Aluminum alloys have high specific strength, high fatigue strength, good fracture toughness and low crack propagation rate, and also have excellent forming processability and good corrosion resistance in aviation, aerospace, automotive, machinery manufacturing, marine and The chemical industry has been widely used. The wide application of aluminum alloys has promoted the development of aluminum alloy welding technology, while the development of welding technology has expanded the application of aluminum alloys.
However, the characteristics of aluminum alloy itself make its related welding technology face some problems to be solved: surface refractory oxide film, joint softening, easy to produce pores, easy thermal deformation and thermal conductivity is too large and other traditional aluminum alloy welding TIG welding or MIG welding processes are generally used. Although these two welding methods have large energy densities and can obtain good joints when welding aluminum alloys, they still have the disadvantages of poor penetration, large welding distortion, and low production efficiency. The search for new welding methods began, and laser technology gradually began to be applied to the industry in the middle and late 20th century. The Airbus A340 aircraft fuselage produced by the European Airbus Company uses laser welding technology instead of the original riveting process, which reduces the weight of the aircraft body by about 18% and reduces the manufacturing cost by nearly 25%. The German Audi A2 and A8 all-aluminum cars also benefit from the development and application of aluminum alloy laser welding technology. These successful cases have greatly promoted the research of laser welding of aluminum alloys, and laser technology has become the main development direction of future aluminum alloy welding technology. Laser welding has the advantages of high power density, low welding heat input, small welding heat affected zone and small welding distortion, which makes it particularly important in the field of aluminum alloy welding.
Second, aluminum alloy laser welding problems and countermeasures
1. High reflectivity and high thermal conductivity of aluminum alloy surface
This feature can be explained by the microstructure of aluminum alloys. Due to the existence of dense free electrons in the aluminum alloy, the free electrons are forced by the laser (strong electromagnetic waves) to generate secondary electromagnetic waves, resulting in strong reflected waves and weak transmitted waves, so the surface of the aluminum alloy has a higher laser The reflectivity and very small absorption rate. At the same time, the Brownian motion of the free electrons becomes more intense and so the aluminum alloy also has a high thermal conductivity.
Aiming at the high reflectivity of aluminum alloy on the laser, a lot of research has been done at home and abroad. The test results show that appropriate surface pretreatments such as sandblasting, sandpaper grinding, surface chemical etching, surface plating, graphite coating, Oxidation and the like in the air furnace can reduce the beam reflection and effectively increase the absorption of the beam energy by the aluminum alloy. In addition, considering the design of welded structures, aluminum alloys can be increased by artificially making holes in the aluminum alloy surface or using connectors in the form of light collectors, opening V-shaped grooves, or using split welding (splicing gaps are equivalent to artificial holes). Absorption of laser light to achieve greater penetration. In addition, it is also possible to use reasonable design of the welding gap to increase the laser energy absorption on the aluminum alloy surface.
2. Effect of small hole and plasma on laser welding of aluminum alloy
In the aluminum alloy laser welding process, the appearance of small holes can greatly improve the laser absorption rate of the material, welding can obtain more energy, while aluminum, magnesium, zinc and lithium in the aluminum alloy have low boiling point, easily evaporate and steam. The pressure is high, although this helps the formation of small holes, but the cooling effect of the plasma (the shielding and absorption of energy by the plasma, reducing the laser's energy input to the parent metal) makes the plasma itself "overheated" but hinders The continuous presence of small holes, prone to blow holes and other welding defects, thus affecting the weld forming and the mechanical properties of the joints, so the induction and stability of the hole to ensure the quality of laser welding is a key point.
Due to the high reflectivity and high thermal conductivity of aluminum alloys, lasers are required to have a higher energy density to induce the formation of pinholes. Because the energy density threshold is controlled by the alloy composition in essence, the stable welding process can be obtained by controlling the process parameters and selecting the laser power to ensure the proper heat input. In addition, the energy density threshold is also affected by the type of protective gas to some extent. For example, when using N2 gas for laser welding of aluminum alloy, it is easier to induce small holes, while using other gases can not induce small holes. This is because an exothermic reaction can occur between N2 and aluminum, and the resulting aluminum NO ternary compound increases the laser absorption.
3. Stomatal problem
Different types of aluminum alloys produce different types of pores. It is generally believed that aluminum alloys produce the following types of pores during the welding process.
1) Hydrogen pores. After the aluminum alloy is melted in a hydrogen atmosphere, its internal hydrogen content can reach 0.69 ml/100 g or more. However, after solidification, the dissolved hydrogen capacity in the equilibrium state was only 0.036 ml/100 g, a difference of nearly 20 times. Therefore, during the transition from the liquid state to the solid state, excess hydrogen in the liquid aluminum must be precipitated. If the precipitated hydrogen does not float smoothly, it will accumulate into bubbles and remain in the solid aluminum alloy to become pores.
2) Protective gas generated pores. In the process of high energy laser welding of aluminum alloy, the protective gas is entrained into the molten pool to form bubbles due to the strong evaporation of the metal at the front of the small hole at the bottom of the pool. When the bubbles escape too late and remain in the solid aluminum alloy, they become air holes.
3) Pores resulting from the collapse of pores. In the laser welding process, when the surface tension is greater than the vapor pressure, the small holes will not be stable and collapse, and the holes will be formed when the metal is too late to fill. There are also many practical measures to reduce or avoid the porosity defects in aluminum alloy laser welding, such as adjusting the laser power waveform, reducing the unstable collapse of the hole, changing the beam focal height and tilting irradiation, applying the electromagnetic field effect during the welding process and in the vacuum Welding and so on. In recent years, there have emerged the technology of using the filler wire or pre-alloy powder, composite heat source and bifocal technology to reduce the generation of pores, which has a good effect.
4. Crack problem
Aluminum alloys are typical eutectic alloys, which are more likely to generate thermal cracks under rapid solidification by laser welding. When the weld metal crystallizes, the low-melting point eutectics such as Al-Si or Mg-Si series formed at the columnar grain boundaries cause cracking. the reason. To reduce hot cracks, laser welding can be performed using fillers or pre-alloy powders. By adjusting the laser waveform, controlling the heat input can also reduce crystal cracks.
Third, the development prospects of aluminum alloy laser welding
Aluminum alloy laser welding is a feature that attracts people's attention because of its high efficiency, and to fully use this high efficiency is to apply it to large-thickness deep-melt welding. Therefore, research and use of high-power lasers for large-thickness deep-melt welding will be an inevitable trend in the future. Large-thickness deep penetration welding more prominently affects the phenomenon of pinholes and the influence on the pores of the welds. As a result, the formation mechanism and control of pinholes become even greater. It will surely become a hot issue that the industry is concerned about and studies.
Improve the stability of laser welding process and weld formation, improve the quality of welding is the goal pursued by people. Therefore, laser-arc composite process, laser welding of the filler wire, pre-powder laser welding, bifocal technology and beam shaping and other new technologies will be It will be further improved and developed.