Galvanized steel typically involves coating a layer of zinc, usually 20 microns thick, onto low-carbon steel. The melting point of zinc is approximately 419°C, with a boiling point of around 908°C. During welding, zinc melts into a liquid state and floats on the surface of the weld pool or at the root of the weld seam. Zinc has a high solid solubility in iron, and the liquid zinc can penetrate and erode the weld metal along grain boundaries, leading to "liquid metal embrittlement" caused by the low melting point of zinc.
Furthermore, zinc and iron can form intermetallic brittle compounds, which reduce the plasticity of the weld metal and can result in cracks under tensile stress.
When welding fillet welds, particularly those in T-joints, penetration cracks are most likely to occur. During the welding of galvanized steel, the zinc layer on the groove surface and edges, under the heat of the arc, undergoes oxidation, melting, evaporation, and even volatilization, releasing white smoke and vapor, which can easily cause porosity in the weld seam.
ZnO formed by oxidation has a high melting point, exceeding 1800°C. If welding parameters are set too low, it can lead to ZnO inclusions. Additionally, as Zn acts as a deoxidizer, it can produce FeO-MnO or FeO-MnO-SiO2 low-melting-point oxide inclusions. Moreover, the evaporation of zinc generates a large amount of white smoke and dust, which is irritating and harmful to the human body. Therefore, it is essential to grind and remove the galvanized layer from the welding area.
