Effects Of Different Deformation Processes On Microstructure And Mechanical Properties Of Mg-Zn-Al-Ca Alloy

In recent years,the development of high strength and toughness magnesium alloys has become an urgent demand for weight reduction in the fields of new energy and aerospace.However,the expensive and scarce rare earth elements limit the large-scale application of high strength and toughness rare earth magnesium alloys in industrial production.A large number of studies have shown that alkaline earth elements can also improve the mechanical properties of magnesium alloys.Therefore,it is very important to study the mechanism of alkaline earth elements on the microstructure and mechanical properties of magnesium alloys,as well as the deformation mechanism and strengthening mechanism of magnesium alloys with different deformation processes.In this paper,alkaline earth element Ca was added to the Mg-Zn-Al alloy,which played a role in refining grains,strengthening the magnesium matrix,and forming Mg₃₂(Al,Zn)₄₉strengthening phase and part of Mg₂Ca phase containing Ca element.The mechanical properties of magnesium alloys were significantly improved.A series of deformation processes were used to explore the strength limit of Mg-Zn-Al-Ca alloy.In this study,Mg-5Zn-2Al-x Ca alloy was prepared by conventional casting method.The effects of different Ca content on the microstructure and properties of Mg-5Zn-2Al alloy were studied.Mg-5Zn-2Al-1Ca alloy with better microstructure and properties was selected for extrusion and multiple forging deformation,as well as multiple forging+equal channel angular extrusion and extrusion+equal channel angular extrusion.The effect of different deformation processes on microstructure and mechanical properties of Mg-5Zn-2Al-1Ca alloy was studied to prepare high strength and toughness magnesium alloy.The main conclusions of this paper are as follows.(1)The second phase of as-cast Mg-5Zn-2Al-1Ca alloy is long strip and round point distribution.The second phase is Mg₃₂(Al,Zn)₄₉containing Ca element which belongs to the heat resistant high temperature phase and its melting point is 535?.The homogenization treatment process at 400?for 30h can dissolve the coarse second phase into the magnesium matrix,thus greatly improving the mechanical properties of the alloy and its tensile strength,yield strength and elongation after fracture are 235 MPa,100 MPa and 16.7%,respectively.(2)The comprehensive mechanical properties of multiple forging Mg-5Zn-2Al-1Ca alloy at 320?are the best and its tensile strength,yield strength and elongation after fracture are 280MPa,170MPa,and 19.5%,respectively.The improvement of the properties is mainly attributed to the dynamic recrystallization and the second phase Mg₃₂(Al,Zn)₄₉with linear distribution.(3)There is a strong shear texture in the multiple forging+equal channel angular pressing composite deformation alloy(E210 alloy)with the texture strength of 36.5,which is much higher than that in the multiple forging alloy with the texture strength of 9.6.The{0001}crystal plane is parallel to the shear plane.The reason for the strong texture is that the shear texture existing in the multiple forging alloy inherited by the E210 alloy is further strengthened after ECAP shear deformation.According to the Schmid law,the basal slip system with a texture of 45°along the tensile direction is easier to activate,resulting in the poor mechanical properties of the composite deformation E210 alloy.(4)Complete dynamic recrystallization occurred in the extruded Mg-5Zn-2Al-1Ca alloy,which refines the grains and the average grain size is 5.5?m.The tensile strength,yield strength and elongation of the extruded alloy are 287 MPa,170 MPa and 20.2%,respectively,which are 34.7%and 39.1%higher than those of the as-cast alloy,respectively.The typical fiber texture in extruded alloy is very important to the mechanical properties of the alloy.Its basal plane is parallel to the ED direction,which makes it difficult to activate the basal slip system and can improve the strength of the alloy.(5)The mechanical properties of Mg-5Zn-2Al-1Ca alloy(A170 alloy)deformed by extrusion and cooling equal channel angular extrusion are the best.The high strength and toughness magnesium alloy can be prepared by this composite deformation process.The tensile strength,yield strength and elongation after fracture are 349MPa,330MPa and 20.6%,respectively,which are mainly attributed to grain boundary strengthening and substructure strengthening.After ECAP deformation,the fiber texture of A170 alloy is weakened,and there is a weak shear texture which basal plane is approximately parallel to the shear plane.The texture strength is 6.3,which promotes the activation of the basal slip system and it interacts with the ultrafine grain structure,thus improving the plasticity of the alloy.Therefore,the strength of A170 alloy is greatly improved,and the elongation after fracture is also maintained at a high level.