Study On Microstructures And Properties Of Mg/Al Dissimilar Alloy Friction Stir Welding Joints

With the development of social economy,energy and environmental have become increasingly problematic.In order to cope with those challenge,material lightweight,as one of the most effective measures,has become a research hotspot.Al alloy has the advantages of high specific strength,high specific stiffness and low density,which is widely used in aerospace,marine,rail transit and other fields.Mg alloy is one of the lightest structural metal materials at present,which is called"green material of the 21st century".Replacing aluminum with magnesium can further reduce the weight of components to a certain extent,so as to achieve energy saving and emission reduction.A large number of intermetallic compounds occurred during traditional fusion welding of magnesium/aluminum alloy.These welding problems are related to the physical and metallurgical properties of Mg alloy and Al alloy,which are difficult to solve fundamentally.Friction stir welding(FSW),as a solid-phase bonding technology,is one of the most effective methods to solve the welding problem of Mg/Al alloy.Therefore,the microstructure and properties of friction stir welded joints of Mg/Al alloy were studied.The microstructure and mechanical properties of 6061 Al alloy/AZ31B Mg alloy FSW joint were systematically studied.The results show that the joint is well formed,and the sound joint can be obtained under the proper welding condition.A large number of intermetallic compounds presented in nugget zone,which have two morphologies:dense banding and loose dispersion.And the results showed the former is Al₃Mg₂ and a relatively small amount of Mg₁₇Al₁₂,the latter is(Mg₁₇Al₁₂+Mg)eutectic microstructure.There are two main mechanisms for the formation of intermetallic compounds:diffusion and eutectic reaction.The influence of welding parameters(tool offset and rotation speed)on microstructure and mechanical properties of 6061 Al alloy/AZ31B Mg alloy FSW joint was studied systematically.It is found that a sound joint can be obtained by offsetting the tool to Mg side.With the increase of the rotation speed,the intermetallic compounds in the weld core tended to increase.The tensile strength of the joint increases first and decreases then,and the maximum tensile strength is 104 MPa.From the microhardness of the joint,it can be found that the distribution of hardness in the nugget zone has no obvious change,but the width of the heat-affected zone of the joint tends to increase.The numerical simulation of temperature field in friction stir welding shows that the peak temperature decreases with the increase of heat transfer coefficient,indicating that different cooling media can be used to regulate the heat cycle during welding.The morphology of FSW joint obtained by numerical simulation and the thermal cycling curve of characteristic points agree well with the test results,and can reflect the influence of different cooling media on the temperature cycle during welding process.In order to explore the influence of different thermal boundary conditions on the microstructure and mechanical properties of the joints,Mg/Al friction stir welding were conducted in four coolants:air,60?water,15?water,0?water.The investigation results show that,with the temperature of the media decreases,the heat input in the welding process decreases,and the content of intermetallic compounds shows an obvious decreasing trend.The hardness distribution of the welded joint of underwater friction stir welding decreased to a certain extent,among which the hardness of the welded joint conducted at 0?was the lowest,and the tensile strength of the joint was 168 MPa,reaching 70%of the base metal of Mg alloy.