Re: Tacking Al ***

On 29 Aug 2005 16:50:35 -0700, "Vernon" <vtuck@xxxxxxxxxxxxx> wrote:

>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.

.

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