# Grain-boundary refinement in wire-arc directed energy deposition of duplex stainless steel via in-process ultrasonic vibration

Antoine Queguineur, Olli Wiikinkoski, Mohammad Hallaji, Gaurav Mohanty, Jean-Yves Hascoët, Iñigo Flores Ituarte

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Antoine Queguineur, Olli Wiikinkoski, Mohammad Hallaji, Gaurav Mohanty, Jean-Yves Hascoët, et al. Grain-boundary refinement in wire-arc directed energy deposition of duplex stainless steel via in-process ultrasonic vibration. Welding in the World, 2025, 10.1007/s40194-025-02123-1. hal-05264723

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# Grain-boundary refinement in wire-arc directed energy deposition of duplex stainless steel via in-process ultrasonic vibration

Antoine Queguineur<fnref n="1" />,<fnref n="3" /> ID · Olli Wiikinkoski<fnref n="1" /> · Mohammad Hallaji<fnref n="1" /> · Gaurav Mohanty<fnref n="2" /> · Jean-Yves Hascoët<fnref n="3" /> · Iñigo Flores Ituarte<fnref n="1" />

Received: 2 September 2024 / Accepted: 8 July 2025
© The Author(s) 2025

## Abstract

Wire-arc directed energy deposition (WA-DED) is the subject of extensive research in metal additive manufacturing (AM). This study investigates the influence of ultrasonic vibration (UV) on the material properties of deposited ER2209 duplex stainless steel filler wire and evaluates the effects of UV treatment on walls fabricated both with and without weaving. The results demonstrate the effectiveness of a specially developed UV table prototype, showing clear impacts on grain size and ferrite content in the deposited samples. UV treatment reduces the primary ferrite grain width and enhances phase distribution homogeneity, potentially influencing the ferrite-to-austenite transformation during successive reheating cycles. Although the effects on surface morphology and hardness were minimal, significant microstructural changes occurred within the deposited material. UV-induced grain refinement modifies the austenite content, revealing a beneficial interaction between vibration and phase evolution. The UV table design provides a valuable foundation for reproducibility; nevertheless, further optimization of the setup is required to improve performance and support its integration into advanced industrial applications.

**Keywords** Additive manufacturing · Directed energy deposition · Stainless steel · Ultrasonic vibration · WA-DED · Grain refinement

Recommended for publication by Commission I - Additive Manufacturing, Surfacing, and Thermal Cutting

<figure><img src="image_2.png" /></figure>

Antoine Queguineur
antoine.queguineur@tuni.fi

Olli Wiikinkoski
olli.wiikinkoski@tuni.fi

Mohammad Hallaji
mohammad.hallaji@tuni.fi

Gaurav Mohanty
gaurav.mohanty@tuni.fi

Jean-Yves Hascoët
jean-yves.hascoet@ec-nantes.fr

Iñigo Flores Ituarte
inigo.floresituarte@tuni.fi

## 1 Introduction

Directed energy deposition (DED), a category of additive manufacturing (AM) as per ASTM 52900 [1], utilizes an energy source to melt materials, either in powder or wire form. DED techniques are increasingly used in industrial applications such as naval and energy sectors, where process selection influences deposition rate, capacity, and material quality with benefits in terms of environmental impact [2–6]. Wire-arc directed energy deposition (WA-DED) is a subset of DED that uses an electric arc to melt a metallic filler metal and is based on gas metal arc welding (GMAW). WA-DED achieves a high deposition rate, from 3 to 10 kg/h, while maintaining good material characteristics [7, 8].

Higher deposition rate increases the arc energy (AE), which affects grain size and material properties. Balancing the deposition rate and the mechanical–microstructural properties is crucial. Widely used in industrial applications due to their corrosion resistance and mechanical properties, duplex stainless steels (DSSs) exhibit complex microstructural evolution under additive manufacturing conditions. Most steel grades are sensitive to epitaxial growth in WA-DED

<footnote n="1">¹ Faculty of Engineering and Natural Sciences-Automation
Technology and Mechanical Engineering, Tampere
University, 33720 Tampere, Finland</footnote>

<footnote n="2">² Faculty of Engineering and Natural Sciences-Materials
Science and Environmental Engineering, Tampere
University, 33720 Tampere, Finland</footnote>

