VOID FORMATION IN WIDE-GAP BRAZING USING PREPACKS OF NICKEL-BASE BRAZE MIXES

The various types of void formed in wide gap brazed joints of C1023 ni ckel base superalloy produced using prepacks of nickel base braze mixe s 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 tec hnique. Examination of brazed joints produced under a wide range of co nditions revealed that during heating to the brazing temperature, the braze mix partially sinters together, causing the prepack to shrink to wards the centreline, leaving two channels next to the joint faying su rfaces. At the same time, relatively large pockets of free space are c reated within the partially sintered mass of prepack. At the brazing t emperature, the filler metal deposited at the gap mouth becomes molten and this molten filler is drawn into the gap preferentially through t he 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 mo lten filler metal owing to their lack of capillarity. As a result, the various brazing defects described above will be formed should solidif ication occur before these empty spaces are filled. The effects of mat erials and process variables on the formation of various brazing defec ts are briefly discussed.