Unravelling microstructure evolution mechanism within machined subsurface during turning of laser powder bed fusion-manufactured Inconel 625

Post-machining of metal additive-manufactured (AMed) nickel-based alloy components is one of the efficient approaches to reduce surface roughness and enhance surface quality. Although the white layer formed on the wrought nickel-based alloy surface after machining has been deeply investigated, the formation mechanism of the white and dark layers generated on AMed nickel-based alloy still faces challenges. In this study, the white and dark layer formation on laser powder bed fusion (LPBF)-fabricated Inconel 625 alloy surface after turning was determined. Then various material characterization techniques were adopted to comprehensively analyze the microstructure, texture and phase constituent concerning the white and dark layers. Obvious intragranular misorientation change, great concentration of high angle grain boundaries and grain refinement occurred beneath the machined surface. Strongly refined grains in nanometers and noticeable plastic deformation with slight grain division along with disappeared dense dislocations were revealed correspondingly within the white and dark layers. Phase transformation was absent from the machined surface despite cutting parameters. Dynamical crystallization (DRX) following shear deformation dominated the formation of the white layer while plastic deformation was responsible for dark layer formation. The findings were beneficial to understanding the occurrence of damages initiated from machined surfaces during service.