Additive, subtractive, and formative manufacturing of metal components: a life cycle assessment comparison

Manufacturing processes are typically divided into three categories: formative, subtractive, and additive. While formative and subtractive manufacturing processes are considered more traditional, additive manufacturing (AM) is a family of evolving technologies that are rapidly growing with techniques and constraints yet to be explored. In this paper, a life cycle assessment comparison of casting (formative), machining (subtractive), and three AM methods, namely, binder jetting (BJ), powder bed fusion (PBF), and novel bound powder extrusion (BPE) has been performed. To compare each method from the sustainability standpoint, a life cycle assessment was conducted on a double cardan H-yoke, as a case study, focusing on environmental metrics such as water consumption, energy requirements, and CO₂ emissions. This study focuses on the environmental effects of the novel BPE process with respect to current traditional manufacturing and AM methods. The case study was divided into two scenarios of the original and topology-optimized H-yoke to investigate the potential environmental footprint reduction by utilizing the capability of AM in generating complex geometries. The results proved that casting, as a formative manufacturing process, is the most environmentally friendly option for large-scale production of the investigated processes. Among the AM technologies that have been studied, PBF was the most environmentally friendly choice when coupled with renewable energy, reducing the total CO₂ emission by 9.2% when compared to casting. In contrast, BJ was more environmentally friendly when fossil fuel was assumed as the main source of energy, showing only an 8.7% increase in CO₂ emissions. The novel BPE preformed equal to or just short of BJ in all metrics, showing only a 9.4% increase of CO₂ emission using fossil fuel compared to the 41.7% increase seen by PBF, with respect to BJ. AM environmental metrics were significantly improved when the topology-optimized part was employed. Machining, as a subtractive method, performed the worst from the environmental perspective due to the initial billet size and the amount of material to be removed (wasted). The production time for each process was analyzed to display the feasibility of producing the cast study part in a mass manufacturing scenario. The LCA case study proves that the increased number of BPE manufacturing steps does not negatively affect the environmental impact of the process, based on current LCA data. However, the BPE process is the most time-consuming process and must be considered when selecting the method of manufacture.