# Prediction and Experimental Verification of Distortion due to Residual Stresses in a Ti-6Al-4V Control Arm Plate

Authors: Aayush Bhat; Renold S. Elsen; Daniel Abishai; M. Jayaprakash Narayan; Aravind Raj Sakthivel; Utkarsh Chadha; Bulcha Bukele Hirpha
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

This study evaluates the distortion in an SLM-printed Ti-6Al-4V control arm plate of a double-wishbone suspension linkage. Temperature gradients are a common observation in selective laser melting (SLM). The development of residual stresses can lead to performance-based concerns due to the distortions emerging in part. Despite these shortcomings, SLM has often been employed in manufacturing high-end structural components under its ability to produce complex geometries. A thermomechanical simulation predicts the residual stresses developed in part and the distortions generated by those stresses. The simulation model is validated by reconstructing the printed part using computerized tomography (CT) and aligning it with the original design. The results reveal a close correlation between the predicted and the observed distortion, with the thickness of features and the location of support structures having a significant impact on postprint parts. Additionally, the study on build orientation helps understand the distribution of residual stresses and their redistribution after removing support structures.


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## Body

## 1. Introduction

Additive manufacturing has progressed significantly, especially in the automotive and aerospace structures to fabricate lightweight, structural plastic, and metallic components. Metallic structures with load-bearing requirements are often optimized for material distribution to obtain a meaningful trade-off between the weight of the part and its load-bearing capability. Metal additive manufacturing (MAM) has been incorporated in similar applications, with similar characteristics to details obtained from conventional manufacturing processes. Selective laser melting (SLM) is a MAM process containing powdered particles of the raw material required to manufacture the part [1]. A laser beam is employed to melt layers of particles in an inert environment, as per the desired cross-section, after which another layer is deposited above. SLM’s most influencing process control parameters are the laser beam power, scanning strategy, and layer thickness [2]. The primary advantages are cost control to produce complex structures and minimal production times. In addition to that, the technique’s efficiency is magnified owing to the material used. High-performance metals such as titanium, specifically its alloy Ti-6Al-4V, have been used for their high strength and low density, often in the biomedical and aerospace industry. Their application covers a variety of components, from ordinary fasteners used to assemble a spin motor [3] to topologically optimized brackets printed using SLM [4]. The fatigue performance of Ti-6Al-4V alloy has been investigated by performing a high cycle fatigue (HCF) test on a dog-bone sample [5]. The samples were prepared using SLM and had part porosity less than 0.4%, and the fatigue limit was found to be about 40% of that of a standard limit value for the alloy. It was concluded that polishing could potentially improve its fatigue limit. The high strength and stiffness of the titanium alloy were researched postprinting via SLM [6]. Topologically optimized scaffolds were tested under compression and demonstrated high strength to weight and stiffness to weight ratios. An attempt to increase build rate and reduce cost and build time was made by increasing layer thickness [7]. A larger laser beam was implemented to form a more stable melt pool. The higher thickness resulted in poor surface finish, and the values of yield strength and elongation were similar to samples with small layer thickness due to similarity in the microstructure. The effects of heat treatment on Ti-6Al-4V parts were analyzed [8]. It was observed that the maximum temperature during treatment played a crucial role in determining the mechanical properties of the part, with increasing temperature causing a decline in yield strength of the part.The powder-based printing techniques operate with a significant number of variable printing parameters [9], which make manufacturing parts accurate and without defects challenging. Investigation into the performance of the printed parts due to the defects inherited while printing them has helped understand the reason behind them and their consequent impacts. For instance, the effect of distortions caused due to the residual stresses developed in part under temperature gradients can have severe effects on the operation of the final part. Figure 1 depicts the distortion in part due to the temperature gradient. The heat provided by the laser beam causes the layer being printed to experience compressive stress due to the restriction in expansion caused by the layers under it, which in turn causes the layer to bend away from the beam [10]. In cooling, however, the contraction of the layers causes the layer to experience tensile stresses, which lead to the layer bending towards the beam. These distortions affect the final geometry of the structure to be printed.Figure 1
Temperature gradient mechanisms causing residual stress formation during (a) heating and (b) cooling [10].
(a)(b)The residual stresses developed in SLM printed parts are unevenly distributed in different planes [11]. However, the presence of residual stresses was found to have no impact on the hardness of the part. The dissipation of heat is determined by the thermal conductivity of the metal being printed [12]. Since the different layers have different cooling rates, the contraction of the layers takes place at different rates, thus causing plastic deformations. The type of support structures used and the size of the parts to be printed can significantly impact the residual stresses developed. The influence of scanning patterns on the development of residual stresses has been investigated [13]. A horizontal line scanning strategy was found to cause the effect of directional stress distributions, and the out-in scanning strategies were found to develop deformations in the build directions. Rescanning the part was found to have detrimental impacts due increase in the residual stresses and subsequently increased the build time [14]. The type of powder material can impact the extent to which porosities are formed in an SLM part [15]. Mugwagwa et al. researched the impact of porosities on the residual stresses in a Ti6Al4V component [16]. An increase in the layer thickness was observed to cause an increase in the number of porosities in part. It was also observed that the presence of porosities caused a stress relaxation, due to which the distortions observed in part were minimal. The influence of the printing parameters, such as the energy density of the laser beam on the distortions experienced by parts manufactured by Ti-6A-l4V, was analyzed [17]. Increasing the temperature led to an increase in the thermal expansion of the alloy till the critical value of laser energy density was reached, and beyond this particular limit, high magnitudes of residual stresses and distortions were observed. The impact of scanning speed on the residual stress was found to be greater than that of the power of the laser beam [18].Support structures were found to play a crucial role in influencing the distortions of a part printed using SLM [19]. A part printed on support structures was found to have lower stress values than that of a part printed directly on the base plate but still required heat treatments to reduce the amount of residual stress. A small area of contact with the support structures made the part more compliant, resulting in distortions. A study on the distortions occurring in thin-walled aluminum parts helped us understand its magnitude with the change in dimensions of the sample produced by SLM [20]. The amount of distortion experienced by the part increased with an increase in the size of the sample. In addition to that, the impact of heat treatment did not significantly impact restoring the part’s dimensional accuracy. Before designing a part for MAM, it is essential to consider the effects of the heat source on the delicate features of the components. Thicker geometries are subject to lower distortions due to better heat dispersion. The residual stresses generated were greatly influenced by print parameters considered during manufacturing and the attachment with the base plate. The presence of strong support structures near the base plate results in lower distortions [21]. Properties such as thermal diffusivity have an impact on the final residual stresses, and thereby the distortions, as materials with high thermal diffusivity conducts heat faster, thereby reducing the magnitude of the generated thermal stresses [22].The research provides an insight into determining the deformation of an automotive structural component due to residual stresses after SLM. The work extends a previous study that examined the possibility of developing lightweight control arm plates for a double-wishbone suspension assembly using a generative design methodology [23]. The design developed possessed a complex geometry, as depicted in Figure 2, which would make it difficult to be fabricated via conventional manufacturing techniques. However, with the industrialization of MAM, the possibility of printing detailed profiles is feasible. This work investigates the deformation of the part via a thermomechanical simulation study, followed by CT scanning. The scanned geometry was analyzed and compared with the original one to understand the change in dimensions caused by the heat in the printing process. The research can provide a means of printing challenging structural components using MAM by determining the dimensional changes in the printed part, and to study the impact of those changes on the integrity and assembly with subcomponents.Figure 2
(a) Suspension arm assembly and (b) generatively designed control arm plate.
(a)(b)


