Studies On The Mechanism Of Low Stress No Distortion Welding For A Titanium Alloy

Titanium alloy thin-walled structures have been increasingly used in aerospace industries, especially for new type of aircrafts to improve technical performance, decrease weight and increase the loading capability. However, serious welding residual stresses and distortion problems have presented troublesome obstacles that restrict the more wide applications of such structures. To avoid the negative affects of buckling distortions in fabrication of thin-walled structural elements of titanium alloy, experimental and numerical studies should be carried out systematically and thoroughly on the specific features of welding thermal elastoplastic stress and strain during welding process of titanium alloy. Studies on the specific characteristics and control mechanisms of welding stress and distortion in titanium alloys differing from those for aluminum alloys and steels, are essential for optimizing the control technological parameters and improving the welding quality for thin-walled structures of titanium alloy. These studies also present the efforts to promote the manufacturing technology by information science and to develop the welding processes in the direction of digitization through numerical modeling and simulation.To meet the urgent requirements to solve stress and distortion problems of welded titanium alloy structures of aircrafts, research program and technical path for systematic experimental study were designed. The temperature field, welding residual stress and welding residual strain were measured and compared for conventional and Low Stress No Distortion (LSND) gas tungsten arc welding (GTAW) processes. Based on these experiments, a three-dimensional finite element analysis model was constructed for GTAW process. The finite element analysis code ANSYS was then employed to compute the development histories and distribution characteristics of temperature field and thermal elastoplastic stress and strain during welding process of butt joint of TC4 titanium alloy thin sheets. Comparisons were made between the numerical and the experimental results. The results show that during welding process, incompatible compressive plastic strains are produced in the metals in front of the weld pool and corresponding compressive plastic strains are also formed both sides of the weld pool confined by the surrounding metals. During cooling process, tensile plastic strains are produced in the metals within and nearby the solidified weld pool. The compressive (negative) strains formed during heating are partially counteracted, which reduce the residual incompatible strains in the joints. Based on the experimental and theroratical analysis, a layout diagram of welding thermal elastoplastic stress and strain cycles (WSS) for titanium alloy was performed. The WSS diagram consists of compressive plastic strain zone, melted zone, tensile plastic strain zone, unloaded zone, and compressive elastic strain zone. When the welded specimens are cooled completely, both the weld and the region nearby are unloaded, the residual peak tensile stress in the weld are always below the yield stress of the base metal, it is only about 0.5-0.7 s.With the established three-dimensional thermal elastoplastic finite element model, the stress and distortion control mechanism was analyzed for the Dynamically Controlled Low Stress No Distortion GTAW technique (DC-LSND GTAW). It was found that during the DC-LSND GTAW process, due to the intensive cooling of heat sink, a saddle shape temperature field is formed in the center of the heat sink in transversal cross section, and a valley of temperature field induced by the heat sink is always following the arc. Great temperature difference with high thermal gradient exists in the metals between welding arc and heat sink. Sharp cooling and contractionof metals beneath the heat sink produces intensive tensile affects on the solidified metals between heat sink and weld pool, which decrease the compressive (negative) plastic strains nearby the weld produced during heating process while increase the tensile...