Porosity in laser welds has three distinct causes and three different fixes

Porosity is the most common defect in laser welds and the most frequently misdiagnosed, because voids from different mechanisms look alike after the fact. Distinguishing them matters, since the corrective actions point in different directions.

Keyhole instability porosity

In deep penetration welding, the keyhole is a dynamic cavity held open by vapour pressure. It fluctuates, and if it collapses momentarily the trapped vapour becomes a void as the melt solidifies around it.

These pores are typically larger, irregular, and concentrated toward the weld root. The fix is stabilising the keyhole: adjusting power, travel speed, focus position, or using beam shaping to broaden the keyhole and make collapse less likely. Notably, reducing power sometimes helps, which is counterintuitive if the assumption is that porosity means insufficient energy.

Dissolved gas porosity

Hydrogen and nitrogen dissolve in molten metal and come out of solution as it cools. In aluminium this is a well-known problem: hydrogen solubility drops sharply on solidification, and the rejected gas forms fine, spherical, distributed pores.

Sources are surface moisture, hydrated oxide layers, lubricants and inadequate shielding. The fix is on the material side: cleaning, oxide removal shortly before welding, dry storage, and shielding that excludes atmosphere from the solidifying pool. Process parameters barely help.