Advancements in wire-arc directed energy deposition (DED) have created new opportunities for manufacturing efficient large-scale structures. While wire-arc DED is often viewed as more sustainable and economical due to the potential of producing lighter structures, its higher environmental impact and cost per unit of weight necessitate further considerations during the design phase. This paper explores how sustainability and cost can be integrated into conceptual design through topology optimisation. The approach is demonstrated through a case study, including a parametric study on specific environmental impact and the cost of wire-arc DED versus CM, applicable to current data and future estimates. Findings indicate that beams manufactured solely with wire-arc DED are sensitive to fluctuations in specific environmental impact and cost of wire-arc DED, potentially losing their material saving advantage. Conversely, hybrid beams that combine conventional profiles with wire-arc DED offer a better balance between structural performance, sustainability and economic feasibility.

Wire arc additive manufacturing (WAAM) enables the manufacturing of efficient and lightweight structural elements in which material can be utilised wherever needed in an optimised shape, in contrast to standard prismatic profiles used in construction. However, the specific energy consumption (SEC) of WAAM is higher than that of conventional manufacturing (CM) techniques (i.e., hot-rolling) for standard profiles. Therefore, it is an open question whether the material savings through computational design realised via WAAM is environmentally beneficial or not. This systematic study aims to provide a better understanding of the environmental impact of hybrid manufacturing, which is defined as the combination of WAAM and CM rather than using any of them alone. Topology optimisation (TO) is used to design a series of beams with an identical performance (i.e., stiffness) but with a reduced material consumption depending on the hybrid ratio. The environmental impact of the designs has been used to determine when and how hybridisation can become advantageous. The results show that although the optimal proportions of WAAM and CM are dependent on their relative SEC, the hybrid solutions have always been environmentally superior compared to that of WAAM or CM alone for the realistic SEC values, exhibiting up to a 60% reduction in environmental impact compared to that of CM.