Heterogeneous mechanical properties along the building direction in direct laser deposited 17-4 PH steel

The present work thoroughly investigated the heterogeneous mechanical properties along the building direction in a direct laser deposited 17-4 PH part by tensile tests. From top to bottom (1# - 5#) in the as-built cuboid, the yield strength enhanced from -729 MPa to -1050 MPa, while the elongation decreased from -4.69% to -1.50%. The microstructure of samples was composed of main martensite, reversed austenite, and a large amount of NbC nano-particles. The discrepancy in chemical composition and dislocation density between the phases existed due to the heat accumulation as the increasing building height. The various reduced fraction of austenite in different samples after tensile tests, implying the diverse extent of strain-induced phase transformation. Besides, the average Schmid factor of the 1# sample was higher; thus its grain orientation was more conducive to plastic deformation, leading to lower yield stress. The kernel average misorientation (KAM) was higher and distributed uniformly near the fracture of the 1# sample, and showed better deformation compatibility in the microstructure. Based on the combination of microstructural models and the dislocation density micromechanical model, multiscale finite element analysis further found that larger particles advanced the movements of dislocations, inducing more intense plastic flow within the austenite grains, which enabled greater fractions of transformation in austenite to improve the overall elongation of the sample.

Automated summary

This study thoroughly investigated the heterogeneous mechanical properties along the building direction in a direct laser deposited 17-4 PH part by conducting tensile tests. The results showed significant variations in yield strength and elongation at different positions within the as-built cuboid. The microstructure analysis revealed differences in chemical composition and dislocation density among samples, attributed to heat accumulation during the building process. Additionally, the varying reduction in austenite fractions after tensile tests indicated diverse extents of strain-induced phase transformation.