This question comes up frequently because many people understand that holding parts together in proper alignment prior to brazing is very important, and they assume that tack welding the parts together at room temperature will help maintain proper alignment throughout the entire brazing cycle. That is often just not true.
Tack welding must be done properly or else it can actually prove highly detrimental to brazing quality, sometimes resulting in considerable distortion of parts, accompanied by a very significant lack of braze-fill inside the joints. An example of this is shown in Figure 1, in which you can see how the thinner sheet distorted badly because of the constraining welds at each end.
For this reason, and based on my own experience at many customer locations, I urge people not to tack weld assemblies together unless they fully understand the proper way to tack weld for brazing. This can be very different than tack welding prior to welding or tack welding parts prior to painting, cleaning, etc.
In Figure 1, the proper way to tack weld the thin sheet would have been to place one tiny tack weld halfway along the sheet length. Had that been done, the thin sheet could have expanded and contracted away from that small tack weld without any distortion whatsoever. But once you place two or more tack welds along the length of components of different mass, you can almost guarantee that permanent distortion will occur.
When brazing, the parts that make up the assembly will expand when they are heated. Thinner parts expand much more rapidly than heavier parts, even if they are made out of the same base metal. In the photo, the base metals of the thin strip and the heavier part were both 304L stainless. As the tacked assembly was being cooled down from brazing temperature, the thin strip wanted to cool down and shrink much more quickly than the heavier part. The thin strip, in fact, will shrink faster than the heavier part. But because it is constrained between two welds, it cannot shrink properly, since it can’t pull away from the two welds at each end – it’s forced to stay at that length. Trouble.
Based on this info, imagine what happens to the atoms in the thin part in the photo as it goes from room up to brazing temperature, is held a while there, and is then quickly cooled (furnace cooled) down to room temperature once again. At brazing temperature, the assembly is held there long enough so that both the thin strip and the heavier part are at the same temperature, become stable there, and the spacing between the atoms in the thin strip will be the same as the spacing between the atoms in the heavy part. All is stable.
Then the part is cooled. The thin strip will cool more quickly than the heavier mass, so its atoms will contract and get close together quite quickly compared to the heavier bar. Thus, the strip will shrink in size much quicker than the bar. But … the strip is constrained at both ends in the photo (or between tack welds in any tack-welded assembly). Thus, even though its atoms are coming closer together, the welds gripping the ends of the strip don’t allow the strip to shrink to a shorter length. But the atoms will still get closer during cooling! This creates a nightmare situation. The strip will shrink, but because it’s tacked on each end, the welds will force the strip to stretch (yield) because the welds won’t let the strip go to its normal shrunk size that it had at room temperature. This now-stretched-thin strip will begin to bow as the much slower-cooling bar finally comes down to room temperature, and Figure 1 is evidence of what happens.
Always strive to allow the different sections of a part to grow and shrink on their own, at their own rates, without any undue constraints caused by multiple tack welds.
- Best fixturing. No tack welds at all. Allow the part to expand/contract freely, using only wires or gravity as fixturing, to prevent any distortion resulting from too many welds.
- Good fixturing. This can be achieved if there is only one tack weld, and both the thin section and heavier section can expand away from that spot and contract back to that spot at will, with no constraints.
- Worst fixturing. Multiple tack-welds on parts to hold different components in position.
Remedy: If you are forced to use multiple tack welds for some reason, then both the heating and cooling rates of the whole assembly must be very slow so that the thinner member of the assembly only expands and contracts at the same absolute rate as the heavier part. This is not always easy to do. Rarely have I seen parts heated and cooled slowly enough so that distortion of parts with multiple tack-welds does not occur. It is always best, in my experience and in my opinion, to use as few tack welds as possible. Use only one tack if you can do so.