Figure 1. Fuel-rail fixtured for brazing by using large tack-welds to hold component parts in position for subsequent brazing. Note heavy oxidation around welds.
Highly complex assemblies, for a variety of end-use applications in such diverse fields as automotive, aerospace, medical, electronics, and tooling (just to name a few), can be effectively made via brazing.
For this to happen, however, each of the individual components in these complex assemblies must be able to be held together in proper alignment, have the appropriate brazing filler metal (BFM) applied to it, and this assembly then moved into a brazing-furnace, where it can be heated until the BFM melts and flows into the joints by capillary action, thereby permanently joining the components together to make a complex assembly.
An essential part of this process is the specialized braze-fixturing techniques needed to hold all the component parts of the assembly in proper alignment until the applied BFM has melted, flowed, and solidified.
Tack-welding is one fixturing method that has found broad use in the brazing industry to “temporarily” hold component-parts together in proper alignment until brazing can be completed.
Over the years I have seen many tack-welds used to fixture assemblies together for brazing, of which some welds were excellent, while some others were very poorly made.
Figure 1 shows a photo of an automotive fuel-rail assembly that has been tack-welded to hold the tubing and fittings in proper alignment. Notice the large size of the welds, and also the extensive amount of surface oxidation surrounding each of those welds.
Notice in this same photo that there are two (2) fuel-injector ports on the front side of the rail (facing you, the reader), each of which is an open hole into the fuel rail, through which fuel will be injected into the engine. The flat
flange-faces of those injector ports must be completely leak-tight after brazing.
Notice, too, not only how the tack-welds have fully melted the corner on each of those injector-ports, when joining it to the rail, but also that there is a lot of visible oxidation around the outside of those tack-welds.
Very Important Note: The oxidation caused by those tack-welds is not only on the outside surface of the steel, but is also on the surface of the steel underneath the injector-cup flanges.
Thus, the faying surfaces underneath the injector-cup flanges (which are supposed to be brazed in the furnace in the next operation) are oxidized, and run the strong risk of not being able to be brazed, since molten BFM does NOT like to bond to, or flow over, oxidized metallic surfaces!
Warning: Don’t ever be fooled by statements such as: “Oh, don’t worry about that. The furnace will clean it up”, as if the furnace atmosphere (or vacuum) will be able to reach between those tightly fitted (tack-welded) faying surfaces and remove any surface oxidation on those surfaces inside the joint. IT WON’T!
It is almost impossible for any furnace atmosphere or vacuum to “clean-up” oxidized surfaces inside tightly fitting joints once those internal surfaces have been oxidized when being tacked together.
A number of the fuel-rails (same as the one pictured in Fig. 1) had to be scrapped due to the high incidence of helium leaks that came out from underneath the non-brazed flanges around each of the injector-ports during leak-testing of each rail, the leaks having been caused by the trapped oxidation underneath those flange-surfaces that resulted from the high-heat from the tack-welding process. Those oxidized internal surfaces could not be wetted by the BFM.
The proper way to use fusion-welding for “braze-fixturing” is to be sure that the welds are extremely small, leaving no noticeable oxidiation whatsoever to the naked eye around the weld.
Remember: Any weld-fixturing is ONLY supposed to lightly hold the assembly together in alignment until it can be placed into the furnace for brazing. The braze is what is supposed to create the permanent bond, not the fixture-weld!