NICKEL-BASE WIDE-GAP BRAZING WITH PREPLACEMENT TECHNIQUE .1. EFFECT OF MATERIAL AND PROCESS PARAMETERS ON FORMATION OF MACROVOIDS

Macrovoids are voids resembling notches or cracks that can act as stre ss raisers in blazed joints. These are undesirable and should be elimi nated if physical soundness of the joint is to be achieved. In tile pr esent work, nickel base wide gap brazed joints were produced by the pr eplacement technique, using Nicrobraz LC and Nicrogap 116 as a filler metal and gap filler; respectively. After blazing, the joints were sec tioned depthwise, prepared metallographically, and examined by optical microscopy to detect the presence of macrovoids, the latter being def ined as voids of characteristic length greater than the mean diameter of gap filler particles in joints made with blaze mixes, or larger tha n one tenth of the width of the gap in joints made with filler metals only. The results showed that three major types of macrovoid could be identified: irregularly shaped macrovoids throughout the longitudinal section of the joint (type 1); irregularly shaped macrovoids at the ta il end of the joint (type 2); and near spherical macrovoids in the bla ze filler deposit and adjacent joint area (type 3). Each of these thre e types of macrovoid was found to dominate over a range of brazing tem peratures, braze mixing ingredients, and gap depths. At 1125 degrees C , type 1 macrovoids prevailed regardless of the gap filler content and gap depth. In the temperature range 1150-1200 degrees C, type 2 macro voids were common features in joints produced with braze mixes with ga p filler contents of less than 20%, while type 3 macrovoids were obser ved in joints produced with braze mixes with gap filler contents of gr eater than 50%. Joints free from the above mentioned macrovoids were r eproduced with braze mixes with gap filler contents of 30-40% over the latter temperature range. With increasing gap depth, the tendency for the formation of the the various types of macrovoid increased accordi ngly, all other parameters being equal. The results were compiled in t he form of braze quality control maps for easy reference.