## 2. Physical Mechanism in SLM

The molten material primarily absorbs the heat generated during the print phase. It then is dispersed in radiation to the environment, convection between the molten material, powder bed, and the environment, and conduction between the component, support structures, and print bed. Considering these parameters, the heat losses through conduction can be defined as [24]:(1)Qcond=−kΔT,where Qcond describes the heat loss via conduction, k defines the thermal conductivity, and ΔT defines the temperature difference between the surface temperature T of the component and the ambient temperature Tamb.Similarly, the heat loss due to convectionQconv and due to radiation Qrad can be expressed as [25]:(2)Qconv=hT−Tamb,Qrad=σεT4–Tamb4,where σ represents the Stefan–Boltzmann constant and ε represents the emissivity of the powder bed.The first law of thermodynamics defines the energy balance in a closed system as [26](3)Qinp=Qcond+Qconv+Qrad,where Qinp represents the total laser energy input.Assuming a Gaussian laser intensity distribution, the thermal strainεth can be defined as [27](4)εth=∫TreftαeTdT=αeΔT,where αe defines the coefficient of thermal expansion, t defines the interaction time, and Tref is the preceding layer’s reference temperature.The overall elastic strain can be defined by [28](5)εe=ε∗εth,where {εe} is the elastic strain vector and {ε} is the total strain vector.The relationship between stress and strain can be given by(6)σ=Dεe,where {σ} being the stress vector and [D] representing the matrix of elasticity.Substituting equations (5) into (6) provides the final relation between stress and strain:(7)σ=D∗ε∗εth.


## 3. SLM Modeling

The design features of the control arm plate consist of complex sections with varying thicknesses, such as the pushrod tabs and organic surfaces with varying contours. The requirement of obtaining precise features despite their complexity can be owed to assembly requirements with other components. The presence of an organic body connecting the bearing slot with the control arm inserts and the pushrod tabs is a complex feature, with varying attachment areas with the slot and inserts. An accurate fabrication of this feature is a priority in regards to effective load transfer. Additive manufacturing was considered over conventional manufacturing techniques such as multiaxis milling due to the form of the organic body. A powder bed fusion (PBF) technique to melt powdered particles to form slices of each layer would provide greater accuracy than material subtraction techniques, which would require the use of tools of varying dimensions to provide passage into the narrow features of the plate. In addition to that, the requirement of a multiaxis feature would significantly increase the cost of manufacturing, and since the material of the plate is a titanium alloy Ti-6Al-4V, poor surface quality due to spring back is a common observation. Additionally, the raw material cost would not be justified considering the amount of material removed.The simulation model is developed considering a few assumptions [29]:(a)
Owing to the miniature dimensions of the melt pool under very short heat exposure duration, a majority of the heat transfer is considered to occur due to conduction and the effects of heat loss via convection and radiation are neglected.(b)
The powdered particles are assumed to be homogenous throughout, thus developing consistent melt pools in terms of quality and dimensions.


