SUPERSOLIDUS LIQUID PHASE SINTERING AND GRAIN GROWTH ACTIVATION IN A METAL INJECTION MOLDED NICKEL-BASE SUPERALLOY

Abstract

Metal injection molding (MIM) is a technology that can be used to produce net-shape parts while reducing material waste and manufacturing costs. In the aerospace industry, the application of MIM to nickel superalloys (NiSAs) has been restricted to non-structural engine components due to their limited high temperature properties. State-of-the-art solid state sintering practices for MIM NiSAs generally lead to fine-grain microstructures, which have reduced creep resistance compared to coarser, cast, microstructures. Additionally, due to the use of fine pre-alloyed powders in the MIM process, carbon and oxygen can react with alloying elements such as W and other refractory metals (RMs) to form carbide and oxide phases which can restrict grain growth. In the present work, the behaviour of a MIM NiSA during both solid state sintering and supersolidus liquid phase sintering (SLPS) is evaluated using differential scanning calorimetry (DSC) and dilatometry (DIL). The influence of the sintering atmosphere, temperature, and time on the NiSA’s grain growth, shape retention, and density are quantified. Comparative DSC analysis techniques were also developed to quantify the liquid fraction with time and temperature. Microstructural analysis performed using optical microscopy and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) revealed the grain boundary evolution with time and temperature. The MIM NiSA was sensitive to the level of oxygen present in the sintering atmosphere and the formation of RM oxides at prior particle boundaries (PPBs) was found to inhibit grain growth and densification. It was also determined that RM-Ta mixed MC carbides situated at the PPBs in the MIM NiSA restricted grain growth during sintering. Time above the solidus temperature was found to dissolve the RM-Ta mixed MC carbides, enacting grain growth. Consequently, the dissolution of these MC carbides raised the equilibrium liquid fraction, leading to an increased liquid content, decreased part rigidity, and loss of alloy homogeneity. An existing SLPS model was modified to reflect the time dependency of the liquid phase. Similarly, liquid phase sintering with the commercial braze filler metal BNi-2 enacted grain growth in the MIM NiSA at lower temperatures, proving that grain growth can be activated by multiple means in the MIM NiSA.