Controlling Welding Stress, Deformation And Hot Cracking Of Aluminum Alloy Thin Plates By Welding With Trailing Rotating Extrusion

In this dissertation, a new method named Welding with Trailing Rotating Extrusion (WTRE) is proposed to control welding stress, deformation and hot cracking of aluminum alloy thin-plate weldments. This method makes use of a extrusion head with given form of end face to exert proper pressure, while the extrusion head is rotating, on metals in and around the weld behind the welding arc to produce plastic elongation, which can reduce the level of longitudinal residual tensile stress in the weld and its neighbouring zone, so mitigate or even eliminate welding residual deformation. WTRE can also restrain effectively the germination and expansion of hot cracks in aluminum alloy weldments and so comes forth to be an innovative technology used for solving the buckling distortion and hot cracking problems of high-strength aluminum alloy thin-plate weldments.According to basic principle of WTRE, the design block diagram of WTRE device was set up and a workable WTRE device that meets the experimental requirement well was developed.Three-dimensional elastic-plasticity FEM was adopted to simulate stress and deformation of conventional weldment and WTRE weldment in 2A12T4 aluminum alloy. The mechanism of controlling welding residual deformation by WTRE was disclosed that the plastic elongation of metals in and around the weld due to the action of extrusion head can reduce longitudinal residual tensile stress in the weld and its neighboring zone, and as a result decrease the level of average longitudinal residual compressive stress in the whole weldment. The buckling distortion of thin-plate weldments will disappear if only the average longitudinal residual compressive stress can be decreased to below of critical buckling stress by WTRE treatment. As for thin-plate weldments made of 2A12T4 aluminum alloy sheets of 2mm thickness, 130mm width and 270mm length, WTRE treatment can decrease their maximum longitudinal deflection to below 0.3mm, only 4% of conventional as-welded.The control effect of welding residual deformation depends on some technical parameters, such as the distance between extrusion head and welding gun, the vertical pressure exerted on workpieces, the diameter of extrusion head, the rotary speed of extrusion head, and the force position of extrusion head. The distance between extrusion head and welding gun is determined by the principle that, on the premise of guaranteeing the weldments not to generate overmuch compressive stress that may result in buckling distortion after the rotating extrusion treatment, the distance between extrusion head and welding gun should be as near as possible. The friction heat produced in the process of rotating extrusion influences the control effect of welding deformation from two opposite aspects. A small quantity of friction heat avail the deformation control due to the contribution of heat to the enhancement of metal plasticity, but too much of it will bring about an unexpected stress-strain field, which goes against the mitigation of welding residual deformation. Under the condition of keeping other technical parameters unchanged, the vertical pressure exerted on workpieces has an optimal range. Either excessive pressure or deficient one goes against the control effect of welding residual deformation. The effective force area for WTRE treatment is the weld and its neighbouring zone. Only when metals both in and around the weld are elongated by extrusion head can an ideal control effect of welding residual deformation be attained.On the basis of the generation mechanism of transverse strain in weld metal which temperature is in BTR, a new method that exerts transverse restraint on weldments to reduce the tendency of welding hot cracking is proposed in this dissertation. The mechanism of transverse restraint method includes two aspects: one is to decrease transverse tensile strain in BTR weld metal through restricting externally-expanded deformation of weldments; another is to increase transverse compressive strain in BTR weld metal through heightening the extent of non-uniform heat expansion in metal of heat compressional zone beside molten pool.WTRE can inhibit the germination and expansion of welding hot cracks based on the mechanism of merging the inceptive cracks in the arc-starting end and restricting externally-expanded deformation of weldments. Experimental results showed that the vertical pressure exerted on weldments and the distance between extrusion head and welding gun are important technological parameters that have close relationship with control effect of welding hot cracking. When appropriate technological parameters are adopted, 2A12T4 aluminum alloy welds treated with WTRE have no hot cracks.WTRE has positive effect on surface quality of aluminum alloy welds. The surface of conventional welds is rough and there exists sharp stress concentration at the weld toes, while the surface of WTRE welds is flat and composed of regularly distributed ring stripes, which decreases disadvantageous influence of geometric notch effect.The results of tensile test showed that the tensile strength of 2A12T4 alum-inum alloy WTRE welded joints is about 40MPa more than that of conventional welded joints. The fatigue test revealed that WTRE can enhance fatigue life of aluminum alloy welded joints evidently. The observation of metallurgical structures and tensile fractures confirmed the efficiency of WTRE in refining crystal grains as well as reducing air holes and other welding defects.