Defects in Direct Liquid Metal Deposition of Aluminium Alloys

Common metal additive manufacturing technologies require intensive energy sources such as lasers to achieve the melting and bonding of a metallic feedstock. This leads to a sizeable carbon footprint, especially for reflective materials such as aluminium. Moreover, it induces complex and turbulent melt pool dynamics which lead to stochastic formation of defects which are, hence, difficult to suppress. We explore an alternative approach, which is similar to what is now practiced on a large scale with thermoplastic polymers, namely Fused Deposition Modelling. In our technology, a metallic wire is fed through a nozzle and melted using resistive heating. It then exits the nozzle producing a small drop which bonds and solidifies onto the previously deposited layer. This is challenging for metallic materials due to their combination of low viscosity and high capillary forces, which induce instabilities such as bulging, i.e. the breakup of deposited lines into droplets, or the propensity of metals for chemical reaction, in particular with air. To overcome these instabilities, we take advantage of the 1-5 nm thick passivating oxide skin which forms rapidly on the surface of liquid aluminum alloys. This oxide layer is stable and prevents, below a certain size scale, the flow of metal in ways that would increase its surface area. Aluminium alloy wire is used to manufacture walls using this direct liquid metal deposition process and different defect formation mechanisms, which include the bulging instability or the evolution of hydrogen pores, are studied. It is found that these defect formation levels can be reduced using a combination of proper wire preparation and storage, coupled with appropriate process parameter selection.