant tape (Synthetic rubber)</td><td>-</td><td>0.945</td><td>kg</td></tr><tr><td>Energy</td><td>-</td><td>6.0</td><td>kWh</td></tr></tbody></table>

The same materials and processes described above w considered for the realization of the mold used to produce the moth structure. The inventory data related to the mold production are reported in Table 2. To obtain a consistent analysis it is necessary to divide these values by the number of moth structures produced with a mold during its service life, which is about 100 for the Scenario 1 and 200 for the Scenario 2. As previously mentioned, the production of the master to realize the mold of the moth is considered within the system boundaries of the analysis. Table 3 reports the
<figure><img src="image_2679.png" /><figcaption>Figure 2 System boundaries of analysed scenarios.</figcaption></figure>

In fact, prepreg offers a good control of resin content which cannot be guaranteed by impregnating the fibres in a resin bath. By considering a composite with 60wt% of fibres and 40wt% of matrix, 4.8 kg of epoxy resin and 7.2 kg of carbon fibres are considered in the Scenario 1, while 2.68 kg of epoxy resin and 4.03 kg of carbon fibres are analysed in the Scenario 2. Since the impregnation process included in the Scenario 1 is a manual process, no energy consumption is considered, while in the Scenario 2 an energy consumption of 40 MJ per kg of composite is included for the realization of prepreg [14]. To fabricate the moth structure a manual lamination process is used, so no consumptions are considered. An oven curing process is considered only in the Scenario 2, because the Scenario 1 includes a curing process at room temperature. The consumptions related to this process are the needed energy and the consumable materials as the polyamide (PA66) for the vacuum bag, the polyethylene (PET) for the breather, the polypropylene (PP) for the release film and a synthetic rubber for the sealant tape. Table 1 summarizes the moth inventory data.

<table><caption>Table 1 LCI input data of moth structure.</caption><thead><tr><th rowspan="2">Input</th><th rowspan="2"></th><th colspan="2">Quantity</th><th rowspan="2">Unit</th></tr><tr><th>Scenario 1</th><th>Scenario 2</th></tr></thead><tbody><tr><td rowspan="2">Materials</td><td>Epoxy resin</td><td>4.8</td><td>2.68</td><td>kg</td></tr><tr><td>Carbon fibres</td><td>7.2</td><td>4.03</td><td>kg</td></tr><tr><td>Production</td><td>Prepregging</td><td>-</td><td>40.0</td><td>MJ/kg</td></tr><tr><td rowspan="5">Curing process</td><td>Vacuum bag (PA66)</td><td>-</td><td>3.0</td><td>kg</td></tr><tr><td>Breather (PET)</td><td>-</td><td>1.2</td><td>kg</td></tr><tr><td>Realase film (PP)</td><td>-</td><td>0.4</td><td>kg</td></tr><tr><td>Sealant tape (Synthetic rubber)</td><td>-</td><td>0.945</td><td>kg</td></tr><tr><td>Energy</td><td>-</td><td>6.0</td><td>kWh</td></tr></tbody></table>

The same materials and processes described above w considered for the realization of the mold used to produce the moth structure. The inventory data related to the mold production are reported in Table 2. To obtain a consistent analysis it is necessary to divide these values by the number of moth structures produced with a mold during its service life, which is about 100 for the Scenario 1 and 200 for the Scenario 2. As previously mentioned, the production of the master to realize the mold of the moth is considered within the system boundaries of the analysis. Table 3 reports the
inventory data related to the master production. It includes the material used (polyurethane) and a milling process. As for the mold, to gain consistent results the values reported in Table 3 have to be divided by the number of moth structures produced with a master during its service life, which is about 1000 for Scenario 1 and about 2000 for Scenario 2. All reported data are directly measured except for the energy consumption related to the prepregging process which was retrieved by literature [14]. The quantity of composite material considered in the Scenario 2 is an output of the design phase: the corresponding weight value to the optimal stratification.

<table><caption>Table 2 LCI input data of mold.</caption><thead><tr><th colspan="2">Input</th><th>Quantity</th><th>Unit</th></tr><tr><th colspan="2"></th><th>Scenario 1</th><th>Scenario 2</th></tr></thead><tbody><tr><td rowspan="2">Materials</td><td>Epoxy resin</td><td>19.6</td><td>15.6 kg</td></tr><tr><td>Carbon fibres</td><td>29.4</td><td>23.4 kg</td></tr><tr><td rowspan="2">Production</td><td>Prepregging</td><td>-</td><td>40.0 MJ/kg</td></tr><tr><td>Vacuum bag</td><td>-</td><td>4.2 kg</td></tr><tr><td rowspan="4">Curing process</td><td>Breather</td><td>-</td><td>1.8 kg</td></tr><tr><td>Realase film</td><td>-</td><td>0.6 kg</td></tr><tr><td>Sealant tape</td><td>-</td><td>1.26 kg</td></tr><tr><td>Energy</td><td>-</td><td>5.0 kWh</td></tr></tbody></table>

<table><caption>Table 3 LCI input data of the master.</caption><thead><tr><th rowspan="2">Input</th><th colspan="2">Quantity</th><th rowspan="2">Unit</th></tr><tr><th>Scenario 1</th><th>Scenario 2</th></tr></thead><tbody><tr><td>Polyurethane</td><td>501.8</td><td></td><td>kg</td></tr><tr><td>Polyurethane removed by milling</td><td>144.8</td><td></td><td>kg</td></tr></tbody></table>

## Life Cycle Impact Assessment

The LCIA phase is important to choose the impact categories to allow the achievement of consistent, relevant, and reliable results. In fact, by considering different environmental indicators and analysis methods, a comprehensive view of the environmental effects of the two analysed scenarios can be obtained. This study considered two different impact assessment methodologies: Global Warming Potential (GWP) and ReCiPe. To simplify the translation of inventory data to the chosen impact categories, the LCA SimaPro software was used.

## Results and discussion

The design phase led to the identification of the optimal composite layering of the moth structure which consists in 6 total layers: 4 unidirectional fabrics with CF oriented toward the length of the hull (0°) and 2 plain fabrics with a CF orientation of 45° with respect to the hull length. This layering resulted in a moth structure of about 1 mm thickness and 6.71 kg weight. This value is the input data related to the composite material considered in the Scenario 2. Figure 3 compares the LCA results of all phases included within the system boundaries of two analyzed scenarios, by considering GWP method.
inventory data related to the master production. It includes the material used (polyurethane) and a milling process. As for the mold, to gain consistent results the values reported in Table 3 have to be divided by the number of moth structures produced with a master during its service life, which is about 1000 for Scenario 1 and about 2000 for Scenario 2. All reported data are directly measured except for the energy consumption related to the prepregging process which was retrieved by literature [14]. The quantity of composite material considered in the Scenario 2 is an output of the design phase: the corresponding weight value to the optimal stratification.

<table><caption>Table 2 LCI input data of mold.</caption><thead><tr><th colspan="2">Input</th><th>Quantity</th><th>Unit</th></tr><tr><th colspan="2"></th><th>Scenario 1</th><th>Scenario 2</th></tr></thead><tbody><tr><td rowspan="2">Materials</td><td>Epoxy resin</td><td>19.6</td><td>15.6 kg</td></tr><tr><td>Carbon fibres</td><td>29.4</td><td>23.4 kg</td></tr><tr><td rowspan="2">Production</td><td>Prepregging</td><td>-</td><td>40.0 MJ/kg</td></tr><tr><td>Vacuum bag</td><td>-</td><td>4.2 kg</td></tr><tr><td rowspan="4">Curing process</td><td>Breather</td><td>-</td><td>1.8 kg</td></tr><tr><td>Realase film</td><td>-</td><td>0.6 kg</td></tr><tr><td>Sealant tape</td><td>-</td><td>1.26 kg</td></tr><tr><td>Energy</td><td>-</td><td>5.0 kWh</td></tr></tbody></table>

<table><caption>Table 3 LCI input data of the master.</caption><thead><tr><th rowspan="2">Input</th><th colspan="2">Quantity</th><th rowspan="2">Unit</th></tr><tr><th>Scenario 1</th><th>Scenario 2</th></tr></thead><tbody><tr><td>Polyurethane</td><td>501.8</td><td></td><td>kg</td></tr><tr><td>Polyurethane removed by milling</td><td>144.8</td><td></td><td>kg</td></tr></tbody></table>

## Life Cycle Impact Assessment

The LCIA phase is important to choose the impact categories to allow the achievement of consistent, relevant, and reliable results. In fact, by considering different environmental indicators and analysis methods, a comprehensive view of the environmental effects of the two analysed scenarios can be obtained. This study considered two different impact assessment methodologies: Global Warming Potential (GWP) and ReCiPe. To simplify the translation of inventory data to the chosen impact categories, the LCA SimaPro software was used.

## Results and discussion

The design phase led to the identification of the optimal composite layering of the moth structure which consists in 6 total layers: 4 unidirectional fabrics with CF oriented toward the length of the hull (0°) and 2 plain fabrics with a CF orientation of 45° with respect to the hull length. This layering resulted in a moth structure of about 1 mm thickness and 6.71 kg weight. This value is the input data related to the composite material considered in the Scenario 2. Figure 3 compares the LCA results of all phases included within the system boundaries of two analyzed scenarios, by considering GWP method.
<figure><img src="image_2685.png" /><figcaption>Figure 3 Comparison between LCA results of Scenario 1 and Scenario 2, GWP method.</figcaption></figure>

It emerged that the manual impregnation of carbon fibres considered in the Scenario 1 represents a more impactful solution than the use of prepreg of the Scenario 2 (335.75 kg CO₂ eq vs 228.22 kg CO₂ eq). Such result is consistent to the results obtained by Forcellese et al. in [15]. As highlighted in Figure 3, this result is mainly due to the materials phase, which gives the highest contribution to the total impacts: about 308.66 kg CO₂ eq for Scenario 1 and 178.66 kg CO₂ eq for Scenario 2. All other phases provide a low environmental load: for the Scenario 2 the second most impactful phase is the production of the prepreg, and this is related to the energy consumption of prepregging and curing processes. Anyway, this value is an order of magnitude lower than the value associated with the materials phase. Moreover, it can be observed that the master production and the mold materials and production give negligible contributions to the total impacts, although the high input values. This result is related to the allocation of the generated impacts to the service life of master and mold respectively. The EoL phase irrelevantly contributes to the total impacts, but it provides negative effects due to the landfill disposal. Different EoL strategies (e.g. mechanical, chemical and thermal recycling) and the recovery of secondary raw materials could enhance the scenarios sustainability and reduce the contribution of raw materials. Hence, a scenarios impact reduction is expected as high efficiency recycling technologies are developed and will become the preferred industrial solution for composite disposal. Since the phase of materials is the most impactful one, it is specifically analysed to identify which material is the most responsible of this high impact value. Figure 4 shows the environmental impacts related to the input materials considered and highlights that the high impact value related to the materials phase is mainly due to the use of carbon fibres. The contribution of epoxy resins and the consumable materials of curing process are negligible. The difference between the environmental impacts provided by carbon fibres in the Scenario 1 and 2 arises from a different input quantity of carbon fibres. The same reason explains the lower environmental impact of epoxy resin in Scenario 2 than in the Scenario 1. As a matter of fact, manual impregnation (Scenario 1) requires a higher quantity of material than the prepreg (Scenario 2) to guarantee the same mechanical properties.

<figure><img src="image_2686.png" /><figcaption>Figure 4 Comparison between LCA results of materials phase of two scenarios, GWP method.</figcaption></figure>

Considering the high impacts contribution of raw material, it is clear that an optimal design phase and a minimization of material use are crucial to reduce the overall environmental impacts
<figure><img src="image_2680.png" /><figcaption>Figure 3 Comparison between LCA results of Scenario 1 and Scenario 2, GWP method.</figcaption></figure>

It emerged that the manual impregnation of carbon fibres considered in the Scenario 1 represents a more impactful solution than the use of prepreg of the Scenario 2 (335.75 kg CO₂ eq vs 228.22 kg CO₂ eq). Such result is consistent to the results obtained by Forcellese et al. in [15]. As highlighted in Figure 3, this result is mainly due to the materials phase, which gives the highest contribution to the total impacts: about 308.66 kg CO₂ eq for Scenario 1 and 178.66 kg CO₂ eq for Scenario 2. All other phases provide a low environmental load: for the Scenario 2 the second most impactful phase is the production of the prepreg, and this is related to the energy consumption of prepregging and curing processes. Anyway, this value is an order of magnitude lower than the value associated with the materials phase. Moreover, it can be observed that the master production and the mold materials and production give negligible contributions to the total impacts, although the high input values. This result is related to the allocation of the generated impacts to the service life of master and mold respectively. The EoL phase irrelevantly contributes to the total impacts, but it provides negative effects due to the landfill disposal. Different EoL strategies (e.g. mechanical, chemical and thermal recycling) and the recovery of secondary raw materials could enhance the scenarios sustainability and reduce the contribution of raw materials. Hence, a scenarios impact reduction is expected as high efficiency recycling technologies are developed and will become the preferred industrial solution for composite disposal. Since the phase of materials is the most impactful one, it is specifically analysed to identify which material is the most responsible of this high impact value. Figure 4 shows the environmental impacts related to the input materials considered and highlights that the high impact value related to the materials phase is mainly due to the use of carbon fibres. The contribution of epoxy resins and the consumable materials of curing process are negligible. The difference between the environmental impacts provided by carbon fibres in the Scenario 1 and 2 arises from a different input quantity of carbon fibres. The same reason explains the lower environmental impact of epoxy resin in Scenario 2 than in the Scenario 1. As a matter of fact, manual impregnation (Scenario 1) requires a higher quantity of material than the prepreg (Scenario 2) to guarantee the same mechanical properties.

<figure><img src="image_2681.png" /><figcaption>Figure 4 Comparison between LCA results of materials phase of two scenarios, GWP method.</figcaption></figure>

Considering the high impacts contribution of raw material, it is clear that an optimal design phase and a minimization of material use are crucial to reduce the overall environmental impacts
of the composite structures. The LCA results were also analysed by using ReCiPe method (Figure 5). It can be observed that the results are in line with the ones analysed with GWP method: the most impactful phase is the materials for both Scenario 1 and Scenario 2 in all considered impact categories.

<figure><img src="image_2687.png" /><figcaption>Figure 5 Comparison between LCA results of Scenario 1 (a) and Scenario 2 (b), ReCiPe method.</figcaption></figure>

The other phases included in the Scenario 1, so the master, the mold materials and the end-of-life give a negligible contribution in percentage to the total impacts, as shown in Figure 5a. An exception can be made for the “Freshwaster eutrophication” impact category, in which the contribution of the EoL is consistent, comparable to the one related to materials. This result is probably due to the landfill disposal which provides negative effects, especially for this impact category. Moreover, it can be observed a consistent percentage contribution of the master for the “Human carcinogenic toxicity” impact category, and this is related to the use of polyurethane. The same observations can be done for the Scenario 2: Figure 5 highlights that the trend of the results obtained by using ReCiPe methodology is the same for the Scenario 1 and 2. The only difference between the two scenarios is the presence of two additional phases in the Scenario 2: the production of the moth structure and the mold of the moth. Both phases give low percentage contributions to the total environmental impacts. The contributions of the production of the moth for the various impact categories are higher than the ones related to the production of the mold. A consistent percentage value of the production of the moth is observed for “Water consumption” impact category. This result is due to the use of the nylon for the vacuum bag used for the curing process.

Figure 6 shows the comparison between the resulting impacts related to the materials phase in the Scenario 1 (Figure 6a) and the Scenario 2 (Figure 6b). It can be noticed that the main percentage
of the composite structures. The LCA results were also analysed by using ReCiPe method (Figure 5). It can be observed that the results are in line with the ones analysed with GWP method: the most impactful phase is the materials for both Scenario 1 and Scenario 2 in all considered impact categories.

<figure><img src="image_2682.png" /><figcaption>Figure 5 Comparison between LCA results of Scenario 1 (a) and Scenario 2 (b), ReCiPe method.</figcaption></figure>

The other phases included in the Scenario 1, so the master, the mold materials and the end-of-life give a negligible contribution in percentage to the total impacts, as shown in Figure 5a. An exception can be made for the “Freshwaster eutrophication” impact category, in which the contribution of the EoL is consistent, comparable to the one related to materials. This result is probably due to the landfill disposal which provides negative effects, especially for this impact category. Moreover, it can be observed a consistent percentage contribution of the master for the “Human carcinogenic toxicity” impact category, and this is related to the use of polyurethane. The same observations can be done for the Scenario 2: Figure 5 highlights that the trend of the results obtained by using ReCiPe methodology is the same for the Scenario 1 and 2. The only difference between the two scenarios is the presence of two additional phases in the Scenario 2: the production of the moth structure and the mold of the moth. Both phases give low percentage contributions to the total environmental impacts. The contributions of the production of the moth for the various impact categories are higher than the ones related to the production of the mold. A consistent percentage value of the production of the moth is observed for “Water consumption” impact category. This result is due to the use of the nylon for the vacuum bag used for the curing process.

Figure 6 shows the comparison between the resulting impacts related to the materials phase in the Scenario 1 (Figure 6a) and the Scenario 2 (Figure 6b). It can be noticed that the main percentage
contribution to the total environmental load is provided by the carbon fibres for both scenarios and for all the considered impact categories. The difference between the two scenarios is the presence of the prepregging phase in the Scenario 2, which includes the needed energy consumption to realize the prepreg. However, from Figure 6b emerges that the percentage contribution associated with this phase is much lower than the one related to the carbon fibres for all the impact categories except for the “Ionizing radiation”: this result is due to the energy consumption.

<figure><img src="image_2688.png" /><figcaption>Figure 6 Comparison between LCA results of materials phase of Scenario 1 (a) and Scenario 2 (b), ReCiPe method.</figcaption></figure>

A specific analysis of the resulted environmental impacts provided by carbon fibres was conducted to identify what the high value is due to (Figure 7). A single graph is reported because the same trend was observed between two scenarios. It can be noticed that for almost all impact categories, the polyacrylonitrile (PAN) give the highest percentage contribution (nearly 100%) to the total impacts. In addition, it is observed that the percentage contribution of air emissions is higher than the one related to PAN for few impact categories. This means that for few environmental issues the air emissions give more significant negative effects than the PAN production, differently for all the others considered issue. Instead, the nitrogen and heat emissions
contribution to the total environmental load is provided by the carbon fibres for both scenarios and for all the considered impact categories. The difference between the two scenarios is the presence of the prepregging phase in the Scenario 2, which includes the needed energy consumption to realize the prepreg. However, from Figure 6b emerges that the percentage contribution associated with this phase is much lower than the one related to the carbon fibres for all the impact categories except for the “Ionizing radiation”: this result is due to the energy consumption.

<figure><img src="image_2683.png" /><figcaption>Figure 6 Comparison between LCA results of materials phase of Scenario 1 (a) and Scenario 2 (b), ReCiPe method.</figcaption></figure>

A specific analysis of the resulted environmental impacts provided by carbon fibres was conducted to identify what the high value is due to (Figure 7). A single graph is reported because the same trend was observed between two scenarios. It can be noticed that for almost all impact categories, the polyacrylonitrile (PAN) give the highest percentage contribution (nearly 100%) to the total impacts. In addition, it is observed that the percentage contribution of air emissions is higher than the one related to PAN for few impact categories. This means that for few environmental issues the air emissions give more significant negative effects than the PAN production, differently for all the others considered issue. Instead, the nitrogen and heat emissions
and the electricity consumption do not significantly contribute to the comprehensive environmental impacts.

