Influence mechanism of grain size on chip deformation characteristics in the turning of powder metallurgy superalloys

The difficulty in machining powder metallurgy superalloys is closely related to the deformation characteristics of the chips produced during turning. To elucidate the microscopic mechanisms of chip deformation during turning, this study systematically investigates the effects of grain size and microstructure on the turning performance and chip deformation of powder metallurgy superalloys. The results indicate that the volume fraction of the secondary γ’ phase is an important factor influencing the machinability of powder metallurgy superalloys. The greater difficulty in deforming coarse-grained chips compared with fine-grained ones is mainly attributed to the higher volume fraction of the secondary γ’ phase in coarse-grained powder metallurgy superalloys. Grain size significantly influences the uniformity and regularity of chip deformation. The distribution and properties of strengthening phase particles affect the deformation resistance of the grain boundary region within the chip. The observed microstructural characteristics indicate that deformation of the secondary γ’ phase occurs primarily through a dislocation shear mechanism. The high hardness and brittleness of the secondary γ’ phase, together with its strong interfacial bonding with the matrix, restrict deformation within grains. Based on the findings, a multi-stage heat treatment strategy is proposed to enhance the machining efficiency and tool life of powder metallurgy superalloys.