Metal-Mold Reactions in CMSX-4 Single Crystal Superalloy Castings
AdvisorLaCombe, Jeffrey C.
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Metal-mold reaction (MMR) layers are often found on the surface of as-cast CMSX-4 single-crystal alloy parts. These layers cannot be removed prior to solution heat treatment of cast parts because of the sensitivity of the single-crystal castings to recrystallization (RX) defect formation. Removal of the reaction layer after solution heat treatments is very costly, as the layer is very hard, and requires abrasive water jet and pressure (grit) blasting processes. To address this manufacturing concern, it was desirable to understand the mechanisms of reaction layer formation and hardening after solution heat treatment. With this understanding, we developed methods for minimizing the reaction layer formation, which will potentially bring a big cost saving for the CMSX-4 casting processes.In this work, we confirmed experimentally that silica (SiO<sub>2</sub>) reacts with Al, Hf, and Ti, facilitating surface oxidation and formation of a tenacious surface eutectic phase. To avoid this, elimination of the Si is desired. However, Si is present in many of the refractory and pattern materials used in the casting system, which can transfer into the alloy during the solidification process in both liquid and solid state. A two-path solution was investigated: 1) eliminate all Si sources, and 2) create surface diffusion barriers to prevent reaction of the SiO<sub>2</sub> with the metal. Potential sources of silica/silicon in the casting system include: the CMSX-4 charge material (nominal Si content less than 400 ppm), thermocouple protection quartz tube (100% silica), crucible (4% silica), bushing (80% silica), funnel (if used-60% silica), ash in pattern waxes, mold release (silicone-based), binder for facecoating (includes nanoscale silica-4% in facecoat), and cores (when used-80% silica).For a diffusion barrier, an yttria slurry (short lifetime and high cost of fine yttria flour), was replaced with an yttria aerosol spray coating, applied directly to the wax pattern before normal zircon primary facecoating. This was followed by an yttria binder washing and soaking, applied on top of the spray coating shell after dewaxing for strengthening. This process showed good bonding in casting trials in an argon atmosphere Bridgman casting furnace. Optical microscopy, SEM/EDS, AES and XPS techniques were employed for characterization of MMR interfaces of both the CMSX-4 casting and the shell mold. These characterization methods revealed MMR layers with oxidation, Si and Hf rich features.In this study, the yttria spray (alone) slightly reduces the amount of MMR of CMSX-4, but when the yttria spray was combined with the binder wash, the reaction was further reduced.