Brazing Fundamentals 
Major Factors that Affect Joint Strength

Brazed joint strength is a function of both overlap distance (for lap-joints) and the thickness of the brazed joint itself. Figure 74 shows the effect of the overlap distance on strength and Figure 8 shows the effect of gap-clearance. 

Note: Never rely on so-called raw "filler-metal strength" data to determine how strong an assembly will be after brazing! Such data is very misleading! Here's why:

From a metallurgical point of view, metals typically fail when an applied load is so great that it forces the metal's crystal lattice structure (a term that describes the way a metal's atoms align themselves) to break down and literally slide over each other along certain preferred "slip-planes" in their structure.

If you were to pull apart a Ni-Cr-Si bar in a tensile-testing machine, you would obtain a certain tensile "strength" for the bar (See Figure 9A). However, if you properly braze together two bars of high-strength superalloy material using this same Ni-Cr-Si as the BFM, and then pull the assembly apart in a tensile machine, not only will the tensile strength of the assembly be much higher than that of the Ni-Cr-Si rod by itself, but the failure will not occur in the Ni-Cr-Si BFM, but rather in the superalloy base metal (See Figure 9B)! To achieve this, it is assumed that the joint gap (clearance) is small (less than .003") and the braze has been well diffused. Failure of the assembly in the base metal occurs due to the strong superalloys on each side of the joint that "constrain," or prevent, the BFM from being able to "slip" along its slip-planes. Thus, the BFM can only begin to yield ("slip" along its slip-planes) when the constraining base metals on each side of the joint have moved away (i.e., "slipped" along their own slip-planes).

However, as Figure 8 shows, the tensile strength of this brazed tensile bar decreases dramatically as the joint clearance gets larger, until once again the strength of the assembly is equal to the BFM, and the joint failure now occurs in the BFM rather than in the base metals.

Conclusions: For maximum joint strength and capillary flow, as discussed in this and our previous articles, the parts intended for brazing must be clean (free of oil, dirt, oxides, etc), and the gap-clearance between the parallel (very important!) mating surfaces should be held between.001"-.003" at brazing temperature.

Of all the factors contributing to joint strength, without question the most important one is to keep the joint clearance as small as possible at brazing temperature!

The American Welding Society (AWS) has adopted a standard for the evaluation of joint strength, which is recommended whenever such testing is required. See AWS C3.2, "Standard Method for Evaluating Strength of Brazed Joints."

(1) AWS, Brazing Handbook, 4th ed. (Miami: AWS, 1991), p.35, fig. 2.32.