Effect of laser incidence angle on surface roughness and fatigue properties of additively manufactured 316L

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However, there are numerous factors that can influence the properties of components manufactured by this process. One scarcely investigated factor is the laser incidence angle, which refers to the orientation and positioning of a component relative to the direction from which the laser interacts with the layer to be melted. This study examines the effect of the laser incidence angle on surface roughness and fatigue properties of 316L components. Results confirmed that laser incidence angle significantly affects the surface roughness and can contribute to the formation or reduction of internal defects. Increased surface roughness and internal defects, particularly the latter, adversely impact fatigue strength, with internal defects playing a primary role in crack initiation in this study. Given that the laser incidence angle significantly influences both surface roughness and fatigue strength, careful positioning of components on the build plate relative to the laser incidence angle is crucial for optimizing the mechanical properties of additively manufactured parts. Additive manufacturing Laser powder bed fusion 316L Laser incidence angle Fatigue properties Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1 Introduction Laser powder bed fusion for metals (PBF-LB/M), as defined by DIN EN ISO 52900:2022-03, is an additive manufacturing (AM) process that allows for the precise fabrication of three-dimensional components within a powder bed. The process involves the application of a fine layer of metal powder onto a build platform, followed by selective melting of the material using a laser beam. This layer-by-layer approach continues until the component is fully formed. A wide range of metal alloys, including stainless steel, cobalt-chrome, copper, aluminum, titanium, Inconel, gold, and tool steels, are available for use in PBF-LB/M [1]. Rough surfaces can act as a crack initiation spot, leading to premature failure under cyclic loading conditions. Therefore, it is well established that the fatigue properties are strongly dependent on surface roughness; generally, the higher the surface quality of a specimen, the better its fatigue performance [9]. Although several studies have investigated the interdependency of surface roughness, surface orientation, and laser incidence, and a dedicated parameter has recently been introduced to describe this relationship more accurately [10], further experimental validation and deeper understanding of the underlying mechanisms remain necessary. In this context, Li et al. demonstrated that the build direction significantly influences the fatigue properties of LPBF components [11]. For instance, in an Al-Si alloy, fatigue properties decreased with increasing build angle (i.e., part orientation relative to the horizontal direction) due to differences in defect size and distribution of melting pool boundaries, which influence crack nucleation and growth. Similarly Zhang et al. observed for Ti-6Al-4V that the highest fatigue life was at a 75° build orientation due to optimal surface quality and favorable grain orientation [12]. Conversely, orientations below 30° showed poor surface quality leading to fatigue crack initiation at downward-facing surfaces. 2 Experimental 2.1 Specimen production For the manufacturing of the specimens, a stainless steel powder with the nominal composition outlined in Table 1 was utilized, as identified in a previous study [13]. The 316L stainless steel powder employed in this study exhibited volume-based equivalent diameters d 10 = 18.3 µm, d 50 = 26.2 µm, and d 90 = 37.5 µm. The specimens were fabricated using a LASERTEC 30 SLM 2nd Generation system from DMG MORI, featuring a build volume of 300 × 300 × 300 mm. Argon 4.6 was employed as the inert gas, with a target flow rate of 1050 L/min. Each layer of the specimens was fabricated using a combination of hatch and contour parameters. The contour was applied as a single track around the outer geometry of the specimens. The distance between the contour track and the hatch area was set to 0.04 mm. The laser power for the contour was 140 W, with a scanning speed of 300 mm/s. For the hatch, a laser power of 254 W and a scanning speed of 1000 mm/s were utilized. The laser focus diameter was maintained at 70 µm, while the hatch distance was set to 90 µm. The oxygen concentration within the build chamber was controlled at 0.2 ± 0.05 vol.%, and the layer thickness was fixed at 50 µm. Melting was conducted using a 12 mm stripe pattern with a layer rotation of 31° between successive layers. A total of 18 specimens were fabricated, comprising 9 specimens inclined toward (IT) the laser, and 9 specimens inclined away (IA) from the laser, as illustrated in Fig. 1 (a). The inclination angle relative to the build platform is 45° for both specimen orientations. The geometry of the specimens, based on ASTM E466-15 with tangentially blended fillets between uniform cross-sections, is depicted in Fig. 1 (b). After specimen fabrication, the specimens were machined to ensure that only the surfaces under investigation - the up-skin and down-skin areas - contributed to crack initiation and thus predominantly influenced the fatigue properties. All other external surfaces were machined to a surface roughness of Ra = 0.8 µm. Furthermore, no heat treatment was applied to the specimens, allowing them to be examined in the as-built condition. 2.2 Analysis methods To determine the surface roughness, optical roughness measurements were conducted using an Alicona Infinite Focus SL microscope, which performed a 3D scan of the surfaces after specimen fabrication. The roughness values were then evaluated on both the up-skin and down-skin surfaces of the specimens within the gauge area. The analysis of the surface roughness is essential for evaluating the fatigue life of materials, as surface imperfections, such as roughness, can act as stress concentrators and significantly influence crack initiation and propagation. Consequently, following the surface roughness measurements, axial fatigue loading was applied using an MTS Landmark servo-hydraulic test machine, equipped with a 100 kN load cell, to investigate differences in fatigue life. All tests were conducted at room temperature, under load control, using a sinusoidal signal in uniaxial tension with a frequency of 30 Hz and a load ratio of R = 0.1. The runout limit was set at 2 × 10 6 cycles to assess the fatigue failure mechanisms. For the failed specimens, fracture surface analysis was conducted using a FEI QUANTA 650 FEG scanning electron microscope (SEM) to gain a deeper understanding of the failure mechanisms. 3 Results 3.1 Surface roughness The surfaces of the up-skin and down-skin areas of components manufactured using PBF-LB exhibit distinct characteristics due to the differing thermal conditions during the melting process. A graphical representation of the surface profiles of specimens inclined towards and away from the laser is shown in Fig. 2 . Furthermore, the surface roughness values for both up-skin and down-skin areas were determined for each specimen type, as shown in Table 2. Five measurements were performed for each area, and the mean value along with the standard deviation was calculated. It can be observed that the surface roughness of the up-skin area is superior compared to that of the down-skin areas, as depicted in Fig. 2 . However, it can be observed that the up-skin surfaces of the specimens inclined towards and inclined away from the laser differ. The up-skin surface of the specimen inclined away from the laser exhibits lower surface roughness (R a = 8.4 µm) compared to the up-skin surface of the specimen inclined towards the laser (R a = 14.3 µm). Nevertheless, the difference between the down-skin surfaces of the two specimen types is more pronounced. The down-skin surface of the specimen inclined towards the laser is significantly smoother (R a = 24.4 µm), while that of the specimen inclined away from the laser is noticeably rougher (R a = 33.1 µm). In addition to the arithmetic mean roughness Ra ​ and the mean peak-to-valley height R z ​, Table 2 also presents the values for total roughness height R t ​, skewness of the roughness profile R sk ​, and kurtosis of the roughness profile R ku ​. It is evident that the two down-skin surfaces exhibit a significantly higher average total roughness (R t = 247.4 µm and ​R t = 192.6 µm) compared to the up-skin surfaces (R t = 87.2 µm and R t ​= 114.7 µm). The skewness of the roughness profile (R sk ​) indicates whether a surface has more pronounced peaks or valleys. A positive skewness (R sk ​>0) suggests a predominance of valleys, which can increase the risk of notch effects and crack initiation [14]. All specimens investigated exhibit a positive R sk ​ value, with the highest value observed for specimen type IA (R sk ​ = 0.89). Besides skewness, the kurtosis of the roughness profile (R ku ​) is also highly relevant for fatigue life, as it describes whether the roughness is evenly distributed or characterized by extreme peaks and valleys. High R ku ​ values (>3) indicate numerous sharp peaks and deep valleys, leading to localized stress concentrations and accelerated crack initiation [14]. All examined specimens exhibit R ku ​ values exceeding 3, with the highest value also observed for specimen type IA (R ku ​= 5.81). Figure 2 (a) further illustrates that the surface profile of the up-skin surfaces is characterized by the layer wise build process. This is evident from the elongated ridges orthogonally to the build direction, which appear at intervals corresponding to the layer thickness. Additionally, the up-skin surfaces exhibit little to no unintendedly molten powder