Optimization of hot backward extrusion process parameters for flat bottom cylindrical parts of Mg-8Gd-3Y alloy based on 3D processing maps

摘要

Based on three-dimensional processing maps and numerical simulation, a demo flat bottom cylindrical part (with outer diameter of 235 mm, wall thickness of 34 mm, and height of 255 mm) of high strength Mg-Gd-Y magnesium alloy was hot backward extruded by adding an outer flange to increase the overall deformation amount and strain uniformity. Firstly, on the basis of dynamic material model and Murty instability criterion, isothermal compression stress-strain curves of cast-homogenized Mg-8Gd-3Y alloy were used to construct the processing maps. The processing maps show that the formable domain is relatively narrow: at lower strain rates ranging from 0.001 to 0.006 s(-1), and the suitable temperature is from 350 to 450 degrees C; and at higher strain rates ranging from 0.006 to 0.1 s(-1), and the temperature is from 410 to 450 degrees C. Then, the processing maps were integrated into a finite element software to simulate the forming process of cylindrical parts, and the influences of deformation temperature and velocity on the power dissipation efficiencies of different positions of flanged cylindrical parts were mainly discussed. The simulation results indicate that the average strain of flanged cylindrical parts reaches 30.07% and is larger than that of unflanged cylindrical parts, and the standard deviation of the strain of flanged cylindrical parts is 19.35% and less than that of unflanged cylindrical parts. The optimal process parameters corresponding to the maximum power dissipation efficiency are the temperature of 430 degrees C and velocity of 1 mm/s. Finally, under the optimal forming condition, the hot backward extrusion experiments of flanged cylindrical parts were conducted. The experimental results exhibit that the flanged cylindrical parts could be properly formed with good surface quality, and have relatively uniform microstructures and mechanical properties. The difference of tensile strength between the bottom and cylindrical body is less than 5 MPa, and the hardness difference is less than 1.6 HV.