Study On The Optimization Of Extrusion Processes And The Deformation Mechanism Of Mg-Gd-Y Alloy Tube

Magnesium, the lightest structural metal, is increasingly used forautomotive application due to its low density, espescially hollow sections,offer attractive mass saving opportunities. However, the typicalcommercial AZ31alloy has severe mechanical asymmetry and difficult otbe formed at low temperature. This study is to study on the formability andmechanical property of GW83alloy which has a potential for textureweakened. Optical microscope (OM), scan electronic microscope (SEM),in-situ and ex-situ electron back scatter diffraction (EBSD), transformationelectronic microscope (TEM) were used to examine the microstructure ofthe extruded tubes and the evolution of microstructure during deformation.Tension and compression tests were also carried out to analyse thedeformation behavior. The influence of extrusion parameters on themicrostructure and mechanical properties have been fully discussed as wellas the influence of initial texture, precipitate and deformation twinning onthe deformation behavior. The results are given as following.Good formability is achieved in the range of370~420℃. Uniformmicrostructure is also obtained in using cast billit and air cooling orextruded bar as billets and air cooling or water cooling. The extruded tubeshave much randomized texture which has their c-axis deflected60°fromthe extruded direction. GW83alloy tubes have excellent mechanicalproperties. The yield stress ranges150-195MPa, the ultimate strengthranges270-295MPa and the elongation ranges18-27%. After agingtreatment, the yield stress increases to240-295MPa, the ultimate stressincreases to395-405MPa and the elongation keeps at6-12%. The microstructure evolution process during compression shows thathigh strain hardening rate in AZ31compression test was caused by thegrowth of large friction extension twinning to cover the whole matrix grain.While, the lower volume fraction of twins and multiply dislocation slip inGW83alloy are responsible for the lower strain hardening rate and betteryield symmetry.Evolution process of tension and compression in extruded and agedMg-Gd-Y alloy shows that only small amount of twinning is activated inboth extruded and aged alloys during deformation along differentdirections. And in this case, the tension-compression yield compressionasymmetry is dismissed. However, during deformation when basal slip isfavored, the strength is low and when basal slip is difficult to be operatedthe strength is high. After aging treatment, the effective inhibition of βˊon basal slip further improves the mechanical symmetry.