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The most effective way to obtain an excellent weld by electron beam welding is to simultaneously scan the weld that has just been welded during the welding process. The retrace spacing determines the degree of controllable grain refinement, and the solidified structure can be transformed from coarse columnar crystals into fine equiaxed grains. Scanning welding of AlMg0. 4Si1.2 alloy compared to non-scanning welding, the length of the crystal spindle is reduced to 1 / 5 of the non-scanning welding; the hardness of the weld is increased by 80%, close to the base metal level. The degree of grain refinement of aluminum alloy welds has an important influence on joint performance. Electron beam scanning welding with retrace motion reduces the loss of alloying elements, refines the weld structure, makes it into fine equiaxed grains, and increases hardness. For crystals that have been nucleated, if the electron beam scanning pitch is too small, remelting occurs during electron beam scanning, but the effect of electron beam retracement to refine grains is weakened [15]. Figure 6 shows the relationship between the length of the crystal main axis and the hardness of the weld metal. Figure 7 shows the relationship between the scanning frequency fs and the welding speed v and the length L of the weld metal crystal main axis.
The electron beam welding of aluminum alloy is very sensitive to the electron beam flow, especially for the welding of large-thickness aluminum alloy plates, the electron beam current can not be penetrated for a small time, and when it is large, it will collapse and pits appear. Another difficulty in electron beam welding of aluminum alloys is the welding of air holes. The main components of the oxide film on the surface of the aluminum alloy are Al2O3 and MgO. The easy absorption of a large amount of moisture is the main source of pores in the weld of the aluminum alloy. The specific surface of the aluminum alloy oxide film is close to the substrate, and it is easy to enter the weld to produce inclusions and pores. Especially for rust-proof aluminum alloy electron beam welding, the problem of air holes is more serious. Conventional TIG welding aluminum alloys usually use large heat input and weld at a lower welding speed to promote hydrogen escape from the molten pool. Electron beam welding of aluminum alloy is fast, heat input is small, hydrogen is too late. It escapes from the molten pool and easily forms pores. Generally, electron beam welding of aluminum alloys uses subsurface focusing and narrower welds and scanning remelting to prevent the generation of pores. In addition, electron beam welding is required to be carried out under vacuum conditions, so it is difficult to apply welding to large-sized aluminum structural members. The electron beam is susceptible to the electromagnetic field of the surrounding environment. The equipment is complicated and expensive, so it has not yet reached large-scale industrial production.
The partial vacuum electron beam welding process developed in recent years has solved the problem of large-scale electron beam welding of aluminum alloy components [16]. Drauge2lates and others successfully carried out partial vacuum high-speed electron beam welding on AlMg5Mn and AlMg0. 4Si1.2 alloys. The results show that welding at 60 m/min can produce welds without weld defects, and partial vacuum electron beams can be seen. Welded aluminum alloy has a very good development prospect and is an advanced process for welding aluminum alloy [17].
3 Conclusion
(1) Friction stir welding is a solid-state joining process, so the welded aluminum alloy does not have various defects during welding, and the joint performance is good. For aluminum alloys with poor weldability and previously impossible to weld, it can be effectively welded. .
(2) Laser welding is a high-energy density welding process. Welding aluminum alloy can effectively prevent defects caused by traditional welding processes, and the strength coefficient is greatly improved. However, the laser power is generally small, it is difficult to weld aluminum alloy thick plates, and the absorption rate of the laser beam on the aluminum alloy surface is very low, and there is a threshold problem in the deep fusion welding, so there is a certain difficulty in the process.
(3) Aluminum alloy laser-arc hybrid welding solves the problem of laser welding power, the absorption rate of laser beam on aluminum alloy surface and the threshold value of deep fusion welding. It is one of the most promising aluminum alloy welding processes. The process is still immature and is in the stage of research and exploration.
(4) Electron beam welding is another high-energy density welding process. Welded aluminum alloy can obtain high-quality welded joints. At the same time, electron beam welding has good penetration performance and can weld aluminum alloy thick plates, but electron beam welding requirements It is carried out in a vacuum environment, so the application is limited. Partial vacuum electron beam welding developed in recent years can solve this problem to some extent.
Aluminum alloy advanced welding process (Figure) (2)
Compared with traditional arc-welded aluminum alloys, electron beam welding has a high energy density of 3-4 orders of magnitude, which is comparable to another high-energy density welding process, laser welding. Therefore, the heat affected zone of the welded joint is very small, and the joint strength is much higher than the conventional welding method. The penetration performance of the electron beam is good, and the aluminum alloy of a large thickness can be welded, and the mechanical properties of the joint after welding are good. The crack resistance of aluminum alloy weld metal increases with the increase of welding energy density and the decrease of heat input [13]. 5å€ã€‚ The aluminum alloy electron beam welded joints of the cracking performance is much higher than the welding joint using the traditional welding method, generally more than 1 ~ 1.5 times higher than the argon arc welding joint. After the electron beam welding of aluminum alloy, the residual stress is small and the deformation is small, and it can be almost deformed after the thin plate welding. Electron beam welding is required to be done under vacuum. Vacuum is the best protection. Under these conditions, pure weld metal can be obtained, avoiding air or shielding gas pollution. When the electron beam welding aluminum alloy is vacuum remelted, the impurity content in the weld is negligible, and the weld gas content is reduced by nearly half, so that the weld plasticity and toughness are greatly improved. The electron beam has good controllability, can be easily scanned, deflected, tracked, etc., and is easy to automate the welding process, and the electron beam scanning molten pool can eliminate defects and improve joint quality [14].