Serrated chip characteristics and formation mechanism in high-speed machining of selective laser melted Ti6Al4V alloys

Serrated chips, consisting of extremely uneven plastic deformation, are a prominent feature of high-speed machining of difficult-to-machine materials. This paper focuses on the evolution of chip form, chip morphology features (chip free surface, tool-chip contact surface, and chip edge), and chip segment parameters in subsequent high-speed (vc=50 and 150 m min−1) machining of selective laser melted (SLMed) Ti6Al4V alloys, which are significantly different from conventional Ti6Al4V alloy in microstructure, mechanical properties and machinability. The effect of laser beam scanning schemes (0°, 67.5°, and 90°), machined surfaces (top and front), and cutting speeds on serrated chip characteristics of SLMed Ti6Al4Valloys was investigated. Based on the Johnson-Cook constitutive model of SLMed Ti6Al4V alloys, an orthogonal cutting model was developed to better understand the effect of physical-mechanical properties on the shear localization, which dominates the formation mechanism of serrated chips in post-machining of SLMed Ti6Al4V alloy. The results showed that the critical cutting speed (CCS) for chip serration of SLMed Ti6Al4V alloy is lower than that for serrated chips of conventional Ti6Al4V alloy, and the serrated profile of SLMed Ti6Al4V chips was more regular and pronounced. Besides, due to anisotropic microstructure and mechanical properties of SLMed Ti6Al4V alloys, the serration degree of chips produced on the top surfaces of SLMed Ti6Al4V alloys is more prominent than that of chips generated on the front surfaces. In addition, because of the poor deformation coordination and high plastic flow stresses of needle-like martensite α′, the plastic flow and grain distortion in the adiabatic shear band (ASB) of SLMed Ti6Al4V chips are significantly smaller than those in the ASB of conventional Ti6Al4V with equiaxed grains.