| Refill Friction Stir Spot Welding(RFSSW)is a new type of solid-state welding technology,which makes up for the shortcomings of traditional welding methods such as resistance spot welding and riveting.At present,it is widely used in the connection of light metals such as aluminum alloys.Based on the design of the friction stir tool and its motion control,the friction stir spot welding lap test of 1mm 6061-T6 aluminum alloy sheet was completed at high speed.The macroscopic morphology,microstructure and mechanical properties of solder joints under different welding parameters were studied,and the variation of characteristic parameters during welding was emphatically expounded.Based on the motion model of RFSSW,this thesis adopts the design scheme of double spindles,and cooperates with the designed stirring tools' structure to complete the separate control of the rotation and the plunging movement of sleeve and the pin.PLC+CNC control method is adopted to realized the precise control of the two important process of RFSSW is realized,that is the plunging and refill of the stirring pin and sleeve.The response surface method was used to optimize the test parameters in the test.The results showed that the influence of the plunging depth on the mechanical properties of the welding joint was the most significant,followed by the plunging speed,and the influence of the rotating speed was the smallest.At the same time,the optimal welding process parameters were obtained : rotating speed 6000 rpm,plunging depth 1.4 mm,and plunging speed 0.4mm/s.When the welding parameters are 6000 rpm,1.4mm and 0.4mm/s,the quality of the welding joint is the best,and there is no obvious defect on the surface.The microstructure of welding joint is divided into four regions: Stirring Zone(SZ),Thermo-mechanically Affected Zone(TMAZ),Heat Affected Zone(HAZ)and Base Metal(BM).The size of grain in the SZ is the smallest and the HAZ is the largest.Due to the difference of material flow,Hook defects with different morphologies appear at the welding joints,and ear-shaped morphology appears near the exit line of the sleeve.The hardness distribution of the solder joint shows ' W ' shape.Due to the softening of the material at the solder joint during the welding process,the hardness of the solder joint is lower than that of the base metal.The highest hardness in the stirring zone was 78 HV,which was 82.9% of the base metal,and the lowest hardness was 56 HV at the boundary between the heat affected zone and the heat affected zone,which was 56.9% of the base metal.The failure mode of welded joint under tensile load can be divided into shear fracture and plug fracture according to fracture position With the increase of welding speed,sleeve down depth and the decrease of speed,the shear force increases first and then decreases.The collected temperature field and pressure data show that under different welding parameters,the temperature change trend in the welding process is first increased and then decreased,and the temperature reaches the highest value in the refill stage.The peak temperature of the solder joint is positively correlated with the rotating speed and the plunging depth,and negatively correlated with the plunging speed.The force curves under different welding parameters are also roughly the same,showing a trend of first increase and then decrease.The peak value of force decreases with the decrease of plunging speed,the increase of stirring rotating speed and plunging depth. |
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