DC TIG Welding Aluminum

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I've got a Power MIG 300 welder that I'm using to weld an aluminum boat I'm building.  This welder is capable of TIG welding and I'd like to use a TIG torch to seal up any pin holes in the MIG welds.  The problem is that this welder is DC only.  I've read that it is possible, but not desireable to TIG weld aluminum using DCSP.  I've tried it, but the results aren't pretty.  I do get melting, but the molten pool looks like its got a lot of crap in it and when it solidifies, it looks very pitted and craggy.  I'm preparing the surface by wiping with acetone and stainless streel wire brushing (heavily).  Does anyone have any advice on this?
Reply:All my reference material says AC for aluminum TIG.  An O/A torch might do the job on the pin holes.Be well.hankj
Reply:For DC aluminum, I'd reccomend a helium/argon mix, or pure helium.  Pure argon is out of the question.Sharpen the tungsten, and be careful...it will burn a lot hotter than AC does.I'd reccomend cleaning with a scotchbrite wheel.  Even a stainless brush can contaminate the aluminum.  Aluminum is very porous, and therefore rough (even if at a mircoscopic level).  Excessive brushing, especially with any motorized wire wheel or brush will embed small particles of stainless in the aliuminum....and it can interfere with your efforts to make the best weld possible.  I learned this the hard way..  I kept brushing and brushing parts thinking they weren't clean enough.   I finally figured out I was only making it worse.
Reply:Engloid,  I was wondering what is the reason that you said argon is out of the question.  The main advantage of helium is that it allow greater heat input into the work piece, which is helpful when welding thick sections of high conductance metals such as aluminum or magnesium, but when patching pinholes in a boat which is probably thin, I wouldn't think that that would be needed.  Running on DCSP he is going to have to watch his heat input as it is, because most of the heat will be going into his workpiece.  If AC is not available to him, then I would recomend that he use DCRP with argon.  On thin metal and using a large electrode he will still probably get enough heat and will have the advantage of the plasma ions self cleaning the AL  oxides.
Reply:It's been a while since I last tried DC aluminum with argon, so I forget exactly what it does.however, I do know I have a mental note posted in the back of my mind that says it doesn't work.  I'm thinking it just turns it black like aluminum AC does when your argon runs low.
Reply:Can We see a Picture of the boat when your done?
Reply:i just tried aluminum with dc tig, dcep was tungsten melting up into a ball and arcing out. dcen had a good arc but when i took it away from the metal, it was porous and clumpy....StangnetShop Full Of Stuff. Joey
Reply:Vipermanz,  I think your results are exactly what is to be expected.  On DCEN 70% of your heat goes into your workpiece, so you get a good arc and deep penetration.  The "porous and clumpy" results are do to the lack of Al oxide breakup.  Any time you strike an arc you immediately form a plasma,  which is any gas that is superheated to the point where it will conduct electricity.  When you are using DCEP, the positively charged plasma ions are attracted to the comparatively large negatively charged workpiece with such force that the are physically able to break up the Al oxides that are formed during the welding process.  This is  the AC advantage of having part of your arc DC- for penetration and heat and the other part of you arc DC+ for oxide breakup.  When you tig weld on DCEP 70% of your heat ends up in your relatively small electrode.  It should ball up into a nice sphere, but if it is balling up then melting off, you are using more amps than the electrode can handle.  A pure tungsten electrode will only handle about  1/8 the amount of amps on DCEP as it will on DCEN.  That is why you have to use large electrodes and small amps, and why you are limited to thin workpieces that won't draw your heat out on you.  If I can get to the back of my garage this afternoon, I'm going to try Engloid's experiment on DCEN with argon and see what happens.
Reply:DCEN is used for automatic high speed welding, such as, shielded power cable. Shielded power cable is the cable bundled inside of a corrugated aluminum case that has a plastic skin covering. Used for extremely high voltage entrance cable to mills and plants. The bundled cable is wrapped inside of the aluminum  and then welded. It does not injure the enclosed wires or plastic sheathing. This being a high speed welding operation it generally is NOT used manually because of the effects already noted.DCEP is not recommended. You've seen that result.Helium is used for greater speeds or deeper penetration.Power brushing of any type is not recommended due to the venturi effect created by the high-speed wheel sucking air onto the clean aluminum.  Instant heavy aluminum oxide. Manual cleaning is preferred and recommended.
