Research On The Plastic Deformation Behavior Of Friction Stir Welded2024Aluminum Alloy Tube

Due to the increasingly urgent lightweight demand in the aerospace industry,the application of thin-walled aluminum alloy tube fittings continue to expand inthe aircraft and aerospace industry. Such fittings with complex shape have alarge radius-to-thickness ratio, which put forward more strict requirements fortube geometry and formability. It is difficult to obtain such large diameter thin-walled aluminum tube by the conventional aluminum tubing manufacturingmethod. Thus an urgent need for research and development of hydroformingthin-walled tube is required. In this paper, large diameter thin-walled aluminumwere produced using a hybrid process combining friction stir welding andspinning.2024aluminum alloy was studied as the base materials. Therelationship between the microstructure and plastic deformation behavior of theFSW joints were revealed during FSW tube preparation and the manufacturingproblem of the thin aluminum pipe was solved. The hydroforming characteristicsand wall thickness distribution law of spiral weld pip were given, aiming toprovide theory foundation and technology support to the application of the newprocess.The relationship between the microstructure and the plastic deformationbehavior of the FSW joints was examined. The effect of the post-weld heattreatment on the plastic deformation characteristics of the FSW joints wasrevealed. The microstructure heterostructure of the joints lead to non-uniformmechanical properties and plastical deformation, and the elongation of the jointdecreases44%. Deformation heterogeneity of the joint is improved by the post-weld heat treatment and the joint showed high ductily. The plasticity of the jointincreases firstly and then decreases with increasing the annealing temperature.When the temperature is300℃, the plasticity has a maximum value, which is1.6times higher than that of the welded joint.Combined spinning composite forming process of aluminum alloy FSWtube experiments, the effect spinning thinning rate on microstructure andmechanical properties of FSW pipe was given. It is found that the grain refiningeffect is more obvious with increasing the reduction of spinning. The grain sizeof base metal (BM) is decreased from200μm to3.5μm, with spinning thinning rate of70%. The more uniform distribution of grains in the weld and BM isgiven. The strength of the tube increases significantly with increasing thereduction of spinning.The microstructure of the FSW tube was controlled by heat treatment, andthe effect the annealing temperature on the microstructure of the tube wasobtained. The bulging performance variation of the FSW tube after heattreatment was revealed during the hydroforming. When the temperature is below300℃, the deformed microstructure of the spin state of the pipe change into therecrystallized microstructure of fine equiaxed grain and large-angle grainboundaries. The grains of the weld are approximately1.7μm and the grains ofbase material are4μm. The organization uniformity of the tube has beensignificantly improved. When the temperature is350-400℃, the abnormalgrowth of the tube occurs, and the delamination along the thickness of the tubeis obvious in which the outer layer grain is4-6times higher than that of theinner layer. With increasing temperature, the expansion coefficient of the FSWtube first increases and then decreases. When the temperature is300℃, theexpansion coefficient of the tube has a maximum value, which is1.6timeshigher than that of the spinning. The expansion coefficient of the FSW tube canbe significantly increased by heat treatment, which is due to that the heattreatment can control the organizational stability and further improve theformability of FSW tube.The effect strength matching coefficient on the wall thickness distributionof spiral weld was obtained during the hydroforming. It is found that the BMnear the weld (nearly30°and180°) combined with the area opposite the weldshows a great thinning of23.7%for the tube, which is consist with the wallthickness distribution of the tube with a high-strength matching coefficient. Thetube with a low-strength matching coefficient ruptures in the weld with themaximum thinning of23.7%, which is significantly higher than that of the BM.The mechanical analysis points out that the BM near the weld combined with thearea opposite the weld show larger equivalent stress for the tube with a high-strength matching coefficient, thus above region show seriously thinning.Base on the experimental research of hydroforming multidiameter tube withspiral weld, the thickness distribution law of the FSW tube could be obtained. It is found that the axial wall thinning shows M-shaped distribution. Severethinning is found in the expansion zone of1/4from symmetry plane alonglongitudinal direction, and the thinning in the ends of tube including symmetryplane is smaller. Hoop thinning of tube is similar to the free bulging tube, but itshows more uniform thickness distribution. By pre-forming, the contacting-dieof the cross-section is controlled, and base metal adjacent weld contacts diefirstly. Therefore, the uniformity of thickness distribution is improved.The numerical simulations were conducted to reveal the relationshipbetween thickness distribution, cross-sectional shape and stress state of spiralweld tube during free bulging. It is found that cross-section of the FSW tube isno longer circular. The radius of the base metal adjacent weld is greater thanother BM for η=1.2tube, and suffer higher hoop and axial tensile stress, whichlead to severe thinning. The distribution of the radius for η=0.9tube is oppositeto the η=1.2tube. The hoop and axial tensile stress of base metal adjacent weldis smaller, which lead to less thinning of the tube.