On adiabatic shear localized fracture during serrated chip evolution in high speed machining of hardened AISI 1045 steel

The occurrence of adiabatic shear localized fracture (ASLF) which is the final stage of adiabatic shear evolution usually leads to discontinuously short serrated chip with only a few segments joined together or even isolated segment formation. To develop a deep study on ASLF, damage evolution mechanism of ASLF in chip formation is investigated under optical microscope and scanning electron microscope (SEM) through high speed machining experiment of hardened AISI 1045 steel at a relatively high cutting speed. Considering the energy convergence during damage evolution in shear band, the theory model of ASLF in which adiabatic shear saturation limit and saturation degree are proposed as the ASLF criterion is built and compared with the experimental results. The quantitative influences of shear band properties, material properties and loading conditions on ASLF are also analyzed numerically. The results presented here show that complete ASLF in isolated segment formation is found above cutting speed of 1300 m/min. The mechanism of ASLF in isolated segment formation is a multi-damage evolution process (or ductile–brittle–ductile transition mechanism), including voids coalescence, crack propagation and even fusing. Thermal softening effect plays a significant role in the process of ASLF. A thinner shear band with larger specific heat and thermal conductivity under higher strain and strain rate more easily leads to ASLF. The proposed ASLF theory can effectively assess saturation limit and quantitatively in machining hardened AISI 1045 steel.