The application of magnesium alloy in automobile manufacturing can meet the development needs of low displacement and low fuel consumption in automobile industry. In order to further promote the extensive use of magnesium alloys in the automotive field, it is necessary to carry out further innovative research on welding technology.

1. Weldability analysis of magnesium and magnesium alloys
Compared with other materials, magnesium alloys have remarkable properties and special weldability. Due to the low density of magnesium alloy, low melting point, high thermal and electrical conductivity, high thermal expansion coefficient, strong chemical activity, easy oxidation and high melting point of oxide, a series of difficulties will occur in the welding process of magnesium alloy, mainly in the following aspects:

1. Oxidation and evaporation

Due to the strong oxidation of magnesium, it is easy to form an oxide film (MgO) during the welding process, and MgO has a high melting point (2500℃) and a high density (3.2g/cm3), which is easy to form impurities in the weld, reducing the performance of the weld. At high temperatures, magnesium is also easy to chemically react with nitrogen in the air to produce magnesium nitride, weakening the performance of the joint. The boiling point of magnesium is not high, which will lead to easy evaporation at high arc temperatures.

2. Coarse grain
Due to the high thermal conductivity, it is necessary to use high-power heat source and high-speed welding when welding magnesium alloys, which is easy to cause metal overheating and grain growth in weld and near-weld areas.

3. Thermal stress

The coefficient of thermal expansion of magnesium alloy is large, about 1 ~ 2 times that of aluminum, which is easy to produce large welding deformation and cause large residual stress in the welding process.

4. The weld metal is collapsing

Because the surface tension of magnesium is smaller than that of aluminum, it is easy to produce the weld metal collapse during welding, which affects the weld forming quality.

5. Stomata

Similar to aluminum alloy welding, magnesium alloy is prone to hydrogen porosity during welding. The solubility of hydrogen in magnesium decreases with the decrease of temperature, and the density of magnesium is smaller than that of aluminum, the gas is not easy to escape, and pores will be formed during the solidification process of weld.

6. Hot crack

Magnesium alloy is easy to form low melting point eutectic structure with other metals, and it is easy to form crystal cracks in welded joints. When the temperature at the joint is too high, the low melting point compound in the joint tissue will melt holes at the grain boundaries, or produce grain boundary oxidation, the so-called "overburning" phenomenon.

In addition, magnesium and magnesium alloys are easy to burn, so the protection of inert gas or flux is required during melting and welding.

Second, magnesium alloy welding status and development trend

Magnesium alloys are suitable for many welding methods, such as argon tungsten arc welding, electron beam welding, laser welding, friction stir welding, explosive welding and resistance spot welding. Regardless of the welding method, the microstructure of magnesium alloy after welding mostly contains both dendrites and equiaxed crystals. It is generally accepted that the properties of equiaxed crystals are better than those of columnar crystals or dendrites, so it is desirable to obtain small size equiaxed crystals in the solidification structure of metals, while minimizing the percentage of columnar crystals/dendrites.

1. TIG (Tungsten Argon Arc Welding)

Tungsten argon arc welding (TIG) is one of the most commonly used welding methods for magnesium alloys. Because magnesium alloys are easy to oxidize, TIG arc welding magnesium alloys usually use the cathode cleaning effect of alternating current to remove the oxide film, and DC TIG welding magnesium alloys are rarely used. However, compared with DC, the heat input of AC TIG welding is lower, the heat conduction of magnesium alloy is fast, and the weld weld depth is shallow, so there are some problems in AC TIG welding of magnesium alloy thick plate. Therefore, it is necessary to adopt multi-layer and multi-pass welding or double-sided welding when welding medium thick magnesium alloy plate, which increases the difficulty of welding and reduces the production efficiency.

The main defects of argon arc welding of magnesium alloy are porosity and porosity. In the welding process, the quantity and volume of pores can be significantly reduced by increasing the flow rate of protective gas, and the loss of magnesium content in the weld can be reduced, thus improving the mechanical properties of the joint. In addition, for the prevention and control of pores, the arc can also be kept as low as possible during welding (about 2mm) to give full play to the cathode crushing effect of the arc and stir the molten pool, so that the gas can escape the molten pool.

2. Laser welding

Laser welding of magnesium alloys is an efficient precision machining method that uses high energy density laser beam as a heat source for welding. Its research mainly focuses on laser selection (such as CO2, diode, Nd: YAG and fiber lasers), laser power, focusing position, welding speed, penetration, protective gas types and filling materials.