<footnote n="3">³ Ecole Centrale de Nantes, GeM-UMR CNRS 6183, 1 Rue de La Noé, 44321 Nantes, France</footnote>
with rapid fusion and repeated thermal cycles promoting large grain-oriented structures leading to anisotropic material properties [6, 9]. Using a DSS ER2209 filler metal as defined by the AWS-5.9 standard [10], Zhang et al. [11] have reported a columnar ferrite grains structure with epitaxial growth across the layers leading to anisotropic mechanical properties. The dual phase nature of DSS is of particular interest due to its solidification occurring ferritically, followed by austenite precipitation within the 1300–800 °C temperature range. Zhang et al. [12] have summarized the austenite transformation process as primarily occurring at the grain boundaries, with grain boundary austenite (GBA), followed by the formation of Widmanstätten austenite (WA) and intragranular austenite (IGA) within the ferrite matrix. The numerous reheating cycles during deposition can lead to substantial secondary austenite precipitation as reported by Posch et al. [6]. The objective of 40/60 ferrite to austenite ratio is a typical AM requirement for DSS applications [13, 14]. Excessive ferrite content has been shown to negatively affect impact toughness at low temperatures [15]. Studies on WA-DED using ER2209 filler metal revealed a range of ferrite content from 30 to 65% depending on a variation of deposition parameters [2, 6, 8, 16]. Applying similar AE, ER2209 filler metal exhibits a lower ferrite content than ER2205 [8]. It is attributed to its higher nickel content as nickel is known to promote the austenite formation [17].

In AM applications, various techniques for grain size control are employed either in-process or post-process and include methods such as shot peening, rolling, and thermal control. Ultrasonic shot peening induces plastic deformation in the as-deposited layer, promoting recrystallization at the interlayer bands and resulting in finer grains [18]. Interlayer cold or hot rolling generates many dislocations refining the microstructure, while reducing porosity in the material via plastic deformation [19, 20]. Precise control of the heating and cooling cycles influences grain size and phase distribution [21]. Adding an external system that mechanically affects the as-deposited layers can significantly complicate the manufacturing process, particularly for parts with complicated path planning. Furthermore, extensive post-fabrication heat treatments may introduce considerable part distortion, limiting their applicability for large components. Other techniques that influence grain refinement while maintaining a high deposition rate must address these considerations to facilitate the industrialization of the WA-DED process.

A novel technique emerges with the use of ultrasonic vibration (UV) to affect the microstructure. A few studies have investigated how vibration frequency affects grain size in single or multilayer deposits [22]. Ji et al. [23] demonstrated through computational fluid dynamics (CFD) simulation that UV induces pressure variations in the molten pool, causing cavitation and shock waves, impacting temperature fields and layer morphology in single beads. More research

has focused on austenitic stainless 316L, reporting significant grain refinement, defect reduction, and partial transition from columnar to equiaxed grains [24]. Vorontsov et al. [25] observed a reduction of δ-ferrite volume fraction when using UV, which was associated with a partial transition to an equiaxed microstructure, improving the mechanical properties. Despite previous studies already focusing on various materials, a substantial gap remains in understanding the effect of UV-assisted grain refinement on ER2209 filler metal. Additionally, details regarding the manufacturing of UV systems and their operating parameters are often insufficient for ensuring reproducibility and to enhance the adoption of this technology in AM.

This work aims to evaluate the impact of UV on the microstructure of walls deposited from ER2209 filler wire and to assess its potential for microstructural refinement. The influence of UV in ER2209 could affect the austenite precipitation through finer ferrite microstructure. Using UV to control the microstructure could reduce the reliance on conventional parameters such as voltage (V), current (I), and wire feed speed (WFS), enabling higher deposition rates while maintaining material properties. This introductory study aims to conduct experiments with consistent parameters, while also sharing an open-access prototype for a UV table to support research toward the production of large-scale AM.

