A novel method to predict residual stress by coupling the phase transition effect for LA103Z Mg-Li alloy milling

Machining-induced residual stress is the dominant contributing factor to distortion of Mg-Li alloy thin-walled parts. Phase transition takes a crucial role on inducing the residual stress during milling for duplex LA103Z Mg-Li alloy material. Coupling the phase transition effect to the thermal-mechanical finite element (FE) model is the key to accurately predict the residual stress during milling. In this study, an innovative method to equivalently calculate the complex stress-strain fields induced by phase transition was proposed to establish a thermomechanical-phase transition (TMP) multi-physical field coupling model. In the model, the complicate stressstrain behavior induced by phase transition was quantitatively described as the stress-strain behavior under the dynamic thermal expansibility. The proposed TMP multi-physical field coupling model was embedded into the Abaqus platform via user-defined subroutine VUSDFLD. The good agreement between experimental and simulated residual stress and phase transition volume fraction (VF) verified the numerical simulation model. The phase transition degree and its effect on the residual stress under different milling conditions was analyzed. Meanwhile, the interaction of cutting parameters on the residual stress was also discussed. It was revealed that the phase transition mainly decreased the value of tensile stress within the limited surface layer. The combination process range of cutting speed of 60-70 m/min, feed rate of 0.2-0.3 mm/r, and depth of cut of 0.2-0.4 mm could narrow the tensile stress ranging from 20 to 30 MPa and compressive stress ranging from -30 to -15 MPa. The effectiveness of the process range was verified, with the maximum prediction error is 13.75 %, which further indicates the effectiveness of the proposed method in this study.