### 3.1. Print Orientation Selection

Altair Inspire is employed to create a workspace designed to mimic SLM. The plate is imported into the Print 3D module in Altair Inspire. The design of support structures play an important role in determining the quality of the final component prepared. The support structure design can enable understanding of the extent to which a component could distort, based on the area of contact with the supports [30]. The supports can help prevent the extent of deformation, as they increase the heat transfer rate from the melt pool and dissipate it throughout the component, as well as the base plate. The orientation for building the part is iterated initially by maximizing and minimizing the build height. Usually, the build height is maintained as low as possible to reduce the print time, which consequently reduces the cost of printing as well. In addition to the cost, the parameters taken into consideration are the area of contact between the part and the support structure, support volume, and time required for the printing process. The area of contact and the support volume will help determine the amount of support structure required for reinforcing the overhang features, and the overall cost addition due to additional support structures. The choice of an optimal part orientation will help visualize the location of support structures required and the time required to print the entire part. Table 1 lists the results of varying the parameters and the different support structures generated. The support structures in both cases (a) and (b) indicate lodging of structures in narrow, critical regions, such as the space between the head of the inserts and the bearing slots, as depicted in the figures in Table 1. This can lead to increased postprocessing requirements and can also damage the features due to poor tool passage while removing the supports. In order to prevent this, case (c) indicated in Table 1 was assessed and ultimately incorporated. This custom orientation allowed for a reduction in bulky supports and let several sections be self-supported, and prevented the generated supports from getting wedged in narrow regions. In spite of the increased build height, this orientation is chosen by giving quality a greater priority than the printing cost.Table 1
Parameters for selection of build orientation and support structures.
OrientationSupport area (mm2)Support volume (mm3)Printing time (s)Figures(a) Maximum build height1292.052000.09593.99(b) Minimum build height3162.073000.07150.51(c) Custom orientation1240.051000.09518.11


### 3.2. Thermomechanical Simulation Setup

The thermomechanical analysis allows establishing a temperature field, followed by an estimation of the thermal stresses and associated distortions via a mechanical analysis. The SLM 280 2.0 twin laser printer is assigned for the printing simulation. The parameters associated with the specifications of the printer are listed in Table2. The model helps take into account the effects of thermal expansion of the material as well, thus increasing the accuracy of the final result. The boundary conditions applied for the study are as follows:(a)
The initial thermal boundary is an equal preprinting temperature assigned to the print bed and the powder layer.(b)
All the open surfaces of the part will be subject to radiation and convection.(c)
The degrees of freedom of the lower face of the plate are restricted to accurately simulate the adhesion of the plate to the build platform.Table 2
Specifications of SLM 280 2.0.
Technical specificationsValuesPower (P)400 WExposure (t)120μsBuild envelope280∗ 280 ∗ 365 mmBuild rate113 cm3/hrVariable layer thickness (Lt)20–90μmPoint distance (Pd)60μmBeam focus diameter80–115μmHatch spacing (h)80μmMaximum scan speed10 m/sAverage inert gas consumption5 l/min (argon)The temperature of the base plate is set at ambient temperature (25°C), and the selected print orientation is assigned to the part. Based on the selected parameters, the support geometry depicted in Table1 is developed. Slicing is observed to validate the part geometry during printing.The printer deposits layers of 60μm thickness for the part as well as the support structures. Based on this layer thickness, the energy density requirement for fabricating almost completely dense parts can be calculated to be 166.67 J/mm3, as per the following equation [31]:(8)E=P∗tPd∗h∗Lt.


## 3.1. Print Orientation Selection

Altair Inspire is employed to create a workspace designed to mimic SLM. The plate is imported into the Print 3D module in Altair Inspire. The design of support structures play an important role in determining the quality of the final component prepared. The support structure design can enable understanding of the extent to which a component could distort, based on the area of contact with the supports [30]. The supports can help prevent the extent of deformation, as they increase the heat transfer rate from the melt pool and dissipate it throughout the component, as well as the base plate. The orientation for building the part is iterated initially by maximizing and minimizing the build height. Usually, the build height is maintained as low as possible to reduce the print time, which consequently reduces the cost of printing as well. In addition to the cost, the parameters taken into consideration are the area of contact between the part and the support structure, support volume, and time required for the printing process. The area of contact and the support volume will help determine the amount of support structure required for reinforcing the overhang features, and the overall cost addition due to additional support structures. The choice of an optimal part orientation will help visualize the location of support structures required and the time required to print the entire part. Table 1 lists the results of varying the parameters and the different support structures generated. The support structures in both cases (a) and (b) indicate lodging of structures in narrow, critical regions, such as the space between the head of the inserts and the bearing slots, as depicted in the figures in Table 1. This can lead to increased postprocessing requirements and can also damage the features due to poor tool passage while removing the supports. In order to prevent this, case (c) indicated in Table 1 was assessed and ultimately incorporated. This custom orientation allowed for a reduction in bulky supports and let several sections be self-supported, and prevented the generated supports from getting wedged in narrow regions. In spite of the increased build height, this orientation is chosen by giving quality a greater priority than the printing cost.Table 1
Parameters for selection of build orientation and support structures.
OrientationSupport area (mm2)Support volume (mm3)Printing time (s)Figures(a) Maximum build height1292.052000.09593.99(b) Minimum build height3162.073000.07150.51(c) Custom orientation1240.051000.09518.11


