Effect of intermetallics on susceptibility of Mg alloy welds to liquation cracking

Abstract

The circular-patch welding test was used to study the liquation and liquation cracking of AZseries Mg alloys. A heat treatment was carried out on the as-received AZ91 alloy to dissolve the γ (Mg 17 Al 12 ) particles before welding. The circular-patch welding test was then conducted on the heat-treated, as-received AZ91 alloy using AZ61 and AZ91 filler wires. The results showed that the susceptibility of AZ91 alloy to liquation and liquation cracking was significantly reduced by the dissolution of massive γ (Mg 17 Al 12 ) particles via a heat treatment before welding as the liquation mechanism was changed. Both constitutional liquation and incipient melting occurred in the partially melted zone of the as-received AZ91 welds, while only incipient melting occurred in the heat-treated AZ91 welds.

Critical review on fusion welding of magnesium alloys: metallurgical challenges and opportunities

“… Modification of as-cast microstructure via a post casting/pre-welding homogenisation: Thermal treatment of cast alloys aiming at the dissolution of second phase particles can enhance the resistance to liquation during welding. It is shown that using a prolonged homogenisation treatment before welding of AZ91 can reduce the volume fraction of eutectic Mg 17 Al 12 , which in turn can eliminate the possibility of constitutional liquation mechanism [138] Modification of as-cast microstructure via a friction stir processing (FSP) as a pre-welding treatment: In the situation where there is no possibility to produce a crack-resistance microstructure during the manufacturing process (e.g. casting) within the bulk material, friction stir processing (FSP) can be utilised as a successful strategy for local microstructural modification of the base metal before the welding [139].…”

“… Modification of as-cast microstructure via a post casting/pre-welding homogenisation: Thermal treatment of cast alloys aiming at the dissolution of second phase particles can enhance the resistance to liquation during welding. It is shown that using a prolonged homogenisation treatment before welding of AZ91 can reduce the volume fraction of eutectic Mg 17 Al 12 , which in turn can eliminate the possibility of constitutional liquation mechanism [138] …”

Pre-weld friction stir processing enabling strong and ductile fusion welds on AZ91 magnesium alloy

“…Therefore, the deleterious effect of PMZ on the joint performance remains unchanged. In order to improve the resistance to liquation, Yuan et al [7] performed a pre-weld heat treatment to dissolve eutectic β-Mg 17 Al 12 in the as-cast AZ91. They showed that this technique is successful in eliminating constitutional liquation.…”

Effect of liquation on the tensile properties of cast Mg–9Al–1Zn Alloy fusion welds

“…However, this alloy presents some metallurgical challenges during fusion welding, as a key manufacturing process as well as a viable repair technology. AZ91 exhibits high resistance to solidification cracking, however [4], it is highly susceptible to liquation cracking during fusion welding [5,6]. Liquation cracking in the weld is due to the formation of grain boundary liquid and the inability of the liquid films to support tensile stresses which develop during weld cooling [7].…”