Basic Research On Microstructure And Properties Of Mg-Sn-Zn-Ca Alloy Optimized By The Second Phase Of Rolling Process Regulation

In recent years,due to its light weight and superior mechanical performance,magnesium alloy has become a popular choice of material for the fields of automotive,aerospace and rail transportation.However,due to its low tensile strength and low mechanical plasticity,the mechanical performance of magnesium alloys is often difficult to meet the requirements in actual applications.In order to enhance the mechanical performance of magnesium alloys,researchers have improved their properties by adding suitable alloying elements.Mg-Sn-Zn-Ca alloy is a new type of magnesium alloy with excellent mechanical properties and corrosion resistance.The distributions and structures of the second phases are essential factors in determining the properties of the alloy,and they can significantly improve the mechanical characteristics of the material.The existence of discontinuous phase,namely the second phase,has significant mechanical properties,and their size,shape and distribution will directly affect the mechanical and machining properties of the alloy.Therefore,in this paper,the extrusion Mg-2Sn-2Zn-1Ca magnesium alloy sheet was taken as the research object,and the processing window of the alloy was obtained through Gleeble thermal simulation test.Then,the homogenized alloy was taken as the research object,and processed by two passes of 300℃rolling and the first pass of 300℃and two passes of 400℃rolling respectively.Through the characterization and analysis of the second phase,the effects of the changes of the second phase on the structure and performance were investigated,and the structure was optimized and the optimal mechanical properties were attained through the adjustment of the second phase by the rolling process.Through the regulation of the second phase by rolling technology,the structure is optimized and the best mechanical properties are obtained.Mg-2Sn-2Zn-1Ca alloy with low cost,high strength and toughness coordination was obtained by studying the effect of rolling and annealing processes on the second phase,then optimized the organization and obtained the optimal performance of sheet metal and guiding the production process.The results show that:（1）By XRD characterization of the extruded Mg-2Sn-2Zn-1Ca plate,it is found that in addition toα-Mg,Mg_xZn_y,Mg₂Sn and Mg₂Ca phases are mainly included in the plate.After homogenizing process exploration combined with Mg-Sn,Mg-Ca,Mg-Zn dual phase diagram,the determined treatment at 400℃for 3h can promote the dissolution of the second phase in the matrix to the greatest extent,forming the optimal recrystallization structure dominated by equiaxed grains,improving its processability and providing the original plate for the follow-up window exploration and rolling.（2）The flow stress-strain curve of Mg-Sn-Zn-Ca alloy was constructed through thermal simulation experiment,and the influence law of flow stress on the alloy was analyzed.According to the modified equation,The dynamic DMM diagrams were constructed by computing the thermal energy of activation and rheological stress equations based on the correction equation,and the processing Windows with dissipated power less than 30%were obtained,including:low temperature and low strain zone(250～320℃,0.00092～0.001 s^-1),high temperature and high strain zone(350～380℃,0.37～1 s^-1).Through the experimental verification of the deformation temperature provided in the two processing Windows,300℃is finally determined as the best processing temperature,and the maximum reduction of a single pass is 30%.This is mainly due to the high temperature sensitivity of Mg_xZn_y phase in the plate,which is easy to crack during high temperature rolling process,which is also confirmed by the shape and microstructure of the plate.When the second phase is rolled at300℃,there is no coarse second phase,which promotes the improvement of dynamic recrystallization degree at 300℃,and a large number of recrystallization grains near the second phase appear,which also improves the workability of the sheet at low temperature.（3）In the rolling process,it is found that Mg₂Sn is the main phase in the second phase of the sheet after a single pass.When rolling at 300+300℃,the sheet is difficult to form,and the surface is very prone to tongue defects,impacting the whole performance of the sheet,the performance of its middle sample is only 161.3 MPa,6.6%.The performance of part of the sample is only161.3 MPa,6.6%.When rolling at 300+400℃,the sheet is well formed,the maximum tensile strength is 296 MPa,21%elongation at fracture,a significant increase over the original sheet.Through microstructure characterization and fracture element analysis,it is concluded that this situation is mainly due to the fact that at 300+400℃rolling,the second pass temperature is higher,which provides enough energy to the area subject to a large number of second phase and dislocation entanglement,and promotes it to become a preferred nucleation location for recrystallization,so that it is easy to deformation.The content of Mg_xZn_y phase increases after 300+300℃rolling,which affects the formability of the sheet.Meanwhile,the second phase observed at the fracture is coarse,which leads to the stress concentration and rapid failure of the material in the test process.Therefore,300+400℃process should be selected for plate processing.（4）After annealing treatment of the sheets rolled by taking the optimum process,it was found that the tensile strength of the sheets were all decreased and the elongation was improved by over 5%for all processes with the exception of the process treated at 350°C for 90 min.Among them,the elongation of the plate after treatment at 300℃for 30 min is about 31.7%,and the maximum tensile strength at this time is 249.97 MPa,which is mainly due to the dissolution of Sn and Zn elements and the dispersion strengthening of the second phase caused by annealing treatment.After being treated at 350℃for 90min,its maximum tensile strength is about 236.2 MPa,and its elongation is18.9%.The main reason is that the second phase inside the treated sheet is large,which is easy to become the initiation point of micro-cracks,leading to the deterioration of its performance.Finally,the comprehensive mechanical properties of the sheet are optimal after 300℃for 30 min.