<figure><img src="image_2689.png" /><figcaption>Figure 7 LCA results of carbon fibres, ReCiPe method.</figcaption></figure>

## Conclusions

This study presents the life cycle assessment of a moth structure focusing on the materials and production: two scenarios were investigated. The scenario 1 considers the production of a moth structure starting from a CFRP manually impregnated while the scenario 2 considers the production of a moth structure by using CFRP prepreg. The main results obtained are summarized as follows:

* As a general outcome, the use of prepreg (Scenario 2) represents a more sustainable solution than the use of CFRP manually impregnated (Scenario 1). Specifically, in terms of Global Warming Potential, the Scenario 1 provides 335,75 kg CO₂ eq, while the Scenario 2 shows impacts equal to 228,22 kg CO₂ eq. This result is mainly due to the lower quantity of carbon fibres considered in the second scenario than in the first one.

* The materials phase is the most impactful: it provides 308,66 kg CO₂ eq for Scenario 1 and 178,66 kg CO₂ eq for Scenario 2. The carbon fibres give the highest contribution to the total environmental impacts, and this is related to the production of the PAN.

* The results analyzed by GWP and ReCiPe methods have the same trend.

* No advantages are gained from the EoL phase as no composites recovery processes are considered.

Research activities are in progress to increase the sustainability of this structure, by consider bio-composites as raw material and different end-of-life scenarios. Combustion and mechanical recycling processes can be considered in the analysis to evaluate the associated potential reduction in the total environmental impacts. In this way, it will be possible to identify which process represents the more sustainable solution. Moreover, a cost analysis, through the Life Cycle Costing methodology, can be carried out to identify if the use of prepreg represents a more economically sustainable solution too.

## Acknowledgements

This research was supported by the project titled “NAUTILUS: Progetto di Ricerca e Sviluppo di Nuovi Processi ad Alta Efficienza per la Produzione di Strutture ad Elevate Performance per la
and the electricity consumption do not significantly contribute to the comprehensive environmental impacts.

<figure><img src="image_2684.png" /><figcaption>Figure 7 LCA results of carbon fibres, ReCiPe method.</figcaption></figure>

## Conclusions

This study presents the life cycle assessment of a moth structure focusing on the materials and production: two scenarios were investigated. The scenario 1 considers the production of a moth structure starting from a CFRP manually impregnated while the scenario 2 considers the production of a moth structure by using CFRP prepreg. The main results obtained are summarized as follows:

* As a general outcome, the use of prepreg (Scenario 2) represents a more sustainable solution than the use of CFRP manually impregnated (Scenario 1). Specifically, in terms of Global Warming Potential, the Scenario 1 provides 335,75 kg CO₂ eq, while the Scenario 2 shows impacts equal to 228,22 kg CO₂ eq. This result is mainly due to the lower quantity of carbon fibres considered in the second scenario than in the first one.

* The materials phase is the most impactful: it provides 308,66 kg CO₂ eq for Scenario 1 and 178,66 kg CO₂ eq for Scenario 2. The carbon fibres give the highest contribution to the total environmental impacts, and this is related to the production of the PAN.

* The results analyzed by GWP and ReCiPe methods have the same trend.

* No advantages are gained from the EoL phase as no composites recovery processes are considered.

Research activities are in progress to increase the sustainability of this structure, by consider bio-composites as raw material and different end-of-life scenarios. Combustion and mechanical recycling processes can be considered in the analysis to evaluate the associated potential reduction in the total environmental impacts. In this way, it will be possible to identify which process represents the more sustainable solution. Moreover, a cost analysis, through the Life Cycle Costing methodology, can be carried out to identify if the use of prepreg represents a more economically sustainable solution too.

## Acknowledgements

This research was supported by the project titled “NAUTILUS: Progetto di Ricerca e Sviluppo di Nuovi Processi ad Alta Efficienza per la Produzione di Strutture ad Elevate Performance per la
Nautica", co-financed by the Italian Ministry of Economic Development as part of the Fund for Sustainable Growth - "Fabbrica intelligente" PON I&C 2014-2020 (project F/190047/01-03/X44).

## References

[1] M. Holmes, "Carbon composites continue to find new markets," *Reinforced Plastics*, vol. 61, no. 1, pp. 36-40, Jan. 2017. https://doi.org/10.1016/J.REPL.2016.12.060

[2] W. Jarrett, S. P. Jeffs, F. Korkees, and M. Rawson, "The opportunities and challenges of hybrid composite driveshafts and their couplings in the aerospace industry: A review," *Compos Struct*, vol. 320, p. 117203, Sep. 2023. https://doi.org/10.1016/J.COMPSTRUCT.2023.117203

[3] P. Singh et al., "Composite material: A review over current development and automotive application," *Mater Today Proc*, Nov. 2023. https://doi.org/10.1016/J.MATPR.2023.11.012

[4] F. Rubino, A. Nisticò, F. Tucci, and P. Carlone, "Marine Science and Engineering Marine Application of Fiber Reinforced Composites: A Review". https://doi.org/10.3390/jmse8010026

[5] J. A. Satterwhite et al., "Achieving process flexibility in next-generation carbon fiber prepreg material," in *International SAMPE Technical Conference*, 2017, pp. 2433 – 2446. [Online]. Available: https://www.scopus.com/inward/record.uri?eid=2-s2.0-85044635534&partnerID=40&md5=57781814f30f3f2f85b0db05ecf3b422

[6] A. Aslam Shaikh, A. Anil Pradhan, A. Mahesh Kotasthane, S. Patil, and S. Karuppanan, "Comparative analysis of Basalt/E-Glass/S2-Fibreglass-Carbon fiber reinforced epoxy laminates using finite element method," *Mater Today Proc*, vol. 63, pp. 630-638, Jan. 2022. https://doi.org/10.1016/J.MATPR.2022.04.385

[7] Z. Gürdal, R. T. Haftka, and P. Hajela, "Design and optimization of laminated composite materials," p. 337, Accessed: Dec. 21, 2023. [Online]. Available: https://www.wiley.com/en-us/Design+and+Optimization+of+Laminated+Composite+Materials-p-9780471252764

[8] S. Dong, C. Li, G. Xian, Z. Zhao, X. Zhang, and Q. Yun, "Environmental impact assessment of aircraft elevator made with new lightning protection material; [使用新型雷击防护材料的飞机升降舵环境影响评价]," *Journal of Zhejiang University: Science A*, vol. 23, no. 9, pp. 669 – 682, 2022. https://doi.org/10.1631/jzus.A2200105

[9] I. Bianchi, A. Forcellese, M. Marconi, M. Simoncini, A. Vita, and V. Castorani, "Environmental impact assessment of zero waste approach for carbon fiber prepreg scraps," *Sustainable Materials and Technologies*, vol. 29, p. e00308, Sep. 2021. https://doi.org/10.1016/J.SUSMAT.2021.E00308

[10] E. Peterson, "Technical Challenges to Adopting Large Scale Additive Manufacturing for the Production of Yacht Hulls," *Advances in Intelligent Systems and Computing*, vol. 1269, pp. 15-20, Jan. 2021. https://doi.org/10.1007/978-3-030-58282-1_3

[11] I. Bianchi, A. Forcellese, S. Pappadà, A. Salomi, and G. Zanzarelli, "Numerical analysis of flexural behaviour of nautical components in CFRP composite," *Procedia CIRP*, vol. 118, pp. 805-809, Jan. 2023. https://doi.org/10.1016/j.procir.2023.06.138

[12] S. Castegnaro et al., "A bio-composite racing sailboat: Materials selection, design, manufacturing and sailing," *Ocean Engineering*, vol. 133, pp. 142-150, Jan. 2017. https://doi.org/10.1016/j.oceaneng.2017.01.017

[13] G. Wernet, C. Bauer, B. Steubing, J. Reinhard, E. Moreno-Ruiz, and B. Weidema, "The ecoinvent database version 3 (part I): overview and methodology," *International Journal of Life Cycle Assessment*, vol. 21, no. 9, pp. 1218-1230, Sep. 2016. https://doi.org/10.1007/S11367-016-1087-8/FIGURES/7

[14] Y. S. Song, J. R. Youn, and T. G. Gutowski, "Life cycle energy analysis of fiber-reinforced composites," *Compos Part A Appl Sci Manuf*, vol. 40, no. 8, pp. 1257-1265, Aug. 2009. https://doi.org/10.1016/J.COMPOSITESA.2009.05.020

[15] A. Forcellese, M. Marconi, M. Simoncini, and A. Vita, "Life cycle impact assessment of different manufacturing technologies for automotive CFRP components," *J Clean Prod*, vol. 271, p. 122677, Oct. 2020. https://doi.org/10.1016/J.JCLEPRO.2020.122677
Nautica", co-financed by the Italian Ministry of Economic Development as part of the Fund for Sustainable Growth - "Fabbrica intelligente" PON I&C 2014-2020 (project F/190047/01-03/X44).

## References

[1] M. Holmes, "Carbon composites continue to find new markets," *Reinforced Plastics*, vol. 61, no. 1, pp. 36-40, Jan. 2017. https://doi.org/10.1016/J.REPL.2016.12.060

[2] W. Jarrett, S. P. Jeffs, F. Korkees, and M. Rawson, "The opportunities and challenges of hybrid composite driveshafts and their couplings in the aerospace industry: A review," *Compos Struct*, vol. 320, p. 117203, Sep. 2023. https://doi.org/10.1016/J.COMPSTRUCT.2023.117203

[3] P. Singh et al., "Composite material: A review over current development and automotive application," *Mater Today Proc*, Nov. 2023. https://doi.org/10.1016/J.MATPR.2023.11.012

[4] F. Rubino, A. Nisticò, F. Tucci, and P. Carlone, "Marine Science and Engineering Marine Application of Fiber Reinforced Composites: A Review". https://doi.org/10.3390/jmse8010026

[5] J. A. Satterwhite et al., "Achieving process flexibility in next-generation carbon fiber prepreg material," in *International SAMPE Technical Conference*, 2017, pp. 2433 – 2446. [Online]. Available: https://www.scopus.com/inward/record.uri?eid=2-s2.0-85044635534&partnerID=40&md5=57781814f30f3f2f85b0db05ecf3b422

[6] A. Aslam Shaikh, A. Anil Pradhan, A. Mahesh Kotasthane, S. Patil, and S. Karuppanan, "Comparative analysis of Basalt/E-Glass/S2-Fibreglass-Carbon fiber reinforced epoxy laminates using finite element method," *Mater Today Proc*, vol. 63, pp. 630-638, Jan. 2022. https://doi.org/10.1016/J.MATPR.2022.04.385

[7] Z. Gürdal, R. T. Haftka, and P. Hajela, "Design and optimization of laminated composite materials," p. 337, Accessed: Dec. 21, 2023. [Online]. Available: https://www.wiley.com/en-us/Design+and+Optimization+of+Laminated+Composite+Materials-p-9780471252764

[8] S. Dong, C. Li, G. Xian, Z. Zhao, X. Zhang, and Q. Yun, "Environmental impact assessment of aircraft elevator made with new lightning protection material; [使用新型雷击防护材料的飞机升降舵环境影响评价]," *Journal of Zhejiang University: Science A*, vol. 23, no. 9, pp. 669 – 682, 2022. https://doi.org/10.1631/jzus.A2200105

[9] I. Bianchi, A. Forcellese, M. Marconi, M. Simoncini, A. Vita, and V. Castorani, "Environmental impact assessment of zero waste approach for carbon fiber prepreg scraps," *Sustainable Materials and Technologies*, vol. 29, p. e00308, Sep. 2021. https://doi.org/10.1016/J.SUSMAT.2021.E00308

[10] E. Peterson, "Technical Challenges to Adopting Large Scale Additive Manufacturing for the Production of Yacht Hulls," *Advances in Intelligent Systems and Computing*, vol. 1269, pp. 15-20, Jan. 2021. https://doi.org/10.1007/978-3-030-58282-1_3

[11] I. Bianchi, A. Forcellese, S. Pappadà, A. Salomi, and G. Zanzarelli, "Numerical analysis of flexural behaviour of nautical components in CFRP composite," *Procedia CIRP*, vol. 118, pp. 805-809, Jan. 2023. https://doi.org/10.1016/j.procir.2023.06.138

[12] S. Castegnaro et al., "A bio-composite racing sailboat: Materials selection, design, manufacturing and sailing," *Ocean Engineering*, vol. 133, pp. 142-150, Jan. 2017. https://doi.org/10.1016/j.oceaneng.2017.01.017

[13] G. Wernet, C. Bauer, B. Steubing, J. Reinhard, E. Moreno-Ruiz, and B. Weidema, "The ecoinvent database version 3 (part I): overview and methodology," *International Journal of Life Cycle Assessment*, vol. 21, no. 9, pp. 1218-1230, Sep. 2016. https://doi.org/10.1007/S11367-016-1087-8/FIGURES/7

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# Life cycle assessment (LCA) of a novel solid-state recycling process for aluminum alloy AA6063 chips via direct hot rolling

CARTA Mauro<sup>1,a*</sup>, BEN KHALIFA Noomane<sup>2,3,b</sup>, BUONADONNA Pasquale<sup>1,c</sup>, MELE Alessandro<sup>1,d</sup> and EL MEHTEDI Mohamad<sup>1,e</sup>

<sup>1</sup>DIMCM-University of Cagliari, Via Marengo 2, 09123 Cagliari, Italy

<sup>2</sup>Institute for Production Technology and Systems, Leuphana Universität Lüneburg, Germany

<sup>3</sup>Institute of Material and Process Design, Helmholtz-Zentrum Hereon, Germany

<sup>a</sup>mauro.carta94@unica.it, <sup>b</sup>noomane.ben_khalifa@hereon.de, <sup>c</sup>buonadon@unica.it, <sup>d</sup>alessandro.mele1@outlook.it, <sup>e</sup>m.elmehtedi@unica.it

**Keywords:** LCA, Direct Hot Rolling, Aluminum, Solid State Recycling, AA6063, Chips

**Abstract.** Aluminum utilization has been growing in the last decades thanks to its high strength to weight ratio, corrosion resistance, excellent plasticity, and good weldability. Recently, unconventional processes have been developed for aluminum recycling, called Solid-State Recycling (SSR), by avoiding melting process, allow to save up to 85% of energy needed for secondary aluminum production. The aim of this study is to assess the environmental advantages of a novel recycling technique namely Direct Hot Rolling process. This innovative solid state recycling process will be evaluated for its eco-friendliness. The Life Cycle Assessment (LCA) analysis follows a gate-to-gate approach: collected chips coming from prior cutting process are cold pressed and subsequently hot and cold rolled to obtain the functional unit, defined as the sheet produced through Direct Rolling of 35g of AA6063 chips. Primary data come from direct measurements provided by an industry plant operating in aluminum production field and direct measurements taken during the experimental activity in laboratory. Secondary data were provided by studies available in literature and SimaPro® databases. This LCA comparison between two similar different routes for SSR through rolling suggests that the second one, Direct Hot Rolling + ARB technique, is more suitable to reduce the environmental impact of aluminum recycle industry. However, Accumulative Roll Bonding is considered a low productivity process for worldwide industries, considering the difficulty of automation and repeatability of the procedure.

**Introduction**

Aluminum has gained popularity due to its outstanding properties such as high strength-to-weight ratio, corrosion resistance, and formability and will reach about 180 million metric tons production in 2050 worldwide [1].

A significant innovation in this field is the development of Solid-State Recycling (SSR) methods to recycle aluminum. SSR omits the conventional melting step, potentially saving up to 85% of the energy typically used in recycling aluminum [2]. These methods involve common steps like cleaning, drying, compaction, and hot or cold plastic deformation, with the final stage varying based on the selected method. The last stage consisting of intense plastic deformation could be processes like direct extrusion[3,4], friction stir extrusion [5-7], friction stir consolidation [8,9], high pressure torsion [10], equal channel angular pressing [11,12] or accumulative roll bonding [13].

Solid-State Recycling is particularly effective in recycling aluminum chips, which are commonly produced in machining industries and traditionally recycled through melting, a process prone to high metal loss due to oxidation that occur during the melting step. SSR significantly reduces this loss by eliminating the need for melting [14].

<figure><img src="image_2690.png" /></figure>
# Life cycle assessment (LCA) of a novel solid-state recycling process for aluminum alloy AA6063 chips via direct hot rolling

CARTA Mauro<sup>1,a*</sup>, BEN KHALIFA Noomane<sup>2,3,b</sup>, BUONADONNA Pasquale<sup>1,c</sup>, MELE Alessandro<sup>1,d</sup> and EL MEHTEDI Mohamad<sup>1,e</sup>

**Keywords:** LCA, Direct Hot Rolling, Aluminum, Solid State Recycling, AA6063, Chips

**Abstract.** Aluminum utilization has been growing in the last decades thanks to its high strength to weight ratio, corrosion resistance, excellent plasticity, and good weldability. Recently, unconventional processes have been developed for aluminum recycling, called Solid-State Recycling (SSR), by avoiding melting process, allow to save up to 85% of energy needed for secondary aluminum production. The aim of this study is to assess the environmental advantages of a novel recycling technique namely Direct Hot Rolling process. This innovative solid state recycling process will be evaluated for its eco-friendliness. The Life Cycle Assessment (LCA) analysis follows a gate-to-gate approach: collected chips coming from prior cutting process are cold pressed and subsequently hot and cold rolled to obtain the functional unit, defined as the sheet produced through Direct Rolling of 35g of AA6063 chips. Primary data come from direct measurements provided by an industry plant operating in aluminum production field and direct measurements taken during the experimental activity in laboratory. Secondary data were provided by studies available in literature and SimaPro® databases. This LCA comparison between two similar different routes for SSR through rolling suggests that the second one, Direct Hot Rolling + ARB technique, is more suitable to reduce the environmental impact of aluminum recycle industry. However, Accumulative Roll Bonding is considered a low productivity process for worldwide industries, considering the difficulty of automation and repeatability of the procedure.

**Introduction**

Aluminum has gained popularity due to its outstanding properties such as high strength-to-weight ratio, corrosion resistance, and formability and will reach about 180 million metric tons production in 2050 worldwide [1].

A significant innovation in this field is the development of Solid-State Recycling (SSR) methods to recycle aluminum. SSR omits the conventional melting step, potentially saving up to 85% of the energy typically used in recycling aluminum [2]. These methods involve common steps like cleaning, drying, compaction, and hot or cold plastic deformation, with the final stage varying based on the selected method. The last stage consisting of intense plastic deformation could be processes like direct extrusion[3,4], friction stir extrusion [5-7], friction stir consolidation [8,9], high pressure torsion [10], equal channel angular pressing [11,12] or accumulative roll bonding [13].

Solid-State Recycling is particularly effective in recycling aluminum chips, which are commonly produced in machining industries and traditionally recycled through melting, a process prone to high metal loss due to oxidation that occur during the melting step. SSR significantly reduces this loss by eliminating the need for melting [14].

<figure><img src="image_2685.png" /></figure>

<footnote n="1"><sup>1</sup>DIMCM-University of Cagliari, Via Marengo 2, 09123 Cagliari, Italy</footnote>

<footnote n="2"><sup>2</sup>Institute for Production Technology and Systems, Leuphana Universität Lüneburg, Germany</footnote>

<footnote n="3"><sup>3</sup>Institute of Material and Process Design, Helmholtz-Zentrum Hereon, Germany</footnote>

<footnote><sup>a</sup>mauro.carta94@unica.it, <sup>b</sup>noomane.ben_khalifa@hereon.de, <sup>c</sup>buonadon@unica.it,</footnote>

<footnote><sup>d</sup>alessandro.mele1@outlook.it, <sup>e</sup>m.elmehtedi@unica.it</footnote>
Recent research has shown the effectiveness of various SSR techniques in recycling aluminum chips into functional products. Techniques like direct recycling extrusion and friction stir extrusion have successfully transformed chips into wires, tubes, and similar forms. The addition of reinforcing particles to chips has been shown to improve adhesion and mechanical properties after recycling [15]. Recycling aluminum is far more energy-efficient than producing it from raw materials, potentially saving up to 95% of energy and resulting in just 5% of the greenhouse gas emissions of primary production. Considering that primary aluminum production contributes about 1% of global greenhouse gas emissions [16], this reduction is significant. Furthermore, aluminum recycling conserves water and reduces waste, minimizing landfill usage.