## 3.2. Thermomechanical Simulation Setup

The thermomechanical analysis allows establishing a temperature field, followed by an estimation of the thermal stresses and associated distortions via a mechanical analysis. The SLM 280 2.0 twin laser printer is assigned for the printing simulation. The parameters associated with the specifications of the printer are listed in Table2. The model helps take into account the effects of thermal expansion of the material as well, thus increasing the accuracy of the final result. The boundary conditions applied for the study are as follows:(a)
The initial thermal boundary is an equal preprinting temperature assigned to the print bed and the powder layer.(b)
All the open surfaces of the part will be subject to radiation and convection.(c)
The degrees of freedom of the lower face of the plate are restricted to accurately simulate the adhesion of the plate to the build platform.Table 2
Specifications of SLM 280 2.0.
Technical specificationsValuesPower (P)400 WExposure (t)120μsBuild envelope280∗ 280 ∗ 365 mmBuild rate113 cm3/hrVariable layer thickness (Lt)20–90μmPoint distance (Pd)60μmBeam focus diameter80–115μmHatch spacing (h)80μmMaximum scan speed10 m/sAverage inert gas consumption5 l/min (argon)The temperature of the base plate is set at ambient temperature (25°C), and the selected print orientation is assigned to the part. Based on the selected parameters, the support geometry depicted in Table1 is developed. Slicing is observed to validate the part geometry during printing.The printer deposits layers of 60μm thickness for the part as well as the support structures. Based on this layer thickness, the energy density requirement for fabricating almost completely dense parts can be calculated to be 166.67 J/mm3, as per the following equation [31]:(8)E=P∗tPd∗h∗Lt.


## 4. Experiments

### 4.1. Printing

The printing also employs SLM 280 2.0, which was used for the simulation study. The printer deposits layers of 60μm thickness, by employing a bidirectional powder recoating technology. The entire printing process is conducted in an inert atmosphere, filled with argon gas. Two 400 W IPG fiber lasers scan the layers at a build rate of 113 cm3/hr, with the process parameters followed through the printing operation being similar to those used during the simulation, as depicted in Table 2. SLM provides self-supporting feasibility, as the metal powders also provide support, thus reducing the requirement of extensive support geometry. Support removal is conducted using wire cutting. Figure 3 depicts the printed control arm plate.Figure 3
Control arm plated printed using SLM.


### 4.2. CT Scanning

CT scanning is a diagnostic imaging technique, popularly implemented in biomedical applications. Owing to its accuracy and the ability to scan complex features, CT scanning is implemented over co-ordinate measuring to develop a 3D model of the printed part. This is due to the presence of internal features such as the bearing slots, and the inner surface of the inserts, which cannot be registered using co-ordinate measuring [32]. After positioning the part on the machine table, a SIEMENS sensation 62 scanner scans the component at a 0.65 mm slice interval, at a precision of 0.02 ± 0.09 mm [33]. The entire component is divided into 752 slices, which are stored as DICOM files. The sliced CT images are imported into the Materialise Mimics Medical Version 21.0. The plate is selected using the point viewer, which is aligned to capture the detailed profile of the object on all the planar views. The component can be seen with the point viewer held on it in the plain axial, plain coronal, and plain sagittal views. The placement of the point viewer is paramount to obtain the specific profile of the object, as depicted in Figure 4. Figure 5(a) represents the optimal volume rendering and grey scale contrast modification for the appropriate selection of profile data from the CT scan to acquire a B-rep model in the platform. The selection of the volume rendering and grey scale contrast is absolutely crucial for the perfect acquisition of the B-rep model. Similarly, Figure 5(b) depicts the customized threshold region of mask, which is required to select the crucial mask region for the B-rep rendering. Here, a minimum value of −117 HU and maximum value of 3071 HU has been selected.Figure 4
Selection and placement of point viewer for the selection of the CT scanned part.Figure 5
(a) Optimum volume rendering and grey scale contrast modification and (b) customized threshold region for mask.
(a)(b)The B-rep model acquired from customizing the volume render and grey scale configuration in association with the threshold region for the mask had certain regions of defects that had to be removed manually using the various tools and options provided by the Mimics software. This postprocessing is done keeping in mind not to change the profile dimensions with vast deviations. After the performing this operation, the model was exported to Autodesk Meshmixer, to obtain the perfect iteration of the B-rep component. Figure6 shows the reconstructed B-rep model of the component after cautious postprocessing.Figure 6
Final reconstructed B-rep model.