particles. However, the surface profile of the down-skin surfaces does not exhibit the elevations or indentations characteristic of the layer wise build process. Instead, these surfaces feature a higher number of randomly distributed, unintentionally adhered powder particles, which are partially or fully molten, see Fig. 2 (b). Additionally, Fig. 2 includes a magnified view of the deepest valley for each surface profile. This valley, commonly referred to in the literature as a "killer notch" is significantly more pronounced on the down-skin surfaces compared to the up-skin surfaces of both specimens. Due to the higher surface roughness and the distinct killer notch, it is expected that crack initiation during fatigue testing will predominantly occur on the down-skin surface. Furthermore, given poorer surface quality of Specimen IA, it will be anticipated that under the same applied load, these specimens will have lower fatigue life than specimens IT. 3.2 Fatigue tests The results of the high cycle fatigue tests can be seen in Fig. 3 . For the fatigue tests, three specimens per test series were subjected to the same maximum stress. The maximum stress levels selected for the experiments were 200 MPa, 250 MPa, and 300 MPa. Assuming 2 × 10 6 cycles as the run-out threshold, no run-outs were observed for specimen IA, indicating that its fatigue strength lies below 200 MPa but cannot be precisely determined from the available data. For specimen IT, two out of three specimens at 200 MPa did not fail, suggesting a fatigue strength at or slightly above 200 MPa. This indicates a noticeable reduction in fatigue strength for the IA specimens compared to IT. As shown in Fig. 3 (a), the specimens IT exhibit a higher average fatigue life and lower scatter compared to the specimens IA. A detailed overview of the load levels, average lifetimes, and standard deviations is provided in Table 3. Fig. 3 (b) illustrates that the difference between the IT and IA specimens becomes increasingly pronounced as the applied load decreases. At lower load levels, the influence of surface roughness on fatigue life becomes increasingly significant, as the stress concentrations induced by surface irregularities represent a larger proportion of the applied load. This results in earlier crack initiation and accelerated propagation. Several studies have highlighted the increased susceptibility of fatigue performance to surface roughness under such conditions [15], [16]. 3.3 Fractography tested specimens. It can be observed that specimens subjected to high stress exhibit shorter crack growth until the final fracture compared to specimens subjected to lower stress, as shown in Fig. 4 (a), (c), (e), and (g). For all specimens shear lips were observed along the edges of the final rupture zone. Additionally, fatigue cracks were primarily observed to initiate within the regions along the thickness where sub surface defects were present, and in some cases the initiation occurred from the sub surface defect rather than the surface roughness (see Fig. 4 (c), (d)). Furthermore, the sub surface defects in the IA test series were found to be larger than those in the IT series, as shown in Fig. 4 (d) and (h). Additionally, it was noted that for all specimens, crack initiation occurred at a single location and did not propagate from multiple spots. A notable observation is that for the most specimens, crack initiation occurred at internal defects near the up-skin surface, which exhibited lower surface roughness, rather than from the rougher down-skin surface as might have been expected based on surface analysis due to the more pronounced killer notch. In contrast, cases the specimens shown in Fig. 4 (b) and (h) demonstrate that crack initiation occurred at the surface, particularly at micro-notches, where internal defects located just beneath the surface contributed to an increased local stress concentration. Although internal defects were present, they did not serve as the primary initiation sites. Instead, their presence amplified the stress at the surface, thereby promoting crack initiation at surface irregularities. A closer look at the internal defects reveals that they appear at a consistent distance from the surface. The distance can be defined as 160 µm - 240 µm with an average of 220 µm as shown in Fig. 4 (d). This distance suggests that the internal defects predominantly form at the transition between the outer contour and the inner hatch. 3.4 Defect analysis To gain a more detailed understanding of the presence of the internal defects, polished cross-sections of one specimen from each type, IT and IA, were prepared and analyzed, as shown in Fig. 5 . The rougher down-skin surfaces and smoother up-skin surfaces are clearly distinguishable. As suggested by the fracture analysis, the up-skin surface of the specimen IT exhibits a negligible number of internal defects (see Fig. 5 (a)). In contrast, a significantly higher number of internal defects are observed near the up-skin surface of the specimen IA, extending across the entire length of the test section Fig. 5 (b). To establish a correlation between internal defects and fatigue properties, the defects of all failed specimens were examined using SEM to determine the size of the defect area. The defect area was manually outlined in image analysis software and calculated accordingly. This value was then correlated with the fatigue data, as shown in Fig. 6 . For the two specimens classified as run-outs, no defect area could be determined; these are represented as dots in the figure. It is evident that the defect area for the IT specimen series does not exceed 0.05 mm² for any specimen, as shown in Fig. 6 (a). In contrast, several specimens in the IA series exhibit defect areas greater than 0.05 mm², Fig. 6 (b). The differing sizes of the internal defect areas are in good correlation with the larger scatter in the fatigue life observed for the specimens IA compared to the specimens IT. Furthermore, it can be observed that for the specimens subjected to 250 MPa and 200 MPa, a decrease in defect area correlates with an increase in fatigue life. 3.5 Stress analysis To investigate the influence of internal defects on crack initiation and fatigue life, a simulation using a simplified model is conducted. The setup and the boundary conditions of the simulation can be found in Appendix A . The modeling is conducted to investigate the influence of surface roughness and internal defects on the non-dimensional stress concentration. For this purpose, the non-dimensional stress concentration factor K t ​ is utilized, which is defined as: \(\:{\text{K}}_{\text{t}}=\frac{{\sigma\:}_{max}}{{\sigma\:}_{nom}}\) ​​ (1) Fatigue cracks always initiate at stress raisers; therefore, identifying the point of crack initiation and propagation is essential. For the model generation, a cross-section of the specimens was created, and the geometry of the up-skin and down-skin surfaces was extracted. If internal defects were visible in the cross-section, they were also incorporated into the model. Figure 7 presents the results of the stress analysis. It can be observed that even though up-skin surfaces presented lower surface roughness a higher Kt​ factor was seen in these regions compared to the down-skin surfaces. The highest Kt factor is found on the up-skin surface of specimen IA, with a value of 4.55, whereas the lowest Kt​ factor is observed on the down-skin surface of specimen IA, with a value of 3.44. Furthermore, it is evident that the highest Kt​ factor is associated with an internal defect, while the other locally highest stress indication factors are located at the surface. 4 Discussion The measured surface roughness values of Ra = 8.4 µm for the up-skin surface and 33.1 µm for the down-skin surface show a good correlation with the values reported in the literature, which are typically reported within the range of 5–40 µm [17], [18], [19]. The up-skin surface exhibits lower surface roughness compared to the down-skin surface due to differences in thermal exposure and melt pool dynamics. The up-skin surface is directly exposed to the laser beam, allowing for a more uniform and controlled solidification process. Additionally, surface tension effects and capillary forces contribute to smoother surface formation by promoting material redistribution and minimizing irregularities. In contrast, the down-skin surface is affected by the lack of underlying solid material support, leading to increased partial sintering of unmelted powder particles. These particles adhere to the surface due to insufficient melting, resulting in higher roughness. Moreover, gravity influences the melt pool behavior, causing instability and increased surface irregularities on downward-facing areas [17]. Consequently, the combination of reduced thermal stability, incomplete fusion, and powder adhesion accounts for the higher roughness observed on the down-skin surface in PBF-LB/M. The difference in up-skin surface roughness between the two specimens is due to the more efficient dissipation of laser-induced heat into the bulk material in the specimen inclined away from the laser [20], [21]. Consequently, fewer additional powder particles adhere to the surface, as they are not partially or fully melted. This assumption is in good agreement with the study of Subramanian et al., who take it into account in their study on the fabrication of small internal cooling channels [20]. Furthermore, the difference in the down-skin surfaces is attributed to the laser beam partially or completely melting powder adjacent to the component. The melted powder solidifies on the surface as additional particles, increasing its roughness. For the specimen inclined towards the laser, heat dissipation into the surrounding powder is significantly reduced, and more heat is conducted into the bulk material. Consequently, fewer