Attainment of favorable microstructure for residual stress reduction through high-temperature heat treatment on additive manufactured inconel 718 alloy

Mitigation of residual stress in additive manufactured metal parts through post-heat treatment helps enhancement of life during service. The present work aims to investigate the effect of stress-relieving heat treatment (SRHT) at 1065 °C on microstructural changes, residual stress patterns, and mechanical properties of Inconel 718 (IN718) blocks fabricated through use of the laser-based powder bed fusion (L-PBF) process. The X-ray diffraction technique known as cos α method was used to capture the Debye ring for the measurement of surface residual stress. Dissolution of the columnar dendrites along with laves phases in the interdendritic region was seen after SRHT. The resultant microstructure showed partially recrystallized grains along with the formation of annealing twins and strengthening phases such as γ’, γ’’, and tiny metal carbides. The residual stress on the top surface of as-built sample was found to be 77% higher on the corners compared to the center region. The distribution of residual stress was seen as not uniform in the as-built condition. The SRHT sample showed uniform distribution of compressive residual stresses in all the locations. This phenomenon was due to the diffusion of atoms present in the high-stress regions migrated to low-stress regions until attaining their equilibrium. The combination of face centered cubic (FCC) and hexagonal lattices in as-built microstructure led to misfit strain between the grains. After SRHT, microstructure of IN718 experienced relaxation of residual stresses due to the transformation of hexagonal structured laves phase into the body-centered tetragonal (BCT) γ’’and FCC-structured γ’ phases. Furthermore, SRHT significantly alters the mechanical properties of IN718 alloy causing a 23.03% increase in average hardness and a 21.04% increase in tensile strength.