Formation Mechanism of Lack of Fusion in MAG Welding

Abstract

This paper deals with the reason and the prevention of the lack of fusion formation in MAG welding.
The results obtained are as follows:
(1) The lack of fusion which is the plane defect appears in the weld metal near weld bond, and the inside of the lack of fusion consists of oxides of Mn, Si.
(2) The lack of fusion is formed when the solid surface appears frequently in molten pool. This solid surface becomes wider with an increase in welding speed, and the humping bead is formed when the solid surface spreads out all over the molten pool.
(3) The motive force which forms the solid surface is the arc pressure. Addition of helium into the shielding gas is very effective to prevent the lack of fusion. With addition of helium gas, the arc pressure decreases, and the critical welding speed at which the lack of fusion appears can be shifted to the high speed zone in comparison with argon gas.
(4) The arc fluctuation also promotes the formation of solid surface. The arc fluctuation appears when the metal transfer is globular type in high arc voltage and low welding current range.