Tack weld then silver solder?

[Perko7Participant @perko7]

Had a search through the forum but could not find any previous discussion on this topic. I have an assembly of steel bits that I want to silver solder together, but I am having difficulty holding them in correct alignment to do so. It would be much simpler if I could tack weld them in position with a stick welder and then silver solder. Has anybody done this before and are there any pitfalls to look out for?

I accept that I will need to very carefully clean around the area of the weld and flux well but the tacks will only be small in comparison with the area of the silver soldered joint so even if the solder did not take in that location it would not seriously compromise the strength of the assembly.

Some of the components have quite a bit of machining on them which I would not want to repeat if I don’t have to.

I may need to do a few trial assemblies first to check if it will work acceptably.

Any relevant advice gratefully accepted.

 

[Nigel Graham 2Participant @nigelgraham2]

Surely it would be better to use one or the other, only, rather than a mixture, for quality?

Using an arc-welder suggests these are fairly hefty components that could be welded anyway. The possibility of the solder not taking around the tack-welds suggests the joint does not need be fluid-tight (gas or liquid).

Since the problem appears to be one of work-holding, make simple jigs to hold the parts together.

Another approach you could consider, whether silver-brazing or welding, is to use a few judiciously-placed, small screws tapped into suitable points. This obviously depends on the nature of the components, but the screws can be cut off and finished flush afterwards if required. It also keeps the joint faces clean – which they might not be if partially welded, due to any welding flux that’s penetrated the crevice.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Perko7, I’ve never tried it myself, and never heard or seen of it been done, but cleaning all the oxides around the tack, produced by the heat, may prove to be difficult, and could effect how close the silver solder will flow. There maybe an acid flux that will do the job with success though. Tacking with TIG might be a better bet, as these can be very small tacks, and you can allow the shielding gas to continue to flow when the arc stops, which should help stop oxides forming while the area around the tacks cool. but it would be dependent on ones skill.

Regards Nick.

[JasonBModerator @jasonb]

I have considered it with some of my fabrications but in the end found other ways to hold the parts. I only have a stick welde and although you may be able to clean the visible surfaces it is the mating two faces where the solder should be that you can’t get to and I assume the solder can’t either.

Building it up as more than one assembly that can be brazed and then soldered with different melting point solders is one way. One solder and keeping the heat off previous work another. Also tabs and notches as part of the initial design will often keep parts inplace as the whole thing becomes self locating.

[NealebParticipant @nealeb]

Well, I have used this technique and it works well. At least, for the kinds of thing I was making. The frame stretchers for my Black 5 are fabricated from multiple CNC-cut steel (typically 3mm) components, using tab-and-slot to locate the pieces. On some of these fabrications I have been able to use a strategically-placed through bolt to hold them for silver-soldering but for others, a small touch of MIG weld was enough to keep it together. Wire brush followed by pickling in ultrasonic bath (my usual treatment) seemed to be sufficient before fluxing in the usual way. Didn’t really care about s-s in region of weld as I didn’t need water-tightness and weld replaced s-s for strength in that area anyway.

Why didn’t I just weld the whole thing? Have you seen the quality of my welding? TIG would have been a great choice but I don’t have the kit or the skills to use it!

[Author]

Posts