Microstructure And Properties Of 6061 Aluminum Alloy Thin Plate’s Friction Stir Joining Area

In the new energy automobile manufacturing industry,the 6000 series aluminum alloy is replacing the steel structure as one of the main materials for the new energy automobile manufacturing due to its light weight,good plasticity and other excellent properties.As the body and other large structural parts need to be assembled together,the technology and strength of the joining area of 6000 series aluminum alloy become the research focus.In this paper,the 6061 aluminum alloy plate with higher specific strength was taken as the research object,and FSJ-LS-A02 friction stir joining device was used to splice the 6061 aluminum alloy plate with a thickness of 1mm.The effects of friction stir joining process on mechanical properties,friction and wear properties,corrosion resistance and microstructure of the joining area were systematically studied.Theoretical support is provided for the application of friction stir joining of 6061 aluminum alloy thin plate in automobile manufacturing industry.The main research contents of this paper are as follows:1.The analytical method of the friction stir joining area of 6061 aluminum alloy was studied and the influence of the pressure on the performance of the joint zone was explored.When the fixed stirring pin rotational speed is 8000 rpm and the stirring pin forward speed is 100 mm/min,the optimum pressing amount is 0.1 mm.The joining area is beautifully formed,the surface is smooth,the tensile strength reaches 202 MPa,and the elongation is 7.9%.2.The mechanical properties and friction and wear properties of 6061 aluminum alloy friction stir joining area and base material were studied.The results show that,when the rotational speed of the stirring pin was 15000rpm and the forward speed of the stirring pin was 300mm/min,the overall mechanical properties of the joint were the best,with the tensile strength of 247MPa,reach 74.8%of the base metal,the elongation reached 41.1%of the base metal,the central hardness value of the joining area is about 73.2%of the base metal.According to the fracture morphology of the tensile specimen,the tensile specimen with high elongation is a typical plastic fracture,while the tensile specimen with low elongation is a mixed plastic and brittle fracture.The microhardness distribution curves of the joining area were all of "W" type,and the microhardness of the joining area was significantly lower than that of the base material.The microhardness of the nugget zone was higher than that of thermo-mechanically affected zone and heat-affected zone,and the hardness lowest point appeared in the transition area of the thermo-mechanically affected zone and heat-affected zone.The wear resistance of the joining area is better than that of the base material,and the wear mechanism is a combination of adhesive wear and abrasive wear.3.The corrosion resistance of 6061 aluminum alloy friction stir joining area and base material was studied.The results show that most of the specimens have better corrosion resistance than the base material.When the rotational speed of the stirring pin was 15000rpm and the forward speed of the stirring pin was 300mm/min,the minimum salt spray corrosion rate in the joining area was 0.694g/(m2h).In the electrochemical corrosion process,the self-corrosion potential Ecorr,was the most positive and the self-corrosion current Icor.was the least,only 1.167 μA,indicating that the corrosion resistance of the joining area was the best under this parameter.4.The microstructure and defects of the 6061 aluminum alloy friction stir joining area were studied.The results show that if the process parameters are selected properly,excellent joints will be obtained,and if the process parameters are not properly selected,the joint surface is rough;Nugget zone is composed of fine equiaxed grains,and the grains in the thermo-mechanically affected zone elongate along the direction of stirring flow.The size of the grains in the heat affected zone is different and it is the coexistence zone of the grains of various sizes.When the rotational speed of the stirring pin is low,the common joining defects such as loose structure,incomplete fusion and holes are easy to appear in the joining area.