powder particles adhere to the surface, resulting in a lower surface roughness [21]. The results of the fatigue tests are consistent with values reported in other studies, which indicate fatigue strength for 2 × 10 6 cycles ranging between 130 MPa and 280 MPa for 316L manufactured with PBF-LB with as-built surface and no heat treatment [22], [23]. Furthermore, it is supported by the findings from the surface roughness analysis that in general, the fatigue life of the specimens IA is lower than the fatigue life of specimens IT. Due to their increased surface roughness, the specimens exhibit a higher susceptibility to crack initiation, which is expected to result in a reduced fatigue life [24]. While internal defects were present, they did not act as the primary crack initiation sites. Instead, their proximity to the surface led to a local amplification of stress, thereby promoting crack initiation at surface irregularities. In general, a decline in fatigue life is expected upon an increase in surface roughness [21]. Furthermore, the decreasing difference in fatigue life of the two categories of specimens at higher applied load levels reinforces the hypothesis that this variation is primarily driven by their susceptibility to crack initiation rather than by differences in their microstructure [25]. However, contrary to the expectations based on the surface analysis, crack initiation was mostly not triggered by surface features alone but rather by an interplay of subsurface defects and surface roughness. Crack initiation in additively manufactured components due to internal defects is frequently reported in the literature. For instance, Solberg et al. observed that fatigue specimens with a rectangular cross-section exhibited crack initiation originating from internal defects, as opposed to surface defects. [26]. Furthermore, it was observed that crack initiation in all specimens occurred at a single location rather than multiple sites, resulting in a relatively flat fracture surface with characteristic regions of fatigue propagation and ductile fracture, as also reported in other studies on specimens manufactured using PBF-LB [27]. The wide scatter in the fatigue properties of specimen IA correlates well with the size of internal defects. However, fatigue tests always exhibit outliers that cannot be solely attributed to internal defect size or surface roughness. Instead, other factors play a crucial role, such as geometric deviations or the shape and morphology of defects, as demonstrated in the simplified modelling results. The observed distance of internal defects from the surface, ranging between 160 µm and 240 µm, corresponds approximately to the melt pool width of a single contour melt track. Literature confirms that, for 316L and the contour process parameters applied in this study, a melt pool width of about 200 µm can be expected [28]. It should be noted, however, that the measured defect distance refers to the orientation orthogonal to the fracture surface rather than to the build direction. Since the specimens were built at an angle of 45° relative to the build platform, the actual defect distance within a single layer is smaller. According to the slicing software, the nominal spacing between the contour and hatch laser tracks should be approximately 40 µm. For both laser vectors, a melt pool width of approximately 200 µm is expected, suggesting that the observed internal defects are not a result of erroneous preprocessing. This supports the assumption that the defects are more likely attributable to process-related variations in melt pool geometry. Specifically, as the melt pool depth increases, the melt pool width tends to decrease, which may increase the effective distance between the solidified contour and hatch regions. Additionally, it is conceivable that the hatch area, which is processed first, potentially leading to already melt the surrounding powder. As a result, the area designated for the contour track may lack sufficient loose powder, impairing complete melting during contour exposure and ultimately contributing to the formation of defects in this region. For a better understanding of defect formation, Fig. 8 illustrates the melt pool formation as a function of the incidence angle for the up-skin and down-skin areas. For both specimen types, IT and IA, the up-skin area is not remelted during the subsequent layer deposition, as it is built at an angle of 45° to the build plate. Several studies suggest that internal defects can be mitigated through remelting caused by subsequent layer exposure [29]. However, the primary difference between the two specimen types lies in the formation of the melt pool. For the IT specimen type, the melt pool is slightly orientated toward the up-skin surface, reducing the distance between the inner hatch and the outer contour. This results in a lower number of internal defects, as shown in Fig. 8 (a). In contrast, the melt pool in the specimens IA is slightly tilted away from the up-skin surface, which, as shown in Fig. 8 (b), creates larger gaps between the inner hatch and the outer contour. It should be noted that determining the exact laser incidence angle is challenging, as the installed F-Theta lens is designed to minimize this angle as much as possible. Moreover, assessing its impact on melt pool formation is even more difficult due to the complex interplay of thermal gradients, fluid dynamics, and energy absorption at varying incidence angles. However, the theory of melt pool formation is also supported by various studies, Sendino et al. pointed out in their study that variations in the component's position on the build platform, and consequently in the laser incidence angle, lead to the formation of different melt pools [21]. Therefore, it can be hypothesized that a normally functional offset in the contour parameters is influenced by the variation in the inclination angles which leads to an accumulation of internal defects in the specimen type IA and aligns well with findings from other studies that also examine the influence of the laser incidence angle on melt pool formation. For instance, Subramanian et al. [20] report that the laser incidence angle varies more significantly at the corners of a build plate due to the greater distance from the center point, thereby affecting surface roughness. Furthermore, these changes are reported to cause variations in melt pool formation and heat flow. The impact of internal defects as stress raisers was demonstrated through simplified simulation. The findings from the fracture analysis were confirmed, particularly the crack initiation originating from internal defects, which was successfully simulated and verified. At this point, it should be noted that the size of internal defects in the same feedstock material is only one of many factors influencing fatigue life. It is well-established that other factors, such as the existing microstructure, residual stresses, and the morphology and location of internal defects, also play a significant role in determining fatigue life [30]. Nevertheless, it can be concluded that correlating internal defects with fatigue data provides valuable insights into the observed scatter in the fatigue results. 5 Conclusions The influence of the laser incidence angle on surface roughness and fatigue properties of 316L specimens has been thoroughly investigated. It has been confirmed that the laser incidence angle has a significant impact on surface roughness and fatigue life. Moreover, the laser incidence angle should be carefully considered when investigating internal defects, particularly in the interaction between the inner hatch and the outer contour. The key findings of this study can be summarized as follows: The laser incidence angle is a key parameter influencing surface roughness and must be carefully considered, particularly for specimens featuring overhang areas. A deliberate selection of component positioning and orientation enables a more controlled optimization of surface roughness compared to arbitrary orientations. The laser incidence angle also affects melt pool formation, which, in turn, can influence the occurrence of internal defects. The positioning of the component relative to the laser incidence angle should be optimized to ensure that the laser beam exposure parallel to the surface of the component. An orthogonal configuration should be avoided; however, if this is not feasible, appropriate measures should be considered to mitigate the adverse effects of the incidence angle on both surface and subsurface quality. Fatigue performance in PBF-LB is significantly affected by the orientation of parts relative to the laser incidence direction. Specimens facing the laser showed up to twice the fatigue life compared to those facing away, despite identical process parameters. This highlights the laser incidence angle as a critical but often overlooked factor in build orientation strategies. Declarations CRediT authorship contribution statement Michael Berghaus : Conceptualization, Methodology, Formal analysis, Validation, Investigation, Writing – original draft, Data curation, Visualization, Resources. Nima Razavi : Methodology, Formal analysis, Validation, Writing – review and editing, Supervision, Resources. Hilmar Apmann : Writing – review & editing, Resources. Axel von Hehl : Writing – review & editing. Open Access This article is licensed under a Creative Commons Attri- bution 4.0 International License, which permits use, sharing, adapta- tion, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/ . Funding Open access funding enabled and organized by Project DEAL. Author Contribution CRediT authorship contribution statement:M.B.: Conceptualization, Methodology, Formal analysis, Validation, Investigation, Writing – original draft, Data curation, Visualization, Resources. N.R.: Methodology, Formal analysis, Validation, Writing – review and editing, Supervision, Resources. H. A.: Writing – review & editing, Resources. A.V.: Writing – review & editing. Data availability Data directly available in the paper On behalf of all authors, the corresponding author states that there is no conflict of interest. Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. References T. DebRoy et al., "Additive manufacturing of metallic components – Process, structure and properties," Progress in Materials Science , pp. 112–224, 2018, doi: 10.1016/j.pmatsci.2017.10.001. R. 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Sendino, M. Gardon, F. Lartategui, S. Martinez, and A. Lamikiz, "The Effect of the Laser Incidence Angle in the Surface of L-PBF Processed Parts," Coatings , vol. 10, no. 11, p. 1024, 2020, doi: 10.3390/coatings10111024. W.-J. Lai, A. Ojha, Z. Li, C. Engler-Pinto, and X. Su, "Effect of residual stress on fatigue strength of 316L stainless steel produced by laser powder bed fusion process," Prog Addit Manuf , vol. 6, no. 3, pp. 375–383, 2021, doi: 10.1007/s40964-021-00164-8. X. Liang, A. Hor, C. Robert, F. Lin, and F. Morel, "Effects of building direction and loading mode on the high cycle fatigue strength of the laser powder bed fusion 316L," International Journal of Fatigue , vol. 170, p. 107506, 2023, doi: 10.1016/j.ijfatigue.2023.107506. J. Fathi Sola, R. Kelton, E. I. Meletis, and H. Huang, "Predicting crack initiation site in polycrystalline nickel through surface topography changes," International Journal of Fatigue , vol. 124, pp. 70–81, 2019, doi: 10.1016/j.ijfatigue.2019.02.027. W.-J. Lai, A. Ojha, Z. Li, C. Engler-Pinto, and X. Su, "Effect of residual stress on fatigue strength of 316L stainless steel produced by laser powder bed fusion process," Prog Addit Manuf , vol. 6, no. 3, pp. 375–383, 2021, doi: 10.1007/s40964-021-00164-8. K. Solberg, E. W. Hovig, K. Sørby, and F. Berto, "Directional fatigue behaviour of maraging steel grade 300 produced by laser powder bed fusion," International Journal of Fatigue , vol. 149, p. 106229, 2021, doi: 10.1016/j.ijfatigue.2021.106229. N. Razavi, S. Bagherifard, S. Hafnor, S. Spiller, M. Guagliano, and F. Berto, "Fatigue analysis of as-built and heat-treated severely notched AlSi10Mg alloy specimens made by laser powder bed fusion technology," International Journal of Fatigue , vol. 179, p. 108041, 2024, doi: 10.1016/j.ijfatigue.2023.108041. N. Diaz Vallejo, C. Lucas, N. Ayers, K. Graydon, H. Hyer, and Y. Sohn, "Process Optimization and Microstructure Analysis to Understand Laser Powder Bed Fusion of 316L Stainless Steel," Metals , vol. 11, no. 5, p. 832, 2021, doi: 10.3390/met11050832. C. Shi, V. Schulze, and S. Dietrich, "Influences of laser remelting on mechanical performances of AISI4140 steel," Material Science and Technology , no. 40, 2024. [Online]. Available: 10.1177/02670836231212614 G. Pouget and A. P. Reynolds, "Residual stress and microstructure effects on fatigue crack growth in AA2050 friction stir welds," International Journal of Fatigue , vol. 30, no. 3, pp. 463–472, 2008, doi: 10.1016/j.ijfatigue.2007.04.016. Additional Declarations No competing interests reported. Supplementary Files AppendixA.docx Cite Share Download PDF Status: Published Journal Publication published 22 Dec, 2025 Read the published version in Progress in Additive Manufacturing → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6871000","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":481787101,"identity":"a2cf491b-20ad-4578-b054-04c528c1121d","order_by":0,"name":"Michael Berghaus","email":"data:image/png;base64,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","orcid":"","institution":"Münster University of Applied Sciences","correspondingAuthor":true,"prefix":"","firstName":"Michael","middleName":"","lastName":"Berghaus","suffix":""},{"id":481787102,"identity":"a0b68b82-db61-4654-8b4c-a4fe4604036f","order_by":1,"name":"Nima Razavi","email":"","orcid":"","institution":"Norwegian University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Nima","middleName":"","lastName":"Razavi","suffix":""},{"id":481787103,"identity":"e08bfc7e-230f-40f6-acca-621934fa844d","order_by":2,"name":"Hilmar Apmann","email":"","orcid":"","institution":"Münster University of Applied Sciences","correspondingAuthor":false,"prefix":"","firstName":"Hilmar","middleName":"","lastName":"Apmann","suffix":""},{"id":481787104,"identity":"e6f9385d-e020-4ce2-8708-24e3989b70b0","order_by":3,"name":"Axel von Hehl","email":"","orcid":"","institution":"University of Siegen","correspondingAuthor":false,"prefix":"","firstName":"Axel","middleName":"","lastName":"von Hehl","suffix":""}],"badges":[],"createdAt":"2025-06-11 10:53:13","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6871000/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6871000/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s40964-025-01441-6","type":"published","date":"2025-12-22T15:57:09+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":86343159,"identity":"126c30fc-bf3b-425d-9d53-51e678d96c78","added_by":"auto","created_at":"2025-07-09 14:32:12","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":123709,"visible":true,"origin":"","legend":"\u003cp\u003eManufactured specimens \u003cstrong\u003ea\u003c/strong\u003e position in the build job, \u003cstrong\u003eb\u003c/strong\u003e dimensions and labeling of the as-built and the machined areas\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6871000/v1/b62bb37647dcf218e88bda3f.png"},{"id":86342519,"identity":"c536f81d-5c4c-4c6f-9dd3-3cc16041e2a8","added_by":"auto","created_at":"2025-07-09 14:24:12","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":357241,"visible":true,"origin":"","legend":"\u003cp\u003e3D surface scan of different areas of the specimens \u003cstrong\u003ea\u003c/strong\u003eup-skin surface of the specimen IT, \u003cstrong\u003eb\u003c/strong\u003eup-skin surface of the specimen IA, \u003cstrong\u003ec\u003c/strong\u003edown-skin surface of the specimen IT, \u003cstrong\u003ed\u003c/strong\u003edown-skin surface of the specimen IA\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6871000/v1/033777b6ab04cce74b5fa37d.png"},{"id":86342521,"identity":"4d685fd5-8535-4e79-917c-9d8878a6e647","added_by":"auto","created_at":"2025-07-09 14:24:12","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":28408,"visible":true,"origin":"","legend":"\u003cp\u003eResults of the fatigue tests (R=0.1) \u003cstrong\u003ea\u003c/strong\u003e Fatigue data for different laser incidence angles, \u003cstrong\u003eb\u003c/strong\u003eBar chart with standard deviations\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6871000/v1/eaa877d9fb04b0eb01fea72e.png"},{"id":86343161,"identity":"e2cbf5db-26b4-4b83-b84e-c8c798061c88","added_by":"auto","created_at":"2025-07-09 14:32:12","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":799360,"visible":true,"origin":"","legend":"\u003cp\u003eFatigue fracture surfaces and crack initiation sites of specimens tested at two stress levels.\u003cbr\u003e\n(a, b) Specimen IT at 300 MPa: (a) overall fracture surface; (b) crack initiation region.\u003cbr\u003e\n(c, d) Specimen IA at 300 MPa: (c) overall fracture surface; (d) crack initiation region.\u003cbr\u003e\n(e, f) Specimen IT at 200 MPa: (e) overall fracture surface; (f) crack initiation region.\u003cbr\u003e\n(g, h) Specimen IA at 200 MPa: (g) overall fracture surface; (h) crack initiation region. sdjsdlasdfsdfasasdasfasdafasf\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6871000/v1/2c5fdb91200989332ffb8892.png"},{"id":86342520,"identity":"65333f4a-a244-4cdd-be15-d80ba8b84480","added_by":"auto","created_at":"2025-07-09 14:24:12","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":409851,"visible":true,"origin":"","legend":"\u003cp\u003eCross-section of the fatigue specimens \u003cstrong\u003ea\u003c/strong\u003e Specimen IT with marginal internal defects, \u003cstrong\u003eb\u003c/strong\u003eSpecimen IA with multiple internal defects.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6871000/v1/fde874b451e7601d7f767182.png"},{"id":86342523,"identity":"b34f5ba0-21c8-478e-b8de-029f8185e89d","added_by":"auto","created_at":"2025-07-09 14:24:12","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":74819,"visible":true,"origin":"","legend":"\u003cp\u003eResults of the fatigue tests (R=0.1) \u003cstrong\u003ea\u003c/strong\u003e Fatigue data for specimen IT considering internal defects, \u003cstrong\u003eb\u003c/strong\u003e Fatigue data for specimen IA considering internal defects\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-6871000/v1/b921faa64caa0c8506880b7a.png"},{"id":86342536,"identity":"ba7708d7-c74f-454f-99f9-812cb85f6fd4","added_by":"auto","created_at":"2025-07-09 14:24:13","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":202624,"visible":true,"origin":"","legend":"\u003cp\u003eSimulation results of the stress concentration factor K\u003csub\u003et\u003c/sub\u003e, \u003cstrong\u003ea\u003c/strong\u003e specimen IT, \u003cstrong\u003eb\u003c/strong\u003e specimen IA\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-6871000/v1/b75a3dbc62f3509b1e3d6753.png"},{"id":86342529,"identity":"3c6945a7-8ce7-470c-b7f5-1579ecb361e4","added_by":"auto","created_at":"2025-07-09 14:24:13","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":38174,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic representation of melt pool formation as a function of the incidence angle \u003cstrong\u003ea\u003c/strong\u003e Specimen IT, \u003cstrong\u003eb\u003c/strong\u003e Specimen IA\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-6871000/v1/b68c6589fcbe7d40fb31d097.png"},{"id":99172446,"identity":"305666ea-f43c-4d04-876b-44b2aef36013","added_by":"auto","created_at":"2025-12-29 16:09:36","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2597271,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6871000/v1/32eb00f9-75d0-4448-9b97-31b8e1dbcf62.pdf"},{"id":86343162,"identity":"c704abea-eae2-49dd-8854-789b3fb10afb","added_by":"auto","created_at":"2025-07-09 14:32:12","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":858712,"visible":true,"origin":"","legend":"","description":"","filename":"AppendixA.docx","url":"https://assets-eu.researchsquare.com/files/rs-6871000/v1/e8386aba054b91189b7da33d.