Application Of Laser Welding Technology In Aluminum Alloy Welding

The use of aluminum alloy to replace the traditional steel on the automobile is one of the important ways to realize the lightweight of the automobile. However, due to the characteristics of aluminum alloy with good thermal conductivity and a large coefficient of linear expansion, there are some problems in welding:

1) Aluminum alloy welded joint softening seriously, low strength coefficient;

2) Aluminum alloy is easy to oxidize to produce refractory oxide film (Al2O3, melting point is 2060℃), which needs to use a high-power density welding process;

3) Easy to produce pores;

4) The linear expansion coefficient is large, easy to produce welding deformation and welding cracks;

5) The thermal conductivity and specific heat capacity are large, and the heat input is 2 to 4 times larger than that of welded steel.

Therefore, in order to obtain high-performance aluminum alloy welding joints, high energy density, low welding heat input and high welding speed are needed, among which laser welding is one of the most promising aluminum alloy welding technologies.

Laser welding is an efficient and precise welding method using high energy density laser beam as a heat source, which has the characteristics of fast speed, large penetration, small deformation, good processing flexibility and easy automation when applied to aluminum alloy welding. It has been widely used in aerospace, automobile manufacturing and light industry electronics and other fields, but it is used in aluminum alloy laser welding. There are also some problems and difficulties.

1.1 Aluminum alloy has a very low-absorption rate of laser

Aluminum alloy has a strong reflective effect on laser, which is due to the high density of free electrons in the solid state of aluminum alloy, which is easy to reflect the energy with the photon in the beam. Studies show that the reflectivity of aluminum alloy to gas CO2 laser is as high as 90%, and the reflectivity of solid laser is close to 80%. At the same time, the aluminum alloy has a strong thermal conductivity, resulting in a very low absorption rate of aluminum alloy laser. Therefore, appropriate measures must be taken to improve the laser absorption rate of aluminum alloy.

Optimization measures:

1) By increasing the laser power density to improve the absorption of aluminum alloy laser. The increase of laser power density will cause a small hole effect in the welding pool, which can greatly improve the absorption rate of the material to the laser.

2) Adopt appropriate surface pretreatment process. Electrolytic polishing, anodizing and sandblasting were used to compare the laser absorptivity of aluminum alloy. It was found that anodizing and sandblasting on aluminum alloy surfaces could significantly improve the laser absorptivity.

3) It is also found that the joint form will affect the absorption of laser. V-shaped bevel and square bevel are more conducive to the formation of keyholes than non-bevel joints, and the laser power density increases and the laser absorption rate of aluminum alloy also increases.

1.2 Easy to form pores

Pores are the most common defect in laser welding of aluminum alloy. Pores can be divided into two categories. One is that due to the sharp decline in the solubility of hydrogen in the cooling process of laser welding of aluminum alloy. The hydrogen content of molten aluminum alloy can be 0.69mL/100g. The hydrogen content of aluminum alloy after cooling and solidification is 0.036mL/100g, and the supersaturated hydrogen precipitates to form hydrogen pores. In addition, there is an oxide film on the surface of the aluminum alloy, and the water in the crystal water, air and protective gas on the surface of the aluminum alloy is directly decomposed into hydrogen during welding. These hydrogen pores are too late to escape during the rapid cooling process of laser welding of aluminum alloys, and remain in the weld to form hydrogen pores. The other is due to the instability of the keyhole generated in the laser welding process and collapse, and the liquid metal has no time to fill the hole. Excessive porosity will reduce the density of the weld, reduce the bearing capacity of the joint, and reduce the strength and plasticity of the joint to varying degrees. There are many measures to reduce the porosity defects in laser welding of aluminum alloy, such as changing the walking track of the laser beam, using beam oscillation to stir the molten pool, increasing the possibility of porosity escape from the surface, using wire or alloy powder, and using double-spot technology and laser composite welding can achieve the effect of reducing porosity, but it is difficult to fundamentally eliminate.

1.3 Hot crack tendency

The reason of hot crack in laser welding of aluminum alloy is mainly related to its own characteristics and the welding process. During the solidification of aluminum alloy, the shrinkage rate is large (up to 5%), the welding stress and deformation is large, and the weld metal will produce low melting point eutectic structure along the grain boundary during crystallization, so that the grain boundary binding force is weakened, and hot cracks are formed under the action of tensile stress. The hot crack tendency can be reduced by filling wire or alloy powder, and the hot crack tendency can also be reduced by adjusting the welding process parameters to control the heating and cooling speed.

1.4 Softening of weld structure and heat-affected zone

"Softening" is the phenomenon of reduced strength and hardness of welded joints. When laser welding aluminum alloy joint is used, the weld structure and heat-affected zone of the welded joint also have softening problems. A large number of studies have shown that the softening phenomenon of aluminum alloy welding is difficult to be fundamentally eliminated, but compared with gas-shielded welding, laser welding reduces the heat input and makes the softening zone of the weld more narrow. Compared with MIG welding, the "softening" degree of laser welded aluminum alloy joints is lower, and the tensile strength increases with the increase of welding speed. The influence of plasma on the welding process Aluminum element ionization energy is low, laser welding is easier to form metal plasma, plasma caused by laser refraction and deflection, thereby changing the focus of the laser beam, so that the weld penetration ratio is reduced, affecting the quality of the welding joint. By presetting the powder on the workpiece surface, the expansion and pulsation of the plasma in the height direction is weakened, so that the plasma can maintain a relatively stable pulsation amplitude on the workpiece surface.