## 4.1. Printing

The printing also employs SLM 280 2.0, which was used for the simulation study. The printer deposits layers of 60μm thickness, by employing a bidirectional powder recoating technology. The entire printing process is conducted in an inert atmosphere, filled with argon gas. Two 400 W IPG fiber lasers scan the layers at a build rate of 113 cm3/hr, with the process parameters followed through the printing operation being similar to those used during the simulation, as depicted in Table 2. SLM provides self-supporting feasibility, as the metal powders also provide support, thus reducing the requirement of extensive support geometry. Support removal is conducted using wire cutting. Figure 3 depicts the printed control arm plate.Figure 3
Control arm plated printed using SLM.


## 4.2. CT Scanning

CT scanning is a diagnostic imaging technique, popularly implemented in biomedical applications. Owing to its accuracy and the ability to scan complex features, CT scanning is implemented over co-ordinate measuring to develop a 3D model of the printed part. This is due to the presence of internal features such as the bearing slots, and the inner surface of the inserts, which cannot be registered using co-ordinate measuring [32]. After positioning the part on the machine table, a SIEMENS sensation 62 scanner scans the component at a 0.65 mm slice interval, at a precision of 0.02 ± 0.09 mm [33]. The entire component is divided into 752 slices, which are stored as DICOM files. The sliced CT images are imported into the Materialise Mimics Medical Version 21.0. The plate is selected using the point viewer, which is aligned to capture the detailed profile of the object on all the planar views. The component can be seen with the point viewer held on it in the plain axial, plain coronal, and plain sagittal views. The placement of the point viewer is paramount to obtain the specific profile of the object, as depicted in Figure 4. Figure 5(a) represents the optimal volume rendering and grey scale contrast modification for the appropriate selection of profile data from the CT scan to acquire a B-rep model in the platform. The selection of the volume rendering and grey scale contrast is absolutely crucial for the perfect acquisition of the B-rep model. Similarly, Figure 5(b) depicts the customized threshold region of mask, which is required to select the crucial mask region for the B-rep rendering. Here, a minimum value of −117 HU and maximum value of 3071 HU has been selected.Figure 4
Selection and placement of point viewer for the selection of the CT scanned part.Figure 5
(a) Optimum volume rendering and grey scale contrast modification and (b) customized threshold region for mask.
(a)(b)The B-rep model acquired from customizing the volume render and grey scale configuration in association with the threshold region for the mask had certain regions of defects that had to be removed manually using the various tools and options provided by the Mimics software. This postprocessing is done keeping in mind not to change the profile dimensions with vast deviations. After the performing this operation, the model was exported to Autodesk Meshmixer, to obtain the perfect iteration of the B-rep component. Figure6 shows the reconstructed B-rep model of the component after cautious postprocessing.Figure 6
Final reconstructed B-rep model.


## 5. Results and Discussion

### 5.1. Simulation Results

The distribution of temperature in a printed layer is dependent on the calculated energy density, with a higher energy density causing a quicker temperature rise. It is observed that the regions closest to the substrate had a faster rate of solidification due to a higher rate of conductivity provided by the thick substrate in comparison with the low heat transfer capability provided by the powdered layers above them. The stress distribution is assessed at different stages in the fabrication process, especially during the cooling and support removal phases. After the final layer of the part is printed, the highest value of Von Mises stress was observed at PTF, as depicted in Figure 7(a), which was the first feature to be printed. Since the stresses decrease with an increase in temperature, owing to the modulus of elasticity of the material, the layers printed initially were found to have the highest observable stress. In a study performed by Casavola et al. [34], thin structures such as the pushrod tab, in this case, possess lower magnitudes of residual stresses due to their ability to deform easily. However, since PTF is supported by structures printed prior to it, the layer of the tab in contact with the support is not allowed to completely strain plastically, causing the development of residual stresses.Figure 7
(a) Von Mises stresses and (b) deformation at the overhanging section of the insert at the end of printing.
(a)(b)Furthermore, the support structures connecting the print bed with the PTF have a small contact area with the tab, due to which the flow of heat between them and subsequently to the print bed is slow, which intensifies the residual stresses generated. By virtue of these factors, the residual stresses remain the highest at the tab, for most of the print cycle. This is validated by Figure 8, which depicts the stress cycle followed by different regions of the plate, till the removal of support structures. The curve depicts an increase in stress after point A, which can be attributed to an uneven contraction of different regions after cooling. This increase is intensified the greatest for PTF, due to the restriction to deform freely. The insert overhang, on the other hand, is unsupported, which allows it to deform without restriction, thereby generating a lower rise in stress. The thickness of the plate is the greatest in OB, connecting the slot with the inserts and the tabs, due to which the residual stresses are found to increase as the printing progresses upwards from the tabs and then decreases again from OB to BS. Figure 8 depicts OB to have a lower stress distribution than PTF. Owing to the thickness of OB, the laser source must make a larger number of passes in order to build the entire feature’s thickness. This brings about a type of heat treatment phenomenon, which helps in stress relieving. The lower face of BS is printed last, and being unsupported causes it to contract more than the layers under it, thus generating lower residual stress. After point B, the trapped residual stresses are redistributed to surrounding areas, and the intensity of these stresses are lowered, which is accompanied by stress concentration at the edges of PTF, as observed in Figure 9.Figure 8
Stage-wise (a) stress cycle and (b) deformation cycle curves (A = end of printing, cooling begins; B = end of cooling, support structures removed).
(a)(b)Figure 9
Stress concentration at the pushrod tab post support removal.The distortions are observed to be the highest in regions where the material contraction post printing is unrestricted by the presence of support structures, as depicted in Figure7(b). The overhanging region of the insert being unsupported is allowed to deform freely, causing it to achieve greater distortion than the other features during the printing phase. The opposite is observed in the case of PTF. The stress redistribution after point A brings about a simultaneous deformation throughout the body, with the greatest magnitude observed for BS. Since the slot is the last feature to be printed, the amount of time provided for the heat to be dissipated during the print phase is quite low, due to the immediate onset of cooling. This brings about a sharp temperature gradient resulting in uneven contraction of material. The cooling phase initially causes an increase in deformation of the insert overhang, due to the stress redistribution in regions around it. However, by the end of the cooling phase, the deformation observed initially at the overhang is observed to reduce slightly. The PTF section on cooling behaves similar to the overhang, but with a greater dip in deformation. After point A, the support structures holding PTF deform slightly, thus bringing about a slight deformation in the tab. However, after point B, the removal of support structures releases the stress and allows for a greater deformation within the tab, causing the curve to spike upwards. As per the stress curve for OB, the stress magnitude increases after cooling due to the presence of support structures. Since the thickness of the plate is greatest at OB, the heat transfer rate through its thickness is lower than the other regions. After point A, when the inserts and the BS start cooling faster than OB, the deformation occurring at those regions are initiated prior to the deformation at OB. As a result, the final deformation at OB is a combination of its own deformation and the deformation brought about by virtue of regions surrounding it.


