Temperature-dependent shear localisation and microstructural evolution in machining of nickel-base superalloys

Abstract

Understanding the microstructural evolution mechanisms in machining of advanced materials is essential to achieve excellent surface integrity levels within the manufacture of safety–critical components. However, as thermal and mechanical effects are coupled in conventional cutting operations, it is difficult to attribute their individual role on microstructural evolution and integrity. To investigate the temperature-dependency of microstructural evolution in cutting, a new experimental set-up has been developed to perform machining experiments under controlled temperatures. Results show that an onset in chip shear localisation with nanocrystalline grain refinement can be induced uniquely by an increase in cutting temperature under fixed cutting parameters, which microstructurally controls the transition from continuous to serrated chip formation. Increase in mechanical effects at HT leads to the formation of a continuous chip grain refinement layer, associated to a change in energy partition at the tool-workpiece interface. These small-scale behaviours are found to control the reduction in cutting forces and energy at higher temperatures, with a decrease of ∼ 25–30%. Nevertheless, despite the lower deformation energy, HT cutting induced larger amounts of microstructural deformation because of thermal softening effects, further disclosing the role of thermal effects on the interplay between shear localisation, microstructural evolution and surface integrity.