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effect of laser incidence angle on surface roughness and fatigue properties of additively manufactured 316L","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eLaser powder bed fusion for metals (PBF-LB/M), as defined by DIN EN ISO 52900:2022-03, is an additive manufacturing (AM) process that allows for the precise fabrication of three-dimensional components within a powder bed. The process involves the application of a fine layer of metal powder onto a build platform, followed by selective melting of the material using a laser beam. This layer-by-layer approach continues until the component is fully formed. A wide range of metal alloys, including stainless steel, cobalt-chrome, copper, aluminum, titanium, Inconel, gold, and tool steels, are available for use in PBF-LB/M [1].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eRough surfaces can act as a crack initiation spot, leading to premature failure under cyclic loading conditions. Therefore, it is well established that the fatigue properties are strongly dependent on surface roughness; generally, the higher the surface quality of a specimen, the better its fatigue performance [9]. Although several studies have investigated the interdependency of surface roughness, surface orientation, and laser incidence, and a dedicated parameter has recently been introduced to describe this relationship more accurately [10], further experimental validation and deeper understanding of the underlying mechanisms remain necessary. In this context, Li et al. demonstrated that the build direction significantly influences the fatigue properties of LPBF components [11]. For instance, in an Al-Si alloy, fatigue properties decreased with increasing build angle (i.e., part orientation relative to the horizontal direction) due to differences in defect size and distribution of melting pool boundaries, which influence crack nucleation and growth. Similarly Zhang et al. observed for Ti-6Al-4V that the highest fatigue life was at a 75\u0026deg; build orientation due to optimal surface quality and favorable grain orientation [12]. Conversely, orientations below 30\u0026deg; showed poor surface quality leading to fatigue crack initiation at downward-facing surfaces.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"2 Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Specimen production\u003c/h2\u003e\u003cp\u003eFor the manufacturing of the specimens, a stainless steel powder with the nominal composition outlined in Table\u0026nbsp;1 was utilized, as identified in a previous study [13]. The 316L stainless steel powder employed in this study exhibited volume-based equivalent diameters d\u003csub\u003e10\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;18.3 \u0026micro;m, d\u003csub\u003e50\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;26.2 \u0026micro;m, and d\u003csub\u003e90\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;37.5 \u0026micro;m. The specimens were fabricated using a LASERTEC 30 SLM 2nd Generation system from DMG MORI, featuring a build volume of 300 \u0026times; 300 \u0026times; 300 mm. Argon 4.6 was employed as the inert gas, with a target flow rate of 1050 L/min. Each layer of the specimens was fabricated using a combination of hatch and contour parameters. The contour was applied as a single track around the outer geometry of the specimens. The distance between the contour track and the hatch area was set to 0.04 mm. The laser power for the contour was 140 W, with a scanning speed of 300 mm/s. For the hatch, a laser power of 254 W and a scanning speed of 1000 mm/s were utilized. The laser focus diameter was maintained at 70 \u0026micro;m, while the hatch distance was set to 90 \u0026micro;m. The oxygen concentration within the build chamber was controlled at 0.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05 vol.%, and the layer thickness was fixed at 50 \u0026micro;m. Melting was conducted using a 12 mm stripe pattern with a layer rotation of 31\u0026deg; between successive layers. A total of 18 specimens were fabricated, comprising 9 specimens inclined toward (IT) the laser, and 9 specimens inclined away (IA) from the laser, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a). The inclination angle relative to the build platform is 45\u0026deg; for both specimen orientations. The geometry of the specimens, based on ASTM E466-15 with tangentially blended fillets between uniform cross-sections, is depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(b).\u003c/p\u003e\u003cp\u003eAfter specimen fabrication, the specimens were machined to ensure that only the surfaces under investigation - the up-skin and down-skin areas - contributed to crack initiation and thus predominantly influenced the fatigue properties. All other external surfaces were machined to a surface roughness of Ra\u0026thinsp;=\u0026thinsp;0.8 \u0026micro;m. Furthermore, no heat treatment was applied to the specimens, allowing them to be examined in the as-built condition.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Analysis methods\u003c/h2\u003e\u003cp\u003eTo determine the surface roughness, optical roughness measurements were conducted using an Alicona Infinite Focus SL microscope, which performed a 3D scan of the surfaces after specimen fabrication. The roughness values were then evaluated on both the up-skin and down-skin surfaces of the specimens within the gauge area. The analysis of the surface roughness is essential for evaluating the fatigue life of materials, as surface imperfections, such as roughness, can act as stress concentrators and significantly influence crack initiation and propagation. Consequently, following the surface roughness measurements, axial fatigue loading was applied using an MTS Landmark servo-hydraulic test machine, equipped with a 100 kN load cell, to investigate differences in fatigue life. All tests were conducted at room temperature, under load control, using a sinusoidal signal in uniaxial tension with a frequency of 30 Hz and a load ratio of R\u0026thinsp;=\u0026thinsp;0.1. The runout limit was set at 2 \u0026times; 10\u003csup\u003e6\u003c/sup\u003e cycles to assess the fatigue failure mechanisms. For the failed specimens, fracture surface analysis was conducted using a FEI QUANTA 650 FEG scanning electron microscope (SEM) to gain a deeper understanding of the failure mechanisms.\u003c/p\u003e\u003c/div\u003e"},{"header":"3 Results","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Surface roughness\u003c/h2\u003e\u003cp\u003eThe surfaces of the up-skin and down-skin areas of components manufactured using PBF-LB exhibit distinct characteristics due to the differing thermal conditions during the melting process. A graphical representation of the surface profiles of specimens inclined towards and away from the laser is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Furthermore, the surface roughness values for both up-skin and down-skin areas were determined for each specimen type, as shown in Table\u0026nbsp;2. Five measurements were performed for each area, and the mean value along with the standard deviation was calculated.\u003c/p\u003e\u003cp\u003eIt can be observed that the surface roughness of the up-skin area is superior compared to that of the down-skin areas, as depicted in Fig. \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. However, it can be observed that the up-skin surfaces of the specimens inclined towards and inclined away from the laser differ. The up-skin surface of the specimen inclined away from the laser exhibits lower surface roughness (R\u003csub\u003ea\u003c/sub\u003e = 8.4 \u0026micro;m) compared to the up-skin surface of the specimen inclined towards the laser (R\u003csub\u003ea\u003c/sub\u003e = 14.3 \u0026micro;m). Nevertheless, the difference between the down-skin surfaces of the two specimen types is more pronounced. The down-skin surface of the specimen inclined towards the laser is significantly smoother (R\u003csub\u003ea\u003c/sub\u003e = 24.4 \u0026micro;m), while that of the specimen inclined away from the laser is noticeably rougher (R\u003csub\u003ea\u003c/sub\u003e = 33.1 \u0026micro;m). In addition to the arithmetic mean roughness Ra ​ and the mean peak-to-valley height R\u003csub\u003ez\u003c/sub\u003e​, Table 2 also presents the values for total roughness height R\u003csub\u003et\u003c/sub\u003e​, skewness of the roughness profile R\u003csub\u003esk\u003c/sub\u003e​, and kurtosis of the roughness profile R\u003csub\u003eku\u003c/sub\u003e​. It is evident that the two down-skin surfaces exhibit a significantly higher average total roughness (R\u003csub\u003et\u003c/sub\u003e = 247.4 \u0026micro;m and ​R\u003csub\u003et\u003c/sub\u003e = 192.6 \u0026micro;m) compared to the up-skin surfaces (R\u003csub\u003et\u003c/sub\u003e = 87.2 \u0026micro;m and R\u003csub\u003et\u003c/sub\u003e ​= 114.7 \u0026micro;m). The skewness of the roughness profile (R\u003csub\u003esk\u003c/sub\u003e​) indicates whether a surface has more pronounced peaks or valleys. A positive skewness (R\u003csub\u003esk\u003c/sub\u003e ​\u0026gt;0) suggests a predominance of valleys, which can increase the risk of notch effects and crack initiation [14]. All specimens investigated exhibit a positive R\u003csub\u003esk\u003c/sub\u003e​ value, with the highest value observed for specimen type IA (R\u003csub\u003esk\u003c/sub\u003e​ = 0.89). Besides skewness, the kurtosis of the roughness profile (R\u003csub\u003eku\u003c/sub\u003e​) is also highly relevant for fatigue life, as it describes whether the roughness is evenly distributed or characterized by extreme peaks and valleys. High R\u003csub\u003eku\u003c/sub\u003e​ values (\u0026gt;3) indicate numerous sharp peaks and deep valleys, leading to localized stress concentrations and accelerated crack initiation [14]. All examined specimens exhibit R\u003csub\u003eku\u003c/sub\u003e​ values exceeding 3, with the highest value also observed for specimen type IA (R\u003csub\u003eku\u003c/sub\u003e ​= 5.81).