Partial transient liquid phase bonding of misoriented single crystal superalloys via a mixture of nickel-chromium-tantalum and nickel-based superalloy powders

Abstract

In order to realize high-performance joining of misoriented single crystal superalloys, a rapid partial transient liquid phase (PTLP) bonding process was proposed in this work. By using a mixture of nickel-chromium-tantalum (Ni-Cr-Ta) and nickel-based superalloy (FGH4096) powders as filler material, high-performance PTLP joints of misoriented single crystal superalloys (DD5) were obtained. The joint bonded at 1260 °C consisted of an epitaxial growth zone (EGZ) containing γ/γ' derived from DD5 and a polycrystalline zone (PCZ) located at the center of the joining layer, in which tantalum dichromide (TaCr2) and tantalum carbide (TaC) existed at the grain boundaries. As the Ni-Cr-Ta content increased, the voids in the joints decreased while the Ta-rich compounds increased and the grain size decreased. With the holding time increasing, the Ta-rich compounds decreased while the grain size increasing. The optimum process was 60 min at 1260℃ with the Ni-Cr-Ta content of 60 wt%. Due to the specially designed filler metal without boron (B) and silicon (Si) elements, there was no low-melting eutectic or compound in the PTLP joints, which ensure the joint with high-performance at elevated temperatures. Compared with transient liquid phase (TLP) bonding process, the PTLP bonding process achieved rapid isothermal solidification with the help of the short-distance and large-area diffusion mechanism between the Ni-Cr-Ta (liquid phase) and FGH4096/DD5. The presence of PCZ prevented the formation of penetrating grain boundary, and thus eliminated the effect of misorientation between two base metals on the performance of the joints. The tensile strengths at room temperature and 980 °C of the PTLP joints which obtained with the optimum process were 926.05 MPa and 681.96 MPa, respectively, up to 90.32% and 92.25% of the base metal. The PTLP bonding process proposed in the present work may provide a high efficiency approach to achieve high-performance single crystal superalloy joint, which is also gifted a major advantage of B/Si-free.