Hydrogen solubility drop
The abrupt drop in hydrogen solubility in aluminum during solidification, which easily leads to weld porosity.
Aluminum absorbs hydrogen in the liquid state. When cooling and solidifying, the metal can no longer hold the hydrogen, it forms bubbles and remains as pores in the weld. This happens especially when the melt solidifies quickly and the bubbles have no chance to escape.
Pores make the weld leaky and weaken it. The main cause is almost always moisture or dirt – whether on the workpiece, filler wire, or shielding gas. Therefore: everything must be clean and dry before you start. Remove oxide layers mechanically before welding, preheat the workpiece slightly if needed to drive off moisture, and work with dry, clean shielding gas.
If you guide the weld calmly and controlled, the gas bubbles have more time to rise out of the melt. For critical parts, use a filler material that inherently brings little hydrogen.