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a) further illustrates that the surface profile of the up-skin surfaces is characterized by the layer wise build process. This is evident from the elongated ridges orthogonally to the build direction, which appear at intervals corresponding to the layer thickness. Additionally, the up-skin surfaces exhibit little to no unintendedly molten powder particles. However, the surface profile of the down-skin surfaces does not exhibit the elevations or indentations characteristic of the layer wise build process. Instead, these surfaces feature a higher number of randomly distributed, unintentionally adhered powder particles, which are partially or fully molten, see Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b). Additionally, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e includes a magnified view of the deepest valley for each surface profile. This valley, commonly referred to in the literature as a \"killer notch\" is significantly more pronounced on the down-skin surfaces compared to the up-skin surfaces of both specimens. Due to the higher surface roughness and the distinct killer notch, it is expected that crack initiation during fatigue testing will predominantly occur on the down-skin surface. Furthermore, given poorer surface quality of Specimen IA, it will be anticipated that under the same applied load, these specimens will have lower fatigue life than specimens IT.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Fatigue tests\u003c/h2\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe results of the high cycle fatigue tests can be seen in Fig. \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. For the fatigue tests, three specimens per test series were subjected to the same maximum stress. The maximum stress levels selected for the experiments were 200 MPa, 250 MPa, and 300 MPa. Assuming 2 \u0026times; 10\u003csup\u003e6\u003c/sup\u003e cycles as the run-out threshold, no run-outs were observed for specimen IA, indicating that its fatigue strength lies below 200 MPa but cannot be precisely determined from the available data. For specimen IT, two out of three specimens at 200 MPa did not fail, suggesting a fatigue strength at or slightly above 200 MPa. This indicates a noticeable reduction in fatigue strength for the IA specimens compared to IT. As shown in Fig. \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a), the specimens IT exhibit a higher average fatigue life and lower scatter compared to the specimens IA. A detailed overview of the load levels, average lifetimes, and standard deviations is provided in Table 3. Fig. \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b) illustrates that the difference between the IT and IA specimens becomes increasingly pronounced as the applied load decreases. At lower load levels, the influence of surface roughness on fatigue life becomes increasingly significant, as the stress concentrations induced by surface irregularities represent a larger proportion of the applied load. This results in earlier crack initiation and accelerated propagation. Several studies have highlighted the increased susceptibility of fatigue performance to surface roughness under such conditions [15], [16].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Fractography\u003c/h2\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003etested specimens. It can be observed that specimens subjected to high stress exhibit shorter crack growth until the final fracture compared to specimens subjected to lower stress, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (a), (c), (e), and (g). For all specimens shear lips were observed along the edges of the final rupture zone. Additionally, fatigue cracks were primarily observed to initiate within the regions along the thickness where sub surface defects were present, and in some cases the initiation occurred from the sub surface defect rather than the surface roughness (see Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (c), (d)). Furthermore, the sub surface defects in the IA test series were found to be larger than those in the IT series, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (d) and (h). Additionally, it was noted that for all specimens, crack initiation occurred at a single location and did not propagate from multiple spots.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eA notable observation is that for the most specimens, crack initiation occurred at internal defects near the up-skin surface, which exhibited lower surface roughness, rather than from the rougher down-skin surface as might have been expected based on surface analysis due to the more pronounced killer notch. In contrast, cases the specimens shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e (b) and (h) demonstrate that crack initiation occurred at the surface, particularly at micro-notches, where internal defects located just beneath the surface contributed to an increased local stress concentration. Although internal defects were present, they did not serve as the primary initiation sites. Instead, their presence amplified the stress at the surface, thereby promoting crack initiation at surface irregularities. A closer look at the internal defects reveals that they appear at a consistent distance from the surface. The distance can be defined as 160 \u0026micro;m - 240 \u0026micro;m with an average of 220 \u0026micro;m as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(d). This distance suggests that the internal defects predominantly form at the transition between the outer contour and the inner hatch.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Defect analysis\u003c/h2\u003e\u003cp\u003eTo gain a more detailed understanding of the presence of the internal defects, polished cross-sections of one specimen from each type, IT and IA, were prepared and analyzed, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. The rougher down-skin surfaces and smoother up-skin surfaces are clearly distinguishable. As suggested by the fracture analysis, the up-skin surface of the specimen IT exhibits a negligible number of internal defects (see Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a)). In contrast, a significantly higher number of internal defects are observed near the up-skin surface of the specimen IA, extending across the entire length of the test section Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo establish a correlation between internal defects and fatigue properties, the defects of all failed specimens were examined using SEM to determine the size of the defect area. The defect area was manually outlined in image analysis software and calculated accordingly. This value was then correlated with the fatigue data, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. For the two specimens classified as run-outs, no defect area could be determined; these are represented as dots in the figure. It is evident that the defect area for the IT specimen series does not exceed 0.05 mm\u0026sup2; for any specimen, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a). In contrast, several specimens in the IA series exhibit defect areas greater than 0.05 mm\u0026sup2;, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b). The differing sizes of the internal defect areas are in good correlation with the larger scatter in the fatigue life observed for the specimens IA compared to the specimens IT. Furthermore, it can be observed that for the specimens subjected to 250 MPa and 200 MPa, a decrease in defect area correlates with an increase in fatigue life.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.5 Stress analysis\u003c/h2\u003e\u003cp\u003eTo investigate the influence of internal defects on crack initiation and fatigue life, a simulation using a simplified model is conducted. The setup and the boundary conditions of the simulation can be found in Appendix \u003cspan refid=\"Sec13\" class=\"InternalRef\"\u003eA\u003c/span\u003e. The modeling is conducted to investigate the influence of surface roughness and internal defects on the non-dimensional stress concentration. For this purpose, the non-dimensional stress concentration factor K\u003csub\u003et\u003c/sub\u003e​ is utilized, which is defined as:\u003c/p\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{K}}_{\\text{t}}=\\frac{{\\sigma\\:}_{max}}{{\\sigma\\:}_{nom}}\\)\u003c/span\u003e\u003c/span\u003e​​ (1)\u003c/p\u003e\u003cp\u003eFatigue cracks always initiate at stress raisers; therefore, identifying the point of crack initiation and propagation is essential. For the model generation, a cross-section of the specimens was created, and the geometry of the up-skin and down-skin surfaces was extracted. If internal defects were visible in the cross-section, they were also incorporated into the model.\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e presents the results of the stress analysis. It can be observed that even though up-skin surfaces presented lower surface roughness a higher Kt​ factor was seen in these regions compared to the down-skin surfaces. The highest Kt factor is found on the up-skin surface of specimen IA, with a value of 4.55, whereas the lowest Kt​ factor is observed on the down-skin surface of specimen IA, with a value of 3.44. Furthermore, it is evident that the highest Kt​ factor is associated with an internal defect, while the other locally highest stress indication factors are located at the surface.