Research On The Microstructure, Performance And Temperature Field Of Aluminum Alloy High-speed Friction Stir Welding Joints

Friction stir welding with high rotational speed is a new welding technology for the welding of ultra-thin plate.Its rotational speed can reach ten times or more of the conventional friction stir welding.It is mainly aimed at the material with the thickness of 1 mm.In view of the characteristics of aluminum alloy sheet welding,the high rotational speed friction stir welding process of 1mm thick aluminum alloy sheet is explored.According to its characteristics,a tool with small pin and large shoulder suitable for the welding of thin plate is designed.The influence of process parameters on the surface forming,microstructure,mechanical properties and corrosion resistance of the joint is analyzed.Using thermocouple and infrared radiation thermometer to measure the temperature change of characteristic points,the temperature distribution law of thin plate is studied.It is found that to ensure sufficient heat input and welding quality,friction stir welding of 1mm thick 5083 aluminum alloy sheet requires a rotational tool with large shoulder.Through test and exploration,the shoulder diameter is 9 mm,the inner concave angle is 3 degrees,the shape of the pin is round platform and the pin length is 0.92 mm,and the plunge depth is 0.04 mm.The welding parameters have great influence on the surface forming and microstructure of the weld joint.The arc shape of joint surface is related to the welding speed and rotational speed,and the spacing between arc lines is determined by the ratio of the two.The microstructure of the joint can be observed.From the center to both sides of the joint is the shoulder affected zone,the nugget zone,the transition zone and the parent material respectively,and the transition zone is composed of the thermo-mechanically affected zone and the heat affected zone,and the dynamic recrystallization occurs in the shoulder affected zone and nugget zone to produce fine equiaxed grains.When the rotational speed is constant,the grain size of the nugget zone decreases with the increase of welding speed,and when the welding speed is constant,the grain size increases with the increase of the rotational speed.The relationship between welding parameters and mechanical properties,fracture morphology and defect types is studied.The microhardness distribution of the joint is presented as an inverted "V" distribution,which is gradually reduced from the center of the weld to both sides.When the rotational speed is constant,the hardness of the nugget zone increases with the increase of welding speed,and when the welding speed is constant,the hardness decreases with the increase of the rotational speed.When the process parameters of sheet welding are changed in 9000-12000 rpm and 200-400mm/min,no defect joints can be obtained.The tensile strength of all joints can reach more than 80% of the parent material.The fracture morphology of the joint is observed.The fracture modes of the joint are ductile fracture and ductile brittle fracture.Through polarization curve and intergranular corrosion test,the corrosion resistance of the joint is better than that of the parent material,and the corrosion resistance of the nugget zone is enhanced because of the more homogenization of the material in the nugget zone.The welding parameters have a certain effect on the corrosion resistance of the joint.The joint with lower welding speed has better corrosion resistance at the same rotational speed.During the welding process,all the characteristic points of the welds experienced a thermal cycle of "heating up to peak cooling",and the heating rate was greater than the cooling rate.When the welding speed is constant,the welding temperature increases with the increase of rotational speed.When the rotation speed is constant,the welding temperature decreases with the increase of welding speed.In the process of friction stir welding,the peak temperature at the advancing side at the corresponding position is generally 10~35 degrees higher than that on the retreating side.The longer the distance from the characteristic points to the center of the weld,the lower the temperature peak of the characteristic points.The highest point of the weld temperature is located at the center of the weld.