| The aluminum alloy has low density,low melting point,high thermal conductivity,corrosion resistance and excellent processing performance advantages,For the unique features,the high entropy alloys exhibit promising properties,such as the stiffness,strength,corrosion resistance,abrasion resistance,high temperature stability and oxidation resistance,magnetic aspects show excellent comprehensive performance.In this thesis,the 2mm thick6061 sheet aluminum alloy was used as the base material,and the high speed friction stir welding technology was adopted to fabricate the welded joint by adding Co Cr Fe Ni high entropy alloy powder.The effects of process parameters on the macroscopic morphology,microstructure and properties of welding were studied.The strengthening effect and mechanism of Co Cr Fe Ni high entropy alloy were compared and analyzed.Then,the high entropy alloy coating was prepared on the surface of the alloy by pretreatment with powder laser coating method.Then,the particles were more evenly distributed in the base metal by high speed friction stir.The related microstructure and mechanical properties of the alloy were studied.The test results show that the optimal process parameters of friction stir welding technology are as follows: the spindle speed is 11000r/min,the welding speed is150mm/min,the shaft shoulder pressure is 0.06 mm,and the inclination Angle is 1°.Heat treatment process parameters were designed.Under the process parameters,the joint was heat treated.Within the temperature range tested,the diffusion of alloy particles was positively correlated with the heating temperature and aging time,and the diffusion of alloy at the weld defects was more obvious than in other areas.It was found that B2 phase with ordered BCC structure was formed by the interaction of solid diffusion.The hardness analysis test of the welded joint shows that the overall distribution of the joint hardness is W-shaped,and the hardness shows the anisotropy of different layers.The addition of Co Cr Fe Ni high entropy alloy particles can effectively improve the microhardness and tensile strength of the material,and the maximum tensile strength can reach 206.56 MPa.After the heat treatment of the joint under the optimal process parameters,it was found that the overall hardness of the joint increased after the heat treatment,and the hardness value reached the maximum when the heating temperature was 400℃ and the aging time was 24 h,and when the heating temperature was 300℃ and the aging time was 48 h.This is because under the action of heat treatment,the alloy particles and the aluminum matrix mutually diffuse,forming B2 phase of BCC structure,and the hardness increases accordingly.Under the optimum process parameters,the corrosion resistance of joints with different heat treatment parameters is different.The aging time has little effect on the corrosion resistance,and the corrosion current density and corrosion potential have little change.The joint obtained by pre-coated powder laser has finer particle size and more uniform distribution.In terms of the mechanical properties of the welded joint,compared with the joint without laser treatment,the cross-section distribution is more uniform,and the hardness values of each layer are close,but the difference between the single laser coating and the second laser coating is small.The tensile strength of secondary coating is obviously better than that of single coating and no laser treatment. |
