Analysis of Smut Formation Phenomena on MIG and Plasma-MIG Hybrid Weld of Cryogenic Al-Mg Alloy

Black deposits (smut) are created on MIG welds in cryogenic Al alloys. The smut should be removed because it ruins the appearance of the end product and affects surface treatments such as painting. It was recently reported that plasma–MIG hybrid (PMH) welding controls the formation of smut during welding. In order to determine the reason for this, smut formation using both MIG and PMH welding was investigated through metallurgy and arc phenomena analysis. Smut on MIG welds is a Mg–Al–O amorphous layer that includes nano-sized MgO particles less than 100 nm in diameter and MgO particles 1–2 µm in diameter. Smut on MIG welds is created by large amounts of metal vapor from the arc explosion generated between the welding wire and the weld pool after a short circuit transfer. However, smut on PMH welds is not created owing to the small amount of metal vapor produced from a stable globular transfer rather than a short circuit transfer and arc explosion.