Strength and fracture resistance of in-situ alloyed compositionally-graded Al-Si processed by dual-wire-arc additive manufacturing

Wire-arc additive manufacturing (WAAM) enables processing large, complex components at high deposition rates and low raw materials costs which combined with the ability for in-situ alloying materials to achieve chemical gradients allows processing capabilities beyond those available in other additive manufacturing methods. Here, we investigate micro and mesostructure, and corresponding mechanical properties, of a compositionally-graded Al-Si alloy that has been processed using commercially available AlSi5 and AlSi12 feeding wires with a dual-WAAM system. Microstructure characterization shows a chemical gradient of ~6.5-9.5% Si along the build direction that correlates with increasing hardness values ranging from ~40 HV0.5 to ~65 HV0.5. While tensile strength increases with build height and ductility is affected by testing orientation, micro and mesostructure show only little impact on fracture resistance. Both strength and failure characteristics are associated with a mismatch in local strain deformation capacity between the -Al dendrites and the eutectic phase that affect melt pool boundaries and the interior of melt pools differently. As a result, tensile testing results are controlled by the initiation of failure at the melt pool boundaries while the constant fracture toughness values are mainly controlled by decohesion the two phases and subsequent crack bridging with crack extension.