Research On Equal Channel Angular Rolling And Annealing Techniques Of AZ31 Magnesium Alloy Sheets

With integrated desirable properties, magnesium alloys are praised as the most development perspective green structural materials in the 21st century. Especially, wrought magnesium alloy sheets exhibit much extensive applicable opportunities and the research on them has been the focus in whole world. Aiming at the engineering problem of poor formability of magnesium alloy sheets at room temperature, this dissertation brings forward an academic idea for controlling basal plane orientation and develops a new processing technology, so-called equal channel angular rolling (ECAR). Effects of ECAR and heat treatment parameters on the evolution of microstructure and mechanical properties in AZ31 magnesium alloy sheets were investigated. The main contents and results are as the following.(1) Research on the ECAR parameters of AZ31 magnesium alloy sheetsEvolution of microstructure and mechanical properties in AZ31 magnesium alloy sheets during ECAR was studied. The relationship among the processing parameters, microstructure and mechanical properties was ascertained. According to the study on the ECAR parameters, including the rolling pass, rolling route, channel clearance and mould temperature, the initial (0002) basal plane orientation weakened gradually with the increasing of rolling pass. The (1 01₀) prismatic plane orientation appeared after 1 pass rolling, and the (0002) basal plane orientation coexisted with the (1 01 1) pyramidal plane orientation, followed with elongation of 25% and an increase of strain hardening at room temperature. ECAR with route AC benefited the grain refinement and the weakening of basal plane orientation. The elongation after 1 pass ECAR in route AC arrived to 22%. With the decreasing of mould clearance, the formation of (0002) basal plane orientation was restrained and the non- basal plane such as (1 01₀) and (1 01 1) orientation was promoted despite of neglect effects on flow behaviors. In certain degree, increasing the mould temperature activated the dynamic recrystallization during ECAR and benefited the elongation.(2) Research on the heat treatment parameters of AZ31 magnesium alloy sheetsEvolution of the microstructure and mechanical properties for AZ31 magnesium alloy sheets processed by normal rolling (NR) and ECAR processes was investigated under various annealing conditions. The results showed that the grains were inclined to equiaxed and coarse due to the recrystallization and second recrystallization with the increasing of annealing temperature and time. The disappearance of twins in both sheets was occurred after annealing at 200℃by 15min and recrystallization was completed annealing at 300℃by 60min for ECAR sheets. Although heat treatment strengthened the (0002) basal plane orientation, the elongation and strain hardening exponent increased and the yield ratio. After annealing under 300℃by 60min, the ECAR AZ31 magnesium alloy sheet showed higher elongation of 32% and 24% in rolling direction (RD) and transverse direction (TD), respectively, associated with the decrease of yield ratio from 0.54 to 0.38 in RD and 0.87 to 0.64 in TD, which would greatly benefit the improvement of formability in magnesium alloy sheets.