Many factors influence the maximum permissible joint gap with the nickel filler metals, such as base metal composition, brazing temperature, dwell time and the filler metal composition. To determine the maximum permissible joint gap a variable joint clearance test should be undertaken.
Two pieces of the base metal, approximately 6mm thick by 12mm wide by 100mm long should be used to produce the test specimen. The widths of base metal are placed together with a zero clearance at one end and with an approximate 0.125mm clearance at the other end, then tack welded in place. The specimen is then positioned with the long axis horizontal, the joint vertical, Fig. 1, and filler metal applied on the top of the joint. The filler metal quantity must be sufficient to fill the entire joint with some excess. The brazing temperature will be dependent on the nickel filler metal selected, however, a reasonable dwell time should be used, between 30-60 minutes to allow adequate diffusion.
After brazing, the specimen should then be sectioned longitudinally through the centre. This will allow the variable joint clearance to be polished, then etched using Marbles reagent to metallographically observe the joint area. At the zero clearance end the filler metal should be unetched, as it is considered to have an amorphous structure and the microhardness should be softer than the original filler metal. When moving along the joint, the clearance should increase and at one point there will be a secondary phase showing up in the middle of the brazed joint. This centre phase should increase as the clearance increases, and will contain a composition close to the original filler metal, so will have a high hardness.
Fig.1. Schematic of a variable joint clearance test specimen
This test will determine the maximum clearance (the point when the second phase appears) at which you will have full diffusion within the joint. Producing joints where the clearance is less than this maximum will offer improved joint ductility and reduced susceptibility to intergranular attack. Conversely, joints with a clearance above this maximum allow the metalloids (boron or phosphorus) to precipitate out, so embrittling the joint.