2004 Volume 22 Issue 3 Pages 389-397
Mechanisms of high melting coefficient and bead formation in welding aluminium-magnesium alloy with AC pulsed MIG welding process are investigated. In electrode negative polarity cathode spots clime up to the solid part of wire tip for the existence of oxide and arc root covers uniformly a large area on the surface of wire tip. This polarity prevents a droplet on wire tip from being overheated in part so that the heat transferred from arc plasma to wire tip is effectively used for wire melting. Moreover, as electrode negative ratio increases at a certain welding current, arc plasma transfers more heat to wire tip at the cost of reducing the heat transferred to base metal. As a result, wire melting coefficient becomes greater at a higher electrode negative ratio.
Since the heat input to base metal is a sum of the heat transferred from arc plasma to base metal and the heat brought into weld pool by droplet transfer, it does not change obviously as electrode negative ratio changes drastically at a certain welding current. At a constant wire feed rate, heat input to base metal and penetration depth decrease as electrode negative ratio increases mainly due to the decrease of welding current. Besides, reinforcement becomes high and bead width becomes narrow as electrode negative ratio increases because of the lower heat input to base metal and reduction of heat content of droplets. Accordingly, AC pulsed MIG welding process can successfully solve the burn through problem in welding thin sheet joints and greatly improve the gap bridging ability. Moreover, thin sheet joints can be welded with high speed and small distortion.