Question: Our supplier tells us that they tumble-deburr a number of the stamped alloy-steel parts they manufacture for us before sending them to us to be assembled for subsequent brazing. We used to make these parts ourselves in our plant but have now subcontracted it out. Our supplier says the deburring makes the parts look better and makes them safer to handle (since there are no burrs on the previously sharp edges of the stamped surfaces). However, these components do not braze as well as the parts we used to manufacture in-house. The only difference appears to be the tumble-deburring done by our supplier, which we never did. When we asked them what they used for the deburring they told us they use half-inch pyramids of sintered aluminum-oxide, as well as a little water in their deburring tub. Can this cause problems with our subsequent brazing of these parts?
Answer: Yes, it is very possible that the deburring operation performed by your supplier is hurting your brazing operation on those deburred parts. Let me explain.
Brazing filler metal (BFM) does not like to bond to (or flow over) oils, dirt, grease or oxides. When the molten BFM encounters any of these, it can be stopped in its tracks (so to speak). It will not be able to flow into a joint effectively by capillary action, nor will it be able to diffuse through the oxide-coating and into the base metal. For good brazing to occur, the molten BFM must be able to “wet” the base metal it seeks to join. That is, it must be able to flow over that surface and deep into the joint by capillary action, and it must be able to alloy with the base metal (by diffusing into it while in the molten state).
The fact that oxides can easily form on metal surfaces when they are heated up, and that these oxides resist wetting (brazing) by any molten BFMs, is well known to a number of BFM manufacturers. They use this knowledge to create, and sell, brazing “stop-off” products, which are paint-like substances that can be coated onto a metal surface to prevent that surface from being wetted by any molten BFMs. These brazing stop-off compounds achieve this by literally “stopping” the molten BFM in its tracks, and thus keeping it off critical surfaces where no BFM is allowed. The chemistry of these brazing stop-offs consists of metallic oxides of all sorts (such as titanium-oxide, yttrium-oxide and aluminum-oxide) suspended in a paint-like liquid media.
In the situation that you described in your question, you are not literally painting on a liquid stop-off compound. Instead, however, the tumble-deburring operation done by your supplier is achieving the same thing using a different method. By pounding the surfaces of the alloy-steel parts with aluminum-oxide pellets, those aluminum-oxide pellets will steadily wear away, leaving a thin layer of aluminum-oxide on the metal surfaces. These aluminum-oxide residues pounded into the metal surface cannot be readily removed by degreasing or wiping.
The situation can be described like this: "So I tumble-deburred my parts in brazing stop-off and then attempted to braze them.” It doesn’t seem to make a lot of sense to do that, does it?
Recommendation: You should suggest to your supplier that they use stainless-steel pellets (BBs) instead of the aluminum-oxide pellets. The stainless BBs are non-contaminating, and any of the stainless that rubs off on the parts will be very brazeable! Always try to remember to NOT use any nonmetallic tumble-deburring media on parts that are to be subsequently brazed. Use only metallic media.
I hope this helps.