Mechanism of ultra-high-speed cutting of Ti-6Al-4V alloy considering time-dependent microstructure and mechanical behaviors

High-speed cutting (HSC) and ultra-high-speed cutting (u-HSC) (6.07-32.7 m/s) tests of Ti-6Al-4V alloy were developed based on an improved Split Hopkinson Pressure Bars (SHPB) system. Specific cutting energy was calculated and scaling effect was observed in HSC with different uncut chip thicknesses (20-200 mu m). Considering time-dependent (in situ and ex situ) microstructure and mechanical parameters, chip morphology, adiabatic shear band (ASB), chip-free surface, material separation characteristic at chip-workpiece interface, and surface integrity were investigated. The micro-hardness within ASB was 25% higher than that outside of the ASB. Ultra-fine grains (77% grain sizes less than 330 nm) and micro-texture were observed by electron backscattering diffraction (EBSD) test. Grain refinement in ASB was explained by rotation dynamic recrystallization. The maximum shear strain and the range of shear strain rate in ASB at the condition of v = 10 m/s and t(1) = 100 mu m were approximately 17 and 6.44 x 10(6) s(-1)-8.5 x 10(6) s(-1), respectively. Severe plastic deformation was observed by the dimples at ductile fracture surface along ASB. Mechanism of parabolic and ellipsoid dimples caused by shear and tensile stresses was explained for HSC of Ti-6Al-4V alloy. Additionally, the micro-hardness of machined subsurface and the ASB (ex situ parameter) exhibits a relationship with specific energy (in situ parameter), which can provide an effective way to control the formation of ASB and surface integrity by the control of in situ parameters in HSC and u-HSC conditions.