Is Multi Pass with Tig on aluminum OK

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Question by a rookie here. I have been in class doing some stick welding. Laying multiple passes on 3/8 steel. I will be starting to learn Tig in the next month and when I buy my own machine I was wondering if I can do multi passes on aluminum when the thickness will surpass my welders capabilities? My budget and shop power for a Tig unit will probably limit me from 185 to 225 amps . (Arcmaster or precision Tig 225) . I doubt I will have the need to weld too much that is more than 1/4" , but it would be nice to be able to do something quite a bit thicker if the need arises. Is it possible to do a sound weld on thick stuff  with a 200 Amp Tig? (This is not a production environment, so duty cycle should not be an issue)Thanks, Bob.
Reply:Yes, it is possible.  I have done it with my TA185 enough times.MM350P/Python/Q300MM175/Q300DialarcHFHTP MIG200PowCon300SMHypertherm380ThermalArc185Purox oaF350CrewCab4x4LoadNGo utilitybedBobcat250XMT304/Optima/SpoolmaticSuitcase12RC/Q300Suitcase8RC/Q400Passport/Q300Smith op
Reply:That is good to know Desert Rider, Thanks.
Reply:As with many things in welding, it depends on the application.I was always taught that "double pass" welds in marine fabrication (towers, t-tops, etc) was a definite NO-NO.  Adversely affects the aluminum in the HAZ and leads to premature failure.  I have seen this to be the case in many tower failures.  It's what makes repairing someone else's work so difficult (or at least time consuming).Aluminum reacts to heat differently than mild steel.  You see more of a change in the metalurgy of the base metal to heat than you do with steel.  Piling on more filler, if you didn't get proper penetration in the first place, seldom results in a "good" weld.Personally, I think it would be wrong to make a blanket statement that multi-pass welds are OK in aluminum if maximum strength is the goal.Syncro 250 DX Dynasty 200 DXMM 251 w/30A SG XMT 304 w/714 Feeder & Optima PulserHH187Dialarc 250 AC/DCHypertherm PM 1250Smith, Harris, Victor O/ASmith and Thermco Gas MixersAccess to a full fab shop with CNC Plasma, Water Jet, etc.
Reply:In a properly made multi pass weld, the root has proper penetration before any subsequent passes go in.  Subsequent passes are made to fill the joint to proper throat depth (for a fillet) or flush with the base metal (for a groove).   Trying to do one massive weave pass to fill a big joint is worse than putting in multiple stringers while maintaining acceptable interpass temps.   That is what I was always taught.MM350P/Python/Q300MM175/Q300DialarcHFHTP MIG200PowCon300SMHypertherm380ThermalArc185Purox oaF350CrewCab4x4LoadNGo utilitybedBobcat250XMT304/Optima/SpoolmaticSuitcase12RC/Q300Suitcase8RC/Q400Passport/Q300Smith op
Reply:So Desert Rider,Does that mean that you do a lot of multi-pass welds on the "thick stuff--aluminum" with your TA 185?What kind of strength, relative to the base metal, do you think you achieve?Syncro 250 DX Dynasty 200 DXMM 251 w/30A SG XMT 304 w/714 Feeder & Optima PulserHH187Dialarc 250 AC/DCHypertherm PM 1250Smith, Harris, Victor O/ASmith and Thermco Gas MixersAccess to a full fab shop with CNC Plasma, Water Jet, etc.
Reply:I don't weld alot of THICK aluminum, but I do on occasion, with the TA 185.   Nothing has broken thus far.  If I wasn't sure it wouldn't break, I wouldn't send it out with the customer.MM350P/Python/Q300MM175/Q300DialarcHFHTP MIG200PowCon300SMHypertherm380ThermalArc185Purox oaF350CrewCab4x4LoadNGo utilitybedBobcat250XMT304/Optima/SpoolmaticSuitcase12RC/Q300Suitcase8RC/Q400Passport/Q300Smