Cold metal transfer wire arc direct energy deposition of Mg alloys: A review

In the quest for designing large scale AM of Mg alloy components in automotive and aerospace sectors, wire arc direct energy deposition (WDED) is currently emerging as a popular process due to its enhanced scalability, high production rate, inexpensive setup for fabrication and the flexibility to integrate with a spectrum of auxiliary processing techniques. However, WDED results in higher heat input into the component being built thereby causing substantial accumulation of thermal energy during layer-by-layer deposition. This heat accumulation leads to thermal gradients, which in turn leads to build up of residual stresses, poor surface finish, distortion and the formation of large columnar grains in the finished component. Among the existing pursuits for mitigating heat accumulation, cold metal transfer (CMT) based WDED is most promising. Here we first describe state of the art in CMT WDED of Mg alloys emphasizing practical aspects and physics of this process, followed by a brief description of general trends in Mg alloy solidification with regard to cooling rate. Next, recent investigations on microstructural characteristics of Mg alloys in the as deposited condition are presented. Subsequently, the role of supplementary in situ / ex situ processing techniques and post process heat treatments in refining microstructure for enhanced mechanical performance are discussed. Finally, potential unaddressed research directions in this topic are summarized.