Nickel base wide gap brazing with preplacement techniqee part 1 - Effect of material and process parameters on formation of macrovoids

Abstract

Macrovoids are voids resembling notches or cracks that can act as stress raisers in brazed joints. These are undesirabee and should be eliminated if physical soundness of the joint is to be achieved. In the present work, nickel base wide gap brazed joinss were produced by the preplacement technique, using Nicrobraz LC and Nicrogap 116 as a filler metal and gap filler, respectively. After brazing, the joints were sectioned depthwise, prepared metallographically, and examined by optical microscopy to detect the presence of macrovoids, 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 joinss 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 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 ofmacrovoid 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 joinss produced with braze mixes with gap filler contenss of less than 20%, while type 3 macrovoids were observed in joinss produced with braze mixes with gap filler contenss of greater than 50%. Joints free from the above mentioned macrovoids were reproduced with braze mixes with gap filler contenss 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. © 1995 The Institute of Materials.