Wire Arc Additive Manufacturing of Aluminium Alloys

Abstract

Summary Additive manufacturing of metals has gained considerable attention from the industry and academia in the last decade. The attributes to create stand-alone components with novel designs and enhanced properties are main drivers to implement additive manufacturing into the industrial value chain. Wire arc additive manufacturing, hereafter referred to as WAAM, utilise the inherent properties of arc welding to manufacture components. WAAM is considered a high efficiency, low-cost method with large build envelopes. Big components can therefore be made in a reasonable amount of time. The method is highly suited for a range of industrial sectors for manufacturing of all kinds of metals, including aluminium. The WAAM process is still maturing for industrial exploitation. One objective for further development of WAAM is to benchmark the process against established forming processes. One of these processes is casting. Thus, two different casting methods, i.e., steel mould casting and sand mould casting, were compared against WAAM. Al-Si alloys with near-eutectic composition were used as reference. The materials were assessed in terms of porosity content, secondary dendrite arm spacing, grain size, tensile strength and hardness. The Al-Si alloy processed by WAAM showed superior properties compared to both casting methods. The enhanced properties were accounted for by a higher cooling rate during solidification and hence a finer microstructure. The comparative study demonstrated that a fine microstructure is key for enhanced properties. A finer microstructure implies better mechanical integrity in terms of strength, ductility, anisotropy, among others. Extensive grain refinement can also depress the cracking susceptibility that pester several aluminium alloys during solidification. Thus, measures to refine the microstructure of WAAM aluminium alloys were investigated. In particular, addition of ceramic nanoparticles to refine the aluminium microstructure and to provide particle strengthening was identified as a promising solution. In an attempt to pursue this solution, a new production route for aluminium alloys reinforced with ceramic nanoparticles was developed. The novel solid-state process metal screw extrusion was examined as an all-in-one solution to mix, disperse and extrude wires for WAAM. Produced materials through metal screw extrusion before and after WAAM deposition were examined by a range of techniques. This included light optical microscopy, scanning electron microscopy, hardness testing, computed tomography, tensile testing and hydrogen measurements. Alloys of AA4043 Al-Si, AA5183 Al-Mg, and AA6082 Al-Mg-Si were mixed with nanoparticle additions (TiC, TiN, TiO$_2$) through metal screw extrusion. The ceramic compounds did not survive the melting cycle in WAAM, and various morphologies of the Al$_3$Ti intermetallic were observed. In high-silicon alloys like AA4043, TiC decomposed to Ti$_7$Al$_5$Si$_{12}$. AA5183 mixed with TiC exhibited significant grain refinement after WAAM, due to heterogeneous nucleation on Al$_3$Ti. Remaining TiC provided particle strengthening. All investigated materials with ceramic additions exhibited a highly porous structure after WAAM. Thermogravimetric measurements of nanoparticle powders showed significant mass loss at elevated temperatures, indicating presence of volatile compounds such as moisture. Metal screw extruded materials were therefore produced with an elevated content of hydrogen, which is the primary cause for porosity in aluminium alloys. Vacuum heat treatment of nanoparticles prior to metal screw extrusion was investigated but failed to lower the pore density after WAAM. Measures to protect the input materials and improve the material quality are briefly discussed in this work. In summary, WAAM of aluminium alloys shows great potential for future industrial implementation. WAAM have attractive attributes which include reasonably high deposition rate, large design freedom, and sound mechanical properties. However, the properties can be further enhanced, and the aluminium alloy selection must be expanded. Addition of ceramic nanoparticles to enhance strength and refine the microstructure is a promising, yet challenging, solution. Metal screw extrusion show promising results to produce wires with low carbon footprint and with novel compositions and mixtures. As WAAM requires high-quality materials for successful deposition, the metal screw extrusion setup needs further development related to protection of the input materials