Need help with a wicking-type silver solder

matt_isserstedt · May 4, 2012

Have a part to make where I have a hub to attach to a shaft. Both parts are 1018 steel and I control all the dimensions.

Roughly, 1-1/2" dia shaft, hub is 1" wide.

I have considered welding but the undercut and fillet is undesirable and I may warp the shaft.

As I recall (if recollections are correct) solder does very well from a shear strength perspective when the joint is tightly fitted, and I believe I can get it into the .001" diametral clearance range.

What I am hoping for is a silver-based solder/flux combination that "wicks" like the old lead-based plumbing solders (really my only experience with solders other than electronic solders and standard brazing - which I know does not wick very well). The goal is if I can get to full penetration thru the 1" length it will likely be stronger than fillet-style welding and with less induced stress and no fillet to get in the way. Hopefully a paste-style flux too which I can apply easily to both parts.

Your sage advice is greatly appreciated, I am really a solder dummy

you also might tell me I can't get there from here too...which I can accept...but thought I'd ask first.

Heavey Metal · May 4, 2012

56% silver white flux

Ps (its braze not solder)

adama · May 4, 2012

It most certainly will do it, might need a 1 thou gap alll around though. Would go as easy flow number 2 over here. pre flux the parts first. Will wick just like plumbing solder but needs to be got some were around 600-700 ish Celsius depending on solder. Simply allow to air cool and it should be relatively distortion free.

matt_isserstedt · May 4, 2012

Ok, looks like a couple of options in 56%. THanks for the ideas, sorry about my clashing terminology

adama · May 5, 2012

Silver braze is commonly referred to as silver solder over here in the uk.

courtiron · May 5, 2012

silver braze

The term silver solder in this case is really silver brazing, but the use of the phrase 'silver solder' is kind of like kleenex... generic and interchangeable with the real process which is brazing.

I think that what you were asking about is a flux metal paste mix.
The use of silver brazing alloy/flux mix is a problem on deep penetration joints. BAg alloy materials (~45% silver) alloys require a temperature of around 1400 degrease or so to flow.

If you mix the flux and the filler metal the joint will never reach a 100% fill. The purpose of the flux is to scavenge oxides and allow the brazing metal to flow once the melting/flow temperature is reached. It is also extremely difficult to assemble parts with .001" clearance with flux/metal mixtures.

Once the flux melts and the solder begins to flow, they are both liquids and cannot occupy the same space. The flux is not consumed or absorbed, it is displaced, and the solder will only flow where the parent metals are hot enough to melt the brazing metal. Adding external metal is problematic and is more difficult than just using the flux and brazing metal procedure.

The solder flows between the parent metals in your example by capillary attraction. The entire joint has to be at the flow temperature. The brazing material needs to be supplied to one 'side' of the joint and allow the flux be displaced out the other side, all the while keeping the temperature high enough to allow the molten solder to flow freely to reach 100% fill.

I would think that, as much as I like hot processes, that a more suitable jointing system would be a shrink fit combined with a tig tack weld to lock everything permanently in place. This method is what is used to assemble the crankshafts on very high horsepower motorcycle crankshafts, and would likely be strong enough for your app.

paul

adama · May 5, 2012

Courtion i don't know if you have done much silver soldering, but if every thing is clean fluxed and at temp it just flows in like a plumbing fitting does. Way better and easier than a std brazing spelter. I have never used the powder metal flux, only plain flux (mixed with di water to make a paste) and solder in stick - wire form. So long as your using the flux with the temp range to match your filler its easy with good prep. Getting everything to temp and not overshooting it too much is important. I like a propane roofing type torch, oxy fuel is a bit too hot for this IMHO. But i do like to run a nice big Nozzel on the blow torch to speed it along.

If you do go oxy fuel, make sure its a neutral flame, a oxidising one will over work the flux and cause issues!

As to weather or not its the best way of making that joint, we don't know enough to really comment. But if its a high contact area joint like described then silver solder - braze could well outperform welds, especial with significant cyclic loading.

lazarus · May 5, 2012

This reminds me of the valves and fittings we had in the military. They had a grove in the center of the fitting with just enough silver solder(braze) for the joint. We would clean, flux, slip together and heat the center till solder flowed out. Almost like "Brazing for Dummies". Never had one leak like the ones we were replacing. Don't know if that method will work for your app, but it's awesome on brass.

Ben

gbent · May 5, 2012

Its going to take a lot of heat to get that shaft and hub up to temperature fast enough to keep from burning up your flux. Do you have a forge where you could heat a steel plate first to use to help heat the assembly faster? I'd use at least .002/side clearance, .004 may be better. Relieve the center in the easier of the two pieces to get the clearance, leave edges on both sides for register. Then take a die grinder and cut two notches in each register to allow the solder to flow into the joint.

I'd probably just put it together with about .003/.004 press using a shrink fit. If you are still nervous add a Dutchman.

courtiron · May 5, 2012

Silver Solder

adama said:
Courtion i don't know if you have done much silver soldering, but if every thing is clean fluxed and at temp it just flows in like a plumbing fitting does. Way better and easier than a std <SNIP>

42 years as a plumber, 30 as a master plumber and a ASSE medical gas installer and ASSE medical gas instructor. Also trained by the Copper Institute as a Soldering and Brazing instructor for the United Association of Plumbers and Pipefitters.

The point was not that it was impossible, only that it was a bad idea. The comparison of a copper fitting and tube to a solid steel shaft and a steel hub is nonexistent. It is easy to heat a copper fitting to brazing temperature, and difficult to heat the brazement that was in question.

The fitting, when fluxed, will properly be joined when the flux is displaced by the solder. You cannot get solder to flow into a space that is filled with flux. The solder does not combine with the flux or alloy with it. The flux prepares the surface to be wet with the solder and then gets out of the way by flowing out of the joint space. When you solder a copper fitting, whether soft solder or brazing with flux, the solder displaces the flux as it fills the capillary space between the fitting and the tube.

To braze the cross sectional area that was asked about the ENTIRE piece would have to be heated to the brazing temperature to maintain the fludious temperature within the brazement joint space.

While you can certainly successfully braze copper with a propane torch in smaller sizes, the successful 100% penetration of 1” copper is best achieved with air acetylene, and larger sizes require oxy-propane, which most professionals don't like, or oxy-acetylene, which we do. This is a function of BTU output of the flame of smaller torches which are inadequate to maintain the temperature of the brazement. The braze filler material will not flow past a cold spot.

paul

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Need help with a wicking-type silver solder

matt_isserstedt

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Heavey Metal

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adama

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matt_isserstedt

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adama

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courtiron

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adama

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lazarus

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gbent

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