| Based on the view of mechanics, a new method named Welding with Two Direction Pre-Stress (TDPS) is proposed to comprehensivly control welding stress, deformation and hot cracks of aluminum alloy thin-plate weldments in this dissertation, directed toward the problems of welding stress, deformation and hot cracks of aluminum alloy thin-walled plates. Applied in the direction parallel to welded seam, longitudinal tensile re-stress (LTPS) produces plastic ductility to reduce the longitudinal residual tensile stress of weld zone, resulting in decreasing or even eliminating welding deflection deformation. Applied in the direction verdical to welded seam, transverse compressive pre-stress produces transverse compressive strain to the weld metal which is in brittle temperature range (BTR) during solidification, in order to buck rupture tensile strain from the contration of weld metal. TDPS provides a effective technology for solving the large distortion and hot cracks problems of high-strength aluminum alloy thin-plate weldments.Load transfer mode of screw pair is selected. According to the demands of distortion controlling and cracks preventing, the TDPS device is developed. Suitable for the flat weld, the device can meet the need of testing requirement.Pre-tension technology obtains obvious effect in the aspect of distortion controlling. Finite element method is employed to conduct the numerical simulation calculation of welding stress fields under the condition of LTPS. The relation between the applied pre-stress and the reduction amount of the peak value of residual tensile stress on the cross-section of stress is quantified. On the base of no local yielding for weldment after the superposition of tensile pre-stress and residual stress, the reduction amount of the peak value of residual stress caused by LTPS equals to the tensile pre-stress value. At the same time, the residual compressive stress decreases, which maintain the equilibrium relationship to tensile stress. By the simulation of evolution course of longitudinal stress under the condition of LTPS, the criterion that eliminates completely buckling distortion is deduced. On the principle of equilibrium of inner stress in the section, the minimum pre-stress needed to load to eliminate completely the buckling distortion is determined. The criterion supplies the theory rule for practical production. For the aluminum alloy LY12 thin plates weldments of which the dimension is 300mm×200mm×2mm, the buckling distortion is eliminated completely when the pre-stress is 0.75σs (σs is the yield stress of aluminum alloy). The maximum longitudinal deflection amount is 0.9mm, which account to 5% of conventional welding weldment.Researches find that pre-tension produces extra transverse tensile action to the weld metal in the rear of molten pool in BTR. The tensile action increases transverse tensile strain so as to increase the tendency of hot cracks. For the weldment of 200mm×100mm×2mm, by the use of bead-on-plate, cracks length produced is 10.1mm when the pre-stress is 0.7σs.By term of mechanical condition of hot cracks initiation and pointing at the hot cracks shortcoming of pre-tension, transverse compressive pre-stress (TCPS) method is proposed to prevent welding hot cracks. Compressive pre-stress caused by TCPS produces transverse compressive strain to the weld metal by strain transfer mode for the metal in BTR. This strain reduces or even counteractes transverse tensile strain of weld metal in BTR and make it less than the minimum plasticity of metal. TCPS can effectively control the expansion and initiation of cracks. The test results of fish-bone specimens show that cracks length decreases with the increasing of compressive pre-stress using the whole body loading. On the condition of conventional welding, hot cracks ratio of specimens reach to 45%, but when TCPS is 0.1σs, the hot cracks are completely eliminated.TCPS makes the specimens shorten because of being compressed. At the same time longitudinal elongation is produced. Two deformations meet the relationship of Poisson's ratio. The longitudinal strain produced at the center of plate makes the peak value of welding residual stress decrease. The decreasing amount equals to (1μ-1) times of the same LTPS value. So it contributes to controlling welding distortion to a certain extent. Limited by the buckling, its value is small as possible or it would cause transverse flex upwords, which are proved by the test results.To control the welding distortion and hot cracks simutanneously, two- direction pre-stress comes into being by combining these two directions stress. According to theory, numerical simulation and test, the interaction exiting between LPTS and TCPS is proved on the condition of TDPS. The pre-stress of each diretion can affect the stress or strain of the other direction. The results of comprehensively action are the superposition of two pre-stress. Through the match of two pre-stress reasonably, the weldments with low stress, small distortion and non- cracks are obtained.In respect of straight welding distortion control, LY12 aluminum alloy thin plates are used, and the dimention of the workpieces is 300mm×200mm×2mm. The experimental results show that the deflection deformation of conventional weldment is 9.92mm while decreasing to 0.9mm after TDPS welding with the match ratio of 0.6σsLTPS and 0.1σsTCPS. The residual compressive stress in the cross-section of the weldment decreases below the critical buckling stress. The buckling disappears completely and there are not hot cracks.TDPS has no effect on surface patten of weld seam. Due to natural forming, the weld seam of TDPS holds the same appearance as conventional welding.The results of tensile and bending test show that the tensile strength increases about 8%, the bending stress increase 6%, the rate of elongation dot not decrease after TDPS. The hardness of the weld seam center and weld toe in weld joint improves a little. It illustrates that the softening of weld joint is instensified.
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