# 2 Experimental

## 2.1 Material and manufacturing process

The manufacturing utilized a GMAW machine, TPS CMT-Advanced 4000R (FRONIUS, Wels, Austria) which was integrated with an ABB robot IRB-4600. The chosen synergic program, CMT+P, combines cold metal transfer with a pulsed arc in synergic mode. Four configurations were used, each consisting of a straight monobead wall structure. Each configuration was tested both with and without UV. The experiments included a comparison with and without weaving. To maintain surface uniformity, each layer was applied in alternating directions. The welding torch was kept perpendicular to the substrate, maintaining an average contact tip-to-work distance (CTWD) of 17 mm. The walls were printed with a target height of 40 mm and a length of 120 mm. The number of layers was adjusted as needed to reach this objective. After deposition, samples were extracted from these walls for further analysis. The process parameters, I, V, travel speed (TS), and WFS, are detailed in Table 1. The AE values are derived from the power source signal and calculated using Eq. 1), in accordance with EN 1011-1 [26].

<figure><img src="image_3.png" /></figure>
<table><caption>**Table 1** Nominal values for process parameters</caption><thead><tr><th></th><th>UV (yes/no)</th><th>Weaving (yes/no)</th><th>AE (kJ/mm)</th><th>WFS (m/min)</th><th>I (A)/V (V)</th><th>TS (mm/s)</th><th>Layer height (mm)</th></tr></thead><tbody><tr><td rowspan="4">ER2209</td><td>Yes</td><td>No</td><td>0.34</td><td>5.4</td><td>136/19.9</td><td>8.0</td><td>1.9</td></tr><tr><td>No</td><td>No</td><td>0.34</td><td>5.4</td><td>136/19.9</td><td>8.0</td><td>1.9</td></tr><tr><td>Yes</td><td>Yes</td><td>0.60</td><td>5.4</td><td>136/19.9</td><td>4.5</td><td>1.5</td></tr><tr><td>No</td><td>Yes</td><td>0.60</td><td>5.4</td><td>136/19.9</td><td>4.5</td><td>1.5</td></tr><tr><td colspan="8">Additional parameters</td></tr><tr><td colspan="8">Interpass temperature (IT)</td></tr><tr><td colspan="8">CTWD</td></tr><tr><td colspan="8">Wall height</td></tr><tr><td colspan="8">Gas flow</td></tr><tr><td colspan="8"></td></tr><tr><td colspan="8">Weaving amplitude</td></tr><tr><td colspan="8">Weaving frequency</td></tr><tr><td colspan="8"></td></tr></tbody></table>

$$AE = \frac{U \times I}{TS} \qquad (1)$$

where AE is the arc energy expressed in kJ/mm, U the arc voltage in volts (V), I the current in amperes (A), and TS the travel speed in mm/s.

The selected weaving parameters presented in Table 1 were established with an amplitude of 12 mm and a frequency of 2 Hz. Adopting these settings resulted in a decrease in the TS to 4.5 mm/s from the 8 mm/s associated with a straight bead deposition strategy, which increased the AE to 0.60 kJ/mm. The IT was maintained below 200 °C by measuring the top surface between layers with a contact thermometer.

ER2209 filler metal was used with a 1.2-mm diameter, and the chemical composition is detailed in Table 2. The chosen shielding gas was a mixture of argon (Ar) with 30% helium (He) and 2% carbon dioxide (CO₂). The substrate utilized was S355 low carbon steel, measuring 300 mm by 150 mm and 5-mm thick, without preheating. Before manufacturing, the surface was ground to remove oxidation and then cleaned with ethanol.

## 2.2 Ultrasonic vibration system

Figure 1a shows the manufacturing robotic cell including the vibration table used in this study, along with the welding torch positioned above the part. The technical drawings, bill of materials, and assembly details of the ultrasonic vibration system are openly accessible on GitHub for the research community [27].

The ultrasonic system employed in the experiments featured a 1200 W I1200-WD-35 K generator (SINAPTEC, Lezennes,

France), Fig. 1b. The converter transforms the electrical output from the generator into mechanical movements at high frequency, thereby producing ultrasonic vibrations, here fixed at 35 kHz. The stepped sonotrode, made of TA6V titanium material, is tuned at half wavelength and consists of two different sections, as shown in Fig. 1c, set to maximize the gain. The amplitude gain is calculated based on the dimensions of the two cross-sections of the sonotrode, as expressed in Eq. 2), resulting in a theoretical gain of 3.41. Given the converter's displacement amplitude of 12 µm, the consequent theoretical amplitude at the sonotrode extremity, considering the maximal attainable value of the system, was 41 µm.

$$G_A = \sqrt{\frac{A_i}{A_o}} = \frac{u_o}{u_i} \qquad (2)$$

where $G_A$ is the sonotrode amplitude gain (unitless); $A_o$ is the output cross-sectional area of the sonotrode, at the tip; and $A_i$ is the input cross-sectional area of the sonotrode, at the converter interface, in mm²; $u_i$ is the displacement amplitude at the converter interface, and $u_o$ the amplitude at the sonotrode tip in µm.

