| In order to meet the demanding requirements of different industries for the service performance of structural materials and to reduce the cost of production,Al/Ti structural members have broad prospects.However,there is a huge difference in performance between the Al/Ti dissimilar metals,which leads to the existence of many problems in the two metals welding,and thus restricts its application.The heat input during friction stir welding is low,which has great advantages in realizing the reliable connection of dissimilar metals.In this paper,the friction stir welding test of 3 mm thick 6061 aluminum alloy and 2 mm thick TC4 titanium alloy was carried out.The influence rule of the welding process parameters and the length of the stirring needle on the microstructure and mechanical properties of the joint was analyzed.We also analyzed the variation of the characteristic parameters of the welding process and explored the formation mechanism and plastic flow behavior of Al/Ti dissimilar metal intermetallic compounds.The welding tests of Al/Ti dissimilar metals under different welding process parameters and different stirring needle lengths were studied.Due to the combined effects of welding heat input and mechanical stirring of the stirring head,different areas in the joints were also in the welding process.Different welding heat cycle deformations are experienced and the degree of plastic deformation is also different.Therefore,the cross section of welded joints can be divided into five regions: stir zone,interface zone,thermo-mechanically affected zone,heat affected zone,base material.The interface structure of lap joints is very complex and there are three main types: hook-like structure on both sides of the interface,Al/Ti composite laminate structure in the central region of the interface region,and dispersed particle structure.With the increase of the heat input or the length of the stirring needle,the interface structure is more complicated,and the main intermetallic compound formed at the interface is Ti Al3.The microhardness distribution test and tensile shear test show that the microhardness curve of the lap joint is basically symmetrical about the joint center,the hardness of the stir zone is the highest and the hardness of the heat affected zone is the lowest,but the hardness value of the whole joint is lower than the hardness value of the parent material.When using the stirring needle with a length of 3.1mm,the welding parameter is set as the stirring head speed of 1000 rpm,the welding speed is 100mm/min,the joint has the highest tensile shear performance,and the maximum load can reach 442N/mm.The fracture mechanism of the joint is complicated,and the fracture form is the mixed fracture form of brittle fracture + ductile fracture.In the welding process,the joint is not only affected by the heat cycle of the welding,but also with the intense mechanical agitation,so the measurement of the characteristic parameters of the welding process plays an important role in revealing the forming mechanism of the joint.The change of the welding parameters and the length of the stirring needle will change the interaction between the stirring needle and the material,and then change the distribution of the temperature field of the joint and the torque value of the spindle.Interface compound formation thermodynamics indicates that in the Ti Al3 phase in the solid phase reaction is the easiest to form,and dynamic recrystallization due to friction stir welding process,interface to produce a large number of crystal defect short-circuit diffusion process,promote the atom and recrystallization grain size of the tiny atom diffusion distance,to increase the number to the spread of the interface and significantly improved the compound layer grew up speed. |
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