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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. [ABSTRACT FROM AUTHOR]

The residual stress on the machined surface has a significant impact on the service life and fatigue strength of machined parts. In the former studies, it has been confirmed that both pre-stress cutting process and multi-step cutting process can significantly affect the residual stress distribution of the finishing machined surface, which gives expression to the increasing trend of compressive residual stress. In this paper, a finite element software Abaqus was used to establish a two-dimensional finite element simulation model for titanium alloy cutting. The finite element model was validated by the chip shape characteristics and cutting forces obtained by orthogonal cutting experiments. Based on the validated model, the simulations of single-step cutting, multi-step cutting, pre-stress single-step cutting, and pre-stress multi-step cutting processes were conducted. The chip shape characteristics, cutting forces, and residual stress distribution of the machined surface under four machining processing conditions were comparably investigated. The influence mechanism and coupling effect of multi-step machining and pre-stress machining on residual stress distribution on machined surface was explained. Investigation results indicate that both multi-step cutting and pre-stress cutting processes can increase the compressive residual stress, while multi-step cutting will change the position of minimum residual stress along the depth direction and increase the chip segmentation degree. Among the four cutting processing conditions, the pre-stress multi-step cutting will effectively increase compressive residual stress value to the utmost extent, which should be attributed to the coupled effect of pre-stress cutting and multi-step cutting processes. [ABSTRACT FROM AUTHOR]

In the multi-step cutting process, the final machined surface quality is affected by the entire cutting process, especially the effect of work hardening and thermal softening induced by the previous steps (roughing or semi-finishing machining). In this paper, two-step (roughing-finishing) and three-step (roughing-semi-finishing-finishing) turning operations were designed by a single-factor experiment, and the effect of the change of cutting parameters in the previous steps on finishing surface quality was analyzed. Experimental results indicated that the microhardness in the machined surface layer of semi-finishing was smaller than that of roughing at the same depth, and the depth affected by work hardening of semi-finishing was thinner. Therefore, compared to two-step machining, the surface roughness affected by the work hardening and thermal softening of the previous steps was smaller after three-step machining, and the variation range of surface roughness with the change of the cutting parameters was smaller. Moreover, the relative height between the convex peak and the concave valley was larger. And the spacing between adjacent two convex peaks became larger and more uneven after three-step machining. The disturbed layer depth or the plastic deformation layer (PDL) depth was obviously reduced. However, the grains were severely distorted and stretched. It was because that semi-finishing weakened the work hardening of roughing in the three-step cutting process; therefore, the surface quality was better after finishing. By studying the effect of work hardening and thermal softening induced by the previous steps on finishing surface quality, the cutting parameters of roughing and semi-finishing were optimized to ultimately improve surface quality. [ABSTRACT FROM AUTHOR]