Hybrid additive manufacture of 316L stainless steel with cold spray and selective laser melting: Microstructure and mechanical properties

Fusion based metal additive manufacturing (AM) techniques such as selective laser melting (SLM) offer many advantages when compared to traditional manufacturing techniques, however, are often limited by the low build rates achievable, particularly for the manufacture of large components at scale. This may be alleviated by combining SLM with other more rapid AM processes such as cold spray (CS). Therefore, in this work, a hybrid AM process combining SLM and CS was utilised for the production of CS-SLM hybrid components. The proposed hybrid process allows the fabrication of complex geometries with SLM and the rapid manufacture of simple geometries using CS. The hybrid parts were manufactured by depositing a thick 316L stainless steel structure onto an SLM 316L stainless steel part via CS, followed by heat treatment to modify the microstructure and improve the CS-SLM adhesion. The microstructure, phase composition and mechanical properties of the as fabricated and heat-treated part were studied using various materials characterisation methods. Based on the experimental results and analysis, it was found that the CS part had a grain structure similar to the feedstock in the as-fabricated state, while the SLM part was characterised by cellular subgrains confined in coarse grain structures. Due to the 'fusion' nature of the process, the SLM part delivered improved mechanical properties when compared to the CS part; however, this difference was reduced after heat treatment through the improvement in the tensile strength of the CS part by over 200% for both helium and nitrogen. Also, heat treatment improved the CS-SLM adhesive strength due to enhanced interface diffusion. Overall, this study demonstrates that the proposed hybrid AM is a promising technique for the manufacture of free-standing CS-SLM components.