Due to the increasingly prominent pressures such as environmental protection, energy conservation and emission reduction, carbon peaking, and carbon neutrality, lightweight will be an inevitable trend in automobile manufacturing. The concept of lightweight originated in motorsports, its advantage is that under the premise of maintaining safety performance, weight reduction can bring better handling and acceleration; and the realization of lightweight manufacturing of automobiles is mainly through: structural design, new material application and The new materials are mainly non-ferrous metals such as aluminum and magnesium.
In automobile manufacturing, materials such as aluminum alloys and aluminum-steel composite structures have replaced traditional steel materials in key components, while magnesium alloys, as a new type of structural material, account for a relatively small proportion of automobile manufacturing. At present, each car in Europe and the United States uses 5.8-23.6kg of magnesium alloy parts, and the consumption of a single car in my country is less than 10kg. The reason is that the difficult welding of magnesium alloys is a key technical problem that restricts the large-scale application of magnesium alloy auto parts.
It is very difficult to achieve high-quality welding of magnesium alloys by fusion welding for the following reasons:
1. Due to the strong oxidizing property of magnesium, it is easy to form an oxide film (MgO) during the welding process, and it is easy to form inclusions in the weld, reducing the performance of the weld. At high temperature, magnesium is also easy to chemically react with nitrogen in the air to form magnesium nitride, which weakens the performance of the joint.
2. The boiling point of magnesium is not high, which will cause it to evaporate easily under the high temperature of the arc.
3. Due to the high thermal conductivity, high-power heat source and high-speed welding are used when welding magnesium alloys, which is easy to cause overheating and grain growth of the metal in the weld and near-weld areas.
4. The thermal expansion coefficient of magnesium alloy is large, which is about 1 to 2 times that of aluminum. It is easy to produce large welding deformation during the welding process, causing large residual stress.
5. Since the surface tension of magnesium is smaller than that of aluminum, it is easy to cause the weld metal to collapse during welding, which affects the quality of the weld formation.
6. Similar to welding aluminum alloys, hydrogen holes are easily generated when magnesium alloys are welded. The solubility of hydrogen in magnesium decreases with the decrease of temperature, and the density of magnesium is smaller than that of aluminum, so the gas is not easy to escape, and pores will be formed during the solidification of the weld.
7. Magnesium alloys are easy to form a low melting point eutectic structure with other metals, and crystalline cracks are easy to form in welded joints. When the temperature at the joint is too high, the low-melting compound in the joint structure will melt at the grain boundary to form cavities, or produce grain boundary oxidation, which is the so-called "overburning" phenomenon.