Fig. 1 Strength of the Brazing Filler Metal (BFM) vs. Gap Clearance. (From report by Robert H. Leach, Handy & Harman Research Laboratory, 1939)
One of the most widely used charts in the field of brazing is the strength vs. clearance chart created from work done in the Handy & Harman laboratories in Fairfield, Connecticut back in the 1930’s. This chart is shown above, in Fig. 1.
Notice that as the joint clearance gets tighter and tighter (moving from right to left along the bottom axis), the tensile strength (as shown on the vertical axis on the left-side of the chart) gets higher and higher. Although there is a lot of experience with this over the years, and general acceptance of this information is widespread, it must be pointed out that this chart is very specific only to the actual testing performed in making this particular chart, and may not be identical to tests performed by others using similar materials or conditions. But the general principal of increased joint strength with tighter gaps can be accepted.
What About That Strength “Drop-Off” Below .0015” (.04mm)?
Please notice that something strange happens at the far left of the chart in the area shown by gap-clearances of about 0.0015″ (0.04 mm) or less. There appears to be a drop-off in the tensile strength when the gaps are tighter than 0.0015″ (0.04 mm).
A number of years ago I attended a brazing conference, and a PhD metallurgist was using this chart in his talk that day. He told the audience that, based on that chart, people should not allow their brazements to have gaps tighter than 0.0015″ (0.04 mm) because joint strength obviously falls-off (gets weaker) when gaps are tighter than this. Unfortunately, the speaker was giving incorrect information to his audience. In reality, there is actually nothing wrong in designing joints to have brazed-clearances tighter than 0.0015″ (0.04 mm).
The original Handy & Harman report is, unfortunately, apparently no longer available. However, we do know enough about those original tests from other subsequent reports and articles to understand that the data for the chart shown in Fig. 1 was generated by flame-brazing (torch-brazing) two pieces of 304 stainless together in a butt-joint configuration, using silver-based brazing filler metal (BFM) and a brazing paste-flux (since it was being brazed in air with a torch).
The stainless steel test pieces being brazed were apparently designed so that the cross-sectional area of the stainless on each side of the joint was much greater than that in the brazed-joint (thus the test specimen was tapering down rapidly as it approached the joint area). Such a test-specimen design would insure that failure was designed to always occur within the joint itself, and not in the stainless base-metals involved. Thus, the increased values of “strength” shown in the chart actually represent the tensile strength of the brazing filler metal (BFM) itself, and not that of the “overall joint” (including the stainless, etc.)! This is very significant, and thus, very revealing about what happens to BFM as the braze-joint gaps get tighter!
Note that the tensile strength of the silver-based BFM itself is increased by the constraints of the proximity of the sides of the joint as the joint clearances get tighter. Thus, a silver-based BFM which might have a tensile-strength of up to 40,000 psi if a rod of that material were pulled apart in a tensile-testing machine, will behave very differently when that same BFM is melted into the confines of a brazed joint. The tensile strength of that BFM –in the joint– is modified by the constraints of the faying surfaces on each side of the gap. As the gap-clearance gets smaller, it reaches a point where the normal mode of metallic deformation along preferred slip-planes in the BFM can no longer effectively take place. Within very tight joints, it appears that instead of slip-planes operating, deformation can only occur by actual rupture of molecular-bonding within the BFM, requiring far higher levels of force to accomplish that. Thus, the chart shows rapidly higher and higher “strength” levels for the BFM, up to more than 3-times the levels of force that would be required to break that same BFM in a tensile machine were the BFM in a non-constrained rod-form out in open air.
Why Did That Strength “drop-off” Occur Below .0015” (.04mm)?
Remember that the chart in Fig. 1 was created based on test results of tensile specimens that were torch-brazed in air with flux in the joint. All brazements joined in air using flux WILL contain some entrapped flux residues, except under extreme laboratory conditions. There is really no such thing as a “flux-free” joint when production brazing in air with a flux. Yes, it is possible to reduce, to some extent, the amount of any entrapped flux-voids in a brazed-joint by a process called “wiping the joint”, but even then you would not get rid of 100% of all flux voids.
Therefore, when the test pieces used in creating the chart in Fig. 1 were being torch-brazed with gaps at about 0.0015″ (0.04 mm) or less, the inevitable flux voids remaining in the joint began to become a noticeable percentage of the total joint-volume remaining inside the extremely thin brazed joint, and began to negatively affect the joint strength due to their presence (percentage wise). Had the joints been able to be “wiped” thoroughly (the joint surfaces moved back and forth relative to each other while still being heated with the flame), it might have helped to remove some of those entrapped flux voids, but it wouldn’t have removed all of them.
Now, let’s look at a similar chart, as shown in Fig. 2 below: