Analysis of Difficulties in Laser Welding of Aluminum Alloys

【China Aluminum Industry Network】 1. Aluminum Alloy Welding Technology Aluminum alloys have high specific strength, high fatigue strength, good fracture toughness, and low crack propagation rate. They also have excellent forming process properties and good corrosion resistance. It has been widely used in aviation, aerospace, automotive, machinery manufacturing, marine and chemical industries. 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 properties of the 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. Traditional aluminum alloy welding generally adopts TIG welding or MIG welding process. Although these two welding methods have higher energy density and can obtain good joints when welding aluminum alloy, they still have poor penetration, large welding distortion, and low production efficiency. With such shortcomings, people began to seek new welding methods. In the middle and late 20th century, laser technology gradually began to be applied to the industry. 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 prompted the study of laser-welded aluminum alloys. 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. Aluminum alloy surface high reflectivity and high thermal conductivity of this feature can be used to explain 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.

In view of the high reflectivity of aluminum alloys on lasers, a great deal of research has been done at home and abroad. The test results show that appropriate surface pretreatments such as sand blasting, sanding, surface chemical etching, surface plating, graphite coating, air furnace Oxidation can reduce the beam reflection and effectively increase the energy absorption of the beam. 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 pinhole effect and plasma on laser welding of aluminum alloy In the laser welding process of aluminum alloy, the appearance of small holes can greatly increase the laser absorption rate of the material, welding can obtain more energy, and aluminum and aluminum. The Mg, Zn, and Li in the alloy have a low boiling point, are easy to evaporate, and have a large vapor pressure. Although this contributes to the formation of small holes, the cooling effect of the plasma (the shielding and absorption of energy by the plasma reduces the laser to the base metal). The energy input) makes the plasma itself "overheated", but it prevents the continuous existence of small holes and easily produces welding defects such as blowholes, thereby affecting the weld forming and the mechanical properties of the joints. Therefore, the induction and stability of the small holes ensure the quality of laser welding. One of the key points.

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 stability of the welding process can be obtained by controlling the process parameters and selecting the laser power to ensure 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 He gas can not induce holes. This is because an exothermic reaction can occur between N2 and Al, and the resulting Al-NO ternary compound increases the laser absorption.

3. Porosity problems 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/100g or more. However, after solidification, the dissolved hydrogen capacity in the equilibrium state is only 0.036ml/100g, which is 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 pores.

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 the vacuum Welding and so on. In recent years, there have emerged processes that use filler wire or pre-alloy powder, composite heat source and bifocal technology to reduce the generation of pinholes, and have a good effect.

4. Crack problem Aluminum alloys are typical eutectic alloys, which are more likely to produce hot cracks under rapid solidification by laser welding. When the weld metal crystallizes, low-melting-point eutectics such as AL-Si or Mg-Si are formed at the columnar grain boundaries, causing cracks. Causes. 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 more attractive features is its high efficiency, and to give full play to this high efficiency is to apply it to the deep-thickness welding of large thickness. 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.

Improving the stability of the laser welding process, forming the weld seam, and improving the welding quality are the goals pursued by people. Therefore, new technologies such as laser-arc hybrid technology, wire-filled laser welding, pre-powdered laser welding, dual focus technology and beam shaping will be further improved and developed.

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