Hot Cracking of Metal Inert Gas Arc Welded Magnesium Alloy AZ91D

Abstract

The microstructure and cracking characteristics of MIG welded magnesium alloy (AZ91D) joint, and the effect of welding speed on cracking susceptibility have been investigated. The welded joint consists of primary a-Mg and divorced phases (eutectic a-Mgϩeutectic b-Mg 17 Al 12 ), the latter mainly distributing along the a-Mg grain boundaries. Solidification cracking often occurred in the crater and was also observed at weld center line when welding speed was 300 mm/min, which are associated with segregation of Mn, Al and Zn and high tensile stresses in the welds. Liquation cracking appeared in HAZ immediately adjacent to the fusion line when low welding speed was used (300 mm/min). It is mainly related to the low welding speed resulting in increasing heat input and tensile stresses in the HAZ. It is favorable to decrease heat input for improving the susceptibility of hot cracking during MIG welding of magnesium alloys.

Characterization of Hot Cracking Due to Welding of High-Strength Aluminum Alloys

“…The other way to decrease the sensitivity to hot cracking and thus to increase the weldability is to add a grain-refinement element, such as Zr, Ti, Sc [8][9][10][11]. There has been much research on aluminum hot cracking, and several methods have been devised to test the hot cracks, including the Varestraint test [12,13], Fissure Bend, the Circular Patch test, the Houldcroft test [14,15], the Hot Ductility test, and the Strain-Induced Crack Opening test [16,17], of which the Varestraint test is the most popular method. The advantage of this method is that both welding parameters and stress can be varied independently, which is useful to investigate the metallurgical factors and mechanical properties on the material hot cracks.…”

Welding of magnesium and its alloys: an overview of methods and process parameters and their effects on mechanical behaviour and structural integrity of the welds

“…Postweld heat treatment (PWHT) improved the plasticity of the weld zone by 16% an improved microhardness profile shown in Figure 2. Using welding voltage of 20-22 V, welding current of 60-120 A and welding speed of 5 mm s À1 , the AZ91D alloy was welded without cracks in the butt joints [63]. The butt joints had fine grain sizes contributing to the high strength.…”

“…Metal inert gas welding (MIG) is among the common form of welding metals. It is also known as gas metal arc welding (GMAW) which requires the melting and solidification of base metals together with a wire electrode [63,76,77]. Improvement initiatives for Mg-based alloys are to minimize porosity rate, improve mechanical properties of joints, understand mechanisms impacting porosity and other weld defects [76].…”

Welding and Joining of Magnesium Alloys

“…This is especially true for a repair practice where thick plates are welded using the metal inert gas technique. During MIG welding of 10 mm thick www.intechopen.com plates of AZ91D, two types of hot cracking occurred: solidification cracking of weld and liquation cracking in HAZ (Sun, et al, 2008). In general, solidification cracking takes place within a specific temperature range when a liquid film appears between dendrites.…”

“…1). The microstructure of MIG welded AZ91D alloy with AZ61 as welding wire consisted of solid solution of α-Mg and intermetallic compound of Mg 17 Al 12 (Sun et al, 2008). The weld contained roughly 7% of Al.…”