\u003c/p\u003e\u003c/div\u003e"},{"header":"4 Discussion","content":"\u003cp\u003eThe measured surface roughness values of Ra\u0026thinsp;=\u0026thinsp;8.4 \u0026micro;m for the up-skin surface and 33.1 \u0026micro;m for the down-skin surface show a good correlation with the values reported in the literature, which are typically reported within the range of 5\u0026ndash;40 \u0026micro;m [17], [18], [19]. The up-skin surface exhibits lower surface roughness compared to the down-skin surface due to differences in thermal exposure and melt pool dynamics.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe up-skin surface is directly exposed to the laser beam, allowing for a more uniform and controlled solidification process. Additionally, surface tension effects and capillary forces contribute to smoother surface formation by promoting material redistribution and minimizing irregularities. In contrast, the down-skin surface is affected by the lack of underlying solid material support, leading to increased partial sintering of unmelted powder particles. These particles adhere to the surface due to insufficient melting, resulting in higher roughness. Moreover, gravity influences the melt pool behavior, causing instability and increased surface irregularities on downward-facing areas [17]. Consequently, the combination of reduced thermal stability, incomplete fusion, and powder adhesion accounts for the higher roughness observed on the down-skin surface in PBF-LB/M.\u003c/p\u003e\u003cp\u003eThe difference in up-skin surface roughness between the two specimens is due to the more efficient dissipation of laser-induced heat into the bulk material in the specimen inclined away from the laser [20], [21]. Consequently, fewer additional powder particles adhere to the surface, as they are not partially or fully melted. This assumption is in good agreement with the study of Subramanian et al., who take it into account in their study on the fabrication of small internal cooling channels [20].\u003c/p\u003e\u003cp\u003eFurthermore, the difference in the down-skin surfaces is attributed to the laser beam partially or completely melting powder adjacent to the component. The melted powder solidifies on the surface as additional particles, increasing its roughness. For the specimen inclined towards the laser, heat dissipation into the surrounding powder is significantly reduced, and more heat is conducted into the bulk material. Consequently, fewer powder particles adhere to the surface, resulting in a lower surface roughness [21].\u003c/p\u003e\u003cp\u003eThe results of the fatigue tests are consistent with values reported in other studies, which indicate fatigue strength for 2 \u0026times; 10\u003csup\u003e6\u003c/sup\u003e cycles ranging between 130 MPa and 280 MPa for 316L manufactured with PBF-LB with as-built surface and no heat treatment [22], [23]. Furthermore, it is supported by the findings from the surface roughness analysis that in general, the fatigue life of the specimens IA is lower than the fatigue life of specimens IT. Due to their increased surface roughness, the specimens exhibit a higher susceptibility to crack initiation, which is expected to result in a reduced fatigue life [24]. While internal defects were present, they did not act as the primary crack initiation sites. Instead, their proximity to the surface led to a local amplification of stress, thereby promoting crack initiation at surface irregularities. In general, a decline in fatigue life is expected upon an increase in surface roughness [21]. Furthermore, the decreasing difference in fatigue life of the two categories of specimens at higher applied load levels reinforces the hypothesis that this variation is primarily driven by their susceptibility to crack initiation rather than by differences in their microstructure [25]. However, contrary to the expectations based on the surface analysis, crack initiation was mostly not triggered by surface features alone but rather by an interplay of subsurface defects and surface roughness. Crack initiation in additively manufactured components due to internal defects is frequently reported in the literature. For instance, Solberg et al. observed that fatigue specimens with a rectangular cross-section exhibited crack initiation originating from internal defects, as opposed to surface defects. [26]. Furthermore, it was observed that crack initiation in all specimens occurred at a single location rather than multiple sites, resulting in a relatively flat fracture surface with characteristic regions of fatigue propagation and ductile fracture, as also reported in other studies on specimens manufactured using PBF-LB [27]. The wide scatter in the fatigue properties of specimen IA correlates well with the size of internal defects. However, fatigue tests always exhibit outliers that cannot be solely attributed to internal defect size or surface roughness. Instead, other factors play a crucial role, such as geometric deviations or the shape and morphology of defects, as demonstrated in the simplified modelling results.\u003c/p\u003e\u003cp\u003eThe observed distance of internal defects from the surface, ranging between 160 \u0026micro;m and 240 \u0026micro;m, corresponds approximately to the melt pool width of a single contour melt track. Literature confirms that, for 316L and the contour process parameters applied in this study, a melt pool width of about 200 \u0026micro;m can be expected [28]. It should be noted, however, that the measured defect distance refers to the orientation orthogonal to the fracture surface rather than to the build direction. Since the specimens were built at an angle of 45\u0026deg; relative to the build platform, the actual defect distance within a single layer is smaller. According to the slicing software, the nominal spacing between the contour and hatch laser tracks should be approximately 40 \u0026micro;m. For both laser vectors, a melt pool width of approximately 200 \u0026micro;m is expected, suggesting that the observed internal defects are not a result of erroneous preprocessing. This supports the assumption that the defects are more likely attributable to process-related variations in melt pool geometry. Specifically, as the melt pool depth increases, the melt pool width tends to decrease, which may increase the effective distance between the solidified contour and hatch regions. Additionally, it is conceivable that the hatch area, which is processed first, potentially leading to already melt the surrounding powder. As a result, the area designated for the contour track may lack sufficient loose powder, impairing complete melting during contour exposure and ultimately contributing to the formation of defects in this region.\u003c/p\u003e\u003cp\u003eFor a better understanding of defect formation, Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e illustrates the melt pool formation as a function of the incidence angle for the up-skin and down-skin areas. For both specimen types, IT and IA, the up-skin area is not remelted during the subsequent layer deposition, as it is built at an angle of 45\u0026deg; to the build plate. Several studies suggest that internal defects can be mitigated through remelting caused by subsequent layer exposure [29]. However, the primary difference between the two specimen types lies in the formation of the melt pool. For the IT specimen type, the melt pool is slightly orientated toward the up-skin surface, reducing the distance between the inner hatch and the outer contour. This results in a lower number of internal defects, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(a). In contrast, the melt pool in the specimens IA is slightly tilted away from the up-skin surface, which, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(b), creates larger gaps between the inner hatch and the outer contour. It should be noted that determining the exact laser incidence angle is challenging, as the installed F-Theta lens is designed to minimize this angle as much as possible. Moreover, assessing its impact on melt pool formation is even more difficult due to the complex interplay of thermal gradients, fluid dynamics, and energy absorption at varying incidence angles. However, the theory of melt pool formation is also supported by various studies, Sendino et al. pointed out in their study that variations in the component's position on the build platform, and consequently in the laser incidence angle, lead to the formation of different melt pools [21]. Therefore, it can be hypothesized that a normally functional offset in the contour parameters is influenced by the variation in the inclination angles which leads to an accumulation of internal defects in the specimen type IA and aligns well with findings from other studies that also examine the influence of the laser incidence angle on melt pool formation. For instance, Subramanian et al. [20] report that the laser incidence angle varies more significantly at the corners of a build plate due to the greater distance from the center point, thereby affecting surface roughness. Furthermore, these changes are reported to cause variations in melt pool formation and heat flow. The impact of internal defects as stress raisers was demonstrated through simplified simulation. The findings from the fracture analysis were confirmed, particularly the crack initiation originating from internal defects, which was successfully simulated and verified.