Problems with braising

I have used braising quite a bit in making my repstrap machine Tommelise. Heretofore, where I've used it is in the making of the heated extruder barrel, which on my machine is made from copper. An ordinary copper-phosphorus braising rod and a propane torch works quite satisfactorily.

Recently, however, I've needed to do a spot of braising of aluminum to aluminum and brass to brass joints. I bought the appropriate braising rods for each of these and a new, MAPP gas torch which gets quite a bit hotter than propane.

Here's what happened. In both cases I managed to melt the braising rod onto the joint. Unfortunately rather than settle into the joint, the molten braising rod in both cases just set there like a puddle of mercury and adamantly refused to settle into the joint.

At first, I thought that the rod wasn't melting, thus the MAPP gas torch. Turns out that it was melting, but the surface tension of the molten metal was very high and simply would not meld with the metal joint.

Has anybody had this experience? For somebody who knows how to do this, what am I doing wrong.

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison

Hi Forrest,

when using soldering paste i have the same when the metal-base isn't hot enough.

You have to heat the metal around, and then dip the braising rod into the hot-spot, so the melting front wet the surface and didn't form a blob on the rod itself.

A hint for aluminium: - there are some aluminium soldering pastes around, which can be used with a normal soldering bit ...

Viktor

Edited 1 time(s). Last edit at 08/12/2008 03:19AM by Viktor.

I have never done any brazing but it sounds like exactly what happens when soldering if you melt the solder with the iron instead of heating the joint with the iron and melting the solder with the joint. It burns off the flux before it has chance to deoxidize the joint.

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I made a much bigger effort to get the joint good and hot before trying to braise. That's quite a trick with aluminum, mind. I did get some success that way, meaning that I got the braising rod aluminum to actually bond to the aluminum.

I also found out something else about braising aluminum. It is very hard to braise onto a smooth, extruded surface. The cut end of a piece of aluminum extrusion, however, will accept a braising attempt pretty well. That means that I will have to plan my braised connections more carefully that I'd originally thought, I guess.

I haven't had that sort of problem with copper, though. The copper-phosphorus braising rods will stick to just about any copper surface you try them on.

Thanks guys!

Hi Forrest

Is the extrusion anodised? If so that may be part of the problem. I think you need to get the oxidisation off mechanically just before you braise.

Edited 1 time(s). Last edit at 08/13/2008 05:05AM by Ian Eagland.

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Regards

Ian

What Ian said. Try using some steel wool on it right before you attempt the braise.

If you are after high quality joints you should avoid normal steel wool and use stainless steel wool. You should also use fresh wool each joint. If you use anything to clean surfaces that has been used on different materials you can get cross contamination that can weaken the joint. I have found out the hard way that for good welding and braising you really need to be squeaky clean. I understand that some joints really need chemical cleaning just before welding/braising but that of course brings its own dangers!

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Regards

Ian

emt Wrote:
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> Hi Forrest
>
> Is the extrusion anodised? If so that may be part
> of the problem. I think you need to get the
> oxidisation off mechanically just before you
> braise.

I don't think the side surfaces of extruded aluminum are anodized, though they look it. I used a small rotary rasp in my Dremel tool and roughed that up a bit. That done. The rough surface accepted the aluminum braising rod material beautifully. It looks like it will take an oxy-acetylene rig to braise the brass, though, from what I've read. The mapp gas torch I have simply won't melt the nickle-silver braising rods that are supposed to work with brass and steel.

I'm thinking of trying silver solder with the brass. I'm trying to make a brass extruder barrel instead of copper. The braised copper barrels that I've made have successfully handled the pressures and temperatures needed to handle HDPE. Unfortunately, the thin-walled copper extruder barrels I designed bend rather easily if I don't handle them very carefully. Brass at those wall thicknesses are much more robust.

Edited 1 time(s). Last edit at 08/13/2008 08:32AM by Forrest Higgs.

I managed to braise one of the guide slides for my z-axis early this morning.



Thanks for the suggestions, guys. I think I would have given up without you all.

Hi Forrest, Victor..
I red your comments and see you know the subject.
I am a novice and hobby man currently working with 1.2mm aluminium sheet.(It has nice "stacco"surface)
If I need to attach piece to another, I use reverts, which I do not like.
Can I solder it( lets say joint 10mm, 50mm, 100mm)
I do not need it have to be very strong as there are no pressure on the joint other than light weight of two jointed pieces.

I am going to buy gas torch for this. What do I need?
Thanks Sergei

Sergei,

I used a MAPP gas torch to do the job. I found that the braised joins were fragile and broke easily. Since then I have been using epoxy adhesives to join aluminum parts with much better success.

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison

Sergei,
To quickly answer your question, I would say use a TIGG or acetylene. For a longer answer - keep reading.


If you are going to braze aluminum then rough it up first to remove the aluminum oxide off the surface. You can rough it up with steel wool, a wire brush, sandpaper, bastard file, dremel tool - whatever. The shinier the better, just like when soldering. The same applies to brass.

Heat the base metal as hot as you can and then put your brazing rod to the joint and melt the rod into the joint. If the melted rod doesn't stick to the base metal then you may need to flux the base metal (use flux designed for the base metal - aluminum flux for aluminum, copper flux (plumbers flux) for copper or brass). If it doesn't stick after fluxing it, then you are not getting the base metal hot enough.

Torch Choices:
Oxy-acetylene will braze quickly and easily but it can also melt the base metal if you are not careful.

Acetylene will braze pretty quick too, but it takes it a while to melt the base metal - which is a good thing. Acetylene is my preferred brazing torch.

MAPP (propane and acetylene mix) - works great with copper and brass. It should work with aluminum, but will probably take a long time. I've never tried it with aluminum.

Propane - works fine with copper and brass. It's a little slow with brass, but not too bad. It's not hot enough to braze aluminum.

Soldering gun - will work with very thin copper and brass.

Brazing rod choices:
Choose the rod that is appropiate for your base metal, your torch, and the joint strength that you need. If all you have is a propane torch and you need to braze aluminum - then you may have to use a giant lump of lead to make the joint.

Options besides brazing:
Have someone weld it for you. It may cost $10(US) for something like the z axis above, but you won't have to worry about it breaking.

Use epoxy, liquid metal, plastic metal, or some other type of glue.

Make a molded plastic joint to snap the metal into. For instance with the z axis above, a plastic end cap will hold it together. You could heat the end of the metal up and plunge it into a piece of 10mm (3/8 inch) delrin to make the correct shape, then remove the excess plastic. And delrin is available cheaply in the form of plastic kitchen cutting boards.

Use screws, rivits, or some other form of fastener.