### 5.2. CT Scanning Results

The assembly points of the plate require precise manufacturing, as the alignment with the control arm tubes and the upright is dependent on its positional accuracy. The size of the pushrod tab, bearing slot and tube insert is an essential feature to be considered to ensure that the plate is fit well into its desired position. In regards to this, a direct dimensional comparison of the CT model with the original design is investigated. As predicted by the simulation model, the unsupported overhang of the insert caused it to contract post cooling by an average of 0.02 mm (considering both inserts), which can be considered to be within the tolerance provided for the thickness of the structural adhesive used to bond the plate with the tubes. The tabs were found to have a deviation of 0.06 mm from the desired diameter, with one of the tabs depicting visible surface irregularity as per the CT model. This can be attributed to the diffraction of X-rays caused during the scanning process, which resulted in the said observation. The deviation between the desired bearing slot diameter and the required diameter is similar to the observed deviation of the inserts. The simulation model depicts a maximum deviation of 11.2% from the CT model, at the pushrod tabs. The inclination angle between the inserts is observed to be reduced by an angle of 0.8°, which is critical to the geometry of the linkage. This reduction in the angle can be attributed to the location of the support structures, which primarily support the central regions of the inserts. However, since the edges of the inserts and the surface join the organic body with the inserts overhang, it allows them to contract postprinting and thus distort. This is proven by the lower residual stresses observed in these regions after the removal of the support structures. The reduction in the inclination between the inserts causes the pushrod tabs to be shifted by 0.09 mm. This shift is observed to be uneven, resulting in a minor misalignment in the tab. The shift was found to have no impact on the assembly feature i.e. the presence of the bolt passing through both the holes. An explanation for the difference between the predicted distortion and the observed distortion lies in the process of remelting printed layers [35]. The finite element model takes into account the remelting of a greater number of layers if the printing is simulated in a volume-by-volume method, which heats a number of layers in a single scan. Due to this, the model predicts a greater degree of remelting, which leads to greater stress relaxation. The actual process, however, usually only remelts the previously deposited layer and hence the actual relaxation is lower. This can lead to the actual measured distortion being greater than the predicted distortion.The surface profiles of the scanned model are compared with the actual design using the GOM Inspect software. The workspace allows the alignment of the original design with the scanned model, to help visualize the color-coded difference in surface profiles. A manual alignment is considered at the bearing slot since the actual assembly requires the bearing slot of the plate to be adjusted with the upright first. This alignment automatically positions the other regions of the plate with respect to the original design. In comparison with the AM simulation results, the CT scanned model shows a close correlation in terms of the material distribution. The organic body depicts a nominal material distribution, with most regions showing an optimal amount of material (considering yellow-shaded regions to be nominal, the scale depicts regions with excess material relative to the design in red and those with lack of material in green). The body is observed to have expanded relative to the design slightly on one side as indicated by Figure10(a), whereas the other side is observed to have compressed slightly where the organic body is joined to the insert. The pushrod tab is observed to have deformed, owing to the stress concentration as predicted by the simulation (refer Figure 9). The simulation predicts a high value of stress at the edge of the tab after support removal, which can be observed as a deformation in Figure 10(b). This deformation allows for stress redistribution, in the form of a filleted edge instead of the designed sharp edge. The surface finishing operations performed after printing and support removal were found to have an impact on the surface profile, and deviations from the original design were observed in organic regions as a consequence.Figure 10
(a) Material expansion on the organic body and (b) filleted edge due to stress concentration.
(a)(b)


