Nickel base wide gap brazing with preplacement technique Part 1 - Effect of l11aterial and process parameters on forl11ation of l11acrovoids

Abstract

Macrovoids are voids resembling notches or cracks that can act as stress raisers in brazed joints. These are undesirable and should be eliminated if physical soundness of the joint is to be achieved. In the present work, nickel base wide gap brazed joints were produced by the preplacement technique, using Nicrobraz LC and Nicrogap 116 as afiller metal and gap filler, respectively. After brazing, the joints were sectioned depthwise, prepared metallographically, and examined by optical microscopy to detect the presence of macro voids, the latter being defined as voids of characteristic length greater than the mean diameter of gap filler particles in joints made with braze mixes, or larger than one tenth of the width of the gap in joints made with filler metals only. The results showed that three major types of macro void could be identified: irregularly shaped macrovoids throughout the longitudinal section of the joint (type 1); irregularly shaped macrovoids at the tail end of the joint (type 2); and near spherical macrovoids in the braze filler deposit and adjacent joint area (type 3). Each of these three types of macrovoid was found to dominate over a range of brazing temperatures, braze mix ingredients, and gap depths. At 1125° C, type 1 macrovoids prevailed regardless of the gap filler content and gap depth. In the temperature range 1150–1200° C, type 2 macrovoids were common features in joints produced with braze mixes with gap filler contents of less than 20%, while type 3 macro voids were observed in joints produced with braze mixes with gap filler contents of greater than 50%. Joints free from the above mentioned macrovoids were reproduced 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 various types of macrovoid increased accordingly, all other parameters being equal. The results were compiled in the form of braze quality control maps for easy reference.

MST/3131