Friction Stir Welding On High Strength Aluminum Slab

It is very difficult to weld high strength aluminum slab with friction stir welding, and my research is to solve problems on slab welding.2519 aluminum alloy is a new kind of high strength armor aluminum alloys, which have light specific weight, high mechanical strength, and strong anticollision and anticorrosion properties. It is very difficult to weld 2519 aluminum slab with traditional fusing welding without voids, cracking, distortion, etc. For the 2519 aluminum slab has special chemical composition and physical character, it is also difficult to be welded with friction-stir welding. So my research is how to apply friction-stir welding on 2519 aluminum slab in order to get high strength welds without defects. Therefore, it is important to study the welding technique and mechanism of friction stir welding on 2519 aluminum slab.Through welding experiments, microstructure analysis and tensile experiments, a special thread stirrer which is suitable for the slab welding is obtained, and the problem to weld long welds is solved by using infrared thermoscope and hydraulic pressure controller to regulate the welding pressure. The special stirrer is used to carry out friction stir welding on 2519 aluminum slab with a thickness of 20 mm by a single pass. The weld's tensile strength has reached 305 MPa, and the welding strength coefficient has reached 64%. It is a breakthrough on the welding property of 2519 aluminum slab.Based on computational fluid dynamics, a heat-fluid-solid coupling mathematic model has been established for friction stir welding on 2519 aluminum slab. The model has synthetically used nonlinear friction factor and nonlinear friction sheafing strength which changes with actual friction interface temperature, and nonlinear viscosity which changes with both temperature and strain rate. It is useful to solve the problem that the present welding model can't reflect the influence of friction factor, friction sheafing strength and fluid-solid coupling interface which change with the welding parameters.Based on the analysis of plastic flows, temperature distributions and stress distributions with different stirrers, a special ameliorated stirrer is obtained to get good welds of 2519 aluminum slab.Based on the simulation analysis of the micro-voids' formation and elimination mechanisms in the process of friction-stir welding, and the experimental observation of the influence of stirrer's structure and welding parameters on the welds' micro-voids, some conclusions have been drawed as follows:1. The mechanism that the square pin can eliminate micro- voids in the process of friction-stir welding results from the formation of the revolving compressive stress field around the pin. 2. The welds' micro-voids can be formed by the increasing of the pin's diameter. It is the result that the transient compressive stress is decreased and the compressive stress zone is relatively narrow.3. The mechanisms that regulating welding parameters can eliminate micro-voids are decided by two reasons: the one is to exert fit press in order to produce fit welding heat, and make the yield strength of the weld metals decrease; the other is fit process parameters and stirrer shape, and make the metals form sufficient plastic flow to get good welds without defects.The above reseach of the welding mechanisms illustrates the reason why the welding on 2519 aluminum slab is difficult, and provides methods to ameliorate stirrer's structure and process parameters.