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Let me start by saying I am not the best tube guy, in fact I would say I am around a C-B grade tube welder. Not that I have seen any pictures of master grade aluminium tube welding on this forum but I see bicycle welds that just blow me away & inspire me to do better. The problem is that I just don't do enough a year.
1.6mm or 2.4mm 2% lanthinated sharpened to blunt point,
100% Argon at 7 litres per minute,
No 9 torch adaptor on a flexhead No 26 Profax torch,
No 9 cup,
Mid size tail cap,
75% AC balance (there is no standard for this some times different machine makers reverse this so 25% is the sweet spot),
Amps not sure (set the machine to max and just use the foot control)
2 second preflow &
10 seconds post flow.
I used to preheat the part with Map gas then an air heat gun but now I just strike an arc and let soak the weldment until it looks right before adding filler. If the part is less than 3mm thick I clamp on a scrap of aluminium angle to draw the heat away. Crazy is it not one minute we want heat in the job the next we are drawing it off. The No 9 torch adaptor halfs the size of a No 26 torch allowing me to get it in more places. It also reduces amp carrying capacity. Being an air cooled torch when welding thick aluminium plate it does heat up in the hand if you are on the peddle to long.
Just to add to my last post.
Cleaning aluminium is more important than the settings sometimes.
I used to do all the text book cleaning (SS wire brush) but gave that away as the finished product looks poor, completely scratched in the welded areas. It looks even worse if you intend polishing the part as the spent polish rests in the scratches highlighting them. You can and I have sanded the scartches out to produce a mirror finish but it takes more time than the welding. I now use a super fine scotch brite linishing belt on all weldments including the filler rod (if it has been left out of its packet or just looks dull).
I did not state the filler rod above because that depends on the series of aluminium that you are welding. I mainly use 5356 on 6061 series aluminium. Remember that not all aluminiums can be welded. This can screw you over sometimes. An aluminium that machines great normally can not be welded. I always ask my aluminium supplier when dealing with round bar "Can that be welded".
My TIG welding machine is an inverter type that is 6 years old now and still going strong. But being that old it is not digital nor does it have 2 features which can help when TIG welding aluminium that I would like. The first is AC frequency control. Being able to adjust the frquency tightens the arc so 90 degree fillet welds are easier to do and with a smaller bead width. This tightening effect melts the material right in the interface between the two plates, this has to be better and faster. The second is the spot weld function. Clamping a weldment is time consuming but having the ability to tack a job together without filler would speed things up and free up a hand. I do tack weld projects now but I can see the advantage with this function.