Theoretical prediction of machining-induced residual stresses in three-dimensional oblique milling processes

Residual stress not only has great influence on the fatigue life of a machined component, but also can cause serious distortion in machining of large components, and thus, it is an important characteristic needed to be studied deeply. Previous researches on theoretical calculation of machining-induced residual stresses were mainly carried out for two-dimensional (2D) orthogonal cutting processes, and analytical prediction of residual stresses in three-dimensional (3D) milling is seldom reported. This paper presents a theoretical model to predict the machining-induced residual stresses in 3D milling processes by taking the 3D instantaneous contact status between the mill and part into consideration. A 3D contact model of oblique cutting is established to predict the stress distributions by integrating the cutting effect in shear zone with the ploughing effect in the honed zone of the cutting edge. Instantaneous machining stresses produced by shearing effects are predicted based on the basic principles of cutting mechanism, contact mechanics and J-C constitutive model; while the stresses in the honed zone are evaluated by using slip line theory. Complicated geometrical relationships in both zone are detailed in the model formulation. Instantaneous cutting temperature fields during milling process are predicted to study the influences of thermal loads on residual stresses. Incremental thermo-elasto-plastic method is adopted to predict the thermo-elasto-plastic constitutive behavior of the workpiece in elastic-plastic cyclic loading process. 3D relaxation procedures are proposed to calculate the final residual stresses after the loading process of the cutting edges. Validity of the proposed model is demonstrated by the good agreements between the theoretical predictions and the results by using finite element methods and experimental means. Furthermore, it is also verified that the computation efficiency of this model is significantly higher than that of finite element method.