Effect of plasma current on surface defects of plasma-MIG welding in cryogenic aluminum alloys

An asymmetric undercut and black-colored deposit (smut) are created on the surface of Plasma-MIG Hybrid (PMH) weld in Al alloys. For the purpose of defect formation control, the effect of plasma current on surface defects in the PMH weld was investigated through arc phenomena with high speed imaging and metallurgy analysis as compared with MIG weld. It was found that the asymmetric undercut is created near the left fusion line of PMH welds by the electromagnetic stirring force induced by high plasma currents over 200 A. As the plasma current increased, three types of smut appear in the welding bead: Smut outside the welding Bead (SOB) on the MIG weld, Smut spots on the welding bead (SS) and Smut inside the welding Bead (SIB) on the PMH weld. Three types of smut formed changed from SOB, to SS, and finally to SIB. The smut formation area moved from the base metal onto the welding bead. The amount of smut formed on the PMH weld is less than that of MIG weld. Smut on MIG and PMH weld have an identical chemical composition including aluminum, magnesium, oxygen and carbon from the base metal and the weld metal. The carbon is from a graphite lubricant on the surface of Al 5183 wire. SOB is generated by the arc generated between the wire and edge of the weld pool after short circuit transfer, as well as droplet explosion transfer depending on MIG current. SS is formed by the combination of graphite layers remained on the droplet and the weld pool. SIB is generated by a combination of the MIG welding wire feeding point, the asymmetric undercut and metal vapor from a droplet explosion transfer under a strong plasma forces induced by a plasma arc with plasma currents over 200 A. Consequently, the plasma current influences the weld pool flow and type of smut by changing the metal transfer mode and strong plasma arc forces. (C) 2015 Elsevier B.V. All rights reserved.