I have actually tried gas welding aluminium but I used Tin Man Technologies own flux, unfortunately I've run out now and I can't afford the postage of it so looking for an alternative. SIF might be the way
As above, depends, eg some old bike alloys can catch you out and turn into a blob preheating. Had some good results with J Matthey pre fluxed on small stuff.
What types/size aluminum are you looking at?
Generally Ali bodywork, so thinnish sheet Ali. I work with old cars most of the time. I would love to be able to do castings but I'm guessing that's probably best kept to the Tig?
Generally when we build bodies we use gas welding. Tig welding can leave a harder area around the weld so in a few years time the metal becomes fatigued and cracks. So you have to use gas welding, so far I'm getting it to work but I'm still blowing occasional holes, a lot more practice needed and now I've run out of flux!
"Forget gas welding thin stuff like this on bodywork, you are more likely to melt holes and the distortion would be horrific."
It is how the aluminum bodied cars were done, all with gas. I remember seeing a bodyman at Aston Martin welding two halfs of a wing together, tacked first then torched with no filler.
From what little I've done I found old metal much harder to weld than new.
I did a wheeling course with MPH panels a couple of years ago and part of the course was gas welding aluminium.
Geoff Moss, the tutor, only used gas to weld his alloy bodies and they are stunning.
The lightweight E type nose cones he makes are made up from 9 sections although you wouldn’t know it.
He used the Sif flux mixed into a paste with water then brushed onto the joint and a strip of metal cut from the panel for filler if needed, but mostly he doesn’t use filler.
It is far quicker than tig and works well when you get onto it.
Also check out David Gardiners DVD, he shows the same process.
What beats a lot of people is that there is no colour change as it gets to melting point, plus the melting point is only around 650 ish C depending on the alloy, there is a subtle change in the surface texture just prior to melting point that you need to recognise.
Runs on plate without filler are a good way to learn the technique, you need a neutral flame and the point of the cone almost but not touching the material, the type of flame is very important, what you don't want is an oxidising flame, I used to teach students to use a neutral flame with the slightest hint of a carburising feather to make sure, good quality goggles are also very important so that you can see the metals surface texture as it changes.
Another thing Geoff showed us was to hold the torch almost parallel to the metal so you are not concentrating the heat and pre-heating the metal in front of the weld pool.
If you are used to tig it's counterintuitive and odd at the start, but when you get the pool going it just a matter of keeping up (or trying to)
So currently looking at the SIF fluxes, as far as I can tell it comes in two flavours No 14 or No 36. Ones higher temperature than the other, which would be the better option?
Many, many moons ago, 1978/85 I was taught to gas weld aluminium, and sif-bronze fluxes were used exclusively:
You could (can) also get aluminium brazing rods, I believe these had a higher silicon % the effect was to lower the melting point and increase fluidity:
The fluxes (then) were extremely aggressive by nature, also hygroscopic and any tin left open would result in a paste: I was taught to warm the tip of the filler rod and dip it into flux as required: and all flux residue had to be washed away to prevent further post welding damage to the workpiece:
As I've said, this was a long time ago, so things may have changed, we do not see many oxy-acetylene welders now