Abstract
Brazing may be described as more of an art than a science , there is little comprehension of the basic mechanisms governing brazing in air or vacuum. Consequently the behaviour of the Gold 18% Nickel brazing alloy in vacuum was studied using the novel techniques of hot-stage scanning electron microscopy with simultaneous bulk and surface analysis. The work demonstrates that careful control of experimental conditions allows both Auger electron and X-Ray spectroscopies (AES and EDXA) to be carried out at high spatial and energy resolutions , with good signal to noise in a realistic analyical time. Combining post-brazing examination of samples heated in a UHV oven , with real-time studies of braze powder/substrate combinations on a heating stage in an Auger microprobe , revealed several important mechanisms. Vacuum brazing is a two-stage process; oxide penetration followed by wetting and spreading via a suboxide route. AES analysis showed that oxide penetration is achieved by reduction as a result of low oxygen partial pressures and high carbon activities , in the system as a whole , or locally (for example under or around a molten braze droplet) , leaving an oxide-free surface , or a discontinuous , penetrable oxide. After penetration has occurred wetting and spreading continues along the metal/metal-oxide interface. Flow may be impeded by residual oxide resisting the advancing liquid which is trying to disbond it. Alternatively , interdiffusion of the braze metal and substrate allows pick-up of elements which raise the melting temperature of the braze alloy and causes solidification. EDX analysis and digital element mapping during interdiffusion and solidification illustrates that the periphery of the braze pools are most affected , and that further elevating the temperature initially caused remelting , but eventually resulted in second phase formation in accord with the Au/Ni/Fe equilibrium diagram.