Microstructural and mechanical characterization of stainless steel 420 and Inconel 718 multi-material structures fabricated using laser directed energy deposition

Abstract

Varying material properties are required for components designed to withstand harsh environments with simultaneous thermal and mechanical loading. This has been traditionally achieved by using multiple materials fabricated by traditional processes, followed by permanent or non-permanent joining techniques to realize a full, multi-material and multi-functional component. A potential simpler approach to this is to develop multi-material structures fabricated using additive manufacturing (AM). Additionally, many existing structures in can also be simplified by incorporating multi-material and multi-functional structures. Laser directed energy deposition (laser-DED) AM technique is suitable for fabricating multi-material structures using powdered feedstock material with high material flexibility. However, the fabrication of multi-material structures requires careful investigation into the compatibility and behavior of the constituent material systems. This remains a crucial task for multi-material deposition because there is relatively less knowledge available currently on the optimal multi-material mixtures for achieving adequate structural integrity and mechanical performance. In this study, multi-material structures of stainless steel 420 (SS420) and Inconel 718 (IN718) alloys were studied. These alloys were chose due to their applications in gas turbine components such as shaft assemblies, exhaust components and in the energy industry for fabricating various high strength, corrosion resistant devices. The specific compositions of the multi-materials structures investigated were: 100% SS420, 100% IN718, 75% SS420 + 25% IN718, 50% SS420 + 50% IN718 and 75% IN718 + 25% SS420. These samples were characterized for their microstructural, phase, and mechanical properties in the as-fabricated condition and compared with conventionally fabricated alloys. Significant differences in the microstructure and properties were observed between the laser-DED fabricated 100% alloys and mixtures of the two alloys. The laser-DED fabricated SS420 and IN718 alloys showed rapidly solidified microstructures typical to laser processes and properties comparable or better than the conventionally fabricated alloys. The three mixtures of SS420 and IN718 alloys showed microstructure and phases that were intermediate between the two unmixed alloys, as well as a higher amount of metallic carbide and Laves phases. Hardness and tensile tests indicated that the three mixtures of SS420 and IN718 were weaker than the unmixed laser-DED fabricated alloys due to the brittle carbide and Laves phases. Amongst the mixture, the mixture composition of 25%SS420 + 75%IN718 has showed highest strength and ductility.