There is, however, limited research on the environmental impact of SSR, particularly in the context of producing aluminum sheets [17,18]. This article introduces a novel approach, Direct Hot Rolling, for recycling aluminum chips into sheets. It evaluates the process's effectiveness in reducing waste and energy consumption and its impact on global warming potential and water usage. The paper also compares this technique to another studied by the authors, based on Accumulative Roll Bonding process [13], emphasizing its sustainability advantages. The paper is structured into sections, starting with a detailed description of the Direct Hot Rolling process, followed by a Life Cycle Assessment analysis, and concludes with research findings, contributing to sustainable aluminum recycling practices.

## Experimental methods: the Direct Hot rolling process

In this paper, a widely used AA6063 Al-Mg-Si aluminum alloy was considered. The alloy was produced by Fonderie Pandolfo (Italy), the chemical composition (wt%) is reported in Table 1. The as-received billet material is characterized by an Ultimate Tensile Strength (UTS) of 186 MPa and 22% elongation. The chips were produced milling process without the use of lubricants to avoid any contamination of the chips.

<table><caption>Table 1 - Chemical composition of the alloy (wt.%).</caption><thead><tr><th>Element</th><th>Si</th><th>Fe</th><th>Cu</th><th>Mn</th><th>Mg</th><th>Cr</th><th>Ni</th><th>Zn</th><th>Ti</th><th>Al</th></tr></thead><tbody><tr><td>wt. %</td><td>0.585</td><td>0.231</td><td>0.027</td><td>0.030</td><td>0.489</td><td>0.007</td><td>0.007</td><td>0.033</td><td>0.018</td><td>Bal.</td></tr></tbody></table>

Figure 1 presents a schematic overview of the direct rolling process. The initial step in recycling is cold compaction. Here, 35 grams of chips are compacted under a load of 200 kN using a hydraulic press, resulting in dimensions of 30x60x9.5 mm³. This achieves a density approximately 68% that of the as-cast material. Samples undergo a heat treatment (HT) for 6 hours at 570 °C in a Nabertherm N50 muffle furnace, followed by hot rolling (HR) at 490 °C with several consecutive passes, without the use of lubricants. These temperatures and the heat treatment duration are selected to mirror those used in industrial settings. The rolling machine employed in the experimental campaign is a BW200 two-high rolling mill, manufactured by Carl Wezel. The compacted chips are subjected to a rolling schedule comprising 7 hot rolling passes and a final cold rolling pass, resulting in a final sheet of 1 mm thick. To maintain the initial temperature of 490 °C, samples are returned to the furnace for 10 minutes after each rolling pass. The jagged borders of the samples, a result of rolling, must be trimmed. These trimmings, constituting approximately 10% of the mass, are scraps that need to be recycled again. This paper focuses on the environmental impact of the process, and therefore, will not discuss the properties of the recycled samples in detail. However, it is worth mentioning the most important tensile test mechanical data. When comparing recycled chips to the bulk material undergoing the same rolling schedule, the bulk material demonstrates an ultimate tensile strength of 218 MPa compared to 177 MPa for the recycled chips, with an elongation at break of 7.5 percent for the bulk material and 7 percent for the recycled chips.
Recent research has shown the effectiveness of various SSR techniques in recycling aluminum chips into functional products. Techniques like direct recycling extrusion and friction stir extrusion have successfully transformed chips into wires, tubes, and similar forms. The addition of reinforcing particles to chips has been shown to improve adhesion and mechanical properties after recycling [15]. Recycling aluminum is far more energy-efficient than producing it from raw materials, potentially saving up to 95% of energy and resulting in just 5% of the greenhouse gas emissions of primary production. Considering that primary aluminum production contributes about 1% of global greenhouse gas emissions [16], this reduction is significant. Furthermore, aluminum recycling conserves water and reduces waste, minimizing landfill usage.

There is, however, limited research on the environmental impact of SSR, particularly in the context of producing aluminum sheets [17,18]. This article introduces a novel approach, Direct Hot Rolling, for recycling aluminum chips into sheets. It evaluates the process's effectiveness in reducing waste and energy consumption and its impact on global warming potential and water usage. The paper also compares this technique to another studied by the authors, based on Accumulative Roll Bonding process [13], emphasizing its sustainability advantages. The paper is structured into sections, starting with a detailed description of the Direct Hot Rolling process, followed by a Life Cycle Assessment analysis, and concludes with research findings, contributing to sustainable aluminum recycling practices.

## Experimental methods: the Direct Hot rolling process

In this paper, a widely used AA6063 Al-Mg-Si aluminum alloy was considered. The alloy was produced by Fonderie Pandolfo (Italy), the chemical composition (wt%) is reported in Table 1. The as-received billet material is characterized by an Ultimate Tensile Strength (UTS) of 186 MPa and 22% elongation. The chips were produced milling process without the use of lubricants to avoid any contamination of the chips.

<table><caption>Table 1 - Chemical composition of the alloy (wt.%).</caption><thead><tr><th>Element</th><th>Si</th><th>Fe</th><th>Cu</th><th>Mn</th><th>Mg</th><th>Cr</th><th>Ni</th><th>Zn</th><th>Ti</th><th>Al</th></tr></thead><tbody><tr><td>wt. %</td><td>0.585</td><td>0.231</td><td>0.027</td><td>0.030</td><td>0.489</td><td>0.007</td><td>0.007</td><td>0.033</td><td>0.018</td><td>Bal.</td></tr></tbody></table>

Figure 1 presents a schematic overview of the direct rolling process. The initial step in recycling is cold compaction. Here, 35 grams of chips are compacted under a load of 200 kN using a hydraulic press, resulting in dimensions of 30x60x9.5 mm³. This achieves a density approximately 68% that of the as-cast material. Samples undergo a heat treatment (HT) for 6 hours at 570 °C in a Nabertherm N50 muffle furnace, followed by hot rolling (HR) at 490 °C with several consecutive passes, without the use of lubricants. These temperatures and the heat treatment duration are selected to mirror those used in industrial settings. The rolling machine employed in the experimental campaign is a BW200 two-high rolling mill, manufactured by Carl Wezel. The compacted chips are subjected to a rolling schedule comprising 7 hot rolling passes and a final cold rolling pass, resulting in a final sheet of 1 mm thick. To maintain the initial temperature of 490 °C, samples are returned to the furnace for 10 minutes after each rolling pass. The jagged borders of the samples, a result of rolling, must be trimmed. These trimmings, constituting approximately 10% of the mass, are scraps that need to be recycled again. This paper focuses on the environmental impact of the process, and therefore, will not discuss the properties of the recycled samples in detail. However, it is worth mentioning the most important tensile test mechanical data. When comparing recycled chips to the bulk material undergoing the same rolling schedule, the bulk material demonstrates an ultimate tensile strength of 218 MPa compared to 177 MPa for the recycled chips, with an elongation at break of 7.5 percent for the bulk material and 7 percent for the recycled chips.
<figure><img src="image_2691.png" /><figcaption>Figure 1 - Schematic representation of the Direct Rolling process.</figcaption></figure>

## Life Cycle Assessment (LCA) Analysis

In order to evaluate environmental benefits of the investigated Direct Rolling process, a LCA analysis was performed with SimaPro® software. Next paragraphs indicate all assumptions, methods and reference databases used in the analysis.

### Introduction to LCA

Life Cycle Assessment can be distinguished of four connected phases. First phase is the goal and scope definition: the aim of this analysis is to evaluate the environmental impact of the SSR process discussed in this study. The goal is to demonstrate the viability and suitability of this technique according to the exigence, from aluminum industry, of the reduction of wastes and energy consumptions, and the purpose of development for new ecological ways in aluminum scrap recycle. This analysis follows a gate-to-gate approach: collected chips coming from prior cutting process are cold pressed and subsequently hot and cold rolled to obtain the functional unit, defined as the sheet produced through Direct Rolling of 35 g AA6063 chips.

Second phase is Life Cycle Inventory (LCI) analysis: material and energy flows of the different unit processes are defined in this phase. Primary data come from direct measurements provided by an industry plant operating in aluminum production field and direct measurements taken during the experimental activity in laboratory. Secondary data were provided by studies available in literature and SimaPro® software databases. The first unit process is Cold pressing, and its input flows are aluminum chips and electricity to feed the press, and the output is the compacted sample. In this process, the manufacturing of the steel die for pressing was included. Therefore, further input is electricity for feed the milling machine, and steel scrap is also an output as solid residue of the process. The electricity usage was determined based on the findings of Priarone et al. [19]. The quantity of steel chips produced as solid waste from milling was estimated through geometric analysis. The energy needed for cold pressing was assessed by taking into account the pressure applied and the surface area of contact of the aluminum chips during the compaction process, according to Eq. 1.

$$E_{press} = pA\delta\frac{1}{\eta} \qquad (1)$$

where p is the applied pressure (40 MPa), A is the contact area, δ is the total thickness reduction during pressing and η the electro-mechanical efficiency (0.76 is assumed as a reasonable value).
<figure><img src="image_2686.png" /><figcaption>Figure 1 - Schematic representation of the Direct Rolling process.</figcaption></figure>

## Life Cycle Assessment (LCA) Analysis

In order to evaluate environmental benefits of the investigated Direct Rolling process, a LCA analysis was performed with SimaPro® software. Next paragraphs indicate all assumptions, methods and reference databases used in the analysis.

### Introduction to LCA

Life Cycle Assessment can be distinguished of four connected phases. First phase is the goal and scope definition: the aim of this analysis is to evaluate the environmental impact of the SSR process discussed in this study. The goal is to demonstrate the viability and suitability of this technique according to the exigence, from aluminum industry, of the reduction of wastes and energy consumptions, and the purpose of development for new ecological ways in aluminum scrap recycle. This analysis follows a gate-to-gate approach: collected chips coming from prior cutting process are cold pressed and subsequently hot and cold rolled to obtain the functional unit, defined as the sheet produced through Direct Rolling of 35 g AA6063 chips.

Second phase is Life Cycle Inventory (LCI) analysis: material and energy flows of the different unit processes are defined in this phase. Primary data come from direct measurements provided by an industry plant operating in aluminum production field and direct measurements taken during the experimental activity in laboratory. Secondary data were provided by studies available in literature and SimaPro® software databases. The first unit process is Cold pressing, and its input flows are aluminum chips and electricity to feed the press, and the output is the compacted sample. In this process, the manufacturing of the steel die for pressing was included. Therefore, further input is electricity for feed the milling machine, and steel scrap is also an output as solid residue of the process. The electricity usage was determined based on the findings of Priarone et al. [19]. The quantity of steel chips produced as solid waste from milling was estimated through geometric analysis. The energy needed for cold pressing was assessed by taking into account the pressure applied and the surface area of contact of the aluminum chips during the compaction process, according to Eq. 1.

$$E_{press} = pA\delta\frac{1}{\eta} \qquad (1)$$

where p is the applied pressure (40 MPa), A is the contact area, δ is the total thickness reduction during pressing and η the electro-mechanical efficiency (0.76 is assumed as a reasonable value).
The dataset for electricity in SimaPro® was Electricity, high voltage, IT, APOS S and for steel chips waste was Scrap steel {Europe without Switzerland}, market for scrap steel, APOS S.

The second unit process is Hot Rolling, which input is the compacted sample. Further input is electric energy required for rolling mill (provided by the referenced industrial plant). In addition, thermal energy is required to heat the samples for HT to 570°C for 6 hours and then (after cooling) to heat them again to the rolling temperature at 490°C, and is considered as the electricity required by the muffle furnace. Electric energy required for muffle furnace was calculated according to the industrial procedure. In the hot rolling process, water is used for cooling, as specified in reference [20]. The resulting product is a hot rolled sheet. Additionally, a metal loss of 10% was considered during this process to remove the jagged border, which is classified as solid waste in the output data. The reference dataset for electricity consumption is Electricity, high voltage, IT, APOS S, while for cooling water the dataset Water, cooling, unspecified natural origin, IT was selected.

The third process is Cold Rolling, performed in one pass to reduce the sheet thickness from 1.5 mm to 1.0 mm. Electric energy is required for rolling machine and it was calculated in the referenced industrial plant. Table 2 presents both primary and secondary data for the Life Cycle Assessment (LCA) of the Direct Rolling process.

<table><caption>Table 2 – Input and output flows for the three steps of investigated Direct Rolling process.</caption><thead><tr><th colspan="3">Cold pressing</th><th colspan="3">Hot Rolling</th><th colspan="3">Cold Rolling</th></tr><tr><th>INPUTS</th><th>Quantity</th><th>Unit</th><th>INPUTS</th><th>Quantity</th><th>Unit</th><th>INPUTS</th><th>Quantity</th><th>Unit</th></tr></thead><tbody><tr><td>Chips</td><td>35</td><td>g</td><td>Compacted chips</td><td>35</td><td>g</td><td>HR sheet</td><td>31.5</td><td>g</td></tr><tr><td>Electric energy (pressing)</td><td>0.0017</td><td>MJ</td><td>Electric energy (heating)</td><td>0.0631</td><td>MJ</td><td>Electric energy (rolling)</td><td>0.0091</td><td>MJ</td></tr><tr><td>Electric energy (milling)</td><td>0.0061</td><td>MJ</td><td>Electric energy (rolling)</td><td>0.0176</td><td>MJ</td><td></td><td></td><td></td></tr><tr><td></td><td></td><td></td><td>Water (cooling)</td><td>4.732·10<sup>-5</sup></td><td>m<sup>3</sup></td><td></td><td></td><td></td></tr><tr><th>OUTPUTS</th><th>Quantity</th><th>Unit</th><th>OUTPUTS</th><th>Quantity</th><th>Unit</th><th>OUTPUTS</th><th>Quantity</th><th>Unit</th></tr><tr><td>Compacted chips</td><td>35</td><td>g</td><td>HR sheet</td><td>31.5</td><td>g</td><td>CR1.0 sheet</td><td>31.5</td><td>g</td></tr><tr><td>Steel chips</td><td>405</td><td>g</td><td>Aluminum scrap</td><td>3.5</td><td>g</td><td></td><td></td><td></td></tr></tbody></table>

The third phase of LCA analysis is Life Cycle Impact Assessment (LCIA), and the fourth and last phase is Results analysis. These steps are described in next paragraph.

## LCIA and results

In LCIA analysis, the IMPACT 2002+ method was selected. Characterization analysis shows the impact of the three-unit processes on different impact categories, like Ozone layer depletion, Global warming potential, Terrestrial ecotoxicity, Mineral extraction and others (Figure 2). Hot Rolling process has the highest impact for the most of categories, due to high energy requirement of the muffle furnace for preheating the sample. Electric energy database is referred to the national country mix, and fossil sources like carbon and natural gas plants are considered with their impact on environment. Cold pressing has high impact in some categories like Respiratory organics and Respiratory inorganics, and the reason is the pollution of solid wastes produced during the steel die manufacturing. Cold Rolling has a minor impact on all categories.
The dataset for electricity in SimaPro® was Electricity, high voltage, IT, APOS S and for steel chips waste was Scrap steel {Europe without Switzerland}, market for scrap steel, APOS S.

The second unit process is Hot Rolling, which input is the compacted sample. Further input is electric energy required for rolling mill (provided by the referenced industrial plant). In addition, thermal energy is required to heat the samples for HT to 570°C for 6 hours and then (after cooling) to heat them again to the rolling temperature at 490°C, and is considered as the electricity required by the muffle furnace. Electric energy required for muffle furnace was calculated according to the industrial procedure. In the hot rolling process, water is used for cooling, as specified in reference [20]. The resulting product is a hot rolled sheet. Additionally, a metal loss of 10% was considered during this process to remove the jagged border, which is classified as solid waste in the output data. The reference dataset for electricity consumption is Electricity, high voltage, IT, APOS S, while for cooling water the dataset Water, cooling, unspecified natural origin, IT was selected.

The third process is Cold Rolling, performed in one pass to reduce the sheet thickness from 1.5 mm to 1.0 mm. Electric energy is required for rolling machine and it was calculated in the referenced industrial plant. Table 2 presents both primary and secondary data for the Life Cycle Assessment (LCA) of the Direct Rolling process.

<table><caption>Table 2 – Input and output flows for the three steps of investigated Direct Rolling process.</caption><thead><tr><th colspan="3">Cold pressing</th><th colspan="3">Hot Rolling</th><th colspan="3">Cold Rolling</th></tr><tr><th>INPUTS</th><th>Quantity</th><th>Unit</th><th>INPUTS</th><th>Quantity</th><th>Unit</th><th>INPUTS</th><th>Quantity</th><th>Unit</th></tr></thead><tbody><tr><td>Chips</td><td>35</td><td>g</td><td>Compacted chips</td><td>35</td><td>g</td><td>HR sheet</td><td>31.5</td><td>g</td></tr><tr><td>Electric energy (pressing)</td><td>0.0017</td><td>MJ</td><td>Electric energy (heating)</td><td>0.0631</td><td>MJ</td><td>Electric energy (rolling)</td><td>0.0091</td><td>MJ</td></tr><tr><td>Electric energy (milling)</td><td>0.0061</td><td>MJ</td><td>Electric energy (rolling)</td><td>0.0176</td><td>MJ</td><td></td><td></td><td></td></tr><tr><td></td><td></td><td></td><td>Water (cooling)</td><td>4.732·10<sup>-5</sup></td><td>m<sup>3</sup></td><td></td><td></td><td></td></tr><tr><th>OUTPUTS</th><th>Quantity</th><th>Unit</th><th>OUTPUTS</th><th>Quantity</th><th>Unit</th><th>OUTPUTS</th><th>Quantity</th><th>Unit</th></tr><tr><td>Compacted chips</td><td>35</td><td>g</td><td>HR sheet</td><td>31.5</td><td>g</td><td>CR1.0 sheet</td><td>31.5</td><td>g</td></tr><tr><td>Steel chips</td><td>405</td><td>g</td><td>Aluminum scrap</td><td>3.5</td><td>g</td><td></td><td></td><td></td></tr></tbody></table>

The third phase of LCA analysis is Life Cycle Impact Assessment (LCIA), and the fourth and last phase is Results analysis. These steps are described in next paragraph.

## LCIA and results

In LCIA analysis, the IMPACT 2002+ method was selected. Characterization analysis shows the impact of the three-unit processes on different impact categories, like Ozone layer depletion, Global warming potential, Terrestrial ecotoxicity, Mineral extraction and others (Figure 2). Hot Rolling process has the highest impact for the most of categories, due to high energy requirement of the muffle furnace for preheating the sample. Electric energy database is referred to the national country mix, and fossil sources like carbon and natural gas plants are considered with their impact on environment. Cold pressing has high impact in some categories like Respiratory organics and Respiratory inorganics, and the reason is the pollution of solid wastes produced during the steel die manufacturing. Cold Rolling has a minor impact on all categories.
<figure><img src="image_2692.png" /><figcaption>Figure 2 – Impact categories for the three-unit processes of Direct Rolling.</figcaption></figure>

Damage analysis allows to understand the impact of each unit process on Human health, Climate change, Resources and Ecosystem quality (Figure 3). For the four environmental damage categories, Hot Rolling has the highest impact as energetic requirement is higher than Cold pressing and Cold Rolling: land occupation, CO2 emissions and environmental pollution due to non-renewable sources are very important factors in this analysis. Cold pressing has high impact in Human Health category because of the pollution and the negative effects of steel scrap produced in the tooling phase.

