Design of Graded Transition Interlayer for Joining Inconel 740H Superalloy with P91 Steel Using Wire-Arc Additive Manufacturing

In this work, two graded transition interlayers were designed using a CALPHADCALPHAD-based ICMEICME framework (CALPHADCALPHAD: Calculation of Phase Diagrams; ICMEICME: Integrated Computational Materials EngineeringIntegrated computational materials engineering) for joining Inconel 740H superalloySuperalloys with P91 steel. Successful builds with the designed interlayers (content of P91 steel are 60 and 85 wt.%) sandwiched between the constituent materials were fabricated using wire-arc additive manufacturingWire-arc additive manufacturing. 60% P91 interlayer exhibited an FCC structure with low hardness, while the 85% P91 interlayer had a martensitic structure with high hardness. A two-step post-heat treatmentHeat treatment consisting of homogenization at 1150°C and agingAging at 760°C was designed. 60% P91 interlayer showed no improvement in hardness after agingAging. It agrees with the CALPHADCALPHAD modeling that predicts a lack of effective strengthening precipitates at 760°C, whereas the hardness of 85% P91 increased significantly after agingAging for 8 h due to the precipitationPrecipitation of the M23C6 phase. Mechanical tests equipped with digital image correlationDigital image correlation were performed to determine the location of the failure and tensile propertiesTensile properties. As-built and heat treated 60% P91 build failed in the graded alloy block, whereas the as-built 85% P91 alloy failed at the 85% P91/740H interface, and the aged alloy failed in the pure P91 region. This proves that post-heat treated 85% P91 is much stronger than pure P91, and the alloy designAlloy design strategy used in this work proves successful for developing interlayers for dissimilar joining.