Metal additive manufacturing (AM) processes have gone through a compound growth over the past decade, and the technology is widely applied in industries like aerospace, automobile and bio-medical fields. There is an increasing need to understand and improve its sustainability given the high profile of existing environmental challenges. This paper aims at developing a precise comparative model for the three major metal AM processes (including Laser Powder Bed Fusion (LPBF), Electron Beam Melting (EBM), and Direct Energy Deposition (DED)) with respect to environmental performance assessment with a future goal of providing closed-loop feedbacks for design optimization with elevated sustainability. To improve the precision of previously reported models, new factors including embodied impacts of machine and recycled powder, operation patterns, system lifespan and batch size, are considered. A topologically optimized rocket bracket made of Ti6Al4V is used as an example to investigate the environmental performance of the three processes. The results showed that given the same design solution, the EBM had the lowest environmental impacts for low batch size, while the DED excelled at production efficiency.