What are the factors that affect the performance of laser welding?

Defects in copper alloy laser welding
1. Welding stress
The coefficient of linear expansion and shrinkage of copper are also relatively large. The coefficient of linear expansion of copper is 15% higher than that of iron, and the shrinkage rate is more than twice that of iron. In addition, copper and copper alloys have strong thermal conductivity, which widens the welding heat affected zone. If the stiffness of the welded part is not high and there are no measures to prevent deformation during welding, significant deformation will inevitably occur. When the workpiece stiffness is high, significant welding stress will be generated.

2. Hot cracking
Copper forms various low melting point eutectic with impurities, such as (Cu+Pb) eutectic with a melting point of 326 ℃, (Cu2O+Cu) eutectic with a melting point of 1064 ℃, and (Cu+Cu2S) eutectic with a melting point of 1067 ℃. Oxygen is the most harmful to copper, as it not only exists in the form of impurities during smelting, but also dissolves in the form of cuprous oxide during welding. Cu2O is soluble in liquid copper but insoluble in solid copper, resulting in a melting point lower than that of copper and a fusible eutectic. When the weld seam contains Cu2O with a mass fraction of 0.2% or more (oxygen content of about 0.02%), warm lines will appear.

3. Hydrogen pores
The thermal conductivity of copper (20 ℃) is more than 7 times higher than that of low-carbon steel, so the crystallization process of copper welds is particularly fast. Hydrogen is not easy to precipitate, and the molten pool is easily saturated with hydrogen to form bubbles. When the solidification crystallization process is fast, bubbles are not easy to float and escape, and hydrogen continues to expand into the bubbles, promoting the formation of pores in the weld.

The solubility of hydrogen in copper increases with temperature until it reaches its highest value (saturation solubility) at 2180 ℃. As the temperature further increases, liquid copper begins to evaporate, and the solubility of hydrogen actually decreases. When welding copper, the hydrogen absorption capacity of the high-temperature molten pool is 3.7 times the melting point solubility; The welding process cools quickly, and even without considering the influence of copper thermal conductivity, the hydrogen absorbed in the high-temperature molten pool is not easily precipitated and becomes supersaturated during the cooling process. In order to eliminate diffusion pores, the source of hydrogen should be controlled during welding, and the cooling rate of the molten pool should be reduced (such as preheating) to make the gas easy to precipitate.

Reaction pores are caused by gases generated through metallurgical reactions. At high temperatures, copper has a strong affinity for oxygen and generates Cu2O. It can dissolve in liquid copper at temperatures above 1200 ℃, and begins to precipitate from liquid copper at 1200 ℃. As the temperature decreases, the amount of precipitation increases. The following reactions occur with hydrogen or CO dissolved in liquid copper: Cu2O+2H=2Cu+H2O ↑ Cu2O+CO=2Cu+CO2 ↑ The water vapor and CO2 formed are insoluble in copper. Due to the strong thermal conductivity of copper, the molten pool solidifies quickly, and water vapor and CO2 cannot escape in time, forming pores. When the oxygen content in copper is low, the possibility of the above-mentioned reaction pores occurring is very low. Oxygenated copper is more sensitive to the reaction pores mentioned above than deoxygenated copper. The main way to prevent reaction pores is to reduce the sources of oxygen and hydrogen and appropriately deoxygenate the melt. In addition, taking measures to slow down the melting pool can also prevent porosity.

4. Bottom porosity
This type of porosity is mainly formed by the collapse of the keyhole during laser deep penetration welding, or by the gasification of low melting point impurities below the welding point.

5. Coarse columnar crystals
The microstructure of the fusion zone is coarse columnar crystals, followed by the heat affected zone, and the microstructure of the base material is the thinnest. During the welding process, the base metal grains undergo recrystallization at high temperatures to form fine equiaxed grains. The fusion zone is heated at a higher temperature, and the grains grow rapidly at higher temperatures, forming large columnar crystals. The heat affected zone is far from the center of the weld and is cooled by the base metal, resulting in smaller grains than the fusion zone; The base material still has a fine cold-rolled grain structure. The structure of the fusion zone is the largest and the mechanical properties are the worst in the entire weld seam. When the weld is subjected to external forces, it fractures first.