The vibration waves were perpendicular to the substrate and consequently each printed layer. In this study, the amplitude of the ultrasonic system was initially set to 75% of its maximum capacity, as previously noted. Consequently, the resulting amplitude at the sonotrode was about 31 µm. Figure 1d shows the setup of the pneumatic unit used to maintain a constant contact pressure between the sonotrode and the substrate, essential for transmitting ultrasonic waves to the part during manufacturing. Applying a contact pressure of 5 bars to the pneumatic unit generates a total force of 113.1 N from the 2

<table><caption>**Table 2** Filler wire chemical composition (wt-%)</caption><thead><tr><th></th><th>C</th><th>Si</th><th>Mn</th><th>P</th><th>S</th><th>Cr</th><th>Ni</th><th>Mo</th><th>Cu</th><th>N</th><th>Fe</th></tr></thead><tbody><tr><td>ER2209</td><td>0.014</td><td>0.48</td><td>1.57</td><td>0.018</td><td>0.001</td><td>23.02</td><td>8.74</td><td>3.14</td><td>0.05</td><td>0.146</td><td>Bal</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>Fig. 1 UV system showing (a) the manufacturing setup, (b) a schematic view of the entire system, (c) the sonotrode, and (d) a detailed view of the subsystem generating vibration</figcaption></figure>

pistons of 12 mm each using Eq. 3). This force is transmitted through the sonotrode to the substrate and ensures effective energy transfer and enhances the efficiency of ultrasonic vibrations.

$$F = P/s \qquad (3)$$

where F is the applied force in Newton (N), P the pressure applied in Pascal (Pa), and S the contact area in m².

## 2.3 Characterization

A FeritScope® testing unit, based on the magnetic induction method, was utilized and installed on a 3-axes Ender-3Pro Cartesian motion platform. The measuring probe, FGAB1.3-Fe (Helmut Fischer GmbH, Sindelfingen, Germany), was automatically positioned, maintaining an approach speed of less than 4 mm/s and a lift-off distance of 20 mm between successive measurement points. FeritScope® measurements were performed across the entire cross-section of each sample, with a fixed spatial resolution of 1 mm. The measurements are depicted in two-dimensional colormaps to visually illustrate the data distribution across the samples and potential variations. The values, presented as a percentage

of ferrite, exclude the areas within 2 mm of the edges, which impact the sensitivity of the FeritScope® measurements.

For the microstructural characterizations, the samples were ground and polished to 1 µm. For the optical characterization of ER2209, a Beraha-II mod. color etchant (170 mL H₂O, 30 mL HCl, and 2 g K₂S₂O₅) was used. This etchant selectively colors the ferrite phase, allowing it to be easily distinguished from the austenite phase. All images were taken using a Leica Dmi8 optical microscope. To evaluate the effects of UV on grain structure, cross-sectional sections were analyzed. Ferrite was measured using image analysis with color segmentation using ImageJ software. The method used a similar approach as described in previous research and is based on contrast differentiation in etched cross-sectional samples [8, 28].

A Vickers indenter, mounted on a NEMESIS 5100G2 hardness testing unit (Innovatest, Maastricht, The Netherlands), was used for hardness measurements. Vickers hardness testing (HV10) was performed with a 10 kgf test load and a dwell time of 10 s per indent, in accordance with the recommendations of the EN ISO 6507-1 standard [29]. To obtain comprehensive cross-sectional mapping, spatial resolutions were defined to scan both thin and thick samples in the studied materials. The surface morphology of the deposited walls in all configurations was analyzed using
an Alicona Infinite Focus G5 (Alicona, Graz, Austria) to measure optical surface waviness, achieving a lateral resolution of 4 µm. The inspected areas of the samples measured 20 mm × 2.81 mm.