Nd: YAG laser and CO2 laser were used to study the laser weldability of 6 kinds of cast magnesium alloys and 4 kinds of extruded magnesium alloys. The results showed that the same composition and different composition of magnesium alloys with thickness from 2 to 8mm could be welded by laser, and very narrow weld and large penetration depth could be obtained.

The defects of laser welding magnesium alloy are mainly porosity, hot crack and solidification crack in heat affected zone. In addition, the large reflectivity of magnesium alloy to laser is also a problem that needs attention in laser welding of magnesium alloy, which makes the laser welding of magnesium alloy shallow penetration. In contrast, electron beam welding has the largest penetration, and far more than laser welding.

3. Electron beam welding

The electron beam can weld through the 30mm magnesium alloy plate, and the microstructure of the melting zone is almost all equiaxed crystals of about 10mm. Electron beam welding can avoid many welding defects, such as holes, biting edges, root hollows and wide heat-affected zones. After process optimization, such as adjusting the focusing position to the root, optimizing welding parameters, etc., the ultimate tensile strength of the weld can reach 83% (with surface stress concentration) and 96% (without stress concentration) of the base metal.

Electron beam welding is usually vacuum welding, and the volatilization of metal gas pollutes the vacuum chamber greatly. It is found that non-vacuum electron beam is very suitable for magnesium alloy welding. AZ31 wrought magnesium alloy and AM50A and AZ91D cast magnesium alloy can obtain good joints under proper welding process. The relatively high energy density allows the welding speed to reach 15m/min, so that the heat input is small and the welding efficiency is high. The weld without porosity, shrinkage and porosity can be obtained by filling the wire. The static load of the joint can be equivalent to that of the base material, and the corrosion resistance of the joint is even better than that of the base material. The high speed, high efficiency and high automation of non-vacuum electron beam welding provides a new way for the large-scale application of magnesium alloys.

4. Resistance spot welding

Resistance spot welding has become the most important welding method in the automobile industry because of its extremely low cost and stable process. Magnesium alloy has high thermal conductivity and low resistance value, so it is necessary to pass high current in a short time when resistance spot welding magnesium alloy, so that the heat production rate is much higher than the heat dissipation rate. This performance is similar to that of aluminum alloys, so spot welding equipment that can weld aluminum alloys can also weld magnesium alloys. The cost of the welding machine is directly proportional to the current load of the transformer secondary coil. Under the same plate thickness, the current required for resistance spot welding steel is much smaller than that for magnesium alloy, so the welding equipment for magnesium alloy is expensive. Welding current, welding time and electrode pressure are the three most important parameters for resistance spot welding of magnesium alloy. These three parameters can effectively control the core size and joint strength. The thermal conductivity and electrical conductivity of aluminum alloy are very high, and the welding current required is 2 to 3 times that of steel.

The core growth of magnesium alloy can be divided into three stages: inoculation, growth and stability. Within the first cycle the molten core is incubated and then begins to grow. With the growth of molten core, the conduction channel increases and the current density decreases. With the increase of electrode - plate contact area, heat dissipation increases. These two factors cause the growth rate to gradually slow down. When heat production and heat dissipation reach a balance, the molten core tends to be stable. Aluminum alloy is similar to magnesium, the incubation time is very short, almost melting in the first cycle; The steel does not begin to melt until the fifth cycle, and the simulation structure shows that the contact resistance of the bonding surface is the main reason for this difference. The difference in temperature distribution along the radial is also one of the reasons, among which steel is flatter than aluminum and magnesium, so aluminum and magnesium heat production is more concentrated, which is conducive to the formation of molten core.

Magnesium alloy spot welded joints are usually divided into 4 zones: base metal, heat affected zone, plastic ring and molten core. Recrystallization and grain growth occurred in the heat affected zone. Similar to aluminum alloy, welding heat affected zone of magnesium alloy is also prone to wildflower cracks. The adhesive surface of the plastic ring in the heat affected zone is a special area of resistance spot welding. Due to the high temperature and high pressure (electrode pressure) in this region, dynamic recrystallization often occurs at the plastic ring. There are usually two kinds of structure of molten core: columnar dendrites and equiaxed crystals.

The extensive use of magnesium alloy in automobile makes the connection technology of magnesium alloy become an urgent problem to solve the application of magnesium alloy, and the research of various welding methods will get further attention of the majority of researchers.