2.1 Aluminum alloy laser self-fusion welding

Laser self-fusion welding refers to the high energy density laser beam as a heat source, impacting on the surface of the base material, so that the base material itself melts, forming a welding joint welding method. For laser welding of aluminum alloy, the surface of aluminum alloy has high reflectivity to the laser, and large laser power is required during welding. The diameter of the laser spot is small, the accuracy of the welding equipment is high, and the tolerance value of the gap of the parts is low, and the gap value of the parts is usually required to be below 0.2mm. During the welding process, the heating and cooling speed is fast, the welding porosity defects are many, the laser energy density is concentrated, and the keyhole effect is easy to cause the phenomenon of weld concave and edge biting, so the welding process parameters have higher requirements. Laser self-fusion welding in aluminum alloy welding reflects the advantages of good welding quality, fast welding speed and easy automation, and is widely used in the automotive industry. In the electric vehicle industry, the sealing of the power battery shell is mainly made of aluminum alloy laser self-fusion welding. In the aluminum body of a domestic new energy automobile company, the welding of the door assembly and the side-wall structure is also made of aluminum alloy laser self-fusion welding.

2.2 Aluminum alloy laser wire filling welding

In laser wire filling welding, laser is still used as the main heat source to melt the welded metal, but the automatic wire feeding device is used to continuously feed the filling metal into the melt pool to achieve the metallurgical connection process. Compared with the laser self-fusion welding, the laser wire filling welding relaxed the requirements of the welding process gap accuracy, by filling different components of the welding wire, improve the metallurgical properties of the weld, prevent welding hot cracks and pores, improve the stability of the welding process and joint mechanical properties.

Aluminum alloy laser wire welding has the characteristics of good-appearance quality, process gap accuracy is looser than laser self-fusion welding, etc. It is usually used in the exterior surface of the body, such as between the top cover and the side wall, and between the upper and lower plates of the suitcase cover. There are also some models in order to obtain higher welding quality and use laser wire welding to weld aluminum alloy doors.

2.3 Aluminum alloy laser arc composite welding

Laser-arc composite welding is the combination of laser and arc two kinds of heat sources with different physical properties and energy transmission mechanism, and work together on the welded parts. It Both gives full play to the advantages of the two heat sources, and makes up for each other's shortcomings. In the laser-arc composite welding of aluminum alloy, the arc can guide the laser heat source, improve the absorption capacity of aluminum alloy and the energy utilization rate in the welding process, and the surface formability of the weld is better than that of the laser self-fusion welding. In addition, the introduction of the arc can greatly reduce the clamping accuracy of welding parts, and arc has a diluting effect on laser welding plasma, which can reduce the shielding effect of plasma on the laser. The laser plays an important role in the stability of the arc, so that the arc can stably act on the joint during high-speed welding, which can improve the welding quality of the joint and increase the welding speed.

There are many advantages in the application of laser welding in the automotive industry :

1) Its welding speed is fast, improves the production rhythm, and the welding speed can reach 6m/min, which has incomparable advantages over other connection methods of body in white (such as spot welding, arc welding, riveting);

2) It has small constraints on the body structure, can be applied to different welding structures (lap, corner, T-joint, butt), and is single-sided welding, where the beam can be reached, the design is more flexible;

3) Its laser welding side requirements are low, welding side can be welded in 6 ~ 8mm, compared with spot welding side requirements (16mm) reduced by half, can play a role in lightweight;

4) The laser welding structure of the roof and rear cover can reduce the body quality, and there is no need to apply sealant and no exterior trim plate, saving the body cost;

5) Laser welding weld joint is smooth and neat, good appearance, etc.

Due to the many advantages of laser welding, it has been widely concerned in the automotive industry and has been favored by many automotive companies. It is used in different parts of European models (such as Volkswagen, BMW, Audi, Mercedes-Benz, Peugeot, etc.) and American models (such as Buick, Ford, Cadillac, Chevrolet, etc.) (roof, door assembly, trunk lid outer panel, side structural parts and flow tank welding, etc.), and as one of the symbols of high-quality body in white.

However, due to the high one-time investment cost, sheet metal accuracy and tooling accuracy requirements, it is not widely used among the domestic brand automobile companies.

At present, laser welding of aluminum alloy has been applied in mass production in aluminum alloy. Audi TT, A6/A8 and Cadillac flagship CT6 are welded with aluminum alloy laser welding on the top cover and side walls. Cadillac CT6, Audi A6/A8/Q5, BMW 5 series /7 series, Mercedes-Benz S series /C series and other models of the trunk cover plate adopts aluminum alloy laser welding. The doors of Audi A6/A8, Mercedes-Benz S /C Series, BMW 5 /7 series, etc., adopt aluminum alloy laser welding. Nio also applied a large number of aluminum alloy laser welding in the cover and sidewall as well as the door.

With the continuous progress of automotive technology, industry processing capacity and processing quality, the cost of using laser welding will be greatly reduced. At the same time, with the development of automotive lightweight, the application of aluminum alloy in the automobile body is increasing, laser welding as one of the important connection methods to solve the problem of aluminum alloy welding quality, will be more widely used in the automotive industry.

Xi'an Guosheng Laser Technology Co., Ltd. is a high-tech enterprise specializing in R&D, manufacturing and sales of automatic laser cladding machine, high-speed laser cladding machine, laser hardening machine, laser welding equipment and 3D printing equipment.

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