Phase diagram-guided composition design and property investigation of Ni-based filler metals

Abstract

The fundamental mechanism underlying the formation of brazed joints involves the diffusion of elements between the base metal (BM) and the brazing filler metal (FM). The formation of brittle compounds in FM during the brazing process frequently results in a significant deterioration of the mechanical properties of the brazed joints. Ni and Cu elements exhibit infinite solubility, and the incorporation of Cu element results in a decrease in the liquidus temperature of FM, thereby enhancing its surface wettability on the BM. The present investigation involves the preparation of as-cast and amorphous FMs with varying Cu element concentrations to examine their material properties, such as weldability, wettability, thermophysical behavior, and creep resistance. The thermophysical and mechanical properties of the nickel-based filler metal BNi-2 were evaluated using the JMatPro software for material thermodynamic simulation and compared with data obtained from existing literatures. An analysis is conducted on the creep properties of a novel FM comprising of Ni, Cr, Fe, Si, B, and Cu. The results suggest that the thermophysical properties of BNi-2 filler are minimally impacted by the presence of Cr (6–8 wt%), Fe (2.5–3.5 wt%), Si (4–5 wt%), and B (2.75–3.5 wt%). Furthermore, alterations in the composition of Si and B elements have a significant impact on the distribution of γ and γ′ phases, consequently influencing the creep properties of the material. At 25 °C, the equilibrium phase of BNi-2 with different Cu contents indicates a gradual decrease in the content of γ phase, while the content of γ′ and Cu–Ni phases increases with an increase in Cu content. The BNi-2 filler with 3 % Cu exhibits the most favorable properties in terms of steady creep rate and fracture life compared to other fillers. This is attributed to the competitive equilibrium between the γ phase and Cu–Ni phase. The addition of 6 % Cu element results in the optimal wettability of the filler metal and creep resistance of the brazed joints. It is also observed that as the concentration of Cu reaches 9 %, there is a gradual increase in the FeB phase, a rise in the proportion of brittle compounds, and a decline in the amorphous forming ability.