effects of thermal history on solidification structure, δ-ferrite retention, and microhardness gradients in waam-fabricated 316l stainless steel

Additive manufacturing (am) of metals has emerged as a transformative approach for producing complex components, with wire arc additive manufacturing (waam) andselective laser melting (slm) representing two prominent techniques. in this study, austeniticstainless steel 316l was fabricated using gmaw-based waam to investigate the influenceof thermal history on microstructural evolution and mechanical properties. the as-fabricatedwaam walls exhibited a dual-phase microstructure composed of γ-austenite and 8.5%retained δ-ferrite, consistent with fa (ferritic–austenitic) solidification predicted by theschaeffler diagram. columnar dendritic grains predominantly grew opposite to the heat flowdirection, and no carbide precipitates or manufacturing defects such as cracks, pores, or lackof fusion were observed. microhardness measurements revealed a gradual decrease from thebottom to the top layers, attributed to thermal accumulation, reduced cooling rates, and graincoarsening. comparatively, slm-produced 316l exhibits fully austenitic microstructures orminimal δ-ferrite (<1–2%), characterized by ultrafine cellular/dendritic γ-austenite, higherhardness (220–280 hv), and more uniform spatial distribution due to extremely rapidsolidification and high cooling rates. the observed differences between waam and slmhighlight the role of process-specific thermal gradients and solidification kinetics indetermining phase constitution, grain morphology, and mechanical performance. overall, thefindings highlight the strong relationship between thermal history, microstructural evolution,and mechanical response in wire arc additive manufactured 316l stainless steel components.