Microstructure Control Of Mg-9Al-1Zn Magnesium Alloy Fabricated By Hard Plate Rolling Route

Based on the urgent needs of energy conservation and emission reduction,Mg alloys have been receiving unprecedented attention as one of the promising lightweight structural materials for application in automobile,aerospace and electronic industries.During the last decade,substantial research has been focused on achievements of ultrafine grain size in the magnesium alloy with severe plastic deformation(SPD)method.Ultrafine grained magnesium alloys generally have a higher strength,but a lower tensile ductility at room temperature compared with their coarse grained counterparts,which limits their structural application.At present,researchers have prepared bimodal grain structured metals with both high strength and high ductility.However,there are few studies on the simultaneous improvement of the plasticity and strength of the bimodally structured magnesium alloy.In our previous work,we have developed large reduction hard-plate rolling(HPR),which can obtain a bimodal grain size distribution in magnesium alloy,and can effectively weaken the basal texture,gaining both a high strength and high uniform ductility.What's more,HPR can largely reduce the occurrence of edge cracks during rolling,increasing the reduction in a single pass and improving rolling efficiency.However,we have not achieved controllable preparation of magnesium alloys with bimodal grain structure,and the forming mechanism of bimodal grain structure is not yet clear.We use HPR to produce AZ91 magnesium sheets through changing rolling parameters(temperature,thickness-reduction,rolling passes)and recrystallization annealing parameters.And we obtained bimodal and fine grain size distribution in magnesium alloys with different structural characteristics(size,proportion,texture).Besides,we investigated the superplasticity behavior of bimodal grain AZ91 magnesium.The main conclusions are as follows:(1)The influence of rolling temperature and thickness-reduction on microstructures and mechanical properties of AZ91 magnesium were analyzed.We optimized a better processing temperature for AZ91 is about 350 ?,at which the alloy can be processed through a single pass rolling almost without side-crack.The room temperature mechanical properties(?0.2-152 MPa,?b-374 MPa,?p-19.6%)of AZ91 alloy after annealing,processed by HPR with a thickness reduction of 55% at 350 ?,were comparable to those alloys processed by normal rolling through multi-passes.(2)The AZ91 magnesium alloy with a bio-modal grain size distribution,which are processed at 350 ? through a single thickness reduction of 75%,exhibited superplasticity at 200 ?,250? and 300 ?.The specimen elongated to 505% when tested at 573 K and 1.0×10-3s-1.(3)We optimized the recrystallization annealing process of AZ91 magnesium alloy sheets processed by the same temperature and drop temperature rolling,respectively.The grain size is much smaller in the drop temperature rolling sheets after annealing at the same condition.(4)We processed AT63 alloy without significant cracking through wave-shaped die rolling(WDR).A substantial basal texture weakening was found to occur during WDR after a single pass.A weaken basal texture make a major contribution to the much enhanced strain hardening ability of WDRed AT63 sheets.