Common defects in magnesium alloy welding
Because magnesium alloy has the characteristics of low density and melting point, high thermal conductivity, electrical conductivity and thermal expansion coefficient, strong chemical activity, easy oxidation and high melting point of oxide, the welding of magnesium alloy must solve the following series of problems:
(1) coarse crystal
Magnesium has low melting point and high thermal conductivity, so it is necessary to use high-power welding heat source during welding, which is easy to produce overheating, grain growth, crystallization segregation and other phenomena in the weld and near seam area, reducing the joint performance.

(2) Oxidation and evaporation
Magnesium is highly oxidizing, easy to combine with oxygen, easy to form MgO in the welding process, MgO melting point is high (2 500 ℃), high density (3.2 g/cm-3), easy to form a fine sheet solid slag in the weld, not only seriously hinder the formation of the weld, but also reduce the performance of the weld. At high welding temperature, magnesium is also easy to combine with nitrogen in the air to form magnesium nitride, and magnesium nitride slag inclusion will also lead to the plastic drop of weld metal, so that the joint property becomes bad. The boiling point of magnesium is not high (1100 ℃), and it is easy to evaporate at high arc temperature.

(3) burning through and collapse of thin parts
When welding thin parts, because the melting point of magnesium alloy is low, and the melting point of magnesium oxide is high, the two are not easy to fuse, and it is difficult to observe the melting process of the weld during welding operation. As the temperature increases, the color of the molten pool does not change significantly, and it is easy to produce burn-through and collapse phenomena.

(4) Thermal stress and cracks
The thermal expansion coefficient of magnesium and magnesium alloy is large, about 2 times that of steel and 1.2 times that of aluminum, which is easy to cause greater welding stress and deformation in the welding process. Magnesium is easy to form low melting point eutectic crystals with some alloying elements (such as Cu, Al, Ni, etc.) (such as Mg-Cu eutectic point temperature of 480 ℃, Mg-Al eutectic point temperature of 430 ℃,Mg-Ni eutectic point temperature of 508 ℃), brittle temperature range is wide, easy to form hot cracks. It is found that when w (Zn) > 1%, it will increase the thermal brittleness and may lead to welding cracks. Adding w (Al) ≤ 10% to magnesium can refine the weld grain and improve the weldability. Magnesium alloys containing a small amount of Th have good weldability and no crack tendency.

(5) Stomata
It is easy to produce hydrogen pores when welding magnesium, and the solubility of hydrogen in magnesium also decreases sharply with the decrease of temperature.

(6) Magnesium and its alloys are easy to oxidize and burn when welding in an air environment, and need to be protected by inert gas or flux when welding.