<figure><img src="image_2693.png" /><figcaption>Figure 3 – Damage categories for the three-unit process of Direct Rolling.</figcaption></figure>

Once produced, the steel die could be used several times before it gets damaged. Thus, excluding the tooling phase from the impact analysis the results change. In this case, Cold pressing has the lowest impact in all categories and Hot Rolling, due to high energetic requirements for heat treatment and heat during rolling, has the highest impact (Figure 4).
<figure><img src="image_2687.png" /><figcaption>Figure 2 – Impact categories for the three-unit processes of Direct Rolling.</figcaption></figure>

Damage analysis allows to understand the impact of each unit process on Human health, Climate change, Resources and Ecosystem quality (Figure 3). For the four environmental damage categories, Hot Rolling has the highest impact as energetic requirement is higher than Cold pressing and Cold Rolling: land occupation, CO2 emissions and environmental pollution due to non-renewable sources are very important factors in this analysis. Cold pressing has high impact in Human Health category because of the pollution and the negative effects of steel scrap produced in the tooling phase.

<figure><img src="image_2688.png" /><figcaption>Figure 3 – Damage categories for the three-unit process of Direct Rolling.</figcaption></figure>

Once produced, the steel die could be used several times before it gets damaged. Thus, excluding the tooling phase from the impact analysis the results change. In this case, Cold pressing has the lowest impact in all categories and Hot Rolling, due to high energetic requirements for heat treatment and heat during rolling, has the highest impact (Figure 4).
<figure><img src="image_2694.png" /><figcaption>Figure 4 – Damage categories without the tooling phase.</figcaption></figure>

The dataset considered for electricity needed in Cold pressing, Hot Rolling and Cold Rolling, is referred to Italian electricity mix based on reports and data published in 2016 by IEA World Energy Statistics and Balances and available in the Ecoinvent database. Major energetic sources are natural gas and carbon fossil plants, therefore environmental impact of the Direct Rolling process is higher because of emissions and pollution related to the electric energy demand. A completer, and more interesting overview, was produced by replicating the same LCA analysis, this time considering the energetic requirement satisfied by national country mix for 50% and by photovoltaic energy for the other 50%, and another case with the 100% of electricity requirement satisfied by photovoltaic (PHV) energy. The reference dataset of photovoltaic energy procurement in SimaPro® was Electricity, low voltage {IT}, photovoltaic open ground installation.

<figure><img src="image_2695.png" /><figcaption>Figure 5 – LCA comparison with 50% and 100% of electric requirement satisfied by photovoltaic energy.</figcaption></figure>

This analysis is more compatible with a greener and eco-friendly scenario of modern industrial plants. Direct Rolling technique avoids the production of wastes and emissions typical of melting processes. The case with 50% of electric requirement provided by photovoltaic energy produces a better and more eco-friendly scenario, leading to an overall reduction of pollution and emissions of this SSR process (Figure 5). The case with 100% of electricity provided by PHV plant could be considered as the perfectly eco-sustainable scenario in the circular economy philosophy.
<figure><img src="image_2689.png" /><figcaption>Figure 4 – Damage categories without the tooling phase.</figcaption></figure>

The dataset considered for electricity needed in Cold pressing, Hot Rolling and Cold Rolling, is referred to Italian electricity mix based on reports and data published in 2016 by IEA World Energy Statistics and Balances and available in the Ecoinvent database. Major energetic sources are natural gas and carbon fossil plants, therefore environmental impact of the Direct Rolling process is higher because of emissions and pollution related to the electric energy demand. A completer, and more interesting overview, was produced by replicating the same LCA analysis, this time considering the energetic requirement satisfied by national country mix for 50% and by photovoltaic energy for the other 50%, and another case with the 100% of electricity requirement satisfied by photovoltaic (PHV) energy. The reference dataset of photovoltaic energy procurement in SimaPro® was Electricity, low voltage {IT}, photovoltaic open ground installation.

<figure><img src="image_2690.png" /><figcaption>Figure 5 – LCA comparison with 50% and 100% of electric requirement satisfied by photovoltaic energy.</figcaption></figure>

This analysis is more compatible with a greener and eco-friendly scenario of modern industrial plants. Direct Rolling technique avoids the production of wastes and emissions typical of melting processes. The case with 50% of electric requirement provided by photovoltaic energy produces a better and more eco-friendly scenario, leading to an overall reduction of pollution and emissions of this SSR process (Figure 5). The case with 100% of electricity provided by PHV plant could be considered as the perfectly eco-sustainable scenario in the circular economy philosophy.
## Comparison with Direct Hot Rolling + ARB technique

Another study published by the authors [13] investigated another SSR procedure, indicated as Direct Hot Rolling + ARB technique. The sample produced by cold pressing 35 g of AA6063 chips has been hot rolled at 550 °C to obtain 1.5 mm thick sheet and subsequently Accumulative Roll Bonding has been performed at 450 °C in two steps. During the ARB process, two sheets are cleaned and stacked together, then passed through a rolling mill, usually at elevated temperatures, with a reduction of 50% in thickness. After the first pass through the rolling mill, the bonded sheets, which have been reduced in thickness, are typically cut in half, and then stacked again. In this way, the rolling, cutting and stacking process can be repeated several times [21]. In this process, after Cold pressing the compacted chips underwent directly to Hot Rolling without any heat treatment. This procedure allowed to obtain a 1.5 mm thick sheet with high mechanical properties and a LCA comparison with melting recycle stated this novel SSR process could highly reduce environmental impact of aluminum recycle industry. A comparison between the Direct Hot Rolling + ARB procedure and the Direct Hot Rolling procedure described in this paper is reported in this paragraph.

Although the two methods differ, they are interesting to compare in a Life Cycle Assessment (LCA) for environmental impact. The first method, direct hot rolling, includes a single heat treatment (HT), followed by seven hot rolling steps (HR) passes and one step cold rolling (CR) pass. The second method combines direct hot rolling (without HT) with Accumulative Roll Bonding (ARB). It involves seven HR passes at 550 °C and then two ARB cycles at 450 °C. This comparison aims to determine which SSR route has the least environmental impact. The functional unit in both cases is the aluminum sheet produced by rolling 35 g of AA6063 compacted chips. Direct Hot Rolling + ARB input and output flows are reported in Table 3. Cold pressing step is common to both processes.

An overview of damage categories is reported in Figure 6. Direct Rolling has the highest impact in all categories, and the reason is the amount of heat necessary to both early heat treatment and heating during Hot Rolling. The higher the energetic demand, the higher the overall impact. Direct Hot Rolling + ARB procedure avoids the heat treatment step, and enhances the eco-sustainability of this SSR route, avoiding the surplus energetic demand for heat treating the compacted samples.

<table><caption>Table 3 – Direct Hot Rolling + ARB input and output flows.</caption><thead><tr><th colspan="3">Hot Rolling</th><th colspan="3">ARB</th></tr><tr><th>INPUTS</th><th>Quantity</th><th>Unit</th><th>INPUTS</th><th>Quantity</th><th>Unit</th></tr></thead><tbody><tr><td>Compacted chips</td><td>35</td><td>g</td><td>HR sheet</td><td>31.5</td><td>g</td></tr><tr><td>Electric energy (heating)</td><td>0.0315</td><td>MJ</td><td>Electric energy (heating)</td><td>0.0227</td><td>MJ</td></tr><tr><td>Electric energy (rolling)</td><td>0.0176</td><td>MJ</td><td>Electric energy (rolling)</td><td>0.0050</td><td>MJ</td></tr><tr><td>Water (cooling)</td><td>4.732·10<sup>-5</sup></td><td>m<sup>3</sup></td><td></td><td></td><td></td></tr><tr><th>OUTPUTS</th><th>Quantity</th><th>Unit</th><th>OUTPUTS</th><th>Quantity</th><th>Unit</th></tr><tr><td>HR sheet</td><td>31.5</td><td>g</td><td>ARB2 sheet</td><td>31.5</td><td>g</td></tr><tr><td>Aluminum scrap</td><td>3.5</td><td>g</td><td></td><td></td><td></td></tr></tbody></table>

This LCA comparison between two similar different routes for SSR through rolling suggests that the second one, Direct Hot Rolling + ARB technique, is more suitable to reduce the environmental impact of aluminum recycle industry. However, Accumulative Roll Bonding is considered a low productivity process for worldwide industries, considering the difficulty of automation and repeatability of the procedure.
## Comparison with Direct Hot Rolling + ARB technique

Another study published by the authors [13] investigated another SSR procedure, indicated as Direct Hot Rolling + ARB technique. The sample produced by cold pressing 35 g of AA6063 chips has been hot rolled at 550 °C to obtain 1.5 mm thick sheet and subsequently Accumulative Roll Bonding has been performed at 450 °C in two steps. During the ARB process, two sheets are cleaned and stacked together, then passed through a rolling mill, usually at elevated temperatures, with a reduction of 50% in thickness. After the first pass through the rolling mill, the bonded sheets, which have been reduced in thickness, are typically cut in half, and then stacked again. In this way, the rolling, cutting and stacking process can be repeated several times [21]. In this process, after Cold pressing the compacted chips underwent directly to Hot Rolling without any heat treatment. This procedure allowed to obtain a 1.5 mm thick sheet with high mechanical properties and a LCA comparison with melting recycle stated this novel SSR process could highly reduce environmental impact of aluminum recycle industry. A comparison between the Direct Hot Rolling + ARB procedure and the Direct Hot Rolling procedure described in this paper is reported in this paragraph.

Although the two methods differ, they are interesting to compare in a Life Cycle Assessment (LCA) for environmental impact. The first method, direct hot rolling, includes a single heat treatment (HT), followed by seven hot rolling steps (HR) passes and one step cold rolling (CR) pass. The second method combines direct hot rolling (without HT) with Accumulative Roll Bonding (ARB). It involves seven HR passes at 550 °C and then two ARB cycles at 450 °C. This comparison aims to determine which SSR route has the least environmental impact. The functional unit in both cases is the aluminum sheet produced by rolling 35 g of AA6063 compacted chips. Direct Hot Rolling + ARB input and output flows are reported in Table 3. Cold pressing step is common to both processes.

An overview of damage categories is reported in Figure 6. Direct Rolling has the highest impact in all categories, and the reason is the amount of heat necessary to both early heat treatment and heating during Hot Rolling. The higher the energetic demand, the higher the overall impact. Direct Hot Rolling + ARB procedure avoids the heat treatment step, and enhances the eco-sustainability of this SSR route, avoiding the surplus energetic demand for heat treating the compacted samples.

<table><caption>Table 3 – Direct Hot Rolling + ARB input and output flows.</caption><thead><tr><th colspan="3">Hot Rolling</th><th colspan="3">ARB</th></tr><tr><th>INPUTS</th><th>Quantity</th><th>Unit</th><th>INPUTS</th><th>Quantity</th><th>Unit</th></tr></thead><tbody><tr><td>Compacted chips</td><td>35</td><td>g</td><td>HR sheet</td><td>31.5</td><td>g</td></tr><tr><td>Electric energy (heating)</td><td>0.0315</td><td>MJ</td><td>Electric energy (heating)</td><td>0.0227</td><td>MJ</td></tr><tr><td>Electric energy (rolling)</td><td>0.0176</td><td>MJ</td><td>Electric energy (rolling)</td><td>0.0050</td><td>MJ</td></tr><tr><td>Water (cooling)</td><td>4.732·10<sup>-5</sup></td><td>m<sup>3</sup></td><td></td><td></td><td></td></tr><tr><th>OUTPUTS</th><th>Quantity</th><th>Unit</th><th>OUTPUTS</th><th>Quantity</th><th>Unit</th></tr><tr><td>HR sheet</td><td>31.5</td><td>g</td><td>ARB2 sheet</td><td>31.5</td><td>g</td></tr><tr><td>Aluminum scrap</td><td>3.5</td><td>g</td><td></td><td></td><td></td></tr></tbody></table>

This LCA comparison between two similar different routes for SSR through rolling suggests that the second one, Direct Hot Rolling + ARB technique, is more suitable to reduce the environmental impact of aluminum recycle industry. However, Accumulative Roll Bonding is considered a low productivity process for worldwide industries, considering the difficulty of automation and repeatability of the procedure.
<figure><img src="image_2696.png" /><figcaption>Figure 6 – Damage categories analysis for Direct Rolling and Direct Hot Rolling + ARB.</figcaption></figure>

The single-score analysis reveals that Resources and Climate change are the most concerned categories about the overall damage for both Direct Rolling and Direct Hot Rolling + ARB processes (Figure 7). The reason is the background of energy production, and the high presence of carbon sources and subsequent pollution leads to higher impact in these categories. Green and eco-friendly energetic sources, like photovoltaic and wind turbine plants, are indicated for a better scenario of the national country mix, reducing the emissions and the pollution. In this case, the benefits introduced by Direct Hot Rolling process are enhanced and the overall impact, compared to secondary aluminum industry based on melting process, is drastically reduced.

<figure><img src="image_2697.png" /><figcaption>Figure 7 – Single score analysis for Direct Rolling and Direct Hot Rolling + ARB.</figcaption></figure>

## Conclusions

This study aimed to assess the environmental advantages of the innovative Direct Hot Rolling process in solid-state recycling and to compare its efficacy with the previously researched Direct Rolling + ARB method. The Life Cycle Assessment (LCA) performed here adopts a gate-to-gate approach. This LCA comparison between two distinct SSR rolling methods indicates that the Direct Hot Rolling + ARB technique might be more effective in reducing the environmental impact in the aluminum recycling industry. However, it's noteworthy that Accumulative Roll Bonding is
<figure><img src="image_2691.png" /><figcaption>Figure 6 – Damage categories analysis for Direct Rolling and Direct Hot Rolling + ARB.</figcaption></figure>

The single-score analysis reveals that Resources and Climate change are the most concerned categories about the overall damage for both Direct Rolling and Direct Hot Rolling + ARB processes (Figure 7). The reason is the background of energy production, and the high presence of carbon sources and subsequent pollution leads to higher impact in these categories. Green and eco-friendly energetic sources, like photovoltaic and wind turbine plants, are indicated for a better scenario of the national country mix, reducing the emissions and the pollution. In this case, the benefits introduced by Direct Hot Rolling process are enhanced and the overall impact, compared to secondary aluminum industry based on melting process, is drastically reduced.

<figure><img src="image_2692.png" /><figcaption>Figure 7 – Single score analysis for Direct Rolling and Direct Hot Rolling + ARB.</figcaption></figure>

## Conclusions

This study aimed to assess the environmental advantages of the innovative Direct Hot Rolling process in solid-state recycling and to compare its efficacy with the previously researched Direct Rolling + ARB method. The Life Cycle Assessment (LCA) performed here adopts a gate-to-gate approach. This LCA comparison between two distinct SSR rolling methods indicates that the Direct Hot Rolling + ARB technique might be more effective in reducing the environmental impact in the aluminum recycling industry. However, it's noteworthy that Accumulative Roll Bonding is
seen as a lower productivity process for global industries due to challenges in automation and replicability. The findings indicate that both the Direct Hot Rolling and Direct Hot Rolling + ARB methods provide environmental advantages. However, the Direct Hot Rolling + ARB technique is slightly more beneficial in this regard. Despite this, it faces practical challenges for broad industrial use, making the simpler Direct Hot Rolling process a more viable option for potential industrial production.

## References

[1] D. Raabe, D. Ponge, P.J. Uggowitzer, M. Roscher, M. Paolantonio, C. Liu, H. Antrekowitsch, E. Kozeschnik, D. Seidmann, B. Gault, F. De Geuser, A. Deschamps, C. Hutchinson, C. Liu, Z. Li, P. Prangnell, J. Robson, P. Shanthraj, S. Vakili, C. Sinclair, L. Bourgeois, S. Pogatscher, Making sustainable aluminum by recycling scrap: The science of “dirty” alloys. Prog. Mater. Sci. 128 (2022) 100947. https://doi.org/10.1016/j.pmatsci.2022.100947

[2] S. Shamsudin, M. Lajis, Z. Zhong, Solid-state recycling of light metals: A review. Adv. Mech. Eng. 8 (2016) 168781401666192. https://doi.org/10.1177/1687814016661921

[3] A.E. Tekkaya, M. Schikorra, D. Becker, D. Biermann, N. Hammer, K. Pantke, Hot profile extrusion of AA-6060 aluminum chips. J. Mater. Process. Technol. 209 (2009) 3343-3350. https://doi.org/10.1016/j.jmatprotec.2008.07.047

[4] M. Haase, N. Ben Khalifa, A.E. Tekkaya, W.Z. Misiolek, Improving mechanical properties of chip-based aluminum extrudates by integrated extrusion and equal channel angular pressing (iECAP). Mater. Sci. Eng. A 539 (2012) 194-204. https://doi.org/10.1016/j.msea.2012.01.081

[5] M. El Mehtedi, A. Forcellese, M. Simoncini, S. Spigarelli, A sustainable solid state recycling of pure aluminum by means of friction stir extrusion process (FSE). (2018) 030004. https://doi.org/10.1063/1.5034847

[6] M. El Mehtedi, A. Forcellese, T. Mancia, M. Simoncini, S. Spigarelli, A new sustainable direct solid state recycling of AA1090 aluminum alloy chips by means of friction stir back extrusion process. Procedia CIRP 79 (2019) 638-643. https://doi.org/10.1016/j.procir.2019.02.062

[7] D. Baffari, A.P. Reynolds, X. Li, L. Fratini, Influence of processing parameters and initial temper on Friction Stir Extrusion of 2050 aluminum alloy. J. Manuf. Process. 28 (2017) 319-325. https://doi.org/10.1016/j.jmapro.2017.06.013

[8] D. Baffari, G. Buffa, G. Ingarao, A. Masnata, L. Fratini, Aluminium sheet metal scrap recycling through friction consolidation. Procedia Manuf. 29 (2019) 560-566. https://doi.org/10.1016/j.promfg.2019.02.134

[9] D. Baffari, A.P. Reynolds, X. Li, L. Fratini, Bonding prediction in friction stir consolidation of aluminum alloys: A preliminary study. (2018) 050002. https://doi.org/10.1063/1.5034875

[10] M.I. Abd El Aal, E. Yoo Yoon, H. Seop Kim, Recycling of AlSi8Cu3 alloy chips via high pressure torsion. Mater. Sci. Eng. A 560 (2013) 121-128. https://doi.org/10.1016/j.msea.2012.09.045

[11] J. Krolo, B. Lela, I. Dumanić, F. Kozina, Statistical Analysis of the Combined ECAP and Heat Treatment for Recycling Aluminum Chips Without Remelting. Metals 9 (2019) 660. https://doi.org/10.3390/met9060660

[12] M.A. Taha, A.T. Abbas, F. Benyahia, H.F. Alharbi, B. Guitián, X.R. Nóvoa, Enhanced Corrosion Resistance of Recycled Aluminum Alloy 6061 Chips Using Hot Extrusion Followed by ECAP. J. Chem. 2019 (2019) e3658507. https://doi.org/10.1155/2019/3658507
seen as a lower productivity process for global industries due to challenges in automation and replicability. The findings indicate that both the Direct Hot Rolling and Direct Hot Rolling + ARB methods provide environmental advantages. However, the Direct Hot Rolling + ARB technique is slightly more beneficial in this regard. Despite this, it faces practical challenges for broad industrial use, making the simpler Direct Hot Rolling process a more viable option for potential industrial production.

