Forming Properties And Microstructure Evolution Of Rare Earth Magnesium Alloy By Rotary Reverse Extrusion

Magnesium alloys are the lightest among engineering structural materials,and have excellent properties such as high specific strength and stiffness,good damping and shock absorption,and good machinability.However,magnesium alloys have poor room temperature performance and are prone to deformation and cracking.The integrated control of alloy formability has become a current research hotspot.Compared with the traditional back extrusion,the rotary back extrusion can change the metal stress state in the deformation zone,control the flow direction,refine the grain,weaken the texture,and reduce the forming force.The research of this process lays a foundation for the forming of high-performance components,and has a good application prospect in the forming of structural components.In this paper,for the Mg-13Gd-3.5Y-2Zn-0.5Zr magnesium alloy,the formed cup-shaped parts are taken as the research object,and thermal simulation experiments are carried out on the Gleeble-3500 thermal torsion unit.The microstructure and mechanical properties were characterized by optical microscope(OM),scanning electron microscope(SEM)with energy dispersive analysis(EDAX),X-ray diffractometer(XRD),and microhardness tester..This paper systematically studies the metal flow law,stress-strain,microstructure and mechanical properties of cup-shaped parts formed under different parameters of rotary back extrusion.The strain distribution along the radial direction of the cross section of the cup-shaped part,and the evolution law of the matrix grains and the second phase under different deformation conditions are analyzed.The specific research contents are as follows:(1)The equivalent strain,metal flow law and load of cup-shaped parts with different deformation parameters are analyzed.The results show that the rotary back extrusion can significantly improve the deformation degree of the cup-shaped part and promote the metal flow along the radial direction,and the rotation speed is the main factor affecting the radial equivalent strain.(2)The thermal simulation experiment of rotary back extrusion of Mg-13Gd-3.5Y-2Zn-0.5Zr magnesium alloy cups shows that the grain size of the cross section increases from the inside to the outside along the radial direction,and the degree of dynamic recrystallization decreases.,the degree of fragmentation of the second phase decreases,and the LPSO phase inside the cup has an obvious streamline distribution phenomenon.With the increase of the rotation speed,the DRX further increases,the grain size decreases significantly,and the LPSO phase appears powdery distribution.(3)The micro-texture analysis of the cup-shaped parts formed by rotary extrusion is carried out.It is not difficult to find that the micro-mechanisms of the rotary back-extruded cupshaped parts are obviously different under different deformation parameters.The increase of extrusion speed and rotation speed leads to the weakening of the texture of the alloy,and the generation of DRX grains becomes the main factor for the weakening of the texture.Because the deformation of the alloy under the condition of rotation speed activates multiple slip systems,the effect of texture weakening is obvious.become the main reason for the change of grain orientation.(4)The microstructure and properties of cup-shaped parts under traditional back-extrusion and rotary back-extrusion were compared,and the results showed that the inner grain refinement was more thorough,the microstructure refinement area was enlarged,and the texture was obtained under rotary back-extrusion.significantly weakened.The microhardness of Mg-13Gd-3.5Y-2Zn-0.5Zr magnesium alloy cups at different positions shows that the change trend of microhardness under different deformation conditions is obviously different,which is mainly related to the softening of DRX and the hardening effect of LPSO phase structure,because the inner side is the main deformation area,when the extrusion speed and the rotation speed increase,it can be found that the inner microhardness decreases accordingly,and the hardness value changes most obviously under different rotation speeds.