A real-world test - Durafix/Alumafix and similar no-flux aluminium brazing rods

Well, I finally had a reason to use some of the aluminium brazing rods I'd picked up at one of the woodworking shows; if you've seen the 'welding up a hole in a coke can' demo you'd know the ones I mean.

There's a number of different suppliers of the rods (I've just linked to a cheap supplier!), but essentially they are all a zinc/aluminium alloy, which means they can be used on aluminium, galvanised steel, stainless steel, diecast and similar metals, but not on mild steel.

The rods have a melting point that's about 200 degrees C below the melting point of aluminium, and they don't need flux.

As they don't use flux, the weld area should be well cleaned with a stainless steel wire brush to remove oxides.

Rather than waffle on about the welding process (hit up google or youtube for 'aluminium brazing' if you want to see how it's done), I thought I'd just describe the actual performance of the rods to actually fix something that was broken.

The broken bit:
Attachment 315059
This bit was off the bottom of a Sherwood 18' Scroll Saw. This casting secures one end of the table, so it's a kinda important part of the saw.

The prep work:
Attachment 315061
Paint was removed, and the cracked area was vee'd out. Initial vee-ing (shown) was done with a sanding bit in a Dremel (well, the Ozito version of a Dremel), but I then switched to a file and did some more substantial prep so I'd get better penetration of the braze.

Unlike soldering or silver soldering or brazing, the alumafix/durafix/whatever alloy won't get drawn anywhere by capillary action, so if you want it to penetrate, you've got to vee the join out. I did considerably more vee-ing (to roughly 3/4 of thickness) than shown in the pic, but forgot to take a pic of the final ready-to-braze state.

The weldy-bit:
Attachment 315065
I'm not going to go on about the brazing process, as there's nothing I can really add to some of the Youtube videos, but here's some extra messages:

Heat:

A big torch is good; not oxy-acetalyne big, necessarily, but not a pencil flame propane torch either. It's not because of torch temperature so much, more the sheer capacity of aluminium to conduct the heat away on larger castings.

Consider a mild steel backing plate:

Keep in mind that aluminium/zinc alloy is very similar to aluminium; solid one moment, and an embarrassing puddle the next. As soon as the rod starts to mix with the cast aluminium, the casting's melting point drops locally, so a backing plate will reduce the chance of holes opening up. It's not too difficult to close an accidental hole as the alloy is quite well behaved when building-up, but it's better not to create a hole in the first place.


Final result:
Attachment 315085
After leaving it to cool and then draw filing the weld area there were still a few little pinholes; these were filled with a few drips of superglue and a shake of baking soda.

A final sand, and a quick coat of Holden Orange (a quick 'n dirty match for the Sherwood orange) and the part is back in service, giving me a cheap scrollsaw to play with!

So if you've got a machine with a broken aluminium casting sitting in your 'one day a part will turn up' pile, don't be afraid to find the pack of aluminium brazing rods you picked up at the 1998 Wood Show and give fixing it a go!