Wire-arc additive manufacturing of functionally graded alloy from Inconel 740H superalloy to P91 steel guided by the Calphad-based ICME approach

In advanced ultra-supercritical (A-USC) power plants, the parts operating at lower temperatures (< 600oC) can be fabricated using P91 steels, while those operating at higher temperatures can be made of Inconel 740H superalloy to reduce the material costs. This strategy will involve the joining of P91 steels with 740H superalloys, and hence, dissimilar joining is inevitable during the construction of the A-USC power plant. Joining dissimilar materials using traditional manufacturing techniques impose challenges such as porosity, cracking, and detrimental phase formation. Additive manufacturing is a suitable processing technique that can mitigate those challenges associated with traditional methods. In this work, graded structures of Inconel 740H superalloy and P91 steel with an intermediate gradient block were fabricated using wire-arc additive manufacturing (WAAM). Intermediate blocks were designed based on high-throughput phase fraction, creep index, and martensitic start temperature predictions. The builds were crack-free and devoid of any detrimental phases. The measured composition of the intermediate blocks were close to the nominal composition calculated using the weighted mean of each element. This implies that there is reasonable composition control during multi-material deposition using WAAM. According to microstructure analysis on intermediate blocks, secondary phases such as blocky carbides are rich in Nb and Ti, and the Laves phase has a high content of Nb, Ti, and Mo. The matrix shows a transition from Martensite in P91 to γ-fcc in Inconel 740H. Post-heat treatments are designed using experiments and thermodynamic calculations to achieve the maximum fraction of strengthening precipitates in the intermediate block. The oxidation behavior of these intermediate blocks will also be evaluated.