NICKEL-BASE WIDE-GAP BRAZING WITH PREPLACEMENT TECHNIQUE .2. FORMATION MECHANISMS OF MACROVOIDS

Microstructural inhomogeneities and variations in the extent of erosio n of base metal in nickel base wide gap blazed joints produced by the preplacement technique with blaze mixes of different filler contents w ere investigated, from which the flow behaviour of the braze mix const ituents 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 constituent s of braze mixes during blazing the latter in turn being strongly infl uenced by the blaze mix ingredients, the brazing temperature, and the gap depth. For a wide gap brazed joint to be free from macrovoids, the blaze mix must be sufficiently viscous to bridge the gap faying surfa ces 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 low a ga p filler 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 the joint. With too high a gap fill er content, the molten filler metal available was insufficient to comp letely fill the interstices among the gap filler particles, leading to near spherical macrovoids in the braze mix deposit and adjacent joint al ea. At too low a brazing temperature, the braze mix would be too v iscous to penetrate into the gap freely and local premature solidifica tion would occur; leading to large, irregularly shaped macrovoids thro ughout the longitudinal section of the joint.