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAt this point, it should be noted that the size of internal defects in the same feedstock material is only one of many factors influencing fatigue life. It is well-established that other factors, such as the existing microstructure, residual stresses, and the morphology and location of internal defects, also play a significant role in determining fatigue life [30]. Nevertheless, it can be concluded that correlating internal defects with fatigue data provides valuable insights into the observed scatter in the fatigue results.\u003c/p\u003e"},{"header":"5 Conclusions","content":"\u003cp\u003eThe influence of the laser incidence angle on surface roughness and fatigue properties of 316L specimens has been thoroughly investigated. It has been confirmed that the laser incidence angle has a significant impact on surface roughness and fatigue life. Moreover, the laser incidence angle should be carefully considered when investigating internal defects, particularly in the interaction between the inner hatch and the outer contour. The key findings of this study can be summarized as follows:\u003c/p\u003e\u003cp\u003e\u003col\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eThe laser incidence angle is a key parameter influencing surface roughness and must be carefully considered, particularly for specimens featuring overhang areas. A deliberate selection of component positioning and orientation enables a more controlled optimization of surface roughness compared to arbitrary orientations.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eThe laser incidence angle also affects melt pool formation, which, in turn, can influence the occurrence of internal defects. The positioning of the component relative to the laser incidence angle should be optimized to ensure that the laser beam exposure parallel to the surface of the component. An orthogonal configuration should be avoided; however, if this is not feasible, appropriate measures should be considered to mitigate the adverse effects of the incidence angle on both surface and subsurface quality.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eFatigue performance in PBF-LB is significantly affected by the orientation of parts relative to the laser incidence direction. Specimens facing the laser showed up to twice the fatigue life compared to those facing away, despite identical process parameters. This highlights the laser incidence angle as a critical but often overlooked factor in build orientation strategies.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003c/ol\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eCRediT authorship contribution statement\u003c/p\u003e\u003cp\u003e\u003cb\u003eMichael Berghaus\u003c/b\u003e: Conceptualization, Methodology, Formal analysis, Validation, Investigation, Writing \u0026ndash; original draft, Data curation, Visualization, Resources. \u003cb\u003eNima Razavi\u003c/b\u003e: Methodology, Formal analysis, Validation, Writing \u0026ndash; review and editing, Supervision, Resources. \u003cb\u003eHilmar Apmann\u003c/b\u003e: Writing \u0026ndash; review \u0026amp; editing, Resources. \u003cb\u003eAxel von Hehl\u003c/b\u003e: Writing \u0026ndash; review \u0026amp; editing.\u003c/p\u003e\u003cp\u003e\u003ch2\u003eOpen Access\u003c/h2\u003e\u003cp\u003eThis article is licensed under a Creative Commons Attri- bution 4.0 International License, which permits use, sharing, adapta- tion, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article\u0026rsquo;s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article\u0026rsquo;s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://creativecommons.org/licenses/by/4.0/\u003c/span\u003e\u003cspan address=\"http://creativecommons.org/licenses/by/4.0/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e\u003cp\u003eOpen access funding enabled and organized by Project DEAL.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eCRediT authorship contribution statement:M.B.: Conceptualization, Methodology, Formal analysis, Validation, Investigation, Writing \u0026ndash; original draft, Data curation, Visualization, Resources. N.R.: Methodology, Formal analysis, Validation, Writing \u0026ndash; review and editing, Supervision, Resources. H. A.: Writing \u0026ndash; review \u0026amp; editing, Resources. A.V.: Writing \u0026ndash; review \u0026amp; editing.\u003c/p\u003e\u003ch2\u003eData availability\u003c/h2\u003e\u003cp\u003eData directly available in the paper\u003c/p\u003e\n\u003cp\u003eOn behalf of all authors, the corresponding author states that there is no conflict of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePublisher's\u003c/strong\u003e\u003cstrong\u003eNote\u003c/strong\u003eSpringer\u0026nbsp;Nature\u0026nbsp;remains\u0026nbsp;neutral\u0026nbsp;with\u0026nbsp;regard\u0026nbsp;to\u0026nbsp;jurisdictional\u0026nbsp;claims\u0026nbsp;in\u0026nbsp;published\u0026nbsp;maps\u0026nbsp;and\u0026nbsp;institutional\u0026nbsp;affiliations.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eT. DebRoy\u003cem\u003e et al., \u003c/em\u003e\"Additive manufacturing of metallic components \u0026ndash; Process, structure and properties,\" \u003cem\u003eProgress in Materials Science\u003c/em\u003e, pp. 112\u0026ndash;224, 2018, doi: 10.1016/j.pmatsci.2017.10.001.\u003c/li\u003e\n\u003cli\u003eR. Groarke\u003cem\u003e et al., \u003c/em\u003e\"316L Stainless Steel Powders for Additive Manufacturing: Relationships of Powder Rheology, Size, Size Distribution to Part Properties,\" \u003cem\u003eMaterials (Basel, Switzerland)\u003c/em\u003e, early access. doi: 10.3390/ma13235537.\u003c/li\u003e\n\u003cli\u003eM. J. Heiden\u003cem\u003e et al., \u003c/em\u003e\"Evolution of 316L stainless steel feedstock due to laser powder bed fusion process,\" \u003cem\u003eAdditive Manufacturing\u003c/em\u003e, vol. 25, pp. 84\u0026ndash;103, 2019, doi: 10.1016/j.addma.2018.10.019.\u003c/li\u003e\n\u003cli\u003eG. Mohr, N. Scheuschner, and K. Hilgenberg, \"In situ heat accumulation by geometrical features obstructing heat flux and by reduced inter layer times in laser powder bed fusion of AISI 316L stainless steel,\" \u003cem\u003eProcedia CIRP\u003c/em\u003e, vol. 94, pp. 155\u0026ndash;160, 2020, doi: 10.1016/j.procir.2020.09.030.\u003c/li\u003e\n\u003cli\u003eM. A. Chaudry, G. Mohr, and K. Hilgenberg, \"Experimental and numerical comparison of heat accumulation during laser powder bed fusion of 316L stainless steel,\" \u003cem\u003eProg Addit Manuf\u003c/em\u003e, vol. 7, no. 5, pp. 1071\u0026ndash;1083, 2022, doi: 10.1007/s40964-022-00282-x.\u003c/li\u003e\n\u003cli\u003eG. Mohr, S. J. Altenburg, and K. Hilgenberg, \"Effects of inter layer time and build height on resulting properties of 316L stainless steel processed by laser powder bed fusion,\" \u003cem\u003eAdditive Manufacturing\u003c/em\u003e, vol. 32, p. 101080, 2020, doi: 10.1016/j.addma.2020.101080.\u003c/li\u003e\n\u003cli\u003eK. Solberg, S. Guan, N. 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Reynolds, \"Residual stress and microstructure effects on fatigue crack growth in AA2050 friction stir welds,\" \u003cem\u003eInternational Journal of Fatigue\u003c/em\u003e, vol. 30, no. 3, pp. 463\u0026ndash;472, 2008, doi: 10.1016/j.ijfatigue.2007.04.016.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Additive manufacturing, Laser powder bed fusion, 316L, Laser incidence angle, Fatigue properties","lastPublishedDoi":"10.21203/rs.3.rs-6871000/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6871000/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003ePowder bed fusion using a laser beam (PBF-LB/M) is a widely utilized additive manufacturing technique known for its ability to produce complex geometries with high precision. However, there are numerous factors that can influence the properties of components manufactured by this process. One scarcely investigated factor is the laser incidence angle, which refers to the orientation and positioning of a component relative to the direction from which the laser interacts with the layer to be melted. This study examines the effect of the laser incidence angle on surface roughness and fatigue properties of 316L components. Results confirmed that laser incidence angle significantly affects the surface roughness and can contribute to the formation or reduction of internal defects. Increased surface roughness and internal defects, particularly the latter, adversely impact fatigue strength, with internal defects playing a primary role in crack initiation in this study. Given that the laser incidence angle significantly influences both surface roughness and fatigue strength, careful positioning of components on the build plate relative to the laser incidence angle is crucial for optimizing the mechanical properties of additively manufactured parts.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e","manuscriptTitle":"Effect of laser incidence angle on surface roughness and fatigue properties of additively manufactured 316L","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-09 14:24:07","doi":"10.21203/rs.3.rs-6871000/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"fa308a47-9f89-4cce-9217-10d91dae7d1f","owner":[],"postedDate":"July 9th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-12-29T16:04:42+00:00","versionOfRecord":{"articleIdentity":"rs-6871000","link":"https://doi.org/10.1007/s40964-025-01441-6","journal":{"identity":"progress-in-additive-manufacturing","isVorOnly":false,"title":"Progress in Additive Manufacturing"},"publishedOn":"2025-12-22 15:57:09","publishedOnDateReadable":"December 22nd, 2025"},"versionCreatedAt":"2025-07-09 14:24:07","video":"","vorDoi":"10.1007/s40964-025-01441-6","vorDoiUrl":"https://doi.org/10.1007/s40964-025-01441-6","workflowStages":[]},"version":"v1","identity":"rs-6871000","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6871000","identity":"rs-6871000","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}