Reply:Engloid,  I just finished trying to use DCEN and argon and got terrible results.  Lots of pitting, contamination and porosity.  I don't think it has anything to do with the argon, rather I am pretty sure it is the DCEN that is causing the problem.  I don't have any helium to play with, but I would be willing to bet that you would get the same bad results.  I also welded up some 1/16 inch AL using DCEP and argon and it turned out just fine.  I got good CJP, good fusion and no contamination or porosity.  I was doing butt joints with 1/32" gap and it looked just like it had been welded with AC except that the bead profile was slightly  flatter and wider  which is to be expected since your arc is not as focused on DCEP.  A couple of things to remember when doing this is not to exceed the capacity of your pure tungsten electrode and not to over heat your torch if it is not water cooled.  It also helps to increase your post flow to shield your electrode a little longer.   As mentioned earlier, since about 70% of your heat ends up in your electrode, you have to use a large electrode with small amps and you are limited to light gage metals unless you have an awfully big tig torch.  I was using 1/8" pure W and running about 50 amps with no problems.
Reply:From the dark recesses of my memory, Heliarc, the original patented name for TIG got that name in large part due to the use of Helium as the shielding gas for welding aluminum.The process was very difficult to be kind, and not a lot of people were interested in pursuing it.Switching to AC and adding hi freq to the arc definitely made welding aluminum a lot easier.Appreciation Gains You Recognition-
Reply:Originally posted by docwilson Engloid,  I just finished trying to use DCEN and argon and got terrible results.  Lots of pitting, contamination and porosity.  I don't think it has anything to do with the argon, rather I am pretty sure it is the DCEN that is causing the problem.  I don't have any helium to play with, but I would be willing to bet that you would get the same bad results.
Reply:Well, I can testify DC don't work for damn with Argon on aluminum.  I got thoroughly frustrated trying it till somebody told me to switch the machine to AC.Appreciation Gains You Recognition-
Reply:Thanks a lot for all of your input.  I had been thinking about getting some bigger electrodes and trying DCRP, but I think that even with 1/4" electrodes, I may not be able to run a high enough current.  This is a big boat I'm building (27') so the material is between .16-.25" thick.  That will suck up a lot of heat.  Even with 1/4" electrodes, I can still probably only run at around 125 A.  I'm not sure if this is sufficient.  Plus those electrodes are expensive, so it would be a costly failure if it doesn't work.
Reply:Engloid, you've got me going now.  I may have to go out and buy a bottle of helium just to try it.  I normally never use DC on aluminun because  I have a square wave tig.  Looking forward to your pictures!  I still learn something everyday.
Reply:I've also read that when you do brush you soon to be welded Al joint it should only be brushed in one direction.
Reply:Docwilson...  After more thought, I considered your statements about argon not working and thinking that it was likely the negative polarity instead of the gas...but do consider that the gas is considered an essential variable in all specs (to my knowledge)...and that although nitrogen makes a fair purge, it won't work for crap for the actual welding gas.I actually got out a bottle of helium the other day at work and was going to do a few sample runs, but couldn't get it to run very well...    and then noticed that some idiot had put an empty bottle in the full rack, and it was barely giving any coverage at all.  I didn't have time to get another bottle.  Don't let me forget and I'll give it a shot again.
Reply:hi.  right  now i work in an aerospace plant manufacturing the external fuel tank for the space shuttle and for many years, and also still to this day, we weld EVERYTHING using DC straight polarity.  the tank is manufactured out of 2219 aluminum and 95% of the welds are automated, but yet still, all DC straight.  the MAIN thing is for u to have ur workpiece as clean as possible.  u need to wirebrush the heck out of it and have it as clean as u can, cause DC welding does not have the cleaning action of AC.  also we use 100% helium on all of our welds.    there is also a good bit of manual welding done on the tank and all of that is also DC straight with helium.  everything is x-rayed and has to be water clear, but we do get good results.   like i said...focus on having a CLEAN joint. we weld thicknesses from .090" up to .850" with good success.    we now also have advanced to something called plasma arc welding, and also stir friction welding,so we are advancing to other methods for the future.  i will say that 2219 aluminum will react alot different also than what ur using, but it certainly can be done.
Reply:Engloid did u get any pictures yet?