## References

[1] D. Raabe, D. Ponge, P.J. Uggowitzer, M. Roscher, M. Paolantonio, C. Liu, H. Antrekowitsch, E. Kozeschnik, D. Seidmann, B. Gault, F. De Geuser, A. Deschamps, C. Hutchinson, C. Liu, Z. Li, P. Prangnell, J. Robson, P. Shanthraj, S. Vakili, C. Sinclair, L. Bourgeois, S. Pogatscher, Making sustainable aluminum by recycling scrap: The science of “dirty” alloys. Prog. Mater. Sci. 128 (2022) 100947. https://doi.org/10.1016/j.pmatsci.2022.100947

[2] S. Shamsudin, M. Lajis, Z. Zhong, Solid-state recycling of light metals: A review. Adv. Mech. Eng. 8 (2016) 168781401666192. https://doi.org/10.1177/1687814016661921

[3] A.E. Tekkaya, M. Schikorra, D. Becker, D. Biermann, N. Hammer, K. Pantke, Hot profile extrusion of AA-6060 aluminum chips. J. Mater. Process. Technol. 209 (2009) 3343-3350. https://doi.org/10.1016/j.jmatprotec.2008.07.047

[4] M. Haase, N. Ben Khalifa, A.E. Tekkaya, W.Z. Misiolek, Improving mechanical properties of chip-based aluminum extrudates by integrated extrusion and equal channel angular pressing (iECAP). Mater. Sci. Eng. A 539 (2012) 194-204. https://doi.org/10.1016/j.msea.2012.01.081

[5] M. El Mehtedi, A. Forcellese, M. Simoncini, S. Spigarelli, A sustainable solid state recycling of pure aluminum by means of friction stir extrusion process (FSE). (2018) 030004. https://doi.org/10.1063/1.5034847

[6] M. El Mehtedi, A. Forcellese, T. Mancia, M. Simoncini, S. Spigarelli, A new sustainable direct solid state recycling of AA1090 aluminum alloy chips by means of friction stir back extrusion process. Procedia CIRP 79 (2019) 638-643. https://doi.org/10.1016/j.procir.2019.02.062

[7] D. Baffari, A.P. Reynolds, X. Li, L. Fratini, Influence of processing parameters and initial temper on Friction Stir Extrusion of 2050 aluminum alloy. J. Manuf. Process. 28 (2017) 319-325. https://doi.org/10.1016/j.jmapro.2017.06.013

[8] D. Baffari, G. Buffa, G. Ingarao, A. Masnata, L. Fratini, Aluminium sheet metal scrap recycling through friction consolidation. Procedia Manuf. 29 (2019) 560-566. https://doi.org/10.1016/j.promfg.2019.02.134

[9] D. Baffari, A.P. Reynolds, X. Li, L. Fratini, Bonding prediction in friction stir consolidation of aluminum alloys: A preliminary study. (2018) 050002. https://doi.org/10.1063/1.5034875

[10] M.I. Abd El Aal, E. Yoo Yoon, H. Seop Kim, Recycling of AlSi8Cu3 alloy chips via high pressure torsion. Mater. Sci. Eng. A 560 (2013) 121-128. https://doi.org/10.1016/j.msea.2012.09.045

[11] J. Krolo, B. Lela, I. Dumanić, F. Kozina, Statistical Analysis of the Combined ECAP and Heat Treatment for Recycling Aluminum Chips Without Remelting. Metals 9 (2019) 660. https://doi.org/10.3390/met9060660

[12] M.A. Taha, A.T. Abbas, F. Benyahia, H.F. Alharbi, B. Guitián, X.R. Nóvoa, Enhanced Corrosion Resistance of Recycled Aluminum Alloy 6061 Chips Using Hot Extrusion Followed by ECAP. J. Chem. 2019 (2019) e3658507. https://doi.org/10.1155/2019/3658507
[13] M. El Mehtedi, P. Buonadonna, M. Carta, R. El Mohtadi, A. Mele, D. Morea, Sustainability Study of a New Solid-State Aluminum Chips Recycling Process: A Life Cycle Assessment Approach. Sustainability 15 (2023) 11434. https://doi.org/10.3390/su151411434

[14] J. Gronostajski, H. Marciniak, A. Matuszak, New methods of aluminium and aluminium-alloy chips recycling. J. Mater. Process. Technol. 106 (2000) 34–39. https://doi.org/10.1016/S0924-0136(00)00634-8

[15] S. Al-Alimi, S. Shamsudin, N.K. Yusuf, M.A. Lajis, W. Zhou, D.H. Didane, S. Sadeq, Y. Saif, A. Wahib, Z. Harun, Recycling Aluminium AA6061 Chips with Reinforced Boron Carbide (B4C) and Zirconia (ZrO2) Particles via Hot Extrusion. Metals 12 (2022) 1329. https://doi.org/10.3390/met12081329

[16] A. Wagiman, M.S. Mustapa, R. Asmawi, S. Shamsudin, M.A. Lajis, Y. Mutoh, A review on direct hot extrusion technique in recycling of aluminium chips. Int. J. Adv. Manuf. Technol. 106 (2020) 641–653. https://doi.org/10.1007/s00170-019-04629-7

[17] D. Paraskevas, K. Kellens, Y. Deng, W. Dewulf, C. Kampen, J.R. Duflou, Solid state recycling of aluminium alloys via a porthole die hot extrusion process: Scaling up to production. AIP Conf. Proc. 1896 (2017) 140008. https://doi.org/10.1063/1.5008164

[18] N.K. Yusuf, M.A. Lajis, A. Ahmad, Life Cycle Assessment on the Direct Recycling Aluminium Alloy AA6061 Chips and Metal Matrix Composite (MMC-AlR). 13 (2021) 6.

[19] P.C. Priarone, G. Ingarao, L. Settineri, R. Di Lorenzo, On the Impact of Recycling Strategies on Energy Demand and CO2 Emissions When Manufacturing Al-based Components. Procedia CIRP 48 (2016) 194–199. https://doi.org/10.1016/j.procir.2016.03.044

[20] J.A.S. Green, Aluminum Recycling and Processing for Energy Conservation and Sustainability. (2007)

[21] M. El Mehtedi, D. Lai, R. Almehtedi, M. Carta, P. Buonadonna, F. Aymerich, Bonding of similar AA3105 aluminum alloy by Accumulative Roll Bonding process. ESAFORM (2021) https://doi.org/10.25518/esaform21.942
[13] M. El Mehtedi, P. Buonadonna, M. Carta, R. El Mohtadi, A. Mele, D. Morea, Sustainability Study of a New Solid-State Aluminum Chips Recycling Process: A Life Cycle Assessment Approach. Sustainability 15 (2023) 11434. https://doi.org/10.3390/su151411434

[14] J. Gronostajski, H. Marciniak, A. Matuszak, New methods of aluminium and aluminium-alloy chips recycling. J. Mater. Process. Technol. 106 (2000) 34-39. https://doi.org/10.1016/S0924-0136(00)00634-8

[15] S. Al-Alimi, S. Shamsudin, N.K. Yusuf, M.A. Lajis, W. Zhou, D.H. Didane, S. Sadeq, Y. Saif, A. Wahib, Z. Harun, Recycling Aluminium AA6061 Chips with Reinforced Boron Carbide (B4C) and Zirconia (ZrO2) Particles via Hot Extrusion. Metals 12 (2022) 1329. https://doi.org/10.3390/met12081329

[16] A. Wagiman, M.S. Mustapa, R. Asmawi, S. Shamsudin, M.A. Lajis, Y. Mutoh, A review on direct hot extrusion technique in recycling of aluminium chips. Int. J. Adv. Manuf. Technol. 106 (2020) 641-653. https://doi.org/10.1007/s00170-019-04629-7

[17] D. Paraskevas, K. Kellens, Y. Deng, W. Dewulf, C. Kampen, J.R. Duflou, Solid state recycling of aluminium alloys via a porthole die hot extrusion process: Scaling up to production. AIP Conf. Proc. 1896 (2017) 140008. https://doi.org/10.1063/1.5008164

[18] N.K. Yusuf, M.A. Lajis, A. Ahmad, Life Cycle Assessment on the Direct Recycling Aluminium Alloy AA6061 Chips and Metal Matrix Composite (MMC-AlR). 13 (2021) 6.

[19] P.C. Priarone, G. Ingarao, L. Settineri, R. Di Lorenzo, On the Impact of Recycling Strategies on Energy Demand and CO2 Emissions When Manufacturing Al-based Components. Procedia CIRP 48 (2016) 194-199. https://doi.org/10.1016/j.procir.2016.03.044

[20] J.A.S. Green, Aluminum Recycling and Processing for Energy Conservation and Sustainability. (2007)

[21] M. El Mehtedi, D. Lai, R. Almehtedi, M. Carta, P. Buonadonna, F. Aymerich, Bonding of similar AA3105 aluminum alloy by Accumulative Roll Bonding process. ESAFORM (2021) https://doi.org/10.25518/esaform21.942
# Monitoring of the friction stir welding process: A preliminary study

SILVESTRI Alessia Teresa¹,ᵃ*, COZZOLINO Ersilia¹,ᵇ, DE ALTERIIS Giorgio²,ᶜ, ASTARITA Antonello¹,ᵈ, SCHIANO LO MORIELLO Rosario²,ᵉ and SQUILLACE Antonino¹,ᵉ

¹Department of Chemical, Materials and Production Engineering, University of Naples Federico II, Naples, Italy

²Department of Industrial Engineering, University of Naples Federico II, Naples, Italy

ᵃalessiateresa.silvestri@unina.it, ᵇersilia.cozzolino@unina.it, ᶜgiorgio.dealteriis@unina.it,

ᵈantonello.astarita@unina.it, ᶜrschiano@unina.it, ᵇantonino.squillace@unina.it

**Keywords:** Friction Stir Welding, Monitoring, Aluminum Alloys

**Abstract.** Friction stir welding (FSW) has received great response in both academia and industry. Extensive research has been carried out to investigate the feasibility of the FSW process for different alloys and configurations. However, the growing demand for sustainable and cost-effective manufacturing, coupled with the pursuit of high-quality products, has led to the need for continuous monitoring of the manufacturing process and machine health. Therefore, the development of robust monitoring methods to assess the condition of the welding during the process becomes crucial for sustaining high production efficiency and ensuring quality standards. In this context, this study aims to provide methods for properly monitoring each phase characterizing the FSW process. Power consumption, vibrations, and temperatures were measured and evaluated in dissimilar friction stir lap welding of aluminum alloys.

## Introduction

Friction stir welding (FSW) has met great response in both academia and industry due to its compelling possibilities as a solid-state joining process. Patented in 1991, extensive research has been carried out to explore the feasibility of FSW for different alloys and configurations, experimentally and simulated [1–7]. It is important to highlight that the FSW is considered a more environmentally friendly, energy-efficient, versatile, and economically viable splicing technique in comparison with traditional joining techniques, and it is considered a green technology as it ensures zero gas or radiation emissions to the environment [8]. However, the growing demand for sustainable and cost-effective manufacturing, coupled with the pursuit of high-quality products, has led to the need for continuous monitoring of the manufacturing process and machine health. Monitoring a process involves extracting relevant information from data acquired through sensors. Monitoring begins with the identification of the target parameters or features for analyses, followed by the identification of signals that would be best suited to provide information about specific target parameters or features. Once the signal in question is acquired, a processing technique is chosen to be implemented on the signal to extract the information. These techniques would eventually be used to identify fluctuations occurring within a process and, thus, can help in the control of the above-mentioned fluctuations when required.

For a comprehensive understanding of the FSW process and to ensure high product quality, it is crucial to explore how the various types of sensors are used in other joining processes, such as in fusion welding and, also, in machining processes similar to FSW operation [9]. Traditional fusion welding, like arc welding, involves monitoring welding current, voltage, arc sound, and temperature. In laser welding, on the other hand, monitoring is performed in three stages: the first one is the pre-process, which is dedicated to monitoring the weld bead, then there is the

<figure><img src="image_2698.png" /></figure>
# Monitoring of the friction stir welding process: A preliminary study

SILVESTRI Alessia Teresa¹,*, COZZOLINO Ersilia¹,ᵇ, DE ALTERIIS Giorgio²,ᶜ, ASTARITA Antonello¹,ᵈ, SCHIANO LO MORIELLO Rosario²,ᵉ and SQUILLACE Antonino¹,*f

¹Department of Chemical, Materials and Production Engineering, University of Naples Federico II, Naples, Italy

²Department of Industrial Engineering, University of Naples Federico II, Naples, Italy

ᵃalessiateresa.silvestri@unina.it, ᵇersilia.cozzolino@unina.it, ᶜgiorgio.dealteriis@unina.it,

ᵈantonello.astarita@unina.it, ᶜrschiano@unina.it, ᵇantonino.squillace@unina.it

**Keywords:** Friction Stir Welding, Monitoring, Aluminum Alloys

**Abstract.** Friction stir welding (FSW) has received great response in both academia and industry. Extensive research has been carried out to investigate the feasibility of the FSW process for different alloys and configurations. However, the growing demand for sustainable and cost-effective manufacturing, coupled with the pursuit of high-quality products, has led to the need for continuous monitoring of the manufacturing process and machine health. Therefore, the development of robust monitoring methods to assess the condition of the welding during the process becomes crucial for sustaining high production efficiency and ensuring quality standards. In this context, this study aims to provide methods for properly monitoring each phase characterizing the FSW process. Power consumption, vibrations, and temperatures were measured and evaluated in dissimilar friction stir lap welding of aluminum alloys.

**Introduction**

Friction stir welding (FSW) has met great response in both academia and industry due to its compelling possibilities as a solid-state joining process. Patented in 1991, extensive research has been carried out to explore the feasibility of FSW for different alloys and configurations, experimentally and simulated [1–7]. It is important to highlight that the FSW is considered a more environmentally friendly, energy-efficient, versatile, and economically viable splicing technique in comparison with traditional joining techniques, and it is considered a green technology as it ensures zero gas or radiation emissions to the environment [8]. However, the growing demand for sustainable and cost-effective manufacturing, coupled with the pursuit of high-quality products, has led to the need for continuous monitoring of the manufacturing process and machine health. Monitoring a process involves extracting relevant information from data acquired through sensors. Monitoring begins with the identification of the target parameters or features for analyses, followed by the identification of signals that would be best suited to provide information about specific target parameters or features. Once the signal in question is acquired, a processing technique is chosen to be implemented on the signal to extract the information. These techniques would eventually be used to identify fluctuations occurring within a process and, thus, can help in the control of the above-mentioned fluctuations when required.

For a comprehensive understanding of the FSW process and to ensure high product quality, it is crucial to explore how the various types of sensors are used in other joining processes, such as in fusion welding and, also, in machining processes similar to FSW operation [9]. Traditional fusion welding, like arc welding, involves monitoring welding current, voltage, arc sound, and temperature. In laser welding, on the other hand, monitoring is performed in three stages: the first one is the pre-process, which is dedicated to monitoring the weld bead, then there is the

<figure><img src="image_2693.png" /></figure>
intermediate stage, which is used to monitor the weld pool, weld defects, and spatter, and finally, the post-process stage, for weld geometry and visible defects [10,11].

In a machining process, the various features monitored include gradual tool wear, detection of sudden tool breakage, vibration, or surface roughness. Similarly, monitored features in FSW include defects, positioning of the tool over the joint, temperature in the weld zone, etc.

The challenge then arises regarding the methodology for monitoring these features during the process. Monitoring methods can traditionally be classified into two categories: direct and indirect [12]. The direct method makes use of techniques such as laser, radioactivity, optical system, and camera vision to provide information about a given feature. For example, during the machining operation, tool wear was detected using optical methods, i.e., by analyzing the condition of the tool under a microscope. In addition, particle wear can also be studied through radioactivity and using vision systems [13,14]. Similarly, surface roughness can be determined with optical and vision systems [15]. Machine vision systems have also been used for weld seam tracking and for identifying defects and geometry during welding itself [10]. X-rays are also often used for defect identification [10]. Therefore, direct methods are based on dimensional measurements, providing high accuracy but limited practicality due to various constraints, such as placement in a non-vibration-free environment, their delicate handling, and the need for adequate lighting. Moreover, the wrong use could create an interference with the monitoring system, making the whole system unstable.

To overcome the problems associated with direct monitoring, systems have been developed to detect physical quantities such as current, power, force, torque, and vibration, constituting indirect means of monitoring [12]. These systems are less accurate than direct monitoring methods but are cost-effective, more practical, and suitable for industrial applications. For instance, in the study of Mazlan et al., welding current signals have been monitored to study the quality of welding in Gas-Metal Arc Welding (GMAW) [16]. In another work, Huang et al. reported that acoustic signals have been used during laser welding to monitor weld quality, namely, to differentiate between full-penetration and partial-penetration welds [17].

Therefore, the development of robust monitoring methods to assess the condition of the welding during the process becomes crucial for sustaining high production efficiency and ensuring quality standards [18]. In FSW, force and torque signals have been used specifically to detect defects and monitor tool position [19–21], but a reliable monitoring methodology and setup still require further investigation. In this context, the aim of this study is to provide strategies for monitoring the FSW process that can be used for the control and detection of anomalies, such as defects in the welds, or for the assessment of the tool conditions. Specifically, power, vibrations, and temperatures were measured to achieve a sound dissimilar lap joint of two aluminum alloys. The proposed method can be useful for the improvement of process from an efficiency and sustainable standpoint.

## Materials and methods

In the present work, aluminum sheets served as the workpiece for FSW, with a lap joint configuration. Friction stir welding of aluminum alloys is a robust process that has become increasingly well established in the fabrication of critical components, in particular in the welding of 2xxx and 7xxx alloys for the aerospace industry. Specifically, the materials used are AA 2024-T3 and AA 7075-T6, which are particularly interesting, for example, in the aerospace industry for the fuselage panels of aircraft, which are usually made of AA 2024 skins and strengthened by FSW lap joining of AA 7075 stringers [22–25].

The FSW process was executed utilizing an ADRIA Machine FEL-660HG milling machine equipped with a custom clamping system, shown in Fig. 1. The H13 tool employed was supplied by FPT Industries and characterized by a threaded pin having a truncated cone shape. The dimensions of shoulder and tool diameters are 13 mm and 20 mm, respectively; the major and
intermediate stage, which is used to monitor the weld pool, weld defects, and spatter, and finally, the post-process stage, for weld geometry and visible defects [10,11].

In a machining process, the various features monitored include gradual tool wear, detection of sudden tool breakage, vibration, or surface roughness. Similarly, monitored features in FSW include defects, positioning of the tool over the joint, temperature in the weld zone, etc.

The challenge then arises regarding the methodology for monitoring these features during the process. Monitoring methods can traditionally be classified into two categories: direct and indirect [12]. The direct method makes use of techniques such as laser, radioactivity, optical system, and camera vision to provide information about a given feature. For example, during the machining operation, tool wear was detected using optical methods, i.e., by analyzing the condition of the tool under a microscope. In addition, particle wear can also be studied through radioactivity and using vision systems [13,14]. Similarly, surface roughness can be determined with optical and vision systems [15]. Machine vision systems have also been used for weld seam tracking and for identifying defects and geometry during welding itself [10]. X-rays are also often used for defect identification [10]. Therefore, direct methods are based on dimensional measurements, providing high accuracy but limited practicality due to various constraints, such as placement in a non-vibration-free environment, their delicate handling, and the need for adequate lighting. Moreover, the wrong use could create an interference with the monitoring system, making the whole system unstable.

To overcome the problems associated with direct monitoring, systems have been developed to detect physical quantities such as current, power, force, torque, and vibration, constituting indirect means of monitoring [12]. These systems are less accurate than direct monitoring methods but are cost-effective, more practical, and suitable for industrial applications. For instance, in the study of Mazlan et al., welding current signals have been monitored to study the quality of welding in Gas-Metal Arc Welding (GMAW) [16]. In another work, Huang et al. reported that acoustic signals have been used during laser welding to monitor weld quality, namely, to differentiate between full-penetration and partial-penetration welds [17].

