DCEP GTAW aluminum welding methodology

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Dear Forum Members,I am an engineer who worked at Argonne National Labs during 1997-1999. During my time there, I worked with several physicists with beamline experiments related to x-ray CT scans of micro-scale objects.These experiments were performed courtesy of the Advanced Photon Source, the highest profile brilliant x-ray source in the world. Basically a linac w/wigglers attached to a synchrotron sending extremely high-powered electrons into a magnetic "storage ring" - this thing stores energy at 7 GeV.To get an understanding of how precise the APS is, the storage ring components cannot deviate by over 0.1mm between any of the components making it up - to a total allowable error of 5mm over 1734 METERS.In addition, all of the pressure vessels in the beamline must be welded to UHV specs - that is, be capable of holding a vacuum of 10^-9 millibars.  This means no permeability of welds - no pinholes - no nothing.All of this is meant to convince you of one thing: The welders who made this thing knew what they were doing and no amount of bull**** was going to hold this thing together.----Now read this:http://www.aps.anl.gov/Facility/Tech...254/ls254.htmlThis document shows very clearly that 470 welds were made using the following equipment:200 amp GTAW, DC Electrode Positive polarity, on 0.180" thick aluminum with full penetration. Welding material: 6063 aluminum with 4043 filler. Welding gas: 75% helium/25% argon.---Comments?
Reply:Interesting.It sounds like these things have to be so precise that weldors can't be involved in the process.  I have heard of folks using dc tig for aluminum, but I have to say that nobody I know in the field uses it.  I have also never seen a mix 75/25 mix of helium and argon used in aluminum tig, but then again, there are lots of mixes I haven't seen or used.  I read the article pretty carefully and noticed that some relatively sophisticated equipment is used in the automated process.  "The welding parameters were integrated with the motion programs, resulting in repeatable settings so necessary in a production environment. All welding parameters (arc voltage, weld current, wire feed speed, and axis velocity) were controlled within +/- 1% of the programmed values by real-time process monitoring. The stability of the welding arc was guaranteed by a stable and responsive Automatic Voltage Control (AVC) head which allowed precise welding torch travel normal to the work while welding, in order to maintain a constant distance between the tungsten electrode and the parts being welded."Is all this simply to automate, for quality, or is it necessary for other reasons?  Also, dc has been used very frequently in mig applications on aluminum.  The weld quality is lower, but that can also be due to other factors than the type of current used.  In fact, the real difference, as I understand it, between your basic dc mig process and the dc tig process you mention here is the source of the heat (tungsten electrode versus the filler metal itself).  Can you explain why y'all didn't use mig instead?  Seems like it could be just as effective, and maybe simpler to set up and control (fewer objects to control).One issue that stands out in this article is that dc current forces you to exert more effort in hand-cleaning the materials to be welded.  Would you say this is correct?  It sounds like this is one of the big tradeoffs of using dc tig, is that you must REALLY...REALLY clean the surface of the metal before welding, because the cathodic cleaning that occurs in AC is not present at all in DC."The APS welding systems do not have inverter controls. Instead, each joint was hand-scraped immediately before welding to remove the heavy oxide layer. Cleanliness must be a high priority to obtain UHV-quality aluminum welds."If even the slightest contamination is an issue, vigorous hand-cleaning and dc might be just the route, versus pushing the crud aside in the AC process.  I had never given that much thought to it before, but if this is the case, I can see why AC is the current of choice for tig weldors, and that though DC is possible and may be preferable in some situations, it might not be the best current to use on metals that are either not perfectly clean or in an environment that is not as controlled as where you worked.Thanks for the information.Smithboy...if it ain't broke, you ain't tryin'.
Reply:Sonoronos,Do you have access to the articles mentioned in the references?Specifically, I am interested in #3, #4, #8 and the full paper of 9.Smithboy...if it ain't broke, you ain't tryin'.
Reply:I knew you can weld in the dCEP setting, have never done it myself, I once new a guy at a old job that showed it to me once75/25 ar/he is widley used in alum welding,( mostly in the mfg/production shops,)  the added helium give you a hotter arc ( helps with penetration), but you loose a little of your cleaning action, I have also used a 50/50 ar/he mix . If you have a small amperage machine these gas mix's will help you a little when welding thicker materialsI have certified with that Mil -Spec that is quoted, actually it is superseeded now( as well as the AMS1595 spec) by the AWS D1.1.
Reply:The 50/50 mix I have used, but only for instructional purposes.  I also used a 98% argon, 2% helium mix, but that's about it.  Since I got my own stuff, I stuck with pure argon for simplicity.  When I started, it seems like there were only a few mixes even available.  Since I have been in a completely different field now for more than a decade, there is a lot I have lost, some things that I never got information about, and still more that has come about in the meantime.  Such is life, I guess.  Since I am pretty comfortable in my job these days, I have found myself seeking out information in areas in welding where I have big gaps.  The more gaps I fill, the more gaps I find.  Who says welding is not interesting?  I do a fair amount of aluminum tig, so, if there is a better means available for doing it, I am definately interested (even if I dont adopt the method).  Then again, I am pretty interested in most anything...goes with the job...or personality, I guess.Smithboy...if it ain't broke, you ain't tryin'.
