Management of Residual Stresses and Surface Properties in Laser Powder Bed Fusion Built 316L Stainless Steel Components by Means of Post Processing

Abstract

Metal Additive Manufacturing (AM) has the potential to revolutionize the fabrication of complex, lightweight, and high-performance components. Among AM techniques, Laser Powder Bed Fusion (LPBF) is widely used for producing metallic components, including 316L stainless steel, due to its excellent corrosion resistance, mechanical properties, and suitability for critical applications. However, LPBF processing introduces several inherent quality challenges in as printed materials, particularly high tensile residual stresses, near-surface porosity, and surface roughness, which could negatively impact mechanical integrity, fatigue life, and stress corrosion cracking (SCC) resistance. Therefore, post-processing is essential to mitigate these challenges and enhance the surface integrity of these components. This thesis systematically investigated two post-processing techniques namely heat treatment (HT) and severe shot peening (SSP), both individually and in combination, to optimize residual stresses and surface properties in LPBF-built 316L stainless steel. The applicability of the optimized HT+SSP protocol was also explored for another metal AM technique, Binder Jetting (BJ), to assess its broader relevance.

Initially, the study evaluated the effects of SSP on LPBF-built 316L stainless steel and compared its response with conventionally manufactured (REF) samples. Results showed that SSP induces deep compressive residual stresses, refines the surface grain structure, and significantly improves roughness. LPBF-built samples exhibited a distinct response to SSP than conventionally manufactured samples, primarily due to their initial microstructure and pre-existing tensile residual stresses. However, unlike REF samples, which did undergo deformation-induced martensitic transformation, LPBF samples experienced only grain refinement without phase transformation. To further optimize mechanical properties, heat treatment at 600°C and 900°C was applied before SSP. While the 600°C HT (2h) provided only partial stress relief (~10 - 20%), the 900°C HT (0.5h) nearly eliminated tensile residual stresses (>90% reduction) but softened the material (204 HV vs. 239 HV in the as- printed state). Applying SSP after HT compensated for this softening, increasing surface hardness (>460 HV) and introducing deep compressive stresses. The 900HT+SSP condition proved to be the most effective, achieving an optimal balance of stress relaxation, surface hardening, and potential enhanced mechanical durability, making it a promising post-processing route for LPBF-built 316L stainless steel.

Beyond surface and subsurface enhancements, this research explored the SCC performance of post-processed LPBF-built 316L stainless steel, using U-bending tests in a boiling 25% NaCl solution. While as-printed and heat-treated samples exhibited no SCC failures, SSP-treated samples (AB+SSP and 600HT+SSP) developed cracks within 1 to 2 weeks, primarily due to high post-bending tensile stresses (~1008 MPa and ~785 MPa, respectively). In contrast, 900HT+SSP samples remained crack-free even after 5 weeks, demonstrating higher SCC resistance potentially due to microstructural homogenization, relatively better stress state, and reduced stress concentration sites. To further assess the applicability of the HT+SSP protocol, this study extended its implementation to BJ built 316L stainless steel, another metal AM technique. However, unlike LPBF, HT900 proved detrimental for BJ components, increasing porosity from 2.5% to 7.5% due to pore coalescence. Conversely, SSP alone proved highly effective for BJ samples, significantly reducing surface porosity (to 0.45%), enhancing surface hardness (>500 HV), and introducing stable compressive residual stresses. These findings highlight that while HT900 is essential for LPBF components, it is unnecessary for BJ parts, where SSP alone is sufficient for improving mechanical properties and surface integrity. Overall, this study establishes 900HT+SSP as the optimal post-processing strategy for LPBF-built 316L stainless steel, while demonstrating that SSP alone is a robust solution for Binder Jetting components.