Residual Stress Distribution in Selective Laser Melting of SS316L Parts

Department of Mechanical Engineering, Sri Venkateswara College of Engineering & Technology (Autonomous), Chittoor 517127, Andhra Pradesh, India Department of Engineering and Management, University of Padua, Stradella San Nicola, Vicenza 36100, Italy Department of ermal and Energy Engineering, School of Mechanical Engineering, Vellore Institute of Technology, Vellore 632014, India Department of Mechanical Engineering, Birla Institute of Technology, Mesra, Ranchi, India Department of Engineering Design and Materials, Norwegian University of Science and Technology, Trondheim 7491, Norway


Introduction
Additive manufacturing techniques, such as selective laser melting (SLM), are used to create complex components that may be customised in a variety of ways, such as topological optimization, lightweight construction, and lattice architectures. e mechanical characteristics of SLM components are almost identical or even better than those of bulk material provided the components are near full density [1]. It is possible to construct functional metal goods using selective laser melting (SLM) in which a computer model serves as a guide for melting metal particles layer by layer [2]. SLM has a broad variety of applications in aerospace, automotive, tool, medical, and power generation sectors because of its promise of quick production. LPBF enables successive layer assembly with a laser beam as the heat source. e localized heat input in micron length and time scale induces rapid melting and consolidation through the formation of a molten metal pool [3]. Unfortunately, the enhanced thermal cycle of quick melting, cooling, and melt-back during SLM leads to higher residual stress gradients, which contribute to molten pool con guration, microstructure and mechanical properties microcracks, delamination, and component deformation, all of which are major challenges in the metal additive manufacturing (AM) community [4]. It is recalled that residual stress (RS) originates from di erential expansion and contraction of the material [5,6].
Residual stress then causes deformation, delamination, and ssures in the component. Additional faults might emerge throughout the SLM process. Apart from RS, the most prevalent faults are balling, warping, and dross development [7][8][9][10][11]. However, RS may have similar e ects to stress concentration that are vital to component life cycle.
e presence of RS, along with additional loading stress, may dramatically weaken fatigue resistance and induce deformation, as well as delamination, fissures, and other structural problems [12][13][14][15]. Residual stresses are self-balancing stresses that remain in a component after manufacture, even in the absence of a temperature gradient or external forces. ese stresses are created by geometric mismatches between components covering many locations and phases inside a part, or even by local variances in elastic constants, thermal, and mechanical properties [16].
In addition to decreased fatigue resistance and critical failure, while in operation, RS has other negative effects, as illustrated in Figure 1. Chemical resistance, magnetization, deformation resistance, and static and dynamic strength are just a few of the properties that are affected [17]. Sintering/ melting and subsequent solidification of metal may be induced by laser, electron, or plasma heating. ere is a change in how each component functions locally as a result [18]. It is possible that rapid cooling and heating may cause distortions and cracks in AM materials. RS is also responsible for additively manufactured components' anisotropic behavior [19]. In Figure 2, the temperature gradient mechanism (TGM) may be observed as a predecessor to residual stresses in AM. is means that, during localized heating, the power source increases stress in the region around it as well as a sequence of local deformations (expressed by its deformation values). During subsequent cooling of the molten top layer, thermal contraction induces tensile residual stresses on the solidified zone and compressive residual stresses at its boundary due to the self-balancing character of the RS field [15,20]. Because of the intricacy of the AM process, the TGM model can only provide a simplified representation of where and how residual stresses are generated [21]. RS intensity and material parameters such as grain size, heat capacity, porosity, and phase composition, as well as process variables such as laser/electron beam power, preheating type, scan techniques and speed, and layer thickness, are all linked [16,18,22]. e influence of the print scan method on residual stresses is studied utilizing an SLM procedure. e bridgeshaped samples made of AISI316 L stainless steel were created and then tested. e current work is focused on analyzing as-printed SLM specimens in order to get sufficient primary knowledge regarding the surface and interior quality of the samples. XRD is used to quantify residual stresses. ese findings would contribute to a better understanding of the evolution of residual stress during SLM using four different pattern techniques.

Materials and Methods
A TruPrint 1000 SLM equipment was utilised to create the stainless steel 316L samples used in this investigation. e laser has a concentrated beam diameter of 20 μm. Both samples were created in a 99.999% pure argon atmosphere to prevent oxidation during the SLM process. e basic material powders had a spherical form and varied in size from 20 to 40 μm. Table 1 collects the values of process parameters employed in this study such as laser power (W), scan speed (mm/s), and layer thickness (μm). Samples were produced with four different scanning strategies as shown in Figure 3. For each one, four samples were made at a time utilizing the built area, and all were subjected to the required test. e result provides the mean value obtained with very negligible standard deviation.
is ensured the reliability and repeatability of the output. Software associated with the additive manufacturing machine used has an option of dry run which suggests the best orientation on the basis of material used, run time, etc. So, while placing the model for building, orientation considered is given with dimensions and geometry same as depicted in Figure 4(a). In this case, support material was used in the first few layers and below the hump in the product [23,24]. eir bridge-like shape is specially designed to evaluate residual stresses of metal parts built via SLM. e photos in Figure 4 produce them. According to the ASME SEC-VIII standard, the additively manufactured specimens were analyzed to search for macrodefects using the X-ray radiography technique. Potential radiographic black patches are caused by internal defects such as porosity, blowholes, or fractures. ese flaws were not detected in the analyzed samples as shown in Figure 5. Vickers hardness was performed on the components using the ASTM E-384-16 standard. e cold mounted specimens were used for testing. 1 kg load was employed with an indentation dwell time of 8 sec.
In this study, RS is measured on the lateral surface at different positions using the Pulstec μ-X360n portable X-ray residual stress analyzer. Figure 6 shows both the instrument and the camera image in the three different positions of the specimen. e sample surface was cleaned using emery sheets before measurement. cos(α) measuring method with a spot size of 2 mm was adopted. Sample measuring parameters are the diffraction angle (156.396 o ), interplanar spacing (d � 1.170Å), X-ray wavelength (Cr), K-alpha (2.29093Å), and K-beta (2.08480Å).

