Simultaneous enhancement in deposition efficiency and nano-scale precipitation of high-strength AZ31 Mg alloy via water cooling assisted wire-arc directed energy deposition

The inter-layer cooling is crucial for adjusting microstructures and mechanical properties of wire-arc directed energy deposition (WA-DED) Mg alloys. In this study, AZ31 Mg alloy thin-wall components have been fabricated via WA-DED under different cooling modes: natural air cooling (AC) and side wall water cooling (WC) by an innovative WC equipment that provides water cooling on both sides of thin-wall components. The temperature variation during deposition process, microstructures and mechanical properties of AZ31 thin-wall components under different cooling modes (AC and WC components) have been systematically compared and analyzed. It shows that for thin-wall components with the same total deposition layers, the deposition time for the WC component is significantly reduced, i.e., the deposition efficiency of the WC mode has been much improved. Compared with the AC component, the size of both equiaxed and columnar grains in the WC component is finer, i.e., the average grain sizes of top regions in AC and WC components are ∼106.8 μm and ∼74.3 μm, respectively. Apart from (sub)micro-scale secondary phase particles formed during the solidification stage of molten pool, a large number of nano-sized precipitates have formed in the WC component because the cooling rate of molten pool of WC component is large enough in the post-solidification stage, while the AC component contains almost no nano-scale precipitates. Accordingly, the WC component exhibits excellent tensile properties, i.e., the yield strength and ultimate tensile strength along the build direction are ∼170 MPa and ∼250 MPa, respectively. At the same time, the elongation reaches 9.3 %. This study provides references for enhancing the deposition efficiency and improving mechanical properties of Mg alloy components.