Grain boundary character evolution in stainless steel 316L upon laser powder-bed fusion and post-process heat treatment

Abstract

Grain boundary engineering (GBE) has emerged as a promising method for improving mechanical properties and diminishing the susceptibility to corrosion in polycrystalline materials, via engineering recrystallized microstructures with high fractions of low-energy grain boundaries (GBs). Conventional GBE utilizes complex cycles of deformation and annealing to provide the necessary driving forces for recrystallization. This is not applicable to near-net-shape manufacturing, such as metal additive manufacturing (AM). To overcome this limitation, an alternative approach involves adjusting the strain energy introduced during the AM process instead of mechanical deformation, to generate the required driving forces for recrystallization. This requires thorough understanding of the evolution of the solidification microstructure, dislocation structure and the GB character as functions of the AM processing parameters. In this study, we systematically demonstrate the impact of processing parameter variations during laser powder bed fusion and heat treatments on GB evolution in stainless steel 316L. We provide comprehensive analyses of the texture, grain structure, GB habit planes, cellular structure, and micro-segregation, and make a link to mechanical properties. The differences in recrystallization response as a function of AM processing parameters are attributed to variations in the densities of dislocations and the chemical heterogeneity in the as-solidified microstructures. We also introduce a novel concentric scanning technique to achieve site-specific control over the recrystallization response. This facilitates the design of microstructures with both superior thermal stability and GBE-related advantages, offering a pathway towards making high-performance alloy AM parts with engineered and possibly site-specific microstructures, superior mechanical properties, and complex shapes.