Reply:I routinely use DC- with pure Helium to weld thick sections of aluminum. Very deep penetration, very narrow bead. Edges of weld will be smutty with a very shiny middle. This procedure is so hot that you don't have time to feed the rod. Just lay the rod in the seam and fuse it all as you travel along. Also very fast.
Reply:Hmm  thats all very fascinating.  I do believe that the mig is good for high speed, but is much more difficult for them "perfect" welds.  The GTAW process I reember hearing was used on the rotors of some bell helicopters and many of thigns.  Company near here called chromolloy,  I heard they use GTAW to build up turbine blades which were 4140 I beleive.  Anway, its just intersting to me all this neat stuff.  I do know the AC does cleaning and digging in its cycle.  I cant remember which cycle does what.  I stick with DC straight for the tig myself.  Always have, always will.   Im just redneck like that.IF it Catches...Let it Burn
Reply:i had to tig the back of a boat at work one time we did not have a tig so i got my #26 tig torch from home hooked it up to the stick welder and a tank of argon and welded a new 3/16 alum plate on the back so my boss could put his motor back on after that he got me a tig welder for the shop chuck
Reply:Originally Posted by sonoronosIn addition, all of the pressure vessels in the beamline must be welded to UHV specs - that is, be capable of holding a vacuum of 10^-9 millibars.  This means no permeability of welds - no pinholes - no nothing.
Reply:Engloid,can you clarify regarding your Helium leak testing experience?  The manual for our Varian 959 states that it will detect a leak down to 2x10-10 cc/sec (1cc in 30 yrs).Also, the article states: "TIG welding can be done using AC TIG or DC TIG. For the storage ring vacuum chamber, DC TIG was chosen because it results in better penetration welds with a smaller Heat Affected Zone (HAZ) due to less heat input to the metal."Also, in reply to Smithboy's question on the additional references, these should be available through a decent library, maybe at the local University, or maybe you can contact AWS for a copy.  AWS has the Welding Journal research supplement articles on-line, starting in late 1998.  http://www.aws.org/w/s/
Reply:Sonoronas,thank you for the interesting aluminum welding article.  There is alot of really good information about aluminum welding.  The chart of hydrogen solubility in aluminum is great data to support the discussion of porosity in aluminum.I really appreciate when welding issues are explained in terms of the science involved.But, if there is anything science can't explain, just ask the seasoned welder.
Reply:Originally Posted by pulserBut, if there is anything science can't explain, just ask the seasoned welder.
Reply:I use DC- with pure Helium frequently, even in the field. Recently, I had to weld  some quarter inch aluminum angle to a square fence post. Started with a/c but did not have enough heat or was too impatient to wait. So, switched over to DC- with pure Helium, was a breeze. Each piece of angle had two flat welds and one vertical seam. Of course, you have to be ready to move on those vertical seams. By the way, I welded those seams with 90 amps--plenty of heat.
Reply:That's good information.  Most of the aluminum I do is non-critical (agricultural), so I had thought seriously about getting a tank of HE and giving it a go on a project or two, just to re-acquaint myself with it.  When I was learning to weld, I had a chance to try it, but at the time it was not suggested as the best way to get the job done.  So, you used DCEN not EP?  How thick was the angle and the square post?  You say quarter inch, but I am assuming you mean width, not thickness, right?Smithboy...if it ain't broke, you ain't tryin'.
Reply:yeah smith,  unless youre using some really huge tungstens you ll always want dcen or ac.  All I can use is dcen because I aint got hf.  It works    By running dcep you will burn your tunsten out real fast..too much heat or current or someting I forget how that works. TO do with the direct the electrons are flowing.   Good luckIF it Catches...Let it Burn
Reply:Originally Posted by broberts5I use DC- with pure Helium frequently, even in the field. Recently, I had to weld  some quarter inch aluminum angle to a square fence post. Started with a/c but did not have enough heat or was too impatient to wait. So, switched over to DC- with pure Helium, was a breeze. Each piece of angle had two flat welds and one vertical seam. Of course, you have to be ready to move on those vertical seams. By the way, I welded those seams with 90 amps--plenty of heat.
Reply:Originally Posted by pulserEngloid,can you clarify regarding your Helium leak testing experience?  The manual for our Varian 959 states that it will detect a leak down to 2x10-10 cc/sec (1cc in 30 yrs).
Reply:.......................IF it Catches...Let it Burn
Reply:When running dc neg. electrode , about 70 percent heat is in the work and 30 percent in the electrode.  It only works well with helium.  You want a sharp electrode.  The percentages are reversed when using positive electrode.  Thats why it is easy to blow away your tungsten with this setting.  Of course we are talking about aluminum here.
Reply:I use DC when I weld on fuel cell domes for the Delta IV rockets.  The first time I was told that was how I was going to do it I laughed because I had never heard of it and figured they were just messing with me.  Now that I have welded that way I actually prefer to run dc over ac.  As long as you get into the habit of cleaning both the material and the filler your good to go and it brushes up nice afterwards.  I also like the fact of working off of a sharpened tungsten.  Guess its just what you are used to in the long run.