Will The Zinc Layer Evaporate During The Welding of Galvanized Sheet? Will There Be Pores Or Cracks After Welding?

During the welding of galvanized sheet, the zinc layer may evaporate and there may be pores or cracks after welding. Below is a detailed analysis for you:

Evaporation of Zinc Layer

The boiling point of zinc is relatively low, about 907 ℃, and high temperatures are generated during the welding process. When the welding heat source acts on the galvanized sheet, its temperature far exceeds the boiling point of zinc. Therefore, during the welding process, the zinc in the galvanized layer will quickly evaporate. Taking common arc welding as an example, the center temperature of the arc can reach up to 5000-8000 ℃. At such high temperatures, zinc will rapidly evaporate to form zinc vapor.

Reasons for pore formation

The influence of zinc vapor: If the zinc vapor formed by zinc evaporation cannot escape in time during the cooling and solidification process of the molten metal, it will form pores in the weld seam. The generation of zinc vapor increases the gas content in the molten pool, and the rapid cooling of the molten pool prevents the gas from being discharged in time, resulting in porosity defects.

Hydrogen pores: Moisture and oil stains in the welding area decompose at high temperatures to produce hydrogen gas, while zinc vapor may also react with moisture in the surrounding air to produce hydrogen gas. The solubility of hydrogen decreases sharply during the cooling of the molten pool, and if it cannot escape sufficiently, hydrogen pores will form.

Reasons for crack formation

Hot cracking: Zinc and iron will form low melting point eutectic, which will form a liquid film at the grain boundary when the weld metal cools and shrinks, weakening the bonding force between grains. When the weld metal is subjected to a certain tensile stress, it is easy to generate hot cracks in these weak areas.

Cold cracking: The welding stress generated during the welding process and the influence of zinc element on the microstructure and properties of the weld metal can increase the brittleness of the weld metal. When the weld is cooled to a lower temperature, cold cracks may occur due to stress. Especially in structures with high rigidity or when welding process parameters are not properly selected, cold cracks are more likely to occur.

Preventive measure

Removal of Zinc Layer: Before welding, methods such as mechanical polishing and chemical corrosion can be used to remove the zinc layer from the welding area, reducing the generation of zinc vapor and thus reducing the probability of porosity and cracking.

Choosing appropriate welding methods, such as laser welding, tungsten inert gas welding, and other welding methods with high energy density and relatively low heat input, can reduce zinc evaporation and welding heat affected zone, and reduce the possibility of porosity and cracking.

Control welding parameters: Reasonably adjust welding current, voltage, welding speed and other parameters to avoid excessive welding heat input, reduce zinc evaporation and overheating of weld metal, and prevent the formation of pores and cracks.

Preheating and Slow Cooling: Proper preheating of welded parts can reduce welding stress and minimize the occurrence of cold cracks. After welding, slow cooling measures should be taken, such as covering the weld with insulation material to allow the weld to cool slowly, which is beneficial for gas escape and reduces the formation of pores and cracks.

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