## 3 Results

Microstructural observations of the walls produced with weaving, with and without UV treatment, are shown in Fig. 2. The two samples responded differently to the etchant; the UV-treated sample appeared slightly more blurred despite identical etching times. Structural variations are visible across both cross-sections after etching. Both samples exhibit columnar grain structures consisting of primary ferrite and austenite, IGA, WA, and localized secondary austenite formed by reheating from subsequent passes. Although the complex phase distribution, especially at low

magnification, makes this difficult to discern clearly, the UV-treated sample shows narrower primary grains. The average ferrite content in the UV-treated, woven sample was 47.8 ± 3.0%, compared to 36.0 ± 2.4% in the regular configuration. These values reflect averages across all layers, excluding the final deposited layer.

The microstructure of the final deposited layer is noteworthy, exhibiting a higher ferrite content. Although this layer is typically removed by milling in applications, it provides a practical opportunity to examine the effect of UV treatment on the microstructure, as the vibrations primarily influence the molten pool and, consequently, the solidification process. The higher ferrite content is attributed to its as-built condition, without reheating cycles, and to the fact that vibration influences the material during solidification rather than affecting the already solidified metal. The microstructural analysis of the ferrite grain width in the final layer of each wall shows an average spacing of 144 ± 64 µm with UV treatment, compared

<figure><img src="image_5.png" /><figcaption>**Fig. 2** Cross-sections of multilayer ER2209 walls performed using weaving: (a) with UV and (b) regular deposition (red dots indicate areas where image analysis was used for ferrite measurements)</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 3** Cross-sections of multilayer ER2209 walls performed without weaving: (a) with UV and (b) regular deposition (red dots indicate areas where image analysis was used for ferrite measurements)</figcaption></figure>

to 331 ± 110 µm without UV treatment. Under similar process parameters, the average grain width is significantly reduced under the influence of the UV process.

Cross-sections of walls manufactured without weaving are shown in Fig. 3. The stitched macroscopic image reveals the effect of UV treatment on the grain structure, with a thinner and shorter ferritic columnar structure. In contrast, the wall without UV treatment exhibits large columnar grains. The regularly deposited wall shows a wider grain structure compared to its weaving equivalent. The average ferrite content measured in the samples with UV was 45.0 ± 2.4% and 42.8 ± 4.0% in the regular configuration, with the values falling within the standard deviation, showing no significant statistical difference.

Focusing on the analysis of the last layer in its as-deposited structure, similar measurements of the grain width were conducted. The microstructural analysis shows, in walls without weaving, an average grain width of 113 ± 34 µm with UV treatment and 228 ± 157 µm in the regularly deposited sample.

The wall surface morphologies are presented in Fig. 4 for the samples deposited without weaving. The waviness analysis shows that the surface with UV treatment has a similar overall profile, measuring max. 78 µm compared to 83 µm without UV. The surface undulations show slight variation, reaching a maximum of 400 µm with UV treatment and 380 µm without UV.

The evaluation of the ferrite content in the complete cross-section of the walls with a FeritScope® is shown in Fig. 5. No significant variation is observed, with similar ferrite content variations showing a higher percentage in the last layer of each of the four-sample walls studied. The average values for ferrite content were 35.4 ± 1.3% and 36.3 ± 1.3%, without weaving, and 34.2 ± 1.5% and 34.5 ± 1.6%, for weaving samples, respectively, with and without UV treatment.

The 2-dimensional hardness mapping of the sample cross-section is presented in Fig. 6. Both sets of experiments, with and without weaving, exhibited a wide scatter.

<figure><img src="image_7.png" /></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 4** Surface morphologies of as-deposited ER2209 walls, acquired using an optical profilometer: (a) scanned wall surface with UV and (b) scanned regular wall surface</figcaption></figure>

These results did not indicate significant statistical differences, on average, between the samples. Without weaving, the hardness was 248.3 ± 4.6 HV₁₀ with UV treatment and 251.1 ± 5.7 HV₁₀ in regular deposition, while with weaving, it was 244.4 ± 7.4 HV₁₀ with UV treatment and 247.5 ± 6.3 HV₁₀ in regular deposition. The hardness was higher in the last deposited layer of the walls, which aligned with the increased ferrite content observed in this region.

