WIDE GAP BRAZING WITH PRE-PACKS

Abstract

This project attempts to identify the major types of physical defects and their formation mechanisms in nickel-base wide gap brazes with pre-packs of braze mixes. The range of gap filler contents used varied from 0% to 100%. Brazing temperatures of 1125°C, 1150°C, 1175°C and 1200°C were used. Three gap depths: 4 mm, 6 mm and 8 mm were attempted with a gap width of 0.5 mm. Limited microstructural studies were also carried out by means of optical and scanning electron microscopy coupled with standard less energy dispersive X-ray microanalysis and micro hardness measurements.

The major physical defects found were constituent voids including shrinkage, interfacial and interstitial voids and defects due to deficient pre-packing. The effects of various materials and process parameters on the formation of interfacial and interstitial voids were described and the findings were compiled in the form of braze quality control charts.

The effects of materials and process parameters on the degree of base metal erosion were also investigated and the results presented in pictorial form in which the degree of erosion was plotted against the braze mix ingredients for the different brazing temperatures and gap depths used.

From the results obtained, the physical events occurring during brazing using this technique can be deduced as follows: during heating to the brazing temperature, the pre-pack sinters partially and shrinks in volume. This causes the formation of open channels in between the pre-pack and the joint faying surfaces. Upon melting of the filler metal deposited at the gap mouth, the molten metal ls drawn preferentially into the gap through the partially sintered mass of pre-pack, leaving the open channels along the faying surfaces to be filled last. Interfacial voids would thus form when solidification of the molten filler occurs before large interstitial spaces on the shrunk pre-pack surfaces are completely filled.

Likewise, large pockets of empty space within the pre-pack can be formed due to particle conglomeration effect upon the addition of liquid binder and as a result of differential sintering during heating to the brazing temperature. These pockets, being of lower capillarity, will also be filled last, giving rise to the possibility of the formation of interstitial voids in the pre-pack. The formation of interfacial and interstitial voids are generally promoted by a high gap filler content in the braze mix, a low brazing temperature and a large gap depth.

Microstructurally, the present work shows that for brazes with a high gap filler content carried out at 1175°c, the major constituent in the braze main body is the partially melted and resolidified gap filler phase. Four phases were identified in between the resolidified gap filler phase. They are: a chromium-rich phase identified as chromium boride; a nickel-rich phase identified as nickel boride; 7-nickel with fine silicide precipitates and the eutectic of nickel boride and 7-nickel. The microstructural evolution of these phases during the brazing process was also discussed.