High-temperature observations of braze alloy spreading by oxide penetration

Abstract

The authors describe an investigation of the wetting mechanism of a gold–nickel (82–18 wt-%) vacuum brazing alloy on an oxidation-resistant stainless steel by the combined use of hot stage scanning electron microscopy (HSSEM), electron probe microanalysis (EPMA), and Auger electron spectroscopy (AES). With HSSEM the melting process could be recorded under a vacuum of 13 mPa and with EPMA and AES the surface of a specimen could be explored analytically. In this way the authors have greatly increased the information available on interaction between the substrate and the braze alloy when it melts. It has been well documented that this Au–Ni alloy is an extremely reliable alloy for use in the aerospace industry and consequently the observations made are of importance in identifying the properties of a successful brazing alloy. Photography during the melting and flowing process reveals that as the Au–Ni alloy flows, it is preceded by a halo ahead of the edge of the molten mass. These haloes have been identified clearly by photography at high-tilt angle to be a lifting of a surface layer on the steel by the molten alloy flowing beneath it. Analysis of the cold specimen with AES and X-ray photoelectron spectroscopy reveals that this layer is the oxide, mainly Cr₂O₃. It is shown that an important step in initiating the flow is the breakdown of the surface oxide layer at points where the presence of the molten Au–Ni has reduced the local oxygen potential and that silicon in the steel plays a role in this process. Suboxide spreading is the only mechanism of flow that could be discovered and a mathematical model is derived to account for the spreading characteristics based on the authors' observations and gives some indication of the factors which are involved in controlling the spreading process.