AISI4140 processed by powder bed electron beam melting with subsequent surface layer treatment - From process strategy to material properties

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Quenched and tempered steels are materials widely employed for mechanical engineering applications. These steels are suitable for hardening due to their chemical composition and, in the quenched and tempered state, have a relatively high tensile strength. A heat treatment adapted to the intended use, i.e. quenching and tempering, which usually consists of hardening and the subsequent single step tempering, enables a targeted adaptation to technical requirements. In the past few years, additive and generative manufacturing processes (AM) have become a promising and competitive alternative to conventional manufacturing and shaping processes. Powder based AM processes, i.e. laser metal deposition, laser-based powder bed fusion of metals (PBF-LB/M), also referred to as selective laser melting (SLM), and electron-beam powder bed fusion of metals (PBF-EB/M), also referred to as selective electron beam melting (EBM), have proven themselves to be mature to manufacture components of highest geometric complexity on different length scales. In particular, the processes comprising a powder bed enable the production of filigree structures, whereby good control of the process conditions can be guaranteed by processing in shielded build envelopes. In contrast to PBF-LB/M, the PBF-EB/M process offers the advantages of manufacturing components under vacuum and elevated temperature, the latter being related to the required preheating of the powder material. Research results published to date have shown that suitable exposure strategies in the PBF-EB/M process allow processing of different materials, e.g. stainless steels, tool steels, Ni and Co-based superalloys, hard metals, intermetallic compounds, aluminum, copper, beryllium and niobium. These materials can be processed in very high quality. Nevertheless, the PBF-EB/M technology continues to focus on titanium alloys. Thus, additive manufacturing of conventional steels via the PBF-EB/M process has so far been out of focus. The present project addresses elementary questions in the field of additive manufacturing of heat-treatable steels and at the same time considers a resource- and cost-efficient surface layer process, which has not yet been studied thoroughly in this context in the international community. The aim is to increase the service life under relevant loading conditions and, eventually, to improve the structural integrity of the alloys processed.