## 4 Discussions

This study investigated how UV treatment applied during deposition affects the microstructure of duplex ER2209 wire deposits fabricated by WA-DED under different processing conditions. While UV treatment did not lead to substantial changes in surface morphology or hardness, it affected the microstructure with a significant refinement of primary ferrite grains and, in some cases, a shift in the ferrite-to-austenite balance. These microstructural changes were especially apparent in the final layers and under weaving conditions, where thermal gradients and reheating cycles differ from standard deposition. No cracks and only minor isolated gas pores (<0.2 mm) were observed in all samples, indicating that UV treatment did not influence defect formation under the investigated processing conditions. These findings highlight the potential of UV as a process control tool for tailoring microstructure in multi-pass WA-DED applications.

Additionally, this study contributes to the field through the development of an open-source ultrasonic vibration system. Future investigations may explore the effect of UV on intermetallic phase formation, optimize vibration parameters, and further examine localized transformation behavior during WA-DED.

### 4.1 Surface quality and hardness

Surface morphology analysis revealed only minor variations in waviness between UV-treated and regular samples, indicating a limited influence of UV on the overall surface topography under the investigated conditions. Ji et al. [23] studied the influence of UV on deposition layer morphology in WA-DED, demonstrating that it promotes a more uniform and refined surface profile. This effect was attributed to enhanced melt pool fluidity and reduced surface waviness resulting from UV-induced cavitation. However, in the present study, this mechanism did not lead to a significant modification of surface waviness at the scale of the wall comprising multiple layers. Further research could explore the impact of different ultrasonic vibration frequencies and amplitudes on surface morphology to better understand their influence.

Hardness measurements did not demonstrate statistically significant differences between samples. Hardness variability can be influenced by several factors, including local ferrite-austenite ratios, grain morphology, and potential presence of intermetallic phases. The slight variations observed likely reflect local microstructural heterogeneities rather than systematic effects of UV treatment.

### 4.2 Grain refinement and solidification features

Microstructural observations confirmed columnar grain structures composed of ferrite and primary austenite across all samples. The solidification was fully ferritic, with epitaxial growth of the primary ferrite grains, which occurs perpendicular to the solidification front. During cooling, the solid-state transformation from ferrite to austenite begins, influenced by the cooling rate [30]. This transformation initiates at the grain boundaries forming the GBA followed by the precipitation of austenite within
the primary ferrite grains [12]. This eventually forms IGA and WA structures, as observed in Fig. 2. The presence of secondary austenite is attributed to the multiple reheating cycles experienced during the multilayer deposition process. UV-treated samples exhibited noticeably narrower and shorter ferrite grains, particularly in the walls with weaving.

Microstructural variations can be influenced by wall geometry, particularly through differences in wall thickness that affect heat dissipation and, consequently, grain structure. However, the results indicate that the influence of UV treatment remained consistent across the different wall geometries tested. This consistency suggests that the primary effects of UV treatment are likely linked to modifications in melt pool dynamics, rather than being significantly impacted by geometric variations.

In deposited DSS, solidification typically follows the grain orientation of the underlying layer, with competitive grain growth governing the evolution of the dual-phase microstructure [31–33]. Todaro et al. [34] investigated solidification under UV treatment and reported the formation of a wider constitutional supercooling (CS) zone, a region of supercooled liquid ahead of the solidification front. In this zone, the temperature drops below the equilibrium solidification point, promoting nucleation and the growth of new grains. The presence of a wider CS zone in UV-assisted deposition was found to facilitate more uniform grain nucleation and refinement. Although that study focused on ER316L, the fundamental principles are

applicable to DSS, as ER2209, supporting the observation of finer ferrite grains in the present work.

Although grain refinement was confirmed, the UV treatment did not significantly alter the overall grain growth direction or columnar morphology, implying that the fundamental solidification mode remains similar but with finer structural features.

