As an engineering metal material that has risen rapidly in recent years, aluminum alloy has been widely used in aerospace, automobiles, ships and other fields due to its low density, high specific strength and specific stiffness, and good corrosion resistance. .
However, a series of problems such as poor weldability and poor performance of the forming layer in welding restrict the development of aluminum alloy structural parts. Therefore, aluminum alloy welding technology has become one of the main research directions of many scholars at home and abroad.
Aluminum alloy performance overview
Aluminum is a very light metal material with a density of only 2.7g/cm3, which is about 36% of the density of steel. Aluminum alloy is used to manufacture mechanical parts, which can significantly reduce weight and achieve the effects of light weight, energy saving and emission reduction.
The specific strength and specific stiffness of aluminum alloy are higher than 45 steel and ABS plastic. The use of aluminum alloy materials is conducive to the manufacture of integral components with high rigidity requirements.
Aluminum alloy has excellent thermal conductivity, electrical conductivity and corrosion resistance. The performance parameters of A380 aluminum alloy and other materials are shown in Table 1.
Aluminum alloy has good machinability and recyclability. If it is assumed that the cutting resistance coefficient of the most easily cut magnesium alloy is 1, the cutting resistance of other metals is shown in Table 2. It can be seen that the cutting resistance of aluminum alloy is less than that of copper, iron and other materials, and the cutting process is relatively easy.
Aluminum alloy welding characteristics
Affected by the physical and chemical properties of aluminum alloys, there are certain difficulties in the welding process. The current aluminum alloy welding mainly has the following problems: thermal stress, ablation evaporation, solid inclusions, pore collapse, etc.:
Thermal Stress
Aluminum alloys have a higher coefficient of thermal expansion and a smaller modulus of elasticity. During the welding process, due to the large deformation and large linear expansion coefficient of the aluminum alloy, the volume shrinkage rate during solidification is about 6%, and the cooling rate and the primary crystallization rate of the molten pool are fast, resulting in the internal stress of the weld and the rigidity of the welded joint. Larger, it is easy to cause greater internal stress in the aluminum alloy joint, causing greater welding stress and deformation, forming defects such as cracks and wave deformation.
Ablation evaporation
Aluminum has a melting point of 660°C and a boiling point of 2647°C, which is lower than other metal elements such as copper and iron. During the welding process, if the welding temperature is too high, it is easy to cause explosion and form spatter, especially in high-energy beam welding, as shown in Figure 1. In addition, some of the alloying elements added to the aluminum alloy have a low boiling point, which is very easy to evaporate and burn at the instantaneous high temperature of welding, and the splash generated by the explosion will also take away part of the liquid droplets, which inevitably changes the weld area. The chemical composition is not conducive to the performance regulation of the welded joint. Therefore, in order to compensate for high temperature ablation, welding wire or other welding materials with a higher boiling point element content than the base metal are often used during welding.
solid inclusion
The chemical properties of aluminum are very active and easily oxidized. During the welding process, the surface of the aluminum alloy is oxidized to form Al2O3 with a high melting point (about 2050 ° C, while the melting point of aluminum is 660 ° C, which is very different). The oxides are dense and have high hardness, and are mixed in the molten alloy liquid with low density in the molten pool area, which is easy to form fine solid slag and is difficult to discharge, which not only affects the structure of the weld, but also easily produces electrochemical corrosion, which will cause The mechanical properties of welded joints decrease, and Al2O3 covers the molten pool and groove, which seriously affects the welding of alloys and reduces the microstructure and properties of welded joints.
Stomatal collapse
The melting point of aluminum alloy is much lower than that of its oxide, and its nature is lively and easy to oxidize. During the welding process, the aluminum alloy forms a molten pool due to high temperature melting. The aluminum on the surface of the molten pool is oxidized to form an oxide film, which covers the molten pool in a solid state. Since the color of the oxide film after melting is not much different from that of the molten aluminum alloy, and because of the coverage of the oxide film, it is difficult to observe the degree of melting of the aluminum alloy molten pool during the welding process, so it is easy to cause the temperature to be too high, causing welding heat influence The bulk of the area collapses, destroying the shape and properties of the weld metal.
Under the action of the instantaneous high power of the welding heat source, a large amount of hydrogen gas is dissolved in the alloy liquid. After the welding is completed, as the temperature of the molten pool decreases, the solubility of the gas also gradually decreases, which becomes the main cause of pores in the welding process. reason. Because the solidification speed of the aluminum alloy is too fast and the density is low, hydrogen pores of different sizes are formed during the rapid solidification of the weld. These pores will continue to accumulate and expand during the welding process, eventually forming visible large pores and reducing the structural properties of the joint. Of course, the pores are not necessarily formed during the welding process. Due to the influence of the casting process technology, the base metal itself will also produce pores during the casting process. During welding, the heat input and internal pressure are constantly changing, causing the original pores in the base metal to expand or combine with each other to form weld pores. As the welding heat input increases, the pores will also increase. Therefore, in order to control the source of hydrogen, the welding material needs to be strictly dried before use. During welding, the current is appropriately increased to prolong the existence time of the molten pool and give enough time for hydrogen to precipitate, thereby controlling the formation of pores.
