Investigation of High Temperature Flowability of Nickel-Based Braze Using Capillary Holes

The ratio of high-melt superalloy material to low-melt braze material in wide gap brazing of nickel superalloy turbine components has a major influence on high temperature viscosity of the overall braze mixture. The filling and mechanical integrity of wide gap joints larger than 250µm can be directly affected by this ratio. Too much high-melt can lead to poor melting and flow of braze, while too much low-melt can result in the formation of a large number of detrimental, brittle eutectic phases in the joint. A standard sample was developed to assess the flowability of braze at temperatures above 1000˚C. This new sample design also had the ability to highlight the effects of braze wetting, secondary phase formations and poor mixing of braze constituents. Sample design consisted of two sets of four laser-drilled capillary holes of varying diameters between 300µm and 900µm in an Inconel 738 nickel superalloy substrate. These samples were used to assess the previously mentioned properties of three different Inconel 738 high-melt to Amdry DF4B low-melt braze ratios - 80H:20L, 50H:50L and 20H:80L. As the ratio of high-melt to low-melt decreased, the filling of the capillary holes improved. However, excessive low-melt material resulted in braze liquation in the larger capillary holes. Brittle boride phases were observed in the braze material of each braze ratio. The high quantity of boron melting point depressant in the 20H:80L braze mixture resulted in the highest volume of these secondary boride phases in the braze material of all three braze types. The novel capillary hole sample design proved very capable in assessing high temperature braze performance. High temperature flowability, wettability and secondary phase formations were each accounted for, with further potential of the design to detect heterogeneous mixing of braze constituents.