<b>SURFACE INTEGRITY IN LASER POWDER BED FUSION-MANUFACTURED IN718, 316SS AND TI-64: REMEDIAL ROLE OF SHOT PEENING</b>

<p dir="ltr">Critical aerospace components like turbine blades and landing gears are increasingly produced using additive manufacturing (AM), especially laser powder bed fusion (LPBF) due to its design flexibility and material efficiency. However, LPBF inherently results in high surface roughness, primarily due to AM-induced features such as spatter, partially melted particles, and scan tracks. These AM-deterministic features consist of significant surface slope gradients and additional textural area and can act as stress concentrators that compromise part performance and service life. Additionally, high temperature gradients during the layer-by-layer manufacturing techniques generates large tensile stresses. These promote the formation of sub-cellular grain structures and microscopic strain localization zones, especially near melt-pool boundaries. Internal misfit strain and dislocation pile-up generate inhomogeneous stress-localized microstructure. Moreover, the non-uniform distribution of thermal strains across the build results in macroscopic shape distortions (warping and/or residual bowing) which ultimately degrade structural reliability. In-process mitigation of these issues by optimizing printing parameters has limited improvements. Surface post-processing techniques such as machining, electro- and chemical polishing offer partial mitigation to minimizing roughness but compromise geometry dimensions (i.e., material-removal mechanisms) and fails to address the tensile residual stress profiles (surface and sub-surface). Therefore, shot peening is implemented as a post-processing method.</p><p dir="ltr">Shot peening, a mechanical surface work-hardening process that induces severe plastic deformation through repeated high-velocity impacts of spherical metallic media enhancing surface integrity and strength while retaining bulk ductility. This study investigates the influence of shot peening on the surface integrity of LPBF-fabricated IN718, 316SS, and Ti64, across surface orientations from 0–90°.</p><p dir="ltr">Specifically, the research objective is to investigate surface and mechanical property enhancement through shot peening on complex-oriented AM parts. Furthermore, material- and orientation-specific peening parameter optimization including exposure time (coverage), intensity, and impingement angle was performed to achieve uniform roughness reduction by elimination of AM-deterministic features across different surface orientations. Additionally, downskins are peened twice the duration as that of the upskins to compensate the inherent differential factors due to effect of gravity, meltpool-powder interaction, and partial powder adhesion. This work considers two specific peening techniques, gravity-assisted shot peening (GASP) and pneumatic shot peening (PSP), targeting surface modification under controlled low (έ ≈ 10³ s^-1) and high (έ ≈ 10⁵ s^-1) strain rate conditions, respectively. These conditions govern the dislocation mobility and surface plasticity in metallic alloys during shot peening.</p><p dir="ltr">Comparative evaluation of shot peening on three LPBF-manufactured IN718, 316SS, and Ti64 materials are performed for broader applicability. Optimized peening conditions effectively reduced average areal roughness (S_a) by <a href="" target="_blank">≈</a> 70%, and generated significant compressive stresses (≈ -400 MPa), while at the same time, increasing surface strength and hardness (≈ 35%). GASP achieved minimum roughness (S_a ≈ 1.6 µm for IN718 and ≈ 5.7 µm for 316SS) and maximum hardness (65% for IN718 and 55% for 316SS), and PSP resulted in maximum compressive stress (≈ -1000 MPa in IN718, 316SS, and Ti64). Significant AM-deterministic feature reduction validated through surface slope gradient (S_dq↓ ≈ 70%) and additional textural area (S_dr↓ ≈ 90%) minimization. The anisotropic nature of LPBF (bottom-up material addition approach) is critically evaluated through comparative analysis of areal and hybrid roughness metrics for upskins and downskins. As the industry applies AM for complex performance-critical parts, these analyses provide a pathway to achieve industry applicable surface quality through shot peening.</p>