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Fig.2 Formation and convergence of stomata
Classification of aluminum alloy welding technology
With the expansion of the application range of aluminum alloys, more and more problems are highlighted. With the progress of research, the welding technology of aluminum alloy has been greatly developed. At present, there are mainly tungsten argon arc welding (TIG), molten inert gas welding (MIG), laser welding (LBW), friction stir welding (FSW) wait.
Gas tungsten arc welding
Tungsten Inert Gas Welding (TIG) is a typical inert gas shielded welding and is the most commonly used welding method. When welding, the tungsten electrode and the welding surface are used as electrodes, and helium or argon gas is passed between the two electrodes as a protective gas to protect the arc, and the wire and base metal are melted by instantaneous high-voltage discharge, and the aluminum alloy parts are welded and formed, and Welding and repairing of casting defects.
It mainly has the following technical characteristics:
Easy to operate, flexible and controllable, adaptable to various working conditions and environments, and low in cost;
The heat-affected zone is narrow, and the deformation of the welded joint is small under the condition of sufficient wire feeding, and the comprehensive performance of the joint is high;
The performance of the welding process is good and stable, and the weld seam is dense and beautiful.
MIG welding
Both MIG (GMA-Gas Metal Arc Welding) and TIG are inert gas shielded welding. The difference is that TIG welding uses tungsten electrodes as fixed electrodes, while MIG welding uses the filled wire material itself as electrodes.
In the metal inert gas shielded welding process of aluminum alloy, the voltage and current act on the end of the electrode of the welding wire, and an instantaneous high pressure is generated between the electrode and the base metal, which melts the base metal and the groove, and the droplet at the end of the wire falls off and transitions vertically to the base metal. On the molten pool of the material, a weld zone is formed.
However, the application process of aluminum alloy MIG welding is relatively limited, because the softness of the aluminum wire leads to poor wire feedability, and the molten aluminum is prone to form a phenomenon of "hanging but not dripping" during welding, which is easy to cause droplets to splash. The advantage is that MIG welding is faster than TIG welding, and the welding movement range is small when welding large workpieces. By adjusting the wire feeding speed, the welding efficiency can reach several meters per minute.
laser welding
Laser beam welding (Laser Beam Welding LBW) uses high-energy laser pulses to locally heat the material in a small area. The energy of the laser radiation diffuses to the inside of the material through heat conduction, and the material is melted to form a specific molten pool. After solidification, the material is connected into One.
The advantage of laser welding is that the welding action point is small, the high-power heat source is concentrated, it is capable of welding thick plates, the heat-affected zone is narrow, and the welding deformation is small. But at the same time, laser welding has high requirements for welding positioning, expensive welding equipment, and high welding costs. For metal materials such as aluminum and magnesium, the laser reflectivity is high, and direct welding is difficult.
Irradiating materials with lasers with different power densities shows that when the power density on the workpiece reaches more than 107W/cm2, the metal in the heating zone will be gasified in a very short time, and the gas will converge into a small hole in the molten pool and form a The small hole is the center for heat transfer, and a molten pool is formed near the small hole, which is the "keyhole" effect of laser deep penetration welding. In order to avoid the unevenness of the molten pool caused by this phenomenon, it is possible to reduce the laser energy, increase the welding speed or control the remelting of the nugget area to remove the bubbles in the fusion zone and reduce the generation of pores.
friction stir welding
Friction stir welding (Friction stir Welding, FSW) is a new type of solid phase connection technology based on traditional friction welding technology. At the interface to be welded, when the stirring head advances along the weld seam, the temperature of the welding material rises, and the plasticized metal undergoes strong plastic deformation under the action of mechanical stirring and upsetting, and forms a dense solid-phase connection after diffusion and recrystallization.
Compared with traditional welding methods, FSW technology has the following advantages:
Low welding temperature and small welding deformation;
Good mechanical properties of the weld;
The welding process is simple, economical and environmentally friendly.
The main problems and research focus
With the application of aluminum alloys in more and more industries, the problem of its repair connection has also attracted the attention of more and more scholars. Through various welding tests on aluminum alloys, it is found that the maturity of the repair technology has not yet met the development needs of the industry, and there are still various problems in it.
Gas tungsten arc welding and metal inert gas shielded welding are the two most widely used welding methods at present, but these two technologies have a wide heat-affected zone, and the weld metal needs to be melted and then solidified, which has an impact on the structure. Larger, and the residual stress is high, resulting in a serious impact on the mechanical properties of the joint. Laser welding energy beam density is high, and the depth-to-width ratio of the weld is large, but it is very easy to form pores, and its expensive cost also limits the popularization of applications. Friction stir welding provides a solution to the problem of heat, but friction stir welding requires relatively large upsetting pressure and forward driving force, and the equipment is generally complicated and bulky, which limits its development.
The focus of future research on related topics should be on the following aspects:
Starting from the basis of fusion welding, adjust the welding wire formula, add rare earth elements or select a suitable amount of welding activator to control the welding deformation, reduce stress, and reduce the formation of pores.
Due to the expansion of the scope and application of alloys, they are usually used in conjunction with dissimilar materials, so it is necessary to carry out lap welding experiments between dissimilar metals to obtain high-quality joints.
Carry out research on the weldability of composite heat sources, such as TIG-laser hybrid welding, laser composite friction stir welding, in order to obtain the optimal weld performance.
