Study On Extrusion Process And Microstructures And Properties Of ZK60+Y Magnesium Alloys

As the most promising lightweight structural materials,Mg-based alloys are playing an important role in the aircraft industries because of their high specific strength,high specific stiffess and excellent damping vibration.But the magnesium alloy has a limitation of the low strength.At present,to improve the strength,reserchers put much rare-earths into the Mg-based alloys,which rising the cost.This work studied the micrustrctures of the cast alloys,and the effect of homogenization treatment before extrusion and extrusion parameters on the microstructures and mechanical properties of ZK60+Y magnesium alloys were also investigated by optical microscope(OM,POLYVAR MET),scanning electron microscopy(SEM,SSX-550)and Energy spectrum analysis(EDS).The main results obtained are as follows:As-cast ZK60+Y magnesium alloy are mainly composed of a-Mg,W-Mg3Y2Zn3,I-Mg3YZn6 and MgZn phases;after the homogenization treatment at 400? for 10 h,MgZn dissolved into the matrix and the I-Mg3YZn6 decreased,at the same time,some random oriented rod-shaped precipitated appeared in the grains..Alloys have better extrusion forming performance after homogenization treatment,remarkably increase the alloys yield percentages.The ZK60+Y magnesium alloy without homogenization treatment extruded under high speed with low and higher temperature appeared circular crack in the surface compared to that with homogenization extruded under high speed and high temperature.Compared with alloys without homogenization treatment,alloys with homogenization treatment have a more uniform microstructure and the recrystallization conducts more sufficiently.The ingots with homogenization treatment recrystallization area fraction is as high as 83%when extruded at 350? with 6mm/s,while the recrystallization area fraction in ingots without homogenization treatment is up to 66%.The ZK60+Y magnesium alloys extruded under the temperature of 250? at the speed of 1mm/s without homogenization treatment achieve higher strength,the tensile strength and yield strength is 433MPa and 400MPa,respectively,compared to that with homogenization treatment,they increase by 53MPa and 60MPa,respectively,however,the elongation is only 5.7%.To the ZK60+Y magnesium alloys,the higher the extruded temperature,the bigger the recrystallization fraction and recrystallization grain size,the smaller the second phase volume fraction;as extruded temperature and speed increase,the rod strength decreases but the elongation increases.The ZK60+Y magnesium ingot with the homogenization treatment at 400?for 10h present the best formability and excellent mechanical properties when extruded at 300? with lmm/s,the tensile strength,the yield strength and the elongation are 380MPa,335MPa and 8.9%,respectively.The hardness of ZK60+Y magnesium alloy rods extruded at 400? by T5 treatment remarkably increases,while there didn't appear obvious change in the hardness by T6 treatment.The yield strength of the alloy extruded at 2500?with 6mm/s by T5 treatment obviously increases by 15MPa around,the elongation decreases slightly;while by the T6 treatment,the tensile strength and the yield strength of the alloy decrease by 50-120MPa and 60-80MPa,respectively,but the elongation amplitude ratio significantly increase by 50?180%.