In situ tailoring microstructure of martensitic stainless steel during wire-arc directed energy deposition via intrinsic heat treatment

Additive manufacturing enables precise control over process parameters during fabrication, thereby influencing the resulting microstructure. In this study, the microstructure of AISI 420 martensitic stainless steel was tailored during wire-arc directed energy deposition by strategically manipulating intrinsic heat treatment, i.e., rapid reheating from subsequent depositions. By reducing the interpass dwell time and maintaining the interpass temperature above the martensite start (Ms) temperature, the martensitic transformation in the deposited layer was inhibited, preserving the austenite phase. The austenite remained largely unaffected by intrinsic heat treatment and primarily transformed into martensite after the final pass, yielding high hardness (similar to 410 HV) and ultimate tensile strength (similar to 1090 MPa), albeit with reduced elongation (similar to 5 %). In contrast, increasing the interpass dwell time and keeping the interpass temperature below the martensite finish (Mf) temperature led to predominant martensitic transformation upon solidification and cooling. Subsequent intrinsic heat treatment facilitated in situ tempering of martensite, promoting Cr-rich M7C3 carbide precipitation, reducing dislocation density, and increasing the effective grain size. This microstructure exhibited lower hardness (similar to 290 HV) and ultimate tensile strength (similar to 760 MPa) but improved elongation (similar to 14 %). By dynamically adjusting the interpass temperature and controlling intrinsic heat treatment during wire-arc directed energy deposition, locally tailored microstructures with tunable mechanical properties were successfully achieved.