## 5.1. Simulation Results

The distribution of temperature in a printed layer is dependent on the calculated energy density, with a higher energy density causing a quicker temperature rise. It is observed that the regions closest to the substrate had a faster rate of solidification due to a higher rate of conductivity provided by the thick substrate in comparison with the low heat transfer capability provided by the powdered layers above them. The stress distribution is assessed at different stages in the fabrication process, especially during the cooling and support removal phases. After the final layer of the part is printed, the highest value of Von Mises stress was observed at PTF, as depicted in Figure 7(a), which was the first feature to be printed. Since the stresses decrease with an increase in temperature, owing to the modulus of elasticity of the material, the layers printed initially were found to have the highest observable stress. In a study performed by Casavola et al. [34], thin structures such as the pushrod tab, in this case, possess lower magnitudes of residual stresses due to their ability to deform easily. However, since PTF is supported by structures printed prior to it, the layer of the tab in contact with the support is not allowed to completely strain plastically, causing the development of residual stresses.Figure 7
(a) Von Mises stresses and (b) deformation at the overhanging section of the insert at the end of printing.
(a)(b)Furthermore, the support structures connecting the print bed with the PTF have a small contact area with the tab, due to which the flow of heat between them and subsequently to the print bed is slow, which intensifies the residual stresses generated. By virtue of these factors, the residual stresses remain the highest at the tab, for most of the print cycle. This is validated by Figure 8, which depicts the stress cycle followed by different regions of the plate, till the removal of support structures. The curve depicts an increase in stress after point A, which can be attributed to an uneven contraction of different regions after cooling. This increase is intensified the greatest for PTF, due to the restriction to deform freely. The insert overhang, on the other hand, is unsupported, which allows it to deform without restriction, thereby generating a lower rise in stress. The thickness of the plate is the greatest in OB, connecting the slot with the inserts and the tabs, due to which the residual stresses are found to increase as the printing progresses upwards from the tabs and then decreases again from OB to BS. Figure 8 depicts OB to have a lower stress distribution than PTF. Owing to the thickness of OB, the laser source must make a larger number of passes in order to build the entire feature’s thickness. This brings about a type of heat treatment phenomenon, which helps in stress relieving. The lower face of BS is printed last, and being unsupported causes it to contract more than the layers under it, thus generating lower residual stress. After point B, the trapped residual stresses are redistributed to surrounding areas, and the intensity of these stresses are lowered, which is accompanied by stress concentration at the edges of PTF, as observed in Figure 9.Figure 8
Stage-wise (a) stress cycle and (b) deformation cycle curves (A = end of printing, cooling begins; B = end of cooling, support structures removed).
(a)(b)Figure 9
Stress concentration at the pushrod tab post support removal.The distortions are observed to be the highest in regions where the material contraction post printing is unrestricted by the presence of support structures, as depicted in Figure7(b). The overhanging region of the insert being unsupported is allowed to deform freely, causing it to achieve greater distortion than the other features during the printing phase. The opposite is observed in the case of PTF. The stress redistribution after point A brings about a simultaneous deformation throughout the body, with the greatest magnitude observed for BS. Since the slot is the last feature to be printed, the amount of time provided for the heat to be dissipated during the print phase is quite low, due to the immediate onset of cooling. This brings about a sharp temperature gradient resulting in uneven contraction of material. The cooling phase initially causes an increase in deformation of the insert overhang, due to the stress redistribution in regions around it. However, by the end of the cooling phase, the deformation observed initially at the overhang is observed to reduce slightly. The PTF section on cooling behaves similar to the overhang, but with a greater dip in deformation. After point A, the support structures holding PTF deform slightly, thus bringing about a slight deformation in the tab. However, after point B, the removal of support structures releases the stress and allows for a greater deformation within the tab, causing the curve to spike upwards. As per the stress curve for OB, the stress magnitude increases after cooling due to the presence of support structures. Since the thickness of the plate is greatest at OB, the heat transfer rate through its thickness is lower than the other regions. After point A, when the inserts and the BS start cooling faster than OB, the deformation occurring at those regions are initiated prior to the deformation at OB. As a result, the final deformation at OB is a combination of its own deformation and the deformation brought about by virtue of regions surrounding it.


## 5.2. CT Scanning Results

The assembly points of the plate require precise manufacturing, as the alignment with the control arm tubes and the upright is dependent on its positional accuracy. The size of the pushrod tab, bearing slot and tube insert is an essential feature to be considered to ensure that the plate is fit well into its desired position. In regards to this, a direct dimensional comparison of the CT model with the original design is investigated. As predicted by the simulation model, the unsupported overhang of the insert caused it to contract post cooling by an average of 0.02 mm (considering both inserts), which can be considered to be within the tolerance provided for the thickness of the structural adhesive used to bond the plate with the tubes. The tabs were found to have a deviation of 0.06 mm from the desired diameter, with one of the tabs depicting visible surface irregularity as per the CT model. This can be attributed to the diffraction of X-rays caused during the scanning process, which resulted in the said observation. The deviation between the desired bearing slot diameter and the required diameter is similar to the observed deviation of the inserts. The simulation model depicts a maximum deviation of 11.2% from the CT model, at the pushrod tabs. The inclination angle between the inserts is observed to be reduced by an angle of 0.8°, which is critical to the geometry of the linkage. This reduction in the angle can be attributed to the location of the support structures, which primarily support the central regions of the inserts. However, since the edges of the inserts and the surface join the organic body with the inserts overhang, it allows them to contract postprinting and thus distort. This is proven by the lower residual stresses observed in these regions after the removal of the support structures. The reduction in the inclination between the inserts causes the pushrod tabs to be shifted by 0.09 mm. This shift is observed to be uneven, resulting in a minor misalignment in the tab. The shift was found to have no impact on the assembly feature i.e. the presence of the bolt passing through both the holes. An explanation for the difference between the predicted distortion and the observed distortion lies in the process of remelting printed layers [35]. The finite element model takes into account the remelting of a greater number of layers if the printing is simulated in a volume-by-volume method, which heats a number of layers in a single scan. Due to this, the model predicts a greater degree of remelting, which leads to greater stress relaxation. The actual process, however, usually only remelts the previously deposited layer and hence the actual relaxation is lower. This can lead to the actual measured distortion being greater than the predicted distortion.The surface profiles of the scanned model are compared with the actual design using the GOM Inspect software. The workspace allows the alignment of the original design with the scanned model, to help visualize the color-coded difference in surface profiles. A manual alignment is considered at the bearing slot since the actual assembly requires the bearing slot of the plate to be adjusted with the upright first. This alignment automatically positions the other regions of the plate with respect to the original design. In comparison with the AM simulation results, the CT scanned model shows a close correlation in terms of the material distribution. The organic body depicts a nominal material distribution, with most regions showing an optimal amount of material (considering yellow-shaded regions to be nominal, the scale depicts regions with excess material relative to the design in red and those with lack of material in green). The body is observed to have expanded relative to the design slightly on one side as indicated by Figure10(a), whereas the other side is observed to have compressed slightly where the organic body is joined to the insert. The pushrod tab is observed to have deformed, owing to the stress concentration as predicted by the simulation (refer Figure 9). The simulation predicts a high value of stress at the edge of the tab after support removal, which can be observed as a deformation in Figure 10(b). This deformation allows for stress redistribution, in the form of a filleted edge instead of the designed sharp edge. The surface finishing operations performed after printing and support removal were found to have an impact on the surface profile, and deviations from the original design were observed in organic regions as a consequence.Figure 10
(a) Material expansion on the organic body and (b) filleted edge due to stress concentration.
(a)(b)


## 6. Conclusion

The study investigates the residual stress distribution and the distortion of a generatively designed component manufactured via SLM. The research analyzes the impact of build orientation on the stress distribution and consequently the deformation accompanying it via a thermomechanical simulation setup. The comparison between the actual design and the printed part is investigated, by overlapping the design with the CT scanned model of the printed part.(i)
The profiles achieved by SLM are observed to be quite accurate in terms of a direct dimensional and surface profile comparison. The potential impact of residual stresses and consequential deformation is observed to be quite minimal, and is found to have no large scale implications on the performance of the part. Additionally, the simulation setup predicts the possible distortions, for instance, the actual change in the shape of the pushrod tab edge due to stress concentration. Figure8 provides a stage wise representation for the development of stresses and associated distortions throughout the body. The graphs help in accurately predicting the rise and fall in stresses, during printing and after the removal of support geometry. As a result, the stress curves can be used to predict the deformation curves, at different stages during the print cycle.(ii)
The study regarding build orientations can be crucial in analyzing residual stresses. Build orientation (b), as depicted in Table1 has a larger area exposed to the laser source, which can lead to a greater magnitude of stress development. While orientation (a) and (c) both have a smaller area exposed to the laser source, the density of support structures in (a) pose an issue regarding their removal after printing. Orientation (c) allows for greater self-supporting feasibility, and thus the necessity of support in tight areas is eliminated.(iii)
Dimensional analysis provides great insights into the accuracy of the manufacturing process. The linear dimensional analysis concludes the part to be within allowable tolerance limits, with no impact on the assembly. The surface profiles are observed to be consistent with the predicted deformations, keeping aside minor distortions caused during postprocessing. Micro CT scanning can also be suggested owing to its ability to provide an enhanced resolution, especially for complex geometries such as the organic body.(iv)
The limitation of computational power is a restricting factor when it comes to solving the simulation setups. The model is considered to be an accurate tool to determine possible deformations in designs post printing and can provide an opportunity to take them into account and modify the design accordingly before printing. As a result, it can help save a significant amount of time and effort during testing and prototyping. The ability to predict these distortions can massively impact the printing of thin-walled structures that are usually most affected by them. Moreover, the ability to define and create support geometry based on the predicted stresses and distortions in a design iteration can be of significance and can be of reference in further iterations to reduce them suitably.



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*Source: 5211623-2022-07-30.xml*