At the same time, because the temperature of friction stir welding is relatively low, the residual stress or deformation of the structure after welding is much smaller than that of fusion welding. Especially when the aluminum alloy sheet is welded by melting, the out-of-plane deformation of the structure is very obvious. It is very troublesome to adopt non-deformation welding technology or hot and cold sizing technology after welding, which increases the manufacturing cost of the structure.
Friction stir welding is mainly used for nonferrous metals with low melting point, such as aluminum, copper and other alloys. This is related to the material selection and working life of the mixing head. Of course, this is also related to the relative difficulty of melting welding of non-ferrous metals, forcing people to find non-melting welding methods when welding non-ferrous metals. For black materials with good ductility and easy plastic deformation, friction stir welding can also be realized by auxiliary heating or using its superplasticity, but it depends on which technical and economic index is more reasonable: fusion welding or friction stir welding.
Friction stir welding has been successfully applied to the connection of non-ferrous metals, but due to the limitation of welding methods, it is limited to the welding of components with simple structure, such as straight structure or cylindrical structure, and the workpiece should have good support or buffer during welding. In principle, friction stir welding can be carried out in various positions, such as flat welding, vertical welding, overhead welding and prone welding; It can complete various forms of welding joints such as butt joint, fillet joint and lap joint, even the connection of multi-layer material structures with variable thickness, and can also weld dissimilar metal materials.
In addition, as a solid-state welding method, friction stir welding has little pollution to the environment before and during welding. Before welding, the workpiece does not need strict surface cleaning preparation requirements. Friction and stirring during welding can remove the oxide film on the surface of the workpiece, and there is no smoke and splash during welding, with low noise. Friction stir welding is more energy-saving than fusion welding or even conventional friction welding because it only depends on the rotation and movement of the welding head to gradually realize the welding of the whole weld.
Because the heat input in friction stir welding is less than that in fusion welding, no metal melts at the joint. It is a solid-state welding process, which can maintain the metallurgical properties of the base metal in the alloy, and can weld metal matrix composites, rapidly solidified materials and other materials that will have adverse reactions when fusion welding is used. Its main advantages are as follows:
(1) The microstructure of the heat affected zone of the welded joint changes little, the residual stress is low, and the welded workpiece is not easy to deform.
(2) The welding of long seam, large section and different positions can be completed at one time. Seam height:
(3) The operation process is convenient to realize mechanization and automation, with simple equipment, low energy consumption, high efficacy and low requirements for working environment;
(4) There is no need to add welding wire when welding aluminum alloy, and the oxide film and shielding gas are removed before welding, so the cost is low;
(5) It can weld hot crack sensitive materials, which is suitable for welding dissimilar materials;
(6) The welding process is safe, pollution-free, smokeless and radiation-free.
Friction stir welding also has some disadvantages: the workpiece to be welded must be rigidly fixed, and the reverse side should have a bottom plate; When the workpiece is hoisted out by the stirring probe after welding, a small hole is formed at the end of the weld, and the weld is difficult to repair: the tooling design, process parameters and mechanical performance data are only available in a limited alloy range; In some cases, such as when corrosion performance, residual stress and deformation need to be considered in special fields, the performance needs to be further improved before practical application; When connecting the plates once, the welding speed is not very high: the stirring head wears too fast.