## 4.3 Ferrite content variation

The most prominent effect of UV treatment was observed in the variation of ferrite content. Analysis of phase balance from both magnetic (Feritscope®) and optical image analysis techniques highlighted method-specific limitations. Magnetic measurements, though useful for general trends over larger surface areas, lack the resolution needed for precise characterization of localized microstructural variations. Optical image analysis remains the high-resolution reference method, despite its limited applicability to extensive surfaces, as previously confirmed by Putz et al. [35]. Quantitative image analysis revealed that, across the full wall thickness, excluding the final layer, UV treatment led to an increase in ferrite content in samples produced with weaving, while only a slight, statistically insignificant increase was observed in the configuration without weaving. However, the final deposited layer, which was not affected by reheating from subsequent passes, exhibited a markedly higher ferrite content compared to the underlying layers. In the configuration without weaving, UV treatment had a limited effect, resulting in a slightly lower ferrite content (49.5% ± 1.9%) compared to regular deposition (51.9% ± 1.7%), with values falling within the standard deviation. In the samples with weaving, no statistically significant difference was observed, indicating that the influence of UV in the final layer was primarily limited to grain refinement rather than altering the phase balance.

These findings suggest that the effect of UV treatment on phase balance is strongly dependent on the thermal history of the material. In the bulk of the walls, where multiple reheating cycles occur due to subsequent layer deposition, the application of UV may promote localized changes in melt pool dynamics and solute redistribution, leading to a slight increase in ferrite content, especially in configurations with weaving, more thermally affected. UV treatment contributes to grain refinement, but its influence on phase balance is more noticeable in regions subjected to repeated thermal cycles, highlighting the complex interaction between process parameters, heat flow, and phase transformations in DSS.

<figure><img src="image_9.png" /><figcaption>Fig. 5 FeritScope® measurements of a sample (a) without weaving and (b) with weaving</figcaption></figure>

<figure><img src="image_10.png" /></figure>
<figure><img src="image_11.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 6** Hardness (HV₁₀) of UV-treated and regularly deposited ER2209, (a) without weaving and (b) with weaving—and their associated 2-dimensional mapping</figcaption></figure>

**5 Conclusions**

This study investigates the influence of ultrasonic vibration (UV) treatment during wire-arc directed energy deposition (WA-DED) of duplex stainless steel (DSS) ER2209 wire on microstructure, phase balance, surface features, and hardness. The positive effects of ultrasonic vibration (UV) on the microstructure and phase balance were successfully demonstrated. The main contributions of the study are as follows:

* All samples were free of cracks and showed only minor, isolated porosity, indicating that UV treatment did not adversely affect defect formation.

* UV treatment had minimal impact on surface morphology, with negligible differences in surface waviness across all tested configurations.

* Hardness measurements exhibited significant variability, with no consistent differences observed between UV-treated and untreated samples. This is attributed to
local fluctuations in phase balance and potential micro-
structural heterogeneities.

* Ferrite grain refinement was clearly observed in UV-treated samples. UV resulted in reduced primary ferrite grain width, including finer and shorter ferrite grains. However, the overall grain growth direction was not significantly affected.

* UV treatment positively influenced the phase distribu-
tion with an increase in ferrite content, particularly when
applying weaving. This effect is presumably linked to
acoustic streaming mechanisms, which enhance ele-
mental mixing, promote homogeneity, and subsequently
influence elemental diffusion.

Future research will focus on investigating the potential
influence of ultrasonic vibration on the solidification direc-
tion, particularly its effect on grain orientation and competi-
tive growth mechanisms in duplex stainless steel.

**Author contribution** The first draft of the manuscript was written by Antoine Queguineur and Iñigo Flores Ituarte, and all authors commented on previous versions of the manuscript. Antoine Queguineur conceived and designed the experiments. The prototype has been designed and assembled by Olli Wiikinkoski. Material preparation, data collection, and analysis were performed by Antoine Queguineur, Olli Wiikinkoski, and Mohammad Hallaji. All authors read and approved the final manuscript. Iñigo Flores Ituarte, Gaurav Mohanty, and Jean-Yves Hascoët have supervised this study.

**Funding** Open access funding provided by Tampere University (including Tampere University Hospital). The contributions of Antoine Queguineur, Olli Wiikinkoski, and Mohammad Hallaji acknowledge partial funding support from the Business Finland project TANDEM (4056/31/2021), granted under the SMART EUREKA cluster on Advanced Manufacturing, as well as from the Academy of Finland Research Fellowship (346874)—D2M (Multi-disciplinary Digital Design and Manufacturing).

**Data availability** The data supporting the findings of this study are
available from the corresponding author upon reasonable request.

Declarations

**Competing interests** The authors declare no competing interests.

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