Many existing structures can be simplified by incorporating multi-material and multifunctional structures. Laser directed energy deposition (DED) additive manufacturing (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, initially five samples of stainless steel 420 (SS420) and Inconel 718 (IN718) alloys ranging from 100% SS420 to 100% IN718 were fabricated using laser DED, three of which had fixed composition. 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. 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. Among the mixture, the mixture composition of 25%SS420 + 75%IN718 has showed highest strength and ductility.℗ Second phase of the study, laser DED fabrication and heat treatment of SS420 and IN718 multi-material structures (MMS) is investigated. The study improved this multi-material regions' performance by devising a specialized heat treatment for the SS420 and IN718 multi-material structures. Five different coupons were fabricated using laser DED process. The mechanical performance was correlated to the microstructural features, before and after heat treatment, using destructive and non-destructive techniques. The result was a novel SS420+IN718 mixture chemistry that had properties comparable to the parent alloys, thus eliminating a weak link in the MMS.Third phase of the study was to test the interface between parent alloys. For this purpose, SS420 and IN718 metals were bonded using the laser-directed energy deposition manufacturing technique. Two different transitions were tailored to investigate the bonding mechanism. The first one is direct bonding, where the IN718 is manufactured on top of the SS420. In the second one, a fixed composition transition is used, where a 25% SS420 + 75% IN718 mixture composition is fabricated as a transition between the two metals. All samples were heat treated using previous studies' findings. Post processing such as HIP and heat-treatment ware also investigated. These samples and the transition zones were characterized for their hardness and strength. In the "as-processed" state, the hardness transition between the two alloys significantly drops at the transition zone for both bonding techniques. After heat treatment, gradual transition in the interfacial regions' hardness were observed for the fixed composition transition zone. The fixed composition interface showed higher strength than the direct interface.℗ The last phase of the study, a Ni-20%Cr alloy was used to improve the fabricability of the Tribaloy 800 (T800) alloy via laser DED. Different mixture compositions (20%, 30%, 40% NiCr by weight) were investigated. The multi-material T800 + NiCr alloys were heat treated at two different temperatures. These alloy chemistries were characterized for their microstructural, phase, and mechanical properties in the as-processed and heat treated conditions. SEM and XRD characterization indicated the stabilization of ductile phases and homogenization of the Laves phases after laser DED fabrication and heat treatment. In conclusion, the NiCr addition improved the fabricability and structural integrity of the T800 alloy.
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