As an engineering metal material that has risen rapidly in recent years, aluminum alloys have been widely used in aerospace, automobiles, ships and other fields due to their low density, high specific strength and specific stiffness, and good corrosion resistance. .
However, a series of problems such as poor weldability and poor forming layer performance 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.
Overview of Aluminum Alloy Properties
Aluminum is a very light metal material with a density of only 2.7g/cm3, which is about 36% of the density of steel. The use of aluminum alloy to manufacture mechanical parts can significantly reduce the weight and achieve the effect 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 alloys have good machinability and recyclability. If the cutting resistance coefficient of the most machinable magnesium alloy is assumed to be 1, the cutting resistance of other metals is shown in Table 2. It can be seen that the cutting resistance of aluminum alloy is smaller than that of copper, iron and other materials, and the cutting process is easier.
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 lower 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 generate large internal stress in the aluminum alloy joint, causing large welding stress and deformation, forming cracks, wave deformation and other defects.
Ablative evaporation
The melting point of aluminum is 660°C and the boiling point is 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 explode 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 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 droplets, which inevitably changes the welding seam area. The chemical composition is not conducive to the performance regulation of welded joints. Therefore, in order to compensate for high temperature ablation, welding wires or other welding materials with a higher boiling point element content than the base metal are often used during welding.
Solid inclusions
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 they are mixed in the molten alloy liquid with low density in the molten pool area, which is easy to form small solid slag inclusions that are not easy to discharge, which not only affects the microstructure of the weld, but also is prone to 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.
Stoma collapse
The melting point of aluminum alloy is much lower than that of its oxide, and it is very active and easy to oxidize. During the welding process, the aluminum alloy is melted at high temperature to form a molten pool. The aluminum on the surface of the molten pool is oxidized to form an oxide film, which covers the molten pool in a solid form. Since the color of the molten oxide film is not much different from the molten state of the 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 and cause welding heat. Large collapses in the area destroy 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 is dissolved in the alloy liquid. After the welding is completed, as the temperature of the molten pool decreases, the solubility of the gas gradually decreases, which becomes the main cause of pores during the welding process. reason. Due to the rapid solidification rate and low density of aluminum alloys, 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 pores and reducing the structural properties of the joint. Of course, the generation of pores is not necessarily formed during the welding process. Due to the influence of the casting technology, the base metal itself will also generate pores during the casting process. During welding, the constant change of heat input and internal pressure causes the original pores in the base metal to expand by heat or combine with each other to form weld pores. With the increase of welding heat input, the pores will also increase. Therefore, in order to control the source of hydrogen, the welding material needs to undergo strict drying treatment before use. During welding, the current should be appropriately increased to prolong the existence time of the molten pool and allow sufficient time for hydrogen to precipitate, thereby controlling the formation of pores.
Aluminum alloy welding technology classification
With the expansion of the application range of aluminum alloys, more and more problems are highlighted. With the progress of research, aluminum alloy welding technology has made great progress. At present, there are mainly tungsten argon arc welding (TIG), molten inert gas welding (MIG), laser welding (LBW), friction stir welding (FSW) Wait.
TIG welding
Tungsten Inert Gas Welding (TIG) is a typical inert gas arc welding and is the most commonly used welding method. During 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 shielding 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 Weld repair and repair of casting defects.
Mainly have the following technical characteristics:
Easy to operate, flexible and controllable, adaptable to various working conditions, and low cost;
The heat-affected zone is narrow, 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 welding process performance is good and stable, and the welding seam is dense and beautiful.
MIG Welding
MIG (GMA-Gas Metal Arc Welding) and TIG are both inert gas shielded welding, the difference is that TIG welding uses a tungsten electrode as a fixed electrode, while MIG welding uses the filler wire material itself as an electrode.
However, the application process of aluminum alloy MIG welding is greatly limited, because the soft aluminum wire leads to poor wire feeding, and the molten aluminum tends to form a phenomenon of "hanging without 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 motion 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 into the interior of the material through heat conduction, and the material is melted to form a specific molten pool. After solidification, the material is connected as one.
The advantages of laser welding are that the welding point of action is small, the high-power heat source is concentrated, and it has the ability to weld thick plates, with a narrow heat-affected zone and small welding deformation. However, at the same time, laser welding has high requirements for welding positioning, expensive welding equipment, and high welding cost. 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 vaporized in a very short time, and the gas will converge into a small hole in the molten pool, This 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 problem of uneven 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 solid-phase joining technology formed on the basis of traditional friction welding technology. At the interface to be welded, when the stirring head advances along the weld, the temperature of the welding material increases, 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;
The mechanical properties of the weld are good;
The welding process is simple, economical and environmentally friendly.
