Comparison Of Microstructure And Properties Of TIG Welding And Friction Stir Welding Of AZ31 And ME21 Magnesium Alloys

As the lightest metal structural material,magnesium alloy is promising for commercial application in 3C products,defence contractors,aerospace and car ships.Magnesium alloy welding technology is also often involved in industrial manufacturing.This paper studied that the effect of different process parameters on the microstructure and properties of argon tungsten-arc welding?TIG?joints and friction stir welding?FSW?joints in the same material of AZ31 magnesium alloy and ME21 magnesium alloy.And then we explored effect of post-weld heat treatment on Microstructure and properties of welded joint of AZ31 magnesium alloy and ME21 magnesium alloy.By means of microstructure observation,scanning electron microscope and energy spectrum analysis,micro-hardness analysis,room temperature tensile test and XRD analysis,etc.Finally we got the conclusions as followed:?1?AC-TIG welding experiments on AZ31 magnesium alloy plate with 3?m thickness under five welding currents?70A,80A,90A,100A,110A?were carried out.The experimental results showed that:The welding zone consists of Small isometric?-Mg matrix phase and a fine Mg₁₇Al₁₂ phase precipitated along the grain boundary.The heat-affected zone microstructure is coarse and uneven.Tensile strength increased first and then decreased with current increased.Welding microstructure grain size increased with current.When the welding current is 90 A,the maximum tensile strength was 190 MPa,the weld formation was good,and the welded joint coefficient was 73%.The fractures were brittle and mixed fractures,the micro-hardness of the joints was in a"W"shape,and the hardness distribution in the weld fusion zone was uneven.After heat treatment in 400°C×1h,the tensile strength of AZ31 magnesium alloy TIG welding joints reached 230MPa,the coefficient of welded joints was 89.2%,and the micro-hardness of joints still showed a"W"type distribution.The fusion zone has a more uniform hardness with gentle undulations.The fracture was dominated by ductile fractures.After heat treatment,the microstructure and properties of AZ31 magnesium alloy TIG welded joints have been improved.?2?The FSW of 3mm thick AZ31 magnesium alloy was carried out at a rotating speed of 1200r/min for the stirring head,and 60mm/min,80mm/min,and 100mm/min for the welding speed.The experimental results showed that:When the welding speed is80mm/min,the down pressure is 0.25mm,and the inclination angle is 2.5°,the maximum coefficient of the welded joint reaches 81%,the maximum tensile strength reaches 186MPa,the maximum yield strength reaches 122MPa,and the elongation of the welded joint is 3.2%.After heat treatment in 250?×1h,the maximum tensile strength,yield strength,and elongation of welded joints were 197 MPa,126 MPa,and3.5%.These values increased 5%,4%,and 0.3%,respectively.And the joint coefficient reaches 86%.After the heat treatment,the micro-hardness of the weld zone decreased,but the hardness of the weld zone was more uniform than before the heat treatment.The analysis of XRD and EDS spectra before and after heat treatment showed that no new phase was formed after heat treatment.?3?AC-TIG welding experiments on ME21 magnesium alloy plate with 3?m thickness under five welding currents?105A,115A,125A,135A?were carried out.The experimental results showed that:The mechanical properties of welded joints increased first and then decreased with the increase of current.When the current was 115A,the surface quality of the weld was good and no obvious defect,the maximum tensile strength reaches 166 MPa,and the maximum yield strength was 107 MPa.After heat treatment at 470°C for 4 hours,the maximum tensile strength increased from 166 MPa to 184 MPa,an increase of 11%,and the coefficient of welded joints was 84%,which was 10%higher than that before heat treatment.The grain boundary of heat-affected zone is clearer than that before heat treatment,compound between grain boundaries is eliminated,and the hardness value of fusion zone is even.?4?The friction stir welding of 3mm thick AZ31 magnesium alloy was carried out at a rotating speed of 1500r/min for the stirring head,and 10mm/min,20mm/min,30mm/min and 40mm/min for the welding speed.The test results show that:when the rotation speed is 1500r/min and the welding speed is 10mm/min,the maximum tensile strength of the welding joint was 152MPa,the welded joint coefficient was 69%,and the maximum yield strength was 106MPa.The base metal yield strength was 75%.After heat treatment at 470°C for 4h,A small amount of Mn particles precipitated and the mechanical properties have decreased.