(1) Coarse crystal
Magnesium has a low melting point and high thermal conductivity. A high-power welding heat source is required during welding. The weld and near-seam areas are prone to overheating, grain growth, crystal segregation and other phenomena, which reduce joint performance.
(2) Oxidation and evaporation
Magnesium is extremely oxidizing and easily combines with oxygen. It is easy to form MgO during the welding process. MgO has a high melting point (2 500 ℃) and high density (3. 2 g/cm-3), and it is easy to form small flakes in the weld. Solid slag inclusions not only seriously hinder the formation of the weld, but also reduce the performance of the weld. At high welding temperatures, magnesium can easily combine with nitrogen in the air to form magnesium nitride. Magnesium nitride slag inclusion will also cause a decrease in the plasticity of the weld metal and worsen the joint performance. The boiling point of magnesium is not high (1100 ℃) and it is easy to evaporate under the high temperature of arc.
(3) Burning through and collapse of thin parts
When welding thin parts, due to the low melting point of magnesium alloy and the high melting point of magnesium oxide, the two are not easily fused, making it difficult to observe the melting process of the weld seam during welding operations. As the temperature rises, the color of the molten pool does not change significantly, making it prone to burning through and collapse.
(4) Thermal stress and cracks
Magnesium and magnesium alloys have a relatively high coefficient of thermal expansion, about twice that of steel and 1 Twice, it is easy to cause significant welding stress and deformation during the welding process. Magnesium easily forms low melting point eutectic with some alloying elements (such as Cu, Al, Ni, etc.) (such as Mg Cu eutectic temperature of 480 ℃, Mg Al eutectic temperature of 430 ℃, Mg Ni eutectic temperature of 508 ℃), with a wide brittle temperature range and easy formation of hot cracks. Research has found that when w (Zn)>1%, it increases thermal brittleness and may lead to welding cracks. Adding w (Al) ≤ 10% to magnesium can refine the grain size of the weld and improve weldability. Magnesium alloys containing a small amount of Th have good weldability and no tendency to crack.
(5) Stomata
Hydrogen pores are easily generated during magnesium welding, and the solubility of hydrogen in magnesium also decreases sharply with decreasing temperature.
(6) Magnesium and its alloys are prone to oxidation and combustion during welding in an air environment, and require inert gas or flux protection during fusion welding·