Surface quality adjustment and controlling mechanism of machined surface layer in two-step milling of titanium alloy

In the two-step cutting process, due to the combined effects of mechanical and thermal deformation, the microstructure and residual stress of the workpiece are changed with the change of roughing parameters, which affects the machined surface quality. In this paper, the two-step milling (roughing and then finishing) experiments of Ti-6Al-4V titanium alloy were designed to analyze the effect of different roughing parameters on the cutting force of roughing and finishing, the residual stress of finishing surface, and the microstructure. The microstructural characteristics in terms of residual stress, XRD patterns, phase composition and content, and nano-scale crystallite size of machined surface layer were characterized to reveal the machined surface layer quality adjustment and controlling mechanism from the prospective of the microscopic scale. The experimental results showed that the cutting force and compressive residual stress were larger at low roughing cutting speed than that at high roughing cutting speed because of the combined effects of mechanical and thermal deformations. In the two-step machining process, with the increase of roughing cutting speed, the β phase on the finishing surface firstly increased and then decreased. Meanwhile, the high roughing cutting speed weakened the microcrystal refinement of the finishing surface because of the work hardening effect. Therefore, the appropriate roughing machining parameters will contribute to the improvement of finishing machined surface quality in the terms of lower cutting force, higher compressive residual stress, and better grain refinement, phase content, and preferred crystallite orientation, thus increasing the microscopic mechanical properties of machined surface layer. This provides a reference for optimizing the cutting parameters to adjust and control the quality and integrity of the machined surface layer in two-step milling of Ti-6Al-4V alloy.