For a long time, aluminum alloy laser welding has been a difficult and hot area for scientific research institutes and enterprises to continue to carry out technical research and application. With the continuous growth of market demand for lightweight structures and the gradual development of high-power lasers and laser head products, The cost of laser welding systems is on a downward trend.

In this context, the key bottleneck that restricts the expansion of aluminum alloy laser welding applications will shift from cost input to welding technology.

Breakthroughs in new or difficult-to-weld aluminum alloy materials and laser welding technology in specific applications with complex structures of thick plates will become aluminum alloy laser welding new technology.

1 Aluminum alloy laser welding characteristics
With the continuous emergence of high-power and high-performance laser welding machines, aluminum alloy laser welding technology has developed rapidly and has become the most promising aluminum alloy welding method. This method has distinct characteristics in terms of welding heat cycle, chemical metallurgy, production efficiency, and weld shape.
Aluminum alloy laser welding has the following four advantages:
1) Welding specific energy is small. Welding specific energy refers to the energy required to weld a unit surface. Comparing the welding specific energy of laser welding and argon arc welding, it can be found that the specific energy of aluminum alloy laser welding is small, and the heat affected zone is small.
2) Welding deformation is small. The spot diameter of the focused laser beam is very small, so that the laser beam has a small action area on the material, the heat-affected zone is relatively small, and the deformation is relatively small.
3) Production efficiency is high. The laser spot diameter is small, the power density is high, allowing high welding speed, good weld quality and high production efficiency.
4) The grain is fine. In the laser welding process, the peak temperature of the weld is high, the high temperature residence time is short, the cooling rate is fast, the degree of undercooling is large, and the weld structure is fine.

Due to the physical properties, chemical composition and structural characteristics of aluminum alloy, this method also has some problems, mainly in the following five aspects.
1) The reflectivity is high. When the initial temperature of the incident surface of the beam is low, the reflectivity of aluminum alloy laser welding is very high. As the temperature increases, the surface of aluminum alloy melts and vaporizes, and its absorption rate increases rapidly.
2) Easy to produce pores. The laser welding seam is deep and narrow, and the cooling speed of the aluminum alloy is fast, and the pores do not have time to overflow.
3) Welding assembly requirements are high. After the laser is focused, the spot diameter is very small. For a laser welding machine without swing function, the beam is easily shifted on the weld seam, which may easily lead to poor weld formation.
4) The alloying element burns out. Elements such as Mn, Mg, and Zn in aluminum alloys have a strong ability to absorb laser light, which will cause evaporation and burning, resulting in a decrease in weld performance.
5) Welding spatter. The reflection effect of aluminum alloy on the laser is obvious. In order to ensure the effective penetration, the parameters of higher power are usually selected for welding. The high energy density and high peak temperature make it easy to generate spatter when a large amount of steam passes through a narrow space in a short period of time, especially in high-power pulsed laser welding.

2 Development of Aluminum Alloy Laser Welding Technology
2.1 Oil pipeline
1) The aluminum alloy oil pipeline can effectively increase the diameter of the pipeline and the thickness of the pipe wall, so that the pipeline can transport more oil in the rated time. Pipeline transportation of petroleum is very dangerous and prone to leakage. Once a leakage occurs, it may cause immeasurable property losses, casualties, environmental pollution, water pollution and other major hazards.
2) When welding aluminum alloy pipes, it is the key to improve welding quality and avoid welding defects.
3) Laser welding aluminum alloy pipes can produce very large benefits, and at the same time can effectively control the welding quality. During welding, it can ensure that the weld seam is formed once and the welding quality is relatively high, which can avoid the oil leakage hazard caused by weld seam defects.

2.2 Laser welding of aluminum alloy battery case for new energy vehicles
In the new energy vehicle industry, due to the increase in the weight of battery packs, there is a higher demand for lightweight structures. Therefore, compared with carbon fiber reinforced composite materials with higher costs and high-strength steel with higher density, aluminum and aluminum alloys are undoubtedly important.
It has become the material of choice for various battery shell structures, from battery shells and tabs, modules and connectors, to battery trays, aluminum alloy plates, profiles, and cast aluminum alloys have been widely used.
Square shell cells are the most popular products for aluminum alloy laser welding applications, including shell seals, explosion-proof valves, poles, liquid injection holes and soft connections, etc. The materials used include pure aluminum and 3-series aluminum alloys, with good weldability, especially Under the oscillating laser welding process, almost defect-free welded joints that meet the sealing conditions are formed.
The above process adopts conventional fiber laser and scanning galvanometer welding head, which can realize high-quality and high-efficiency laser welding. At present, a complete customized laser welding production line equipment has been formed in the market. New energy vehicle battery modules and battery trays have a high degree of individualization, and mainly use 6-series aluminum alloys with high strength, and some use 5-series aluminum alloys.