Therefore, the development of robust monitoring methods to assess the condition of the welding during the process becomes crucial for sustaining high production efficiency and ensuring quality standards [18]. In FSW, force and torque signals have been used specifically to detect defects and monitor tool position [19–21], but a reliable monitoring methodology and setup still require further investigation. In this context, the aim of this study is to provide strategies for monitoring the FSW process that can be used for the control and detection of anomalies, such as defects in the welds, or for the assessment of the tool conditions. Specifically, power, vibrations, and temperatures were measured to achieve a sound dissimilar lap joint of two aluminum alloys. The proposed method can be useful for the improvement of process from an efficiency and sustainable standpoint.

## Materials and methods

In the present work, aluminum sheets served as the workpiece for FSW, with a lap joint configuration. Friction stir welding of aluminum alloys is a robust process that has become increasingly well established in the fabrication of critical components, in particular in the welding of 2xxx and 7xxx alloys for the aerospace industry. Specifically, the materials used are AA 2024-T3 and AA 7075-T6, which are particularly interesting, for example, in the aerospace industry for the fuselage panels of aircraft, which are usually made of AA 2024 skins and strengthened by FSW lap joining of AA 7075 stringers [22–25].

The FSW process was executed utilizing an ADRIA Machine FEL-660HG milling machine equipped with a custom clamping system, shown in Fig. 1. The H13 tool employed was supplied by FPT Industries and characterized by a threaded pin having a truncated cone shape. The dimensions of shoulder and tool diameters are 13 mm and 20 mm, respectively; the major and
minor pin diameters are 3.96 mm and 5.40 mm, respectively. As previously mentioned, the joint configuration is a lap joint, with AA 2024 as the upper plate and AA as the lower plate. The chosen parameters for the present experimental campaign are summarized in Table 1.

<figure><img src="image_2699.png" /><figcaption>Figure 1. FSW monitoring system setup realized with a power measurement system, an accelerometer, and four thermocouples.</figcaption></figure>

<table><caption>Table 1. Parameters used in the FSW experimental campaign.</caption><thead><tr><td>Parameter</td><td>Value</td></tr></thead><tbody><tr><td>Tilt angle</td><td>1 [deg]</td></tr><tr><td>Tool feed rate</td><td>100 [mm/min]</td></tr><tr><td>Tool rotation</td><td>1800 [rpm]</td></tr><tr><td>Tool plunge</td><td>4.1 [mm]</td></tr><tr><td>Welded length</td><td>90 [mm]</td></tr></tbody></table>

Several sensors were employed to monitor the process. Accelerations during the procedure were acquired using a tri-axial MEMS (Micro Electro-Mechanical System) accelerometer, the technical specifications of which are reported in Table 2. The accelerometer is STIM318 from Sensoror™ [26,27], used to monitor the vibrations of plates during the welding process; in particular, the three sensing axes of the accelerometer are oriented one perpendicular to the welding plane (X-axis), one along the welding direction (Y-axis), and one outward from the plane (Z-axis), respectively. Temperature data was gathered with the use of type K thermocouples and a RS 200 Pro data logger. The thermocouples were inserted in 2 mm deep holes drilled on the top surface of the plates. In total, four thermocouples were strategically positioned to map the temperature within the workpiece, with two thermocouples for the upper plate and two thermocouples for the lower plate. Additionally, power consumption was measured by means of a power monitoring device, i.e., the Montronix Power sensor PS200, which is connected in such a way as to measure the spindle motor
minor pin diameters are 3.96 mm and 5.40 mm, respectively. As previously mentioned, the joint configuration is a lap joint, with AA 2024 as the upper plate and AA as the lower plate. The chosen parameters for the present experimental campaign are summarized in Table 1.

<figure><img src="image_2694.png" /><figcaption>Figure 1. FSW monitoring system setup realized with a power measurement system, an accelerometer, and four thermocouples.</figcaption></figure>

<table><caption>Table 1. Parameters used in the FSW experimental campaign.</caption><thead><tr><td>Parameter</td><td>Value</td></tr></thead><tbody><tr><td>Tilt angle</td><td>1 [deg]</td></tr><tr><td>Tool feed rate</td><td>100 [mm/min]</td></tr><tr><td>Tool rotation</td><td>1800 [rpm]</td></tr><tr><td>Tool plunge</td><td>4.1 [mm]</td></tr><tr><td>Welded length</td><td>90 [mm]</td></tr></tbody></table>

Several sensors were employed to monitor the process. Accelerations during the procedure were acquired using a tri-axial MEMS (Micro Electro-Mechanical System) accelerometer, the technical specifications of which are reported in Table 2. The accelerometer is STIM318 from Sensoror™ [26,27], used to monitor the vibrations of plates during the welding process; in particular, the three sensing axes of the accelerometer are oriented one perpendicular to the welding plane (X-axis), one along the welding direction (Y-axis), and one outward from the plane (Z-axis), respectively. Temperature data was gathered with the use of type K thermocouples and a RS 200 Pro data logger. The thermocouples were inserted in 2 mm deep holes drilled on the top surface of the plates. In total, four thermocouples were strategically positioned to map the temperature within the workpiece, with two thermocouples for the upper plate and two thermocouples for the lower plate. Additionally, power consumption was measured by means of a power monitoring device, i.e., the Montronix Power sensor PS200, which is connected in such a way as to measure the spindle motor
only. The full system setup is shown in Fig. 1, where a Personal Computer acquires the measured power, acceleration, and temperature through Serial Communications that is established by means of a Matlab code in such a way as to acquire synchronized samples from the realized monitoring system.

<table><caption>Table 2. STIM318 accelerometer technical specifications.</caption><thead><tr><th colspan="2">STIM318</th></tr></thead><tbody><tr><td>Measurement Range</td><td>±10 [g]</td></tr><tr><td>Bandwidth (-3dB)</td><td>225 [Hz]</td></tr><tr><td>Bias Instability</td><td>0.003 [mg]</td></tr><tr><td>Velocity Random Walk</td><td>0.015 [m/s/√h]</td></tr><tr><td>Resolution</td><td>1.9 [µg]</td></tr><tr><td>Sample Rate</td><td>2000 [samples/s]</td></tr></tbody></table>

Characterization of the joint surface was carried out with the use of the HIROX RH-2000 Digital Microscope.

## Results and discussion

The tests conducted aim to evaluate the power variations, vibrations, and temperature changes in each phase of the FSW process. The monitoring of the process is essential to obtain an efficient process and, therefore, a more sustainable one, and it could be a fundamental element for the preliminary detection of welding defects and tool wear. The first variable was monitored using the power monitoring sensor, i.e., Montronix PS200, connected only to the spindle motor, whereas the vibrations, intended as accelerations in one of the three main axes, were analyzed through an accelerometer, i.e., STIM318, and referred to the vibrations of the aluminum sheets to be joined. Power variations and accelerations are shown in Fig. 2 and relate to the friction stir lap joint welding performed with a welding speed of 100 mm/min and a tool rotational speed of 180 rpm. Both the above-mentioned components are strongly affected by the material flow, and their variation during the process can be strictly related to the processing stage, and as reported in previous authors' studies, the entire procedure can be divided into three regions [28–30]: Phase I, II, and III.

The first phase refers to the initial stage, which includes turning on the machine, the motor, and the tool plunging phase. Therefore, in Phase I, it is possible to observe three peaks, indicated in Fig. 2 as A, B, and C, both in the power and acceleration. Specifically, the blue arrows refer to the power peaks, and the orange arrows show the acceleration peaks. The recording started before the machine turned on; therefore, the reported values are at their minimum in the initial seconds but not zero. Then, when the motor turned on, a first peak was recorded, both by the Montronix PS200 and the STIM 318 (points A). Subsequently, power and vibration decrease as the tool goes down until the second peak is reached, corresponding to the first part of the tool plunging (point B). It happens when the rotating pin reaches the AA 2024-T3 top surface, generating a peak, and ends when the sheets are completely penetrated. Finally, a third peak was reported as an outcome of the accelerometer and the power sensor, i.e., points C, when the shoulder comes in contact with the top surface.

Motor power and accelerations do not show significant variations between points A and B, unlike what happens between points B and C, and this is in agreement with the physical process, considering that in the time range delimited by points A and B, there is only the descent of the tool, but still no interaction between the sheets to be welded and the tool itself. While between points B and C, it is possible to notice an increasing trend in both power and accelerations, again
only. The full system setup is shown in Fig. 1, where a Personal Computer acquires the measured power, acceleration, and temperature through Serial Communications that is established by means of a Matlab code in such a way as to acquire synchronized samples from the realized monitoring system.

<table><caption>Table 2. STIM318 accelerometer technical specifications.</caption><thead><tr><th colspan="2">STIM318</th></tr></thead><tbody><tr><td>Measurement Range</td><td>±10 [g]</td></tr><tr><td>Bandwidth (-3dB)</td><td>225 [Hz]</td></tr><tr><td>Bias Instability</td><td>0.003 [mg]</td></tr><tr><td>Velocity Random Walk</td><td>0.015 [m/s/√h]</td></tr><tr><td>Resolution</td><td>1.9 [µg]</td></tr><tr><td>Sample Rate</td><td>2000 [samples/s]</td></tr></tbody></table>

Characterization of the joint surface was carried out with the use of the HIROX RH-2000 Digital Microscope.

## Results and discussion

The tests conducted aim to evaluate the power variations, vibrations, and temperature changes in each phase of the FSW process. The monitoring of the process is essential to obtain an efficient process and, therefore, a more sustainable one, and it could be a fundamental element for the preliminary detection of welding defects and tool wear. The first variable was monitored using the power monitoring sensor, i.e., Montronix PS200, connected only to the spindle motor, whereas the vibrations, intended as accelerations in one of the three main axes, were analyzed through an accelerometer, i.e., STIM318, and referred to the vibrations of the aluminum sheets to be joined. Power variations and accelerations are shown in Fig. 2 and relate to the friction stir lap joint welding performed with a welding speed of 100 mm/min and a tool rotational speed of 180 rpm. Both the above-mentioned components are strongly affected by the material flow, and their variation during the process can be strictly related to the processing stage, and as reported in previous authors' studies, the entire procedure can be divided into three regions [28–30]: Phase I, II, and III.

The first phase refers to the initial stage, which includes turning on the machine, the motor, and the tool plunging phase. Therefore, in Phase I, it is possible to observe three peaks, indicated in Fig. 2 as A, B, and C, both in the power and acceleration. Specifically, the blue arrows refer to the power peaks, and the orange arrows show the acceleration peaks. The recording started before the machine turned on; therefore, the reported values are at their minimum in the initial seconds but not zero. Then, when the motor turned on, a first peak was recorded, both by the Montronix PS200 and the STIM 318 (points A). Subsequently, power and vibration decrease as the tool goes down until the second peak is reached, corresponding to the first part of the tool plunging (point B). It happens when the rotating pin reaches the AA 2024-T3 top surface, generating a peak, and ends when the sheets are completely penetrated. Finally, a third peak was reported as an outcome of the accelerometer and the power sensor, i.e., points C, when the shoulder comes in contact with the top surface.

Motor power and accelerations do not show significant variations between points A and B, unlike what happens between points B and C, and this is in agreement with the physical process, considering that in the time range delimited by points A and B, there is only the descent of the tool, but still no interaction between the sheets to be welded and the tool itself. While between points B and C, it is possible to notice an increasing trend in both power and accelerations, again
in agreement with the welding process, since the rotating pin is progressively entering the material, involving greater power consumption and increasing the acting forces, which in turn involve an increase in vibrations.

Phase II is related to the dwell time, characterized by a power-decreasing trend due to the softening of the material caused by the heat generated by the rotating tool. The same happens to the accelerations after an initial increment.

Phase III starts with the other two recorded peaks. Specifically, this phase is related to the welding process, and point D indicates the movement of the rotating pin. Therefore, there is a steady state, followed by a decrease in power and acceleration due to the end of the welding.

<figure><img src="image_2700.png" /><figcaption>Figure 2. Power and acceleration vs time during the FSW process.</figcaption></figure>

The frequency domain analysis of the acceleration along the three axes (Fig. 3) highlights three different vibration effects, which were evaluated separately by means of the Fast Fourier Transform (FFT); in particular, the acquired signal was divided and analyzed into three parts in such a way to identify the different contributions: (i) the motor vibration that is equal to 31Hz, according to the tool rotation (Table 1), this effect can be observed on all axes but is most evident on the Y-axis where the acceleration amplitude is equal to 0.3 m/s²; (ii) the Dwell time where the frequency of about 100Hz and acceleration amplitude of 0.05 m/s² on the Z-axis have been measured, and (iii) in the welding process, the frequency evaluated is about equal to 160Hz, and the acceleration amplitude is equal to 0.01 m/s². For the sake of clarity, the motor vibration is also present in the Dwell time, and during the welding process, the motor vibration and a residual contribution of the Dwell time correspond to the complete insight of the sheets.
in agreement with the welding process, since the rotating pin is progressively entering the material, involving greater power consumption and increasing the acting forces, which in turn involve an increase in vibrations.

Phase II is related to the dwell time, characterized by a power-decreasing trend due to the softening of the material caused by the heat generated by the rotating tool. The same happens to the accelerations after an initial increment.

Phase III starts with the other two recorded peaks. Specifically, this phase is related to the welding process, and point D indicates the movement of the rotating pin. Therefore, there is a steady state, followed by a decrease in power and acceleration due to the end of the welding.

<figure><img src="image_2695.png" /><figcaption>Figure 2. Power and acceleration vs time during the FSW process.</figcaption></figure>

The frequency domain analysis of the acceleration along the three axes (Fig. 3) highlights three different vibration effects, which were evaluated separately by means of the Fast Fourier Transform (FFT); in particular, the acquired signal was divided and analyzed into three parts in such a way to identify the different contributions: (i) the motor vibration that is equal to 31Hz, according to the tool rotation (Table 1), this effect can be observed on all axes but is most evident on the Y-axis where the acceleration amplitude is equal to 0.3 m/s²; (ii) the Dwell time where the frequency of about 100Hz and acceleration amplitude of 0.05 m/s² on the Z-axis have been measured, and (iii) in the welding process, the frequency evaluated is about equal to 160Hz, and the acceleration amplitude is equal to 0.01 m/s². For the sake of clarity, the motor vibration is also present in the Dwell time, and during the welding process, the motor vibration and a residual contribution of the Dwell time correspond to the complete insight of the sheets.
<figure><img src="image_2701.png" /><figcaption>Figure 3. One-side spectrum of the measured acceleration along the three axes during the FSW process.</figcaption></figure>

Finally, temperatures were also evaluated to assess the main parameters useful for monitoring the FSW process, as shown in Fig.4. The thermal trend is measured by means of four thermocouples, two placed on the upper aluminum sheet and two placed on the lower one. The temperature variations highlight in such a way the different phases above described for the power and acceleration. In particular, the recording starts at room temperature, about 35 °C, followed by an increasing trend corresponding to phases I and II, that is, the penetration of the tool into the sheets and the dwell time, and related to the heat generated by the rotating tool. During the final phase III, i.e., the welding phase, a second peak is registered, it was caused by the passage of the tool in correspondence with the position of the four thermocouples.
<figure><img src="image_2696.png" /><figcaption>Figure 3. One-side spectrum of the measured acceleration along the three axes during the FSW process.</figcaption></figure>

Finally, temperatures were also evaluated to assess the main parameters useful for monitoring the FSW process, as shown in Fig.4. The thermal trend is measured by means of four thermocouples, two placed on the upper aluminum sheet and two placed on the lower one. The temperature variations highlight in such a way the different phases above described for the power and acceleration. In particular, the recording starts at room temperature, about 35 °C, followed by an increasing trend corresponding to phases I and II, that is, the penetration of the tool into the sheets and the dwell time, and related to the heat generated by the rotating tool. During the final phase III, i.e., the welding phase, a second peak is registered, it was caused by the passage of the tool in correspondence with the position of the four thermocouples.
<figure><img src="image_2702.png" /><figcaption>Figure 4. Thermal trend measured by the four thermocouples in the FSW process.</figcaption></figure>

In summary, the monitoring methodology presented herein provides a means to properly identify the phases of the welding process in case of overlap welding by taking advantage of multiple sensors. Moreover, it enables the detection of anomalies during the welding operations, as evidenced by deviations such as further peaks, valleys, or oscillations in power, vibrations and temperature data acquisition, distinct from those previously observed and described. An illustration of this is depicted in Fig. 2, where minimal variations, denoted by point D, are indicative of the presence of defects. Specifically, it is possible to observe a valley in the acceleration, indicated by the orange arrow, and oscillation in power consumption, indicated by the blue arrow. The presence of the defect is also detected in Fig. 4, wherein the highest temperature is recorded by the thermocouple K-1, and so by the thermocouple positioned on the upper plate. The anomalies correlated to these variations are illustrated in Fig. 5, where it is possible to note excessive flash and waves of softened material associated with elevated temperatures, reflecting issues in the material flows. Notably, temperature emerges as the most influential characterizing parameter for these surface defects. Moreover, this method could help to check for variations with prolonged use of the tool, investigating the influence of the tool wear on the aforementioned outputs. In conclusion, it is possible to use the proposed method to identify optimal process parameters and enhance overall welding efficiency. While traditionally, the selection of process parameters has been primarily guided by the desired characteristics of the end product, contemporary practice dictates prioritizing combinations that minimize energy consumption, integrating process sustainability into decision-making processes. On these bases,
<figure><img src="image_2697.png" /><figcaption>Figure 4. Thermal trend measured by the four thermocouples in the FSW process.</figcaption></figure>

In summary, the monitoring methodology presented herein provides a means to properly identify the phases of the welding process in case of overlap welding by taking advantage of multiple sensors. Moreover, it enables the detection of anomalies during the welding operations, as evidenced by deviations such as further peaks, valleys, or oscillations in power, vibrations and temperature data acquisition, distinct from those previously observed and described. An illustration of this is depicted in Fig. 2, where minimal variations, denoted by point D, are indicative of the presence of defects. Specifically, it is possible to observe a valley in the acceleration, indicated by the orange arrow, and oscillation in power consumption, indicated by the blue arrow. The presence of the defect is also detected in Fig. 4, wherein the highest temperature is recorded by the thermocouple K-1, and so by the thermocouple positioned on the upper plate. The anomalies correlated to these variations are illustrated in Fig. 5, where it is possible to note excessive flash and waves of softened material associated with elevated temperatures, reflecting issues in the material flows. Notably, temperature emerges as the most influential characterizing parameter for these surface defects. Moreover, this method could help to check for variations with prolonged use of the tool, investigating the influence of the tool wear on the aforementioned outputs. In conclusion, it is possible to use the proposed method to identify optimal process parameters and enhance overall welding efficiency. While traditionally, the selection of process parameters has been primarily guided by the desired characteristics of the end product, contemporary practice dictates prioritizing combinations that minimize energy consumption, integrating process sustainability into decision-making processes. On these bases,
early anomaly detection plays a crucial role, not only in ensuring a robust welding process but also in preventing the production of faulty components, thereby minimizing waste materials and conserving energy resources.

<figure><img src="image_2703.png" /><figcaption>Figure 5. Flash and surface defects.</figcaption></figure>

## Conclusions

The findings of the present work provide reliable strategies for monitoring the welding process. In particular, a suitable monitoring system has been proposed to measure the main quantity of interest during the FSW process, i.e., the power consumption, the plate vibrations, and the welding temperature. To this aim, a power measurement system was connected directly to the tool motor while a tri-axial MEMS accelerometer was mounted on the welding plate, and four thermocouples were on the aluminum sheets. The research carried out highlights that by taking into account these parameters, the different characteristic FSW stages can be identified and divided into three main phases with the aim to help in the early detection of defects, tool conditions, power consumption, and tool failure also during the welding. Specifically, the occurrence of flash and surface defects during the welding resulted in peaks in the recorded temperature and alterations in both accelerations and power consumption. The obtained outcomes can be useful for assessing the state of health of the machine and the tool and, concurrently, for improving the process from a sustainable point of view. Early anomaly detection assumes great importance, ensuring a robust welding process and avoiding the production of defective components, thus minimizing waste materials and preserving energy resources.

