PRACTICAL ASPECTS OF WIDE-GAP BRAZING

Abstract

This project aims at exploring the various practical aspects concerning wide-gap brazing of nickel-base superalloys with nickel-base fillers using the technique of pre-placing the braze filler at the mouth of the gap. All brazings were carried out in a vacuum furnace and the resultant braze joints were examined under an optical microscope. The brazing parameters studied included gap width, gap inclination, gap depth, filler metal type, braze filler ingredients, brazing temperature and brazing cycle. Through careful examination of the physical soundness of the brazes produced under a wide range of conditions, the various types of braze defects were characterised. The braze microstructures were also studied to provide a better understanding of the behaviour of the braze mix constituents during brazing. Causes for the formation of various braze defects were postulated. Semi-quantitative charts showing the braze physical soundness as functions of brazing temperature, braze mix ingredients, gap depth, etc. were constructed. For horizontal gaps brazed with commercial filler metals, flow cessation occurred when the gaps were too fine (0. 1mm or smaller in width). On the other hand, gaps larger than 0.5mm in width, could not be brazed satisfactorily due to poor capillarity, while gaps of width3 ranging from 0.2 to 0.4mm could be brazed successfully. For gaps larger than 0.4mm in width, use of braze mixes containing a high melting point gap filler phase was essential. In this cue, vertical gap orientation offered the best flow conditions for the braze filler as gravitational effect was fully utilised. Gravitational filling had been found to be important in successful brazing of wide, non-capillary gaps. With low gap inclinations of less than 30°, the gap filler particles tended to sinter together at the mouth of the gap, causing problems in flow penetration. Even with vertical gaps, the amount of gap filler in the braze mix also played a significant role in controlling the braze outcome. For braze mixes containing less than 2% (in weight) gap filler, the molten filler metal tended to flow ahead of the mass of braze mix, leading to macrovoid formation at the end of the braze due to the poor bridging properties of the molten filler metal. For braze mixes with 30 to 40% gap filler, the mass of braze mix flowed as a whole and physically sound brazes were repeatedly produced. On the other extreme, for braze mixes containing more than 50% gap filler, even though no differential flow of the constituents occurred, the amount of filler metal was insufficient to completely fill the voids among the gap filler phase during brazing and isolated, near-spherical macrovoids were formed in the braze filler deposit and near the mouth of the gap as a result. The effects of brazing temperature, gap geometry, i.e. gap width and depth. on both the physical soundness of the braze and erosion of base metal were also investigated and discussed.