Nickel base wide gap brazing with preplacement technique Part 2 - Forl11ation mechanisms of macrovoids

Abstract

Microstructural inhomogeneities and variations in the extent of erosion of base metal in nickel base wide gap brazed joints produced by the preplacement technique with braze mixes of different gap filler contents were investigated, from which the flow behaviour of the braze mix constituents and the formation mechanisms of the various types of macrovoid were deduced. The results show that the formation of various types of macrovoid is closely related to the flow behaviour of the constituents of braze mixes during brazing, the latter in turn being strongly influenced by the braze mix ingredients, the brazing temperature, and the gap depth. For a wide gap brazed joint to be free from macrovoids, the braze mix must be sufficiently viscous to bridge the gap faying surfaces and must flow as a whole into the gap. Braze mixes with gap filler contents of 30–40% are ideal for such requirements. With too Iowa gapfiller content, the molten filler would flow preferentially ahead of the mass-of braze mix, leading to the formation of irregularly shaped macrovoids at the tail end of the joint. With too high a gap filler content, the molten filler metal available was insufficient to completely fill the interstices among the gap filler particles, leading to near spherical macrovoids in the braze mix deposit and adjacent joint area. At too Iowa brazing temperature, the braze mix would be too viscous to penetrate into the gap freely and local premature solidification would occur, leading to large, irregularly shaped macrovoids throughout the longitudinal section of the joint.

MST/3132