Development of novel Ni-based brazing filler metals

The capability to join various metallic and non-metallic materials is crucial to the production of a wide array of components across many industries, ranging from the soldering of small scale electronics, to the welding of larger scale automotive and aircraft structures. Brazing is a distinct joining technique, involving the melting of a metallic filler metal introduced in between two or more materials to be joined. While welding requires the melting of the materials being joined, brazing does not, and as such the avoidance of welding-induced defects is important to many industries, including aerospace. Brazing bears similarities to soldering, and is distinguished from soldering in the temperatures at which it is conducted (typically above 450oC). Filler metals used in brazing are usually tailored to the particular materials being joined, in terms of melting temperature and chemistry. Ni-based filler metals are widely used in the vacuum brazing of high temperature materials such as Ni-based superalloys. In order to ensure melting of the filler metal at a temperature below that of the base metal, Ni-based filler metals often contain elements such as B, Si and P, which act as so-called Melting Point Depressants (MPDs). The use of such elements, however, leave brazed joints susceptible to formation of brittle intermetallic phases, which may only be removed by prolonged time at the brazing temperature or lengthy heat treatments post-braze. In many cases, the current commercial Ni-based filler metals have seen little in the way of exploration into compositional changes that may alleviate such issues. In highlighting the need for more work in this area, this project was concerned with the development of novel Ni-based brazing filler metals, primarily for the brazing of Ni-based superalloys, employing alternative MPDs and other compositional changes. Using phase diagrams and CALPHAD (CALculation of PHase Diagrams)-based software, the use of elements In and Ge as alternative MPDs was investigated. Their use as sole alloying additions to Ni (i.e. wholly replacing elements B, Si or P) was deemed unsuitable due to the high liquidus temperatures, which were well in excess of those of commercial Ni-based filler metals. The use of In and Ge in conjunction with reduced B content was also investigated. While this strategy allowed lower liquidus temperatures, the potential for brittle boride formation meant such compositions were deemed unsuitable. CALPHAD predictions of the Ni-In and Ni-Ge systems were of limited accuracy, though were found to be useful in predicting general trends in liquidus as a function of composition. A further strategy, using concepts such as High Entropy Alloys (HEAs) and Multi-Principal Element Alloys (MPEAs), was used to design two Ni-based MPEA filler metals, including novel MPD elements In and Ge. The first filler metal, based on NiCrMnIn, was found to be unsuitable for the vacuum brazing of Inconel-718 (IN718) superalloy due to volatilisation and liquation, though was successfully applied to the belt furnace brazing of carbide-tipped drill bits. The second filler metal, based on NiCrFeGeB, was successful in the vacuum brazing of IN718. Average joint shear strength of 332 ± 15 MPa was recorded following brazing at 1100oC for 60 minutes. While weaker than achievable using commercial AWS BNi-2 under similar conditions, the joint microstructure exhibited isothermal solidification of an MPEA-like NiCrFeGe solid solution, with limited boride formation in the IN718 base metal. Overall, this research demonstrated a design strategy for the development of novel Ni-based MPEA filler metals, and it was found that elements In and Ge may be incorporated into an MPEA-type filler metal, allowing the partial or complete replacement of conventional MPD elements while achieving a liquidus temperature comparable with commercial Ni-based filler metals (and below 1100oC). It is suggested that future work should be focussed on the refinement of the developed compositions to address inferior mechanical properties as compared to joints produced using commercially available AWS BNi-2. Furthermore, it is proposed that the design strategy for such filler metals may be expanded to other brazing applications outside Ni-based superalloys, for example in nuclear reactors.