Laser welding technology

　　 In recent years, laser welding technology, which has been widely used in European and American car factories, has also made a series of targeted improvements for the "new member" of aluminum alloy.

　　 With the addition of alloying elements, eight groups of wrought alloys appeared, extending the overall application of aluminum to a wide range of manufacturing applications. However, regardless of the alloy or the overall application, there are still solderability problems. Fortunately, most alloys can be successfully welded, depending on the alloy filler material. The use of lasers can solve the problems that plague traditional technologies such as metal inert gas arc welding. Compared with metal inert gas arc welding, laser processing has faster welding speed, less heat input, smaller heat-affected area, less distortion, and in many cases self-welding.

　　 However, aluminum and aluminum alloys still have some tricky properties, which can affect welding if not handled properly. The wide range of alloy evaporation and solidification temperatures can cause keyhole instability, porosity, bubbles, loss of mechanical properties, and various defects in welding metallurgy, such as hot cracks. The high degree of hydrogenolysis of molten aluminum can cause a large number of weld pores and bubbles. Molten aluminum with low viscosity and high fluidity can cause sedimentation and sagging at the bottom of the weld bead. Finally, the high reflectivity of aluminum combined with high thermal conductivity can cause light energy to couple to the material. Although the above sounds very frustrating, the history and success stories of laser welding of aluminum are actually the opposite. These thorny features and related welding problems have clear and proven solutions. Let us briefly understand the five most common problems, mechanisms and control measures.

　　 Thermal cracks or welding solidification cracks are the result of solidification pressure acting on the microstructure. The high thermal diffusivity and thermal conductivity of aluminum will exacerbate these cracks. Generally, using suitable filler wires or inlay filler foil materials to change the welding performance and avoid crack sensitivity peaks can avoid thermal crack sensitivity. For example, to obtain good solderability, the typical values of adding silicon and magnesium are greater than 2-3% and greater than 3-4%, respectively. The typical range of these alloys in 2000 series and 6000 series aluminum alloys is 0.4-1.6%, which means that in most cases these alloys require fillers to achieve crack-free welding.

　　 In the past, the high reflectivity of aluminum was a problem for laser welding. However, with the gradual development of high-power, high-beam quality carbon dioxide lasers, and the emergence of high-power, high-brightness solid-state fiber lasers, coupling energy to aluminum is no longer a problem. Here is a misconception that needs attention: Many people now think that solid-state lasers (such as disk lasers and fiber lasers) have shorter wavelengths and are more absorbed by aluminum, so they are the best choice for all applications. This is not the case. For materials with a thickness of about 4 or 5 mm, the wavelength is preferably 1 μm. But if the material thickness is above 6mm, a carbon dioxide laser (10.6μm wavelength) is better. Although the exact physical effect is still controversial, the simple explanation is that the higher absorption rate means that the upper part of the material absorbs more energy from the 1μm wavelength. With a carbon dioxide laser, a wavelength of 10.6μm can be reflected to the keyhole, thereby penetrating the material more deeply.

　　Laser welding has been used in the automotive industry to connect a variety of aluminum parts such as frame, roof, door, trunk, steering column, wheel hub and fuel filter. One notable application is the use of laser termination (butt) welding technology to weld the aluminum doors of BMW 7 Series luxury cars.

　　Aluminum has become the material of choice for BMW designers, not only because of its light weight, but also because it can gain important experience for the application of laser welding aluminum to larger displacement cars in the future. Although the selected alloy (aluminum 5083) is a material that can be automatically welded, manufacturing engineers chose to use end joint design and laser welding, and use filler wire to keep the flange width close to the absolute minimum. This allows engineers to maximize the cross section and use the least amount of material to increase the section coefficient and moment of inertia.

　　The section coefficient of the laser welding door is 1.7 times that of the resistance spot welding door, and the moment of inertia is 2.3 times, which has greatly improved the strength and hardness. The four aluminum doors of each bridge car contain laser welded seams over 15 meters in length, which is about 30% lighter than steel doors. A tighter and more coherent laser welding seam also has the advantage of not requiring adhesives, which further reduces weight and costs.

　　Manufacturers regard aluminum as the ideal metal for their production applications, mainly because of its mass-strength ratio and corrosion resistance. Most aluminum alloys can be fusion welded (with or without fillers), and some common welding problems have been overcome by effective methods in production. Since the 1990s, many industries have used laser welding in production of a large number of aluminum and aluminum alloy parts. The BMW 7-series luxury sedan is a good example, and the vision for the future is that factors such as laser processing, strength, light weight, and cost all converge to create an elegant solution. With the mandatory implementation of fuel economy in the automotive industry, the trend of lightweight cars is inevitable. Aluminum will definitely become an important part of lightweight, and because of its own advantages and performance, laser welding will also enjoy the same status.