Question: I am brazing a 6061 aluminum assembly, the flat bottom of which sits on a 304-stainless plate. After brazing, the aluminum has a slight bow of almost 0.038 inch (about 1 mm). We heat the entire load in our vacuum furnace at 50°F per min (about 38°C/min). What caused this distortion of the part, and how can we prevent that in the future?
Answer: Thank you for your question and for the phone conversion in which you gave me a lot more information about your processing of these 6061 aluminum pieces. You mentioned that the stainless plate is about 0.250-inch thick (about 6.5 mm), and it is used in the as-received condition (you did not grind it or anneal it before brazing).
As you can imagine, the steel, being stronger than the aluminum, will dominate the equation, so to speak. If you are depending on the 304-stainless plates to keep the aluminum flat, then the steel must start out, and remain, perfectly flat throughout the process. That's apparently not happening, and the reason is probably very strongly related to the fact that you are using 304-stainless in the “as-rolled” condition with all its rolling stresses still there. These stresses are then relieved by the heat of the brazing process. This causes the plate to bow slightly, especially if the heating/cooling rate was more than could be handled by the "poor heat conductivity” of the stainless.
Whether it’s a quick “just a few pieces, one time only” or a regular production cycle of lots of parts again and again, the effects of brazing heat will be the same. Distortion will occur when fast heating rates are used (such as 50°F/38°C per minute), and non-machined steel plates will not readily soften to become perfectly flat plates during brazing cycles. This is especially true for the low temperatures used for brazing aluminum (less than 1200°F/650°C). Stainless steels require temperatures above 2000°F/1100°C for a long time to adequately “flatten” during high-temperature brazing cycles (in my experience).
So, you need to make the stainless dead-soft to begin with, then carefully machine it flat (be careful, the material is now fully annealed). Then you really do need to use slower ramp rates so that the steel can fully absorb the heat (steels are very poor conductors of heat) and then be able to transfer it to the buried aluminum. Reverse this heat-transmission process during cooling.
I would recommend that you start with about 10°F/5°C heating rate per minute to allow the heat to penetrate the steel more uniformly and see what results you get. Modify subsequent heating/cooling cycles based on these results.
Conclusion: Many people I’ve worked with tend to assume that rapid heating rates and regular brazing temperatures will be sufficient to give good results, making metals lay flat and braze together well. Not true. Find the heating rate (by experimentation) that will allow all materials in the assembly (including fixturing) to come up to brazing temperature together at about the same rate. No holds will be required during the heat-up portion of the cycle.
When aluminum brazing, never depend on the brazing cycle to “flatten” any stainless plates used as fixturing. Instead, the steel should be first fully annealed, carefully machined flat and then used as a fixture base for your aluminum brazing. Remember, distortion in a brazing run does not just come from the parts you are brazing. It can also come from the fixturing that you are using!