Sensitization and mechanical response of additively manufactured SKD61/ 17-4PH functionally graded steel: Influence of process parameters, post-AM treatment, and insight into reversed austenite formation

초록

This study investigates the phase transformation mechanisms and mechanical behavior of additively manufactured SKD61/17-4 PH graded structure, targeting high-temperature tooling and die applications where thermal stability and strength retention are critical. Using laser powder bed fusion (LPBF), graded structures were fabricated under high- and low-heat-input (HHI and LHI) conditions to examine the effect of processing on solidification and post-annealing response. Electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and first-principles diffusion analyses revealed that LHI processing promotes delta-ferrite retention through rapid cooling, while HHI favors full austenitization and martensitic transformation. Non-equilibrium solidification induces Ni-C partitioning and Cr solute trapping in austenite, which drive recrystallization and partitioning-assisted gamma reversion during annealing at 480 degrees C. Ni-C enrichment at prior austenite grain boundaries enhances gamma nucleation and stress relaxation, whereas Cr-rich delta-ferrite regions foster carbide formation and sensitization upon prolonged exposure. These mechanisms dictate the strength-ductility balance: HHI specimens achieved ultimate tensile strength (UTS) ti 1110 MPa and yield strength (YS) ti 1000 MPa, while LHI specimens reached UTS ti 1250 MPa and YS ti 1080 MPa with superior ductility. The findings provide design insights for thermally stable graded steels in demanding industrial environments.

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