On the site-specific microstructure evolution in 316L stainless steel via tuning concentric laser-powder bed fusion printing strategies

• Best combination of parameters for large site-specific fine grain area in scan plane. • High frequency of horizontal thermal pulses and heat flux disrupt vertical grain growth. • Low hatch spacing results in high energy density, conduction to keyhole mode transition. • Grain size and dislocation cellular structures contribute to hardness of 316L samples. • Texture must be considered when evaluating effect of dislocation cellular structures. The complex thermal history of metal additive manufacturing (AM) is often regarded as a challenge, as it is well-known to promote heterogeneities in microstructures and properties. However, control over the thermal history via tunable AM parameters may enable site-specific microstructure control if the underlying microstructural phenomena can be better understood. This is attractive for next-generation engineering designs, as it may unlock new performance profiles. To date, site-specific microstructures with local variations in phase transformation products and crystallographic texture have been achieved via using modified scan strategies. Building on recent research on AM grain boundary engineering, we showcase site-specific grain refinement via an inward concentric scan in 316L stainless steel, an austenitic steel whose grain structures are sensitive to variations in thermal history. The relationship between tunable L-PBF parameters, the resulting microstructure and hardness is characterized and underpinned by thermal modeling. We discuss how the largest site-specific fine-grain area was achieved via a combination of low hatch spacing and high laser power. This is attributed to the high horizontal thermal pulse frequency and heat flux transfer rate that disrupts epitaxial grain growth vertically. This simple approach to site-specific grain size control in L-PBF that may be suitable for many structural alloys.