Study On Friction Stir Welding And Repair Technology Of ZM5 Magnesium Alloy

ZM5 magnesium alloy is often used in various types of railway equipment parts and external shells.Due to the high specific strength,high specific stiffness and poor chemical stability of magnesium alloy,it is easy to produce cracks and corrosion defects in the process of use.When welding and repairing magnesium alloys using conventional fusion welding,the weld area is very prone to porosity and thermal cracking defects,resulting in a significant reduction in material properties,which is not conducive to the welding and repair of structural parts.Friction stir welding has the advantages of low heat input,low welding deformation and high welding efficiency,and has great potential for development in the field of welding and repair of metal materials.Therefore,in this paper,through the study of ZM5 magnesium alloy stir friction welding joint process for welding process optimization,according to the welding parameters of the joint formation,hardness distribution,tensile properties and other characterization to select the optimal welding process parameters,the optimal welding process of the joint microstructure and EBSD analysis.The crack and corrosion defects were repaired by stir friction welding process,and the forming condition and microstructure of the repaired joints were analyzed,and their hardness and tensile properties were tested to observe the repair effect on the defects.The main conclusions are as follows:（1）Under the welding parameters of stirring head rotation speed of 800 r/min and welding speed of 160 mm/min,the grains at the weld nucleus are uniformly fine and consist mainly ofα-Mg andβ-Mg₁₇Al₁₂ phases.The grain orientation at the weld core is soft orientation and the texture strength is low.The hardness distribution of the weld cross-section shows a"W"pattern,and the hardness of the stirred area in the center of the weld is about 84 HV,equivalent to 90%of that of the base material;its tensile strength is 285.8 MPa,reaching 95%of that of the base material,and the elongation is 5.7%.（2）Successful repair of 1 mm wide and 2 mm deep crack defects by friction stir repair process.In order to reduce the impact of the welding tool on the repaired area,the repair tool is a flow stirring friction tool.In the stirring head rotation speed of 500 r/min,welding speed of120 mm/min,the best filling effect on crack defects,but also residual small gap defects.Its hardness is up to 82 HV,reaching 88%of the base material.It has the best tensile properties and can reach a tensile strength of 295.6 MPa,which is 112%higher relative to the cracked specimen and almost reaches the tensile strength of the base material,with an elongation of 4.4%.Under this parameter,it is beneficial to eliminate the residual gap defects.At four passes,the highest hardness almost reaches the base material hardness.At three passes,the tensile strength is 251.2 MPa and the elongation is 3.6%.Compared with single pass welding,although multi-pass welding helps to eliminate residual gap defects and improve hardness.However,repetitive heat input will aggravate the softening tendency of the repaired joint,resulting in a decrease in tensile strength.（3）Corrosion defects were successfully repaired by friction stir repair process.When the stirring head rotation speed is 800 r/min and the welding speed is 120 mm/min,the bonding degree between Mg-Al alloy plates is the best.However,under the action of axial force,there will be a gap between the Mg-Al alloy plates before welding,resulting in residual holes in the Mg-Al alloy bonding interface of the repaired joint.The transverse hardness distribution of aluminum alloy plate presents an"inverted U"shape,while that of magnesium alloy plate presents an"inverted V"shape.The repaired magnesium alloy stir zone has great hardness,with a hardness value of 98 HV,which is higher than the base material.Its repair joint has the highest tensile strength,with a value of 237.5 MPa,equivalent to 79%of the base material,and the elongation is 1.4%.