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By Dan Kay
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.
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| 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. |
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.
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| Figure 2. Poke-tack welds still showing lots of oxidation. |
CORRECT TECHNIQUE
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!
Shown in fig. 2 is an example of some smaller poke-tack welds that, unfortunately, although quite small, still show a lot of oxidiation around the welds, and thus, inside the joint area that is being tacked together by the welds. Still
not good enough!
Hand-held, capacitive-discharge, resistance spot welders.
To me, the hand-held capacitive-discharge resistance spot welder is an excellent way by which to fixture-weld parts together for subsequent brazing. And example of this is shown in Fig. 3, where a Unitek hand-held unit is shown.
I’ve personally used such units over the years, and they lightly “tack” components together, and leave no visible oxidiation. And certainly when used correctly, there is no noticeable “light” emitted (arcing). Thus no oxidiation.
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| Figure 3. Examples of Unitek hand-held capacitive-discharge resistance spot welders. | Figure 4. Laser tack-welded assembly. A properly made capacitive-discharge spot weld, when properly done, can look virtually identical to this, with no visible oxidation in the joint area. |
Although Fig. 4 shows an example of two tiny laser welds that join the thin metal strip to the back of a small battery, a properly used tiny capacitive-discharge tack will look virtually identical to this, with no visible oxidation around the joint area at all. It will nicely hold the component parts together until the BFM applied to the joint area can melt, flow, and permanently join the components together.
Fig. 5 shows an assembly that was originally held together in proper orientation and alignment using tiny round stainless “BB’s” (approx. 0.032”/0.8mm in diameter) lightly tack-welded along the OD of the flange at three locations, 120-degrees apart from each other.
This fixturing technique uses a modified hand-held capacitive-discharge resistance spot welder, the tip of which has a small suction tube on it, holding the “BB” in place in front of the spot-welder pointed head, as shown in Fig. 6.
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| Figure 5. Brazed assembly that used BB’s for locating and holding flanged-component in place. | Figure 6. Handheld capacitive discharge resistance spot welder with an attached air-hose to hold “BB” in place in front for fixturing via ball-tack technique. |
The welding-current discharged through the head of the hand-held unit bonds the “BB” in the corner with absolutely no distortion of the flange or metal below it.
A very nice video of this process can be seen by going to the following webpage (courtesy of Soudax Mfrg)
CONCLUSION:
Tack-welding can be used quite nicely as a fixturing technique to hold complex assemblies together prior to brazing, as long as certain criteria are closely observed during its use, namely: the weld should be SMALL, just enough to hold the parts together until it can put into the brazing furnace for subsequent brazing. In that way, the following benefits accrue:
1. Damaging oxidation is prevented, and the parts can be successfully brazed without fear of any “leakers” occurring due to any excessive weld-oxidation that can easily occur when using large welds to fixture components together prior to brazing.
2. Distortion of the component parts of the assembly can be effectively prevented, because the larger the weld, the greater the tendency for the metal in the weld-area to distort. Keeping the tack-weld as tiny as possible (very easy with the small Unitek and Soudax equipment) not only eliminates oxidation problems, but also eliminates any issues with distortion of the assembly being fixtured for brazing.
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Dan Kay - Tel: 860-651-5595: - Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Contact information for Dan Kay (e-mail, phone, fax, etc.), can be found by visiting his company’s website at: http://www.kaybrazing.com/
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