Tacking Al sheet

cl,
DC TIG welding in steel is normally done electrode negative. This puts about 70% of the heat into the steel.
The problem with DC TIG welding aluminum is this. Because alunimum dissipates heat so fast you need all the heat you can get into the plate.
However, the problem is, that molten aluminum forms a surface oxide that melts at a much higher temperature than the aluminum.
When welding with negative polarity the arc is actually jumping from the electrode into the puddle. This pulls the oxide down widdit.
You CAN DC TIG aluminum. But unfortunately, this has to be done electrode POSITIVE. As the arc jumps into the ELECTRODE it "parts the water" on the oxide.
However, because 70% of the heat is going into the ELECTRODE you're not getting enough heat into the aluminum unless you crank the amperage way up there.
And if you do THIS you will tend to fry your tungstens unless you use a really big one.
Therefore, I believe the pros in here will confirm that while you can DC weld aluminum this has to be at high amperage with large electrodes. Therefore, you're limited to welding pretty darned thin material with a large electrode at high amperage.
Correct me if I'm wrong folks. Otherwise, that's my story and I'm stickin' to it.
V

what he said....... aluminum oxide is one of the hardest material known, it is used in grinding wheels.
aluminum oxide layers create sandwich layer on top and bottom of your aluminum stock which melts at a higher temp.(soft clean alu being in the middle) one of the reason why it is so hard to wled aluminum with O/A before you have a chance to melt the top layer (aluoxide)the rest of the alu will drop out the bottom like water.
so.... the reason AC "should " be used because alternating current has "cleaning action" , as the current /arc / alternate positive /negative it breakes up this layer of alu-oxide like an imaginery tiny air hammer.

You can also weld aluminum with DCEN, but it is only used on really heavy plate. You have to use a high helium mix or pure helium, and feed in massive amounts of filler. It has insane penetration, and the weld will look terrible on the surface.
It is used for Butt welding 1" aluminum plate using an automated TIG machine, and a cold wire TIG feeder. After the weld the bead is milled off.
I have done it on 1/2" plate and it is UGLY, but does work.

I understand the implications of standard welding of Aluminum. DCEN or DCEP is possible but just more difficult due to having to break the oxide layer. The question I'm seeking is more specific to techniques of tacking work with tig. This is especially a problem if dealing with a lap joint because you do not have access to the puddle. AC does a wonderful job of wetting it out and allowing it to flow and fuse. However, DC is a different beast and I was hoping that someone would have a surefire technique for accomplishing the same feat.
DC welding of Al is all about poking thru the layer of oxide. Once you have it's skin popped with filler, you are good to go. Tacking is causing me more of a problem.
Just to give an idea of my intended purpose. I was hoping that I could use a little TA dragster 85 as a tacking gun and finishing out with OA.