Question: We are brazing 6063 aluminum-sheet components together and were wondering how much of an overlap should be used for good strength. We have heard that the overlap for other types of brazing (silver, nickel, copper, etc.) is supposed to be 3T-6T, where T is the thickness of the thinner member being joined. Should we use the same overlap for our aluminum joints?
Answer: No, you should NOT use the same amount of overlap as is typically used for other forms of brazing. Because ALL other brazing filler metals (BFMs) melt and flow well below the melting point of the base-metals (parent metals) being joined, the BFM can usually flow some distance into the joint being brazed before it stops flowing by capillary action.
With aluminum, however, it is very different! Because the melting temperature for the aluminum BFMs is only a few degrees lower than the melting temperature of the base-metals being joined, the metallurgical reactivity of the BFM with the base metal at the brazing temperature is very intense and diffusion of the BFM into the parent (base) metals quickly takes place, limiting the amount of distance that the BFM can flow into the joint.
For aluminum brazing, therefore, we suggest that an overlap of only 1T-3T be used in braze-joint design so that the aluminum-based BFM has a chance to flow through the joint and be visible on the other side of the joint.
I have seen a number of situations where the designers used the standard 3T-6T overlap criterion for an aluminum joint, and the parts had to be rejected and reworked because the BFM was not able to flow through to the other side of the joint. It could not be seen when being inspected and had to be sent back to the brazing operations with extra BFM applied on the back side of the joint. Unfortunately, this is very poor brazing practice! You should NEVER apply BFM from both sides of the joint and have each BFM flow toward the center from opposite ends. This can trap air in the joint, causing a lot of extra internal voids.
The rule of brazing should always be to “feed the BFM from one end of the joint and inspect on the other end” (i.e. check to see that the BFM has flowed all the way through the joint). For aluminum brazing, this can be achieved very nicely by limiting the sheet overlap (for shear joints) to only 1T-3T (where T is the thickness of the thinner of the two sheets being joined). By doing this, you will achieve good braze quality, full strength and hermiticity. And this will be accomplished the first time through the brazing process!