## References

[1] Thomas, W.M.; Nicholas, E.D.; Needham, J.C.; Murch, M.G.; Temple-Smith, P.; Dawes, C.J. Friction Stir Butt Welding, International Patent Application No. PCT/GB92 Patent application 1991.
early anomaly detection plays a crucial role, not only in ensuring a robust welding process but also in preventing the production of faulty components, thereby minimizing waste materials and conserving energy resources.

<figure><img src="image_2698.png" /><figcaption>Figure 5. Flash and surface defects.</figcaption></figure>

## Conclusions

The findings of the present work provide reliable strategies for monitoring the welding process. In particular, a suitable monitoring system has been proposed to measure the main quantity of interest during the FSW process, i.e., the power consumption, the plate vibrations, and the welding temperature. To this aim, a power measurement system was connected directly to the tool motor while a tri-axial MEMS accelerometer was mounted on the welding plate, and four thermocouples were on the aluminum sheets. The research carried out highlights that by taking into account these parameters, the different characteristic FSW stages can be identified and divided into three main phases with the aim to help in the early detection of defects, tool conditions, power consumption, and tool failure also during the welding. Specifically, the occurrence of flash and surface defects during the welding resulted in peaks in the recorded temperature and alterations in both accelerations and power consumption. The obtained outcomes can be useful for assessing the state of health of the machine and the tool and, concurrently, for improving the process from a sustainable point of view. Early anomaly detection assumes great importance, ensuring a robust welding process and avoiding the production of defective components, thus minimizing waste materials and preserving energy resources.

## References

[1] Thomas, W.M.; Nicholas, E.D.; Needham, J.C.; Murch, M.G.; Temple-Smith, P.; Dawes, C.J. Friction Stir Butt Welding, International Patent Application No. PCT/GB92 Patent application 1991.
[2] Thomas, W.M.; Nicholas, E.D.; Needham, J.C.; Murch, M.G.; Temple-Smith, P.; Dawes, C.J. Improvements Relating to Friction Welding 1993.

[3] Mishra, R.S.; Ma, Z.Y. Friction Stir Welding and Processing. *Materials Science and Engineering: R: Reports* 2005, 50, 1-78. https://doi.org/10.1016/J.MSER.2005.07.001

[4] Dwivedi, M.; Silvestri, A.T.; Franchitti, S.; Krishnaswamy, H.; Narayananperumal, A.; Astarita, A. Friction Welding: An Effective Joining Process for Hybrid Additive Manufacturing. *CIRP J Manuf Sci Technol* 2021. https://doi.org/10.1016/j.cirpj.2021.07.016

[5] Nandan, R.; DebRoy, T.; Bhadeshia, H.K.D.H. Recent Advances in Friction-Stir Welding - Process, Weldment Structure and Properties. *Prog Mater Sci* 2008, 53.

[6] Jain, R.; Kumari, K.; Kesharwani, R.K.; Kumar, S.; Pal, S.K.; Singh, S.B.; Panda, S.K.; Samantaray, A.K. Friction Stir Welding: Scope and Recent Development. In: 2015.

[7] Impero, F.; Scherillo, F.; Silvestri, A.T.; Casarin, R.; Astarita, A.; Squillace, A. Stationary Shoulder Friction Stir Processing: Influence of Tool Wear on Surface Properties. In Proceedings of the Key Engineering Materials; 2019.

[8] Nandan, R.; DebRoy, T.; Bhadeshia, H.K.D.H. Recent Advances in Friction-Stir Welding - Process, Weldment Structure and Properties. *Prog Mater Sci* 2008, 53.

[9] Tucci, F.; Rubino, F.; Carlone, P. Strain and Temperature Measurement in Pultrusion Processes by Fiber Bragg Grating Sensors. In Proceedings of the AIP Conference Proceedings; 2018; Vol. 1960.

[10] Stavridis, J.; Papacharalampopoulos, A.; Stavropoulos, P. Quality Assessment in Laser Welding: A Critical Review. *International Journal of Advanced Manufacturing Technology* 2018, 94.

[11] Stournaras, A.; Stavropoulos, P.; Salonitis, K.; Chryssolouris, G. LASER PROCESS MONITORING: A CRITICAL. *Advances in Manufacturing Technology-XXII* 424.

[12] Sick, B. On-Line and Indirect Tool Wear Monitoring in Turning with Artificial Neural Networks: A Review of More than a Decade of Research. *Mech Syst Signal Process* 2002, 16.

[13] Dan, L.; Mathew, J. Tool Wear and Failure Monitoring Techniques for Turning-A Review. *Int J Mach Tools Manuf* 1990, 30. https://doi.org/10.1016/0890-6955(90)90009-8

[14] D'Emilia, G.; Di Gasbarro, D.; Natale, E. Optical System for On-Line Monitoring of Welding: A Machine Learning Approach for Optimal Set Up. *Acta IMEKO* 5.

[15] Shahabi, H.H.; Ratnam, M.M. In-Cycle Monitoring of Tool Nose Wear and Surface Roughness of Turned Parts Using Machine Vision. *International Journal of Advanced Manufacturing Technology* 2009, 40. https://doi.org/10.1007/s00170-008-1430-8

[16] Mazlan, A.; Daniyal, H.; Mohamed, A.I.; Ishak, M.; Hadi, A.A. Monitoring the Quality of Welding Based on Welding Current and Ste Analysis. In Proceedings of the IOP Conference Series: Materials Science and Engineering; 2017; Vol. 257.

[17] Huang, W.; Kovacevic, R. Feasibility Study of Using Acoustic Signals for Online Monitoring of the Depth of Weld in the Laser Welding of High-Strength Steels. *Proc Inst Mech Eng B J Eng Manuf* 2009, 223. https://doi.org/10.1243/09544054JEM1320

[18] Mishra, D.; Roy, R.B.; Dutta, S.; Pal, S.K.; Chakravarty, D. A Review on Sensor Based Monitoring and Control of Friction Stir Welding Process and a Roadmap to Industry 4.0. *J Manuf Process* 2018, 36, 373-397.

[19] Fleming, P.A.; Lammlein, D.H.; Wilkes, D.M.; Cook, G.E.; Strauss, A.M.; DeLapp, D.R.; Hartman, D.A. Misalignment Detection and Enabling of Seam Tracking for Friction Stir Welding.
[2] Thomas, W.M.; Nicholas, E.D.; Needham, J.C.; Murch, M.G.; Temple-Smith, P.; Dawes, C.J. Improvements Relating to Friction Welding 1993.

[3] Mishra, R.S.; Ma, Z.Y. Friction Stir Welding and Processing. *Materials Science and Engineering: R: Reports* 2005, 50, 1-78. https://doi.org/10.1016/J.MSER.2005.07.001

[4] Dwivedi, M.; Silvestri, A.T.; Franchitti, S.; Krishnaswamy, H.; Narayanaperumal, A.; Astarita, A. Friction Welding: An Effective Joining Process for Hybrid Additive Manufacturing. *CIRP J Manuf Sci Technol* 2021. https://doi.org/10.1016/j.cirpj.2021.07.016

[5] Nandan, R.; DebRoy, T.; Bhadeshia, H.K.D.H. Recent Advances in Friction-Stir Welding - Process, Weldment Structure and Properties. *Prog Mater Sci* 2008, 53.

[6] Jain, R.; Kumari, K.; Kesharwani, R.K.; Kumar, S.; Pal, S.K.; Singh, S.B.; Panda, S.K.; Samantaray, A.K. Friction Stir Welding: Scope and Recent Development. In: 2015.

[7] Impero, F.; Scherillo, F.; Silvestri, A.T.; Casarin, R.; Astarita, A.; Squillace, A. Stationary Shoulder Friction Stir Processing: Influence of Tool Wear on Surface Properties. In Proceedings of the Key Engineering Materials; 2019.

[8] Nandan, R.; DebRoy, T.; Bhadeshia, H.K.D.H. Recent Advances in Friction-Stir Welding - Process, Weldment Structure and Properties. *Prog Mater Sci* 2008, 53.

[9] Tucci, F.; Rubino, F.; Carlone, P. Strain and Temperature Measurement in Pultrusion Processes by Fiber Bragg Grating Sensors. In Proceedings of the AIP Conference Proceedings; 2018; Vol. 1960.

[10] Stavridis, J.; Papacharalampopoulos, A.; Stavropoulos, P. Quality Assessment in Laser Welding: A Critical Review. *International Journal of Advanced Manufacturing Technology* 2018, 94.

[11] Stournaras, A.; Stavropoulos, P.; Salonitis, K.; Chryssolouris, G. LASER PROCESS MONITORING: A CRITICAL. *Advances in Manufacturing Technology-XXII* 424.

[12] Sick, B. On-Line and Indirect Tool Wear Monitoring in Turning with Artificial Neural Networks: A Review of More than a Decade of Research. *Mech Syst Signal Process* 2002, 16.

[13] Dan, L.; Mathew, J. Tool Wear and Failure Monitoring Techniques for Turning-A Review. *Int J Mach Tools Manuf* 1990, 30. https://doi.org/10.1016/0890-6955(90)90009-8

[14] D'Emilia, G.; Di Gasbarro, D.; Natale, E. Optical System for On-Line Monitoring of Welding: A Machine Learning Approach for Optimal Set Up. *Acta IMEKO* 5.

[15] Shahabi, H.H.; Ratnam, M.M. In-Cycle Monitoring of Tool Nose Wear and Surface Roughness of Turned Parts Using Machine Vision. *International Journal of Advanced Manufacturing Technology* 2009, 40. https://doi.org/10.1007/s00170-008-1430-8

[16] Mazlan, A.; Daniyal, H.; Mohamed, A.I.; Ishak, M.; Hadi, A.A. Monitoring the Quality of Welding Based on Welding Current and Ste Analysis. In Proceedings of the IOP Conference Series: Materials Science and Engineering; 2017; Vol. 257.

[17] Huang, W.; Kovacevic, R. Feasibility Study of Using Acoustic Signals for Online Monitoring of the Depth of Weld in the Laser Welding of High-Strength Steels. *Proc Inst Mech Eng B J Eng Manuf* 2009, 223. https://doi.org/10.1243/09544054JEM1320

[18] Mishra, D.; Roy, R.B.; Dutta, S.; Pal, S.K.; Chakravarty, D. A Review on Sensor Based Monitoring and Control of Friction Stir Welding Process and a Roadmap to Industry 4.0. *J Manuf Process* 2018, 36, 373-397.

[19] Fleming, P.A.; Lammlein, D.H.; Wilkes, D.M.; Cook, G.E.; Strauss, A.M.; DeLapp, D.R.; Hartman, D.A. Misalignment Detection and Enabling of Seam Tracking for Friction Stir Welding.
Science and Technology of Welding and Joining 2009, 14.
https://doi.org/10.1179/136217108X372568

[20] Kumar, U.; Yadav, I.; Kumari, S.; Kumari, K.; Ranjan, N.; Kesharwani, R.K.; Jain, R.; Kumar, S.; Pal, S.; Chakravarty, D.; et al. Defect Identification in Friction Stir Welding Using Discrete Wavelet Analysis. *Advances in Engineering Software* 2015, 85.
https://doi.org/10.1016/j.advengsoft.2015.02.001

[21] Kumari, S.; Jain, R.; Kumar, U.; Yadav, I.; Ranjan, N.; Kumari, K.; Kesharwani, R.K.; Kumar, S.; Pal, S.; Pal, S.K.; et al. Defect Identification in Friction Stir Welding Using Continuous Wavelet Transform. *J Intell Manuf* 2019, 30. https://doi.org/10.1007/s10845-016-1259-1

[22] Threadgill, P.L.; Leonard, A.J.; Shercliff, H.R.; Withers, P.J. Friction Stir Welding of Aluminium Alloys. *International Materials Reviews* 2009, 54, 49-93.

[23] Dubourg, L.; Merati, A.; Jahazi, M. Process Optimisation and Mechanical Properties of Friction Stir Lap Welds of 7075-T6 Stringers on 2024-T3 Skin. *Mater Des* 2010, 31, 3324-3330.

[24] Paniti, I.; Viharos, Z.J.; Harangozó, D.; Najm, S.M. Experimental and Numerical Investigation of Single Point Incremental Forming of Aluminium Alloy Foils. *Acta Imeko* 2020, 9, 25-31.

[25] Rubino, F.; Esperto, V.; Paulo, R.M.F.; Tucci, F.; Carlone, P. Integrated Manufacturing of AA6082 by Friction Stir Welding and Incremental Forming: Strain Analysis of Deformed Samples. In Proceedings of the Procedia Manufacturing; 2020; Vol. 47, pp. 440-444.

[26] de Alteriis, G.; Caputo, E.; Moriello, R.S. Lo On the Suitability of Redundant Accelerometers for the Implementation of Smart Oscillation Monitoring System: Preliminary Assessment. *Acta IMEKO* 2023, 12, 1-9. https://doi.org/10.21014/actaimeko.v12i2.1532

[27] de Alteriis, G.; Silvestri, A.T.; Conte, C.; Bottino, V.; Caputo, E.; Squillace, A.; Accardo, D.; Schiano Lo Moriello, R. Innovative Fusion Strategy for MEMS Redundant-IMU Exploiting Custom 3D Components. *Sensors* 2023, 23.

[28] Astarita, A.; Tucci, F.; Silvestri, A.T.; Perrella, M.; Boccarusso, L.; Carlone, P. Dissimilar Friction Stir Lap Welding of AA2198 and AA7075 Sheets: Forces, Microstructure and Mechanical Properties. *International Journal of Advanced Manufacturing Technology* 2021.
https://doi.org/10.1007/s00170-021-07816-7

[29] Tucci, F.; Carlone, P.; Silvestri, A.T.; Parmar, H.; Astarita, A. Dissimilar Friction Stir Lap Welding of AA2198-AA6082: Process Analysis and Joint Characterization. *CIRP J Manuf Sci Technol* 2021, 35, 753-764. https://doi.org/10.1016/j.cirpj.2021.09.007

[30] Boccarusso, L.; Astarita, A.; Carlone, P.; Scherillo, F.; Rubino, F.; Squillace, A. Dissimilar Friction Stir Lap Welding of AA 6082 - Mg AZ31: Force Analysis and Microstructure Evolution. *J Manuf Process* 2019, 44, 376-388. https://doi.org/10.1016/j.jmapro.2019.06.022
*Science and Technology of Welding and Joining* 2009, **14.**
https://doi.org/10.1179/136217108X372568

[20] Kumar, U.; Yadav, I.; Kumari, S.; Kumari, K.; Ranjan, N.; Kesharwani, R.K.; Jain, R.; Kumar, S.; Pal, S.; Chakravarty, D.; et al. Defect Identification in Friction Stir Welding Using Discrete Wavelet Analysis. *Advances in Engineering Software* 2015, **85**. https://doi.org/10.1016/j.advengsoft.2015.02.001

[21] Kumari, S.; Jain, R.; Kumar, U.; Yadav, I.; Ranjan, N.; Kumari, K.; Kesharwani, R.K.; Kumar, S.; Pal, S.; Pal, S.K.; et al. Defect Identification in Friction Stir Welding Using Continuous Wavelet Transform. *J Intell Manuf* 2019, **30**. https://doi.org/10.1007/s10845-016-1259-1

[22] Threadgill, P.L.; Leonard, A.J.; Shercliff, H.R.; Withers, P.J. Friction Stir Welding of Aluminium Alloys. *International Materials Reviews* 2009, **54**, 49-93.

[23] Dubourg, L.; Merati, A.; Jahazi, M. Process Optimisation and Mechanical Properties of Friction Stir Lap Welds of 7075-T6 Stringers on 2024-T3 Skin. *Mater Des* 2010, **31**, 3324-3330.

[24] Paniti, I.; Viharos, Z.J.; Harangozó, D.; Najm, S.M. Experimental and Numerical Investigation of Single Point Incremental Forming of Aluminium Alloy Foils. *Acta Imeko* 2020, **9**, 25-31.

[25] Rubino, F.; Esperto, V.; Paulo, R.M.F.; Tucci, F.; Carlone, P. Integrated Manufacturing of AA6082 by Friction Stir Welding and Incremental Forming: Strain Analysis of Deformed Samples. In Proceedings of the Procedia Manufacturing; 2020; Vol. 47, pp. 440-444.

[26] de Alteriis, G.; Caputo, E.; Moriello, R.S. Lo On the Suitability of Redundant Accelerometers for the Implementation of Smart Oscillation Monitoring System: Preliminary Assessment. *Acta IMEKO* 2023, **12**, 1-9. https://doi.org/10.21014/actaimeko.v12i2.1532

[27] de Alteriis, G.; Silvestri, A.T.; Conte, C.; Bottino, V.; Caputo, E.; Squillace, A.; Accardo, D.; Schiano Lo Moriello, R. Innovative Fusion Strategy for MEMS Redundant-IMU Exploiting Custom 3D Components. *Sensors* 2023, **23**.

[28] Astarita, A.; Tucci, F.; Silvestri, A.T.; Perrella, M.; Boccarusso, L.; Carlone, P. Dissimilar Friction Stir Lap Welding of AA2198 and AA7075 Sheets: Forces, Microstructure and Mechanical Properties. *International Journal of Advanced Manufacturing Technology* 2021. https://doi.org/10.1007/s00170-021-07816-7

[29] Tucci, F.; Carlone, P.; Silvestri, A.T.; Parmar, H.; Astarita, A. Dissimilar Friction Stir Lap Welding of AA2198-AA6082: Process Analysis and Joint Characterization. *CIRP J Manuf Sci Technol* 2021, **35**, 753-764. https://doi.org/10.1016/j.cirpj.2021.09.007

[30] Boccarusso, L.; Astarita, A.; Carlone, P.; Scherillo, F.; Rubino, F.; Squillace, A. Dissimilar Friction Stir Lap Welding of AA 6082 - Mg AZ31: Force Analysis and Microstructure Evolution. *J Manuf Process* 2019, **44**, 376-388. https://doi.org/10.1016/j.jmapro.2019.06.022
# Liquid nitrogen jet impingement cooling of ultra-high strength steel in hot stamping process

CAO Dongsheng<sup>1,2,3,a</sup>, LI Lei<sup>1,2,3,b</sup> and HUANG Haihong<sup>1,2,3,c*</sup>

<sup>1</sup>School of Mechanical Engineering, Hefei University of Technology, Hefei 230009, P. R. China

<sup>2</sup>Key Laboratory of Green Design and Manufacturing of Mechanical Industry, Hefei University of Technology, Hefei 230009, P. R. China

<sup>3</sup>Anhui Province Key Laboratory of Low Carbon Recycling Technology and Equipment for Mechanical and Electrical Products, Anhui, Hefei 230009, China

<sup>a</sup>2021110111@mail.hfut.edu.cn, <sup>b</sup>lei_li@hfut.edu.cn, <sup>c</sup>huanghaihong@hfut.edu.cn

**Keywords:** Hot Stamping Dies, Direct Cooling, Jet Impingement

**Abstract.** In the hot stamping process, elevated sheet temperatures and uneven die cooling contribute to insufficient thickness uniformity in the formed parts, leading to reduced forming efficiency and inducing severe thermo-mechanical fatigue in the die. To address these issues, a novel in-die liquid nitrogen (LN2) jet impingement cooling method is introduced to achieve a low and uniform initial holding temperature. The high-temperature steel sheet is initially cooled to slightly above the martensitic transition temperature through the application of an LN2 jet during the forming stage, followed by pressure-holding quenching. This method demonstrates remarkable outcomes, including a notable 74.19% reduction in holding time and a substantial 24.72% decrease in the maximum thinning rate. Furthermore, the maximum temperature difference between the upper and lower dies sees significant reductions of 34.57% and 25.84%, respectively.

**Introduction**

In the hot stamping process, the efficiency and quality of part formation depend significantly on the cooling performance of the die, specifically in terms of cooling rate and uniformity. Consequently, the top priority for optimizing part quality is the design and optimization of the die's cooling system. Various methods, including optimizing the dimensional parameters of cooling channels [1,2,3], designing a conformal cooling channel structure [4,5,6], selecting the materials of coolant [7,8], calculating flow parameters [9], new structure of cooling systems [10] and upgrading die materials [11,12], have been explored to enhance cooling system performance. However, these approaches primarily focus on optimizing the indirect cooling of traditional hot stamping, yielding relatively modest improvements.

Furthermore, the spatial structure of the formed part often restricts material flow during the forming process, leading to local thinning and resulting in reduced contact pressure with the die or even the formation of gaps. This causes a significant decline in heat transfer performance, poor temperature uniformity, and the need for longer holding times to achieve the desired temperature reduction in the formed part [13,14]. Although improvements can be made through profile trimming, this increases manufacturing costs, and the aforementioned problems are likely to persist with increased wear. The continuous cycling of the hot stamping process also induces large cyclic changes in die temperature, potentially causing severe thermo-mechanical fatigue, increased wear, and even fractures [15]. This phenomenon significantly shortens the service life of the die.

This paper introduces a novel method of in-die liquid nitrogen (LN2) jet impingement cooling for high-temperature ultra-high-strength steel. The method aims to increase the cooling rate of high-temperature steel plates, facilitating the creation of a fully martensitic structure, and to reduce the entry temperature of the plates into the mold, thereby shortening the holding time. Additionally,

<figure><img src="image_2704.png" /></figure>
# Liquid nitrogen jet impingement cooling of ultra-high strength steel in hot stamping process

CAO Dongsheng<sup>1,2,3,a</sup>, LI Lei<sup>1,2,3,b</sup> and HUANG Haihong<sup>1,2,3,c*</sup>

<sup>1</sup>School of Mechanical Engineering, Hefei University of Technology, Hefei 230009, P. R. China

<sup>2</sup>Key Laboratory of Green Design and Manufacturing of Mechanical Industry, Hefei University of Technology, Hefei 230009, P. R. China

<sup>3</sup>Anhui Province Key Laboratory of Low Carbon Recycling Technology and Equipment for Mechanical and Electrical Products, Anhui, Hefei 230009, China

<sup>a</sup>2021110111@mail.hfut.edu.cn, <sup>b</sup>lei_li@hfut.edu.cn, <sup>c</sup>huanghaihong@hfut.edu.cn

**Keywords:** Hot Stamping Dies, Direct Cooling, Jet Impingement

**Abstract.** In the hot stamping process, elevated sheet temperatures and uneven die cooling contribute to insufficient thickness uniformity in the formed parts, leading to reduced forming efficiency and inducing severe thermo-mechanical fatigue in the die. To address these issues, a novel in-die liquid nitrogen (LN2) jet impingement cooling method is introduced to achieve a low and uniform initial holding temperature. The high-temperature steel sheet is initially cooled to slightly above the martensitic transition temperature through the application of an LN2 jet during the forming stage, followed by pressure-holding quenching. This method demonstrates remarkable outcomes, including a notable 74.19% reduction in holding time and a substantial 24.72% decrease in the maximum thinning rate. Furthermore, the maximum temperature difference between the upper and lower dies sees significant reductions of 34.57% and 25.84%, respectively.

**Introduction**

In the hot stamping process, the efficiency and quality of part formation depend significantly on the cooling performance of the die, specifically in terms of cooling rate and uniformity. Consequently, the top priority for optimizing part quality is the design and optimization of the die's cooling system. Various methods, including optimizing the dimensional parameters of cooling channels [1,2,3], designing a conformal cooling channel structure [4,5,6], selecting the materials of coolant [7,8], calculating flow parameters [9], new structure of cooling systems [10] and upgrading die materials [11,12], have been explored to enhance cooling system performance. However, these approaches primarily focus on optimizing the indirect cooling of traditional hot stamping, yielding relatively modest improvements.

Furthermore, the spatial structure of the formed part often restricts material flow during the forming process, leading to local thinning and resulting in reduced contact pressure with the die or even the formation of gaps. This causes a significant decline in heat transfer performance, poor temperature uniformity, and the need for longer holding times to achieve the desired temperature reduction in the formed part [13,14]. Although improvements can be made through profile trimming, this increases manufacturing costs, and the aforementioned problems are likely to persist with increased wear. The continuous cycling of the hot stamping process also induces large cyclic changes in die temperature, potentially causing severe thermo-mechanical fatigue, increased wear, and even fractures [15]. This phenomenon significantly shortens the service life of the die.

This paper introduces a novel method of in-die liquid nitrogen (LN2) jet impingement cooling for high-temperature ultra-high-strength steel. The method aims to increase the cooling rate of high-temperature steel plates, facilitating the creation of a fully martensitic structure, and to reduce the entry temperature of the plates into the mold, thereby shortening the holding time. Additionally,

<figure><img src="image_2699.png" /></figure>
in-die LN2 jet impingement cooling is employed for thermal management of the die, resulting in a reduction in the maximum temperature difference during cyclic changes in die temperature. This approach effectively mitigates thermo-mechanical fatigue, ultimately prolonging the service life of the die.

## Structure of LN2 Jet Impingement Cooling Die

Control of Cooling Rate. The novel approach integrates an LN2 impingement cooling method with a hot stamping die by machining a 1 mm diameter deep hole on the die surface, establishing a connection to a channel inside the die. Fig. 1a schematically depicts the structure of LN2 jet impingement cooling combined with conventional hot stamping dies. The die, comprising an upper and lower component, incorporates uniformly distributed cooling channels on an isometric surface situated at a specific distance from the die surface. Jet holes, evenly distributed on the cooling channels, have their spacing adjusted to ensure uniform cooling.

<figure><img src="image_2705.png" /><figcaption>Fig. 1 Overview of (a) LN2 jet impingement cooling die model and (b) The simplified 2D model</figcaption></figure>

The manipulation of injection parameters for the jet, including jet velocity, is achieved by adjusting the pressure or flow rate at the inlet. In the hot stamping process, the upper die steadily advances toward the sheet at a consistent speed until a snug fit is achieved. While the die is in proximity to the sheet, the surface undergoes impingement cooling through an LN2 jet. Precise regulation of the sheet's cooling rate is achieved by altering both the speed and duration of the jet. Analysis of Heat Transfer Efficiency. In Fig. 1b, the employed Computational Fluid Dynamics (CFD) model is depicted. This model comprehensively characterizes the entire jet phenomenon, considering parameters such as jet velocity, jet height, turbulence, LN2 evaporation, and sheet heat transfer. To simulate the cooling process, a simplified 2D model of the LN2 jet impingement cooling was utilized. In this simulation, LN2, acting as the primary phase, impinges on the high-temperature sheet's surface through a jet hole with a diameter (D).

The finite element analysis software Ansys Fluent was employed to simulate the LN2 jet impingement process through a pressure-based transient analysis. The simulation utilized the widely adopted k-epsilon model with standard wall functions to accurately capture the interaction between the flow and the wall. This model, well-established in industrial flows, ensured a comprehensive representation of the LN2 jet impingement process. The use of Ansys Fluent facilitated a detailed analysis of heat transfer dynamics and fluid-wall interactions during the simulation, contributing to a thorough understanding of the process.
in-die LN2 jet impingement cooling is employed for thermal management of the die, resulting in a reduction in the maximum temperature difference during cyclic changes in die temperature. This approach effectively mitigates thermo-mechanical fatigue, ultimately prolonging the service life of the die.

## Structure of LN2 Jet Impingement Cooling Die

Control of Cooling Rate. The novel approach integrates an LN2 impingement cooling method with a hot stamping die by machining a 1 mm diameter deep hole on the die surface, establishing a connection to a channel inside the die. Fig. 1a schematically depicts the structure of LN2 jet impingement cooling combined with conventional hot stamping dies. The die, comprising an upper and lower component, incorporates uniformly distributed cooling channels on an isometric surface situated at a specific distance from the die surface. Jet holes, evenly distributed on the cooling channels, have their spacing adjusted to ensure uniform cooling.

<figure><img src="image_2700.png" /><figcaption>Fig. 1 Overview of (a) LN2 jet impingement cooling die model and (b) The simplified 2D model</figcaption></figure>

The manipulation of injection parameters for the jet, including jet velocity, is achieved by adjusting the pressure or flow rate at the inlet. In the hot stamping process, the upper die steadily advances toward the sheet at a consistent speed until a snug fit is achieved. While the die is in proximity to the sheet, the surface undergoes impingement cooling through an LN2 jet. Precise regulation of the sheet's cooling rate is achieved by altering both the speed and duration of the jet. Analysis of Heat Transfer Efficiency. In Fig. 1b, the employed Computational Fluid Dynamics (CFD) model is depicted. This model comprehensively characterizes the entire jet phenomenon, considering parameters such as jet velocity, jet height, turbulence, LN2 evaporation, and sheet heat transfer. To simulate the cooling process, a simplified 2D model of the LN2 jet impingement cooling was utilized. In this simulation, LN2, acting as the primary phase, impinges on the high-temperature sheet's surface through a jet hole with a diameter (D).

The finite element analysis software Ansys Fluent was employed to simulate the LN2 jet impingement process through a pressure-based transient analysis. The simulation utilized the widely adopted k-epsilon model with standard wall functions to accurately capture the interaction between the flow and the wall. This model, well-established in industrial flows, ensured a comprehensive representation of the LN2 jet impingement process. The use of Ansys Fluent facilitated a detailed analysis of heat transfer dynamics and fluid-wall interactions during the simulation, contributing to a thorough understanding of the process.
<figure><img src="image_2706.png" /><figcaption>Fig. 2 IHTC transfer between Fluent and Abaqus</figcaption></figure>

Regulation of the Forming Process. Following the analysis of heat transfer efficiency, interfacial heat transfer coefficients (IHTC) for LN2 jet cooling under various parameters were obtained. The forming process was then simulated using Abaqus, incorporating the corresponding IHTC for different regions and time periods, as shown in Fig. 2.

In the simulation of the forming process, the material of the sheet is 22MnB5, and its true stress-strain curve is shown in Fig. 3a. The die was composed of eight-node thermally coupled hexahedral elements of type C3D8T, whereas the sheet was composed of four-node thermally coupled doubly curved thin shell elements of type S4RT utilized in the three-dimensional finite element model. Meshes were finer in the rounded corners of the die and the walls of the cooling channels to improve calculation efficiency, whereas meshes were coarser in the other locations. The upper die, lower die and sheet were meshed using the Abaqus/Explicit mesh module, which contained 11735, 11070 and 3267 nodes as shown in Fig. 3b.

<figure><img src="image_2707.png" /><figcaption>Fig. 3. (a) True stress-strain curve, (b) Meshing in forming models</figcaption></figure>

In contrast to conventional hot stamping, the closing process of jet impingement-cooled hot stamping involves the continuous presence of an LN2 jet, cooling the sheet until the die is fully fitted. Subsequently, as the jet hole becomes blocked, the cooling method transitions primarily to
<figure><img src="image_2701.png" /><figcaption>Fig. 2 IHTC transfer between Fluent and Abaqus</figcaption></figure>

Regulation of the Forming Process. Following the analysis of heat transfer efficiency, interfacial heat transfer coefficients (IHTC) for LN2 jet cooling under various parameters were obtained. The forming process was then simulated using Abaqus, incorporating the corresponding IHTC for different regions and time periods, as shown in Fig. 2.

In the simulation of the forming process, the material of the sheet is 22MnB5, and its true stress-strain curve is shown in Fig. 3a. The die was composed of eight-node thermally coupled hexahedral elements of type C3D8T, whereas the sheet was composed of four-node thermally coupled doubly curved thin shell elements of type S4RT utilized in the three-dimensional finite element model. Meshes were finer in the rounded corners of the die and the walls of the cooling channels to improve calculation efficiency, whereas meshes were coarser in the other locations. The upper die, lower die and sheet were meshed using the Abaqus/Explicit mesh module, which contained 11735, 11070 and 3267 nodes as shown in Fig. 3b.

<figure><img src="image_2702.png" /><figcaption>Fig. 3. (a) True stress-strain curve, (b) Meshing in forming models</figcaption></figure>

In contrast to conventional hot stamping, the closing process of jet impingement-cooled hot stamping involves the continuous presence of an LN2 jet, cooling the sheet until the die is fully fitted. Subsequently, as the jet hole becomes blocked, the cooling method transitions primarily to
indirect cooling. The hot stamping process using LN2 jet cooling and conventional cooling is shown in Fig. 4. This regulatory approach enhances the understanding of the dynamic cooling mechanisms during the forming process. In instances where gaps exist between the plate and the fully closed die, the LN2 can flow through these openings, enhancing local heat transfer.

<figure><img src="image_2708.png" /><figcaption>Fig. 4 Temperature paths for high-strength steel sheets under different cooling methods</figcaption></figure>

## Results and Discussion

The simplification of the CFD model assumes the presence of only LN2 at the injection inlet, excluding any nitrogen. Additionally, it presupposes that only LN2 is present at the injection holes, and the experimental environment consists of nitrogen. Relevant parameters employed in the model are detailed in Table 1. The initial state of liquid nitrogen at the inlet was determined based on its inherent properties, and the jet height was defined in relation to the gap between the mold and the sheet in the actual stamping process. Additionally, it was assumed that the sheet enters the die at a temperature of 800°C after transfer and undergoes cooling to a temperature slightly above the martensitic transition temperature during the forming process. In other words, the sheet's temperature is within the range of 500-800°C.

<table><caption>Table 1 CFD model parameters</caption><thead><tr><td>Parameters</td><td>Value</td></tr></thead><tbody><tr><td>Inlet phase</td><td>LN2 (no gas) at 76 [K] (-196 [°C])</td></tr><tr><td>LN2 jet velocity at inlet, V<sub>inlet</sub></td><td>5-20 [m/s] (uniform)</td></tr><tr><td>Saturation temperature at 1 bar</td><td>77 [K]</td></tr><tr><td>Jet hole diameter, D</td><td>1 [mm]</td></tr><tr><td>Hole-target spacing, S</td><td>5-20 [mm]</td></tr><tr><td>Distance from the intersection of jet centerline along the target wall, r</td><td>10 [mm]</td></tr><tr><td>Temperature of the target wall</td><td>500-800 [°C]</td></tr></tbody></table>

LN2 was injected onto the surface of the high-temperature sheet at a velocity of 20 m/s from the exit of the jet hole, dispersing along the surface to both sides at a velocity of approximately 20.56 m/s after dispersion, as depicted in Fig. 5a. In the proximity of the contact between the center axis of the jet and the sheet, the jet velocity experiences a sharp decrease, creating a stagnation zone and resulting in high static pressure in this region, as illustrated in Fig. 5c. Fig. 5b provides
indirect cooling. The hot stamping process using LN2 jet cooling and conventional cooling is shown in Fig. 4. This regulatory approach enhances the understanding of the dynamic cooling mechanisms during the forming process. In instances where gaps exist between the plate and the fully closed die, the LN2 can flow through these openings, enhancing local heat transfer.

<figure><img src="image_2703.png" /><figcaption>Fig. 4 Temperature paths for high-strength steel sheets under different cooling methods</figcaption></figure>

## Results and Discussion

The simplification of the CFD model assumes the presence of only LN2 at the injection inlet, excluding any nitrogen. Additionally, it presupposes that only LN2 is present at the injection holes, and the experimental environment consists of nitrogen. Relevant parameters employed in the model are detailed in Table 1. The initial state of liquid nitrogen at the inlet was determined based on its inherent properties, and the jet height was defined in relation to the gap between the mold and the sheet in the actual stamping process. Additionally, it was assumed that the sheet enters the die at a temperature of 800°C after transfer and undergoes cooling to a temperature slightly above the martensitic transition temperature during the forming process. In other words, the sheet's temperature is within the range of 500-800°C.

<table><caption>Table 1 CFD model parameters</caption><thead><tr><td>Parameters</td><td>Value</td></tr></thead><tbody><tr><td>Inlet phase</td><td>LN2 (no gas) at 76 [K] (-196 [°C])</td></tr><tr><td>LN2 jet velocity at inlet, V<sub>inlet</sub></td><td>5-20 [m/s] (uniform)</td></tr><tr><td>Saturation temperature at 1 bar</td><td>77 [K]</td></tr><tr><td>Jet hole diameter, D</td><td>1 [mm]</td></tr><tr><td>Hole-target spacing, S</td><td>5-20 [mm]</td></tr><tr><td>Distance from the intersection of jet centerline along the target wall, r</td><td>10 [mm]</td></tr><tr><td>Temperature of the target wall</td><td>500-800 [°C]</td></tr></tbody></table>

LN2 was injected onto the surface of the high-temperature sheet at a velocity of 20 m/s from the exit of the jet hole, dispersing along the surface to both sides at a velocity of approximately 20.56 m/s after dispersion, as depicted in Fig. 5a. In the proximity of the contact between the center axis of the jet and the sheet, the jet velocity experiences a sharp decrease, creating a stagnation zone and resulting in high static pressure in this region, as illustrated in Fig. 5c. Fig. 5b provides
insight into the interaction between LN2 and a high-temperature sheet. At the center of the jet, there exists a very thin gas film between the LN2 and the sheet. With increasing distance from the axis of the jet center, the gas film thickness increases, impeding heat exchange and leading to a gradual decline in the heat transfer coefficient.

<figure><img src="image_2709.png" /><figcaption>Fig. 5 CFD simulation results for (a) Velocity cloud, (b) Volume fraction of LN2, (c) Pressure at the stagnation point and (d) Distribution of IHTC on the sheet surface</figcaption></figure>

Fig. 5d showcases IHTC on the sheet's surface at varying distances from the jet center axis. Interestingly, the maximum heat transfer coefficient occurs at 1.5 mm from the jet center, rather than at the intersection of the jet center axis and the sheet. This is attributed to a severe decrease in LN2 flow rate at the center, weakening the heat transfer effect, while the fluid velocity increases at 1.5 mm from the center of the jet, enhancing heat transfer efficiency.

The interfacial heat transfer coefficient, calculated within a 5mm radius from the center of the jet, serves as a key parameter influenced by different combinations of height, jet velocity, and sheet surface temperature. Fig. 6a illustrates the IHTC corresponding to a jet inlet velocity of 20 m/s, with variations in jet distance between 5-20 mm and sheet surface temperature ranging from 500-800°C. The heat transfer coefficient exhibits an increasing trend with the augmentation of the jet distance, possibly because the increased distance causes the vortex created by the jet to be fully developed. This compels the diverted LN2 to approach the surface of the sheet, thereby enhancing heat transfer efficiency. Fig. 6b showcases interfacial heat transfer coefficient curves at a jet inlet velocity between 5-20 m/s and a sheet surface temperature between 500-800°C, with a fixed jet distance of 20 mm. The interfacial heat transfer coefficient increases gradually with the rise in jet velocity. The heightened jet velocity enhances the heat transfer between LN2 and the high-temperature sheet surface. However, as the surface temperature of the sheet increases, the interfacial heat transfer coefficient gradually decreases. This decrease is attributed to the intensification of vaporization of LN2 due to the increase in superheat, and the generated nitrogen gas hindering heat transfer.