Currently, MIG welding process and friction stir welding technology are mainly used. According to the different needs and design features of the product, there are roughly two types.
The first type is the non-load-bearing module battery case, which is characterized by the existence of aluminum alloy plates with a thickness of ≤1.5mm, and has no sealing requirements for the overall structure. And it is welded in the form of lap penetration welding, butt joint, lap fillet welding, etc..
Using a single laser or a swing laser can meet the needs of melting depth and melting width. The requirements for this type of product are relatively simple, so the process is not difficult, and has been applied in production.
The technical solutions are mainly provided by laser head manufacturers and laser system integrators. However, due to the use of single laser welding, the requirements for product assembly clearance are relatively high, so the consistency of welding quality is greatly affected by the dimensional accuracy of incoming materials and the clamping process.

The second type is that the product has sealing requirements, and some requirements must withstand pressure holding conditions for a certain period of time. The thickness of the sheet is usually 3~5mm, and the assembly with aluminum alloy profiles involves butt joints, corner joints, lap joints, etc.
Due to the smaller size of the product compared with the battery tray and relatively low service conditions, both the manufacturer and the user intend to upgrade the welding process from MIG welding to laser welding. At present, it is in the stage of laser welding process exploration and testing, which is mainly carried out by scientific research institutes, laser suppliers, and parts manufacturers.

2.3 Application of aluminum alloy laser welding technology in ship construction
Due to the low density and lighter weight of the aluminum alloy material, the center of gravity of the ship is lowered while reducing the weight of the ship, which is conducive to improving the safety and stability of the ship. Therefore, it is widely used in some ships such as yachts, cruise ships, submarines, fishing boats, etc. The application of laser welding in the shipbuilding industry is also relatively common. Due to the large size of the hull, welding has played a major role in the shipbuilding industry. The use of laser welding is conducive to obtaining high-strength weldments, thereby reducing the thickness of the aluminum alloy used. To achieve the purpose of light weight and high strength. The United States has calculated that an aircraft carrier built with laser welding technology can save 200 tons of weight. In fact, when you build these large cruise ships in Europe today, the application rate of laser welding has exceeded 20%, and the target utilization rate in the future is 50%.

2.4 Laser welding of aircraft aluminum alloy panel structure

The lightweight of aviation aircraft plays an important role in reducing fuel consumption, increasing cruising range, and prolonging the life of aircraft. Compared with titanium alloys and carbon fiber composite materials, the cost of aluminum alloys is relatively low. Therefore, in the manufacture of aircraft fuselages, aluminum alloys account for a large proportion, mainly 7-series, 6-series, and 2-series aluminum alloys. In the application of the connection between the skin of the fuselage panel and the stringers, the traditional method uses riveting technology, and the skin and stringers adopt a lap joint structure. Due to the extra weight generated by the overlapping edge of the rivet and the stringer, and the production efficiency is low, the stringer and the skin are changed to a T-shaped structure, and laser wire welding is performed on the left and right sides simultaneously to replace the overlapping edge With rivets, it has obvious effects on reducing the weight of the fuselage, improving the connection efficiency and reducing the manufacturing cost. For example, the eight panels of the Airbus A380 model are manufactured using double-sided laser simultaneous welding technology, which reduces the weight of the fuselage by 10%.

To sum up, although aluminum alloy welding has the problems of complicated process and difficult processing, with the increasing demand of aluminum alloy in the process of industrialization and the role it plays in life is becoming more and more important.
With the rapid development of laser welding technology, the application of new welding technology in aerospace welding production will be developed rapidly. Welding automation and the ability to guarantee high quality and high reliability will be the basic requirements of welding technology in the 21st century.
Therefore, when improving and optimizing the welding process of aluminum alloys, it is necessary to fully consider the cost, the advanced degree of the method, and the welding efficiency, in order to suppress the occurrence of pores and cracks to the greatest extent, control the defects of the joints, and form a good weld with excellent quality.