Microstructure And Properties Of Friction Stir Welded Joint Of Aluminum Alloy And Pure Copper

The thermal and electrical conductivities of aluminum are close to those of copper.At the same time,the density of aluminum is 30%of copper,and the cost of Al is 1/3 of copper.The aluminum/copper dissimilar joint components have the advantages of the two materials,i.e.the reduction in cost and structural weight.However,due to the large heat input of traditional fusion welding a large amount of brittle and hard intermetallic compounds are inevitably generated in the Al-Cu joints.Friction stir welding(FSW)as a solid phase welding has lower heat input,which can inhibit the formation of brittle and hard compounds in Al-Cu joints,and also can avoid the generation of defects such as void and crack.At present times,FSW dissimilar Al/Cu mainly focused on lap welding thin plates,while the studies on butt welded joints of to relatively thick plates are relatively few.Therefore,in this dissertation,FSW butting 9 mm thick 6061 aluminum alloy and T2 pure copper was performed,and the microstructure and mechanical properties of the joints are studied.The main findings are as follows:(1)The aluminum alloy is placed on the advancing side,the stirring pin is offset 2.5 and 3mm to the aluminum side,the welding speed is 40 mm/min,and the rotating speed is 800～1200 rpm.The effective connection of Al/Cu dissimilar metals are successfully achieved inweld joint of all parameters and water cooling conditions,except for the weld joint of 2.5 mm offset and 1200 rpm has internal defects,(2)The nugget zone is aluminum based Al-Cu composite structure.A large number of fine copper particles are uniformly distributed on the aluminum substrate.There is a typical"onion ring" structure in the center.The intermetallic compounds(IMC)in the nugget zone include Al2Cu,AlCu and Al4Cu9.With the increase of the rotating speed,the copper particles in the nugget zone are finer and more uniformly distributed.Under water cooling conditions,the morphology of nugget zone is similar to that of air cooling conditions.(3)The Al/Cu interface transition zone consists of three types of IMC layers.Among them,the Al2Cu layer is on the aluminum side,the Al4Cu9 layer is on the copper side,the AlCu layer is located between Al2Cu and Al4Cu9,and the three IMC layers are continuously distributed at the interface in a "sandwich" structure.The heat input gradually increases with the increasing rotating speed.As result,the interdiffusion distance of Al and Cu elements increasesd,leading to the increase of the thickness of IMC layer.Under water cooling conditions,the thickness of the IMC layer at the interface decreases.(4)The tensile properties of the joint under different offsets at air cooling conditionare analysed,it is found that when the offset was 2.5 mm,the tensile strength decreases with the increasing rotating speed.With an offset of 3 mm,the tensile strength increases as the rotating speed increases.In fact,this is mainly related to the thickness of the IMC layer at the Al/Cu interface.The tensile strength is the highest when offset at 2.5 mm,and the rotating speed is 800 rpm,which is 169.7 MPa,reaching 63.3%of the base metal T2 pure copper,and the fracture is a ductile and brittle mixed fracture.All tensile specimens fracture at the Al/Cu interface under different parameters.(5)The tensile properties of joints under water cooling conditions have been researched.It is found that the tensile strength of joints is improved compared to air cooling.Tensile specimens fracture at the Al/Cu interface at a speed of 1200 rpm,and specimensof other rotating speeds fractured in the nugget zone.The tensile strength increases when rotating speed increased from 800 to 1000 rpm.This is mainly because the dispersion strengthening effect of IMC particles in the nugget zone increases with the increasing rotating speed.The highest tensile strength is 197.3 MPa at 1000 rpm,reaching 73.4%of the base metal T2 pure copper,and the fracture is ductile fracture.However,when the rotating speed increased to 1200 rpm,the tensile strength of the joint is low,mainly because the interface IMC layer is too thick.(6)According to the hardness distribution curve of the cross section of the joint,it can be seen that the hardness fluctuations in the nugget zone are relatively large,mainly due to the presence of IMC,and the heat affected zone has the lowest hardness.The hardness of the entire joint is increased under watercooling conditions.