Results and Discussion
3.1. Hardness. Vickers hardness measurements were performed on the top and transverse surfaces of as-fabricated samples shown in Figure 4. e hardness results depend on the four different scanning strategies used to print the parts. e chess field pattern gives the highest hardness average value of 303 HV, and the lowest for anti-parallel horizontal is 227 HV [13,14]. e hardness values were obtained. e increase in the hardness was attributed to the repeated thermal cycles experienced during the multilayered procedure with melting and solidification. SLM samples had higher average hardness values than those of the cast samples (227, 303, 259, and 271 HV) (192 HV). Different studies [10,[15][16][17] have confirmed that additively manufactured SS316 L components have a greater hardness than that of SS316 L components traditionally fabricated.

Residual Stresses.
e residual stress field of metal AM components is affected by scanning techniques, dwell duration, and a variety of other process factors that have a significant influence on thermal history. Wu et al. [25] had measured the residual stress in AISI316 stainless steel produced by SLM by using an imaging method called digital     Advances in Materials Science and Engineering image correlation (DIC) and XRD. ere has been a great deal of research into the dependency of residual stress on aspects such as scanning method, laser power, scanning speed, and construction orientation. A smaller scan island and a higher energy density throughout the length of the scan island would result in a reduced residual stress field. Tensile residual stress in the build direction of a cut-off component in the as-built condition is found immediately under the top surface, followed by a compressive zone in the center and a tensile zone at the bottom surface, according to Mercelis and Kruth [16]. At the build substrate interface, residual stress in SLM manufactured stainless steel and Ti6Al4V small samples was found to be more tensile than that in the perpendicular direction, according to Yadroitsev and Yadroitsava [26]. For the SLM-produced item, Kruth et al. [20] used the bridge curvature technique to measure residual stress. Residual stress of SLM-processed components may be quickly analyzed using this method [27,28]. e angle of curvature of the bridge's two bottom surfaces after it has been removed from the substrate determines the amount of residual stress in the bridge-shaped portion. For decreasing residual stress and deformation during the SLM process, shorter scan vectors and a larger rotation angle were recommended.
As fabricated samples have undergone the residual stresses measured on the surface as shown in Figures 6(b)-6(d)), antiparallel horizontal pattern induces compressive residual stresses at the middle and tensile in nature at other two ends, with a variation from −5 MPa to 14 MPa, as shown in Figure 7(a). In the chess field pattern, the stresses were tensile in nature throughout the measured surface as given in Figure 7 Compared to other scanning strategies, the bidirectional pattern has induced a nonsymmetric residual stress pattern (Figure 7(d)) with a residual stresses value of −25 MPa at the right side and 9 MPa at the left side of the part. Due to the complex thermal history the workpiece undergoes during the additive manufacturing process, it is extremely difficult to predict the residual stress induced in a particular position of the workpiece that was subjected to repeated heating and cooling of different intensities depending on the scanning strategy applied. is work demonstrated that residual stress is highly dependent on the raster strategy, and for the geometry and process parameters used, its values are well below the yield stress of the material under investigation (170 MPa). Despite different works present in the literature covering this topic [16,20,[25][26][27], experimental measurements of residual stress in additively manufactured component are still scarce. is work aims at covering this gap and putting some basis for a better comprehension of this emerging technology. Advances in Materials Science and Engineering

Conclusions
In this study, the formation of residual stress as a function of pattern strategies in metal additive manufacturing was analyzed. e important outcomes of this effort are summarized as follows: (i) Defect-free 3D metal printed parts were produced irrespective to the scanning strategy used. (ii) SLM parts show higher hardness values than those of the cast parts. (iii) Residual stresses in all the printed parts are found within the yield limits of the base materials with a good factor of safety. (iv) Metal AM residual stress may be reduced via preheating, process planning, feedback management, and laser peening. Residual stress relief is best achieved by machining and heat treatment.
e research on residual stress in AM is still in its early stages. In the future, researchers will look into how to incorporate the rotation of the scanning strategy between the layers [29,30] to measure macro-and microresidual stresses, as well as the effects of crystal structure, simulation, postprocess mitigation, fatigue, creep, and corrosion on the size and function of 3Dprinted parts.

Data Availability
All the data or models analyzed during the study are included in this published article.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.  6 Advances in Materials Science and Engineering