Research On Friction Welding Process And Post Weld Heat Treatment Of 7A04 Aluminum Alloy/304 Stainless Steel

For a long time,the connection between aluminum alloy and stainless steel is the difficulty in welding field.Because the physical and chemical properties are very different,it is very difficult to use the common welding method to connect the two materials.Friction welding technology has many advantages,and it is suitable for the welding of dissimilar metals.At present,there is no report on the friction welding of 7A04 aluminum alloy/304 stainless steel.In this paper,the friction welding process of these two materials is studied.The mechanical properties of the welded joints,microstructure and element diffusion was studied,and explores the effect of post welding heat treatment on microstructure and properties of joints.The results show that,for 7A04 aluminum alloy and stainless steel bars with diameter of 20mm,when the friction welding speed is 600r/min,the friction pressure is 90MPa,the axial shortening is 3mm,the forging pressure is 150MPa,the forging time is 10s,the edge fin had good morphology,the weld zone have no defects such as porosity and impurity,the joint’s tensile strength reached 114MPa.The microstructure of the joint can be divided into 3 zones:fine grain zone,deformation zone and base metal zone.The dynamic recrystallization and grain refinement of the organization in fine grain zone are helpful to improve the joint performance.The deformation zone is strip shape because of friction torque and axial pressure.There was no significant change in the microstructure of stainless steel.The weld joint elements mutual diffusion,the thickness of intermetallic compound layer has a great influence on the joint performance.The diffusion distance is shorter,and the thickness of the diffusion layer is about 2.5pm when adopting the above process.It is effective to avoid the generation of too many intermetallic compounds.The intermetallic compounds in the interface are mainly FeAl,Al13Fe4,Al3Ni2 and AINi3.The microhardness of near the weld was higher than the normal level,the microhardness distribution along the radial direction changed little.In order to study the effect of post weld heat treatment on the properties and microstructure of the joints,the joints were annealed by 2h,3h and 4h at the temperature of 350℃,400℃,450℃ and 500℃ respectively.The results show that,after annealing the microstructure of the aluminum alloy on the side of the aluminum alloy has recovered and recrystallized.The microstructure close to the weld is more dense,the grain grows,the higher the annealing temperature is,the longer the holding time is,the larger the grain size is.After annealing,the microstructure of the aluminum alloy was softened,the microhardness decreased,and the stainless steel was slightly reduced.The intermetallic compounds are the same in different annealing process but the content is different.If the thickness of the diffusion layer of the joint can be controlled within 3μm,the performance of the joint can be improved,and the performance of the joint can be reduced by more than 3μm.The thickness of the diffusion layer of the joint annealed at 350℃ and 400℃ is less than 3μm,and the joint performance is improved.The thickness of the diffusion layer of the joint annealed at 400℃ and 450℃ is more than 4.8μm,and the joint performance is reduced.When using the annealing process of 400℃×3h,the tensile strength is the best,increased 61 MPa than no annealing treatment,reached 175MPa.