Void formation in wide gap brazing using prepacks of nickel base braze mixes

Abstract

The various types of void formed in wide gap brazed joints of C1023 nickel base superalloy produced using prepacks of nickel base braze mixes have been investigated and systematically categorised. Of particular interest are interfacial and interstitial voids and unwetted pockets, which are features frequently found in joints brazed using such a technique. Examination of brazed joints produced under a wide range of conditions revealed that during heating to the brazing temperature, the braze mix partially sinters together, causing the prepack to shrink towards the centreline, leaving two channels next to the joint faying surfaces. At the same time, relatively large pockets of free space are created within the partially sintered mass of prepack. At the brazing temperature, the filler metal deposited at the gap mouth becomes molten and this molten filler is drawn into the gap preferentially through the fine capillary paths in the partially sintered mass of prepack. The relatively large pockets of free space in the prepacks as well as the channels adjacent to the joint faying surfaces are bypassed by the molten filler metal owing to their lack of capillarity. As a result, the various brazing defects described above will be formed should solidification occur before these empty spaces are filled. The effects of materials and process variables on theformation of various brazing defects are briefly discussed. © 1994 The Institute of Materials.