Interplay between hierarchical microstructure and graded residual stress in a stainless steel fabricated by laser powder bed fusion

The intrinsic heat treatment during laser powder bed fusion (LPBF) fabrication process restricts the precise control of three-dimensional size and stabilization of properties, thereby hindering the wide application of LPBF parts. This work unveils the formation mechanism of graded microstructure and graded properties along building direction and interplay between them for the AISI 420 stainless steel containing solid phase transformation. The graded microstructure is mainly characterized by increasing volume fraction of martensitic alpha' phase and decreasing thickness of martensitic alpha' lath from bottom substrate to top surface. Martensitic {112} (111)alpha ' compound twin and interfacial co phase exist in the area close to the top surface, while theta-Fe3C cementite was additionally indexed in the area close to the bottom substrate. The hierarchical microstructural featured by the complementary austenitic gamma phase and multi martensitic alpha' variants were revealed. The martensitic variants selection indeed occurs and 5-12 types of martensitic variants form during LPBF. Graded and hierarchical microstructure is responsible for the evolution of graded micro-hardness and graded residual stress. The reduction in thickness of martensitic laths and increase in volume fraction of martensite lead to the increase of micro-hardness and strengthening effect with the gradient of 3.54 +/- 0.16 HV/mm. The coarsened martensitic alpha' lath and reversed phase transformation from martensitic alpha' to austenitic gamma for area close to bottom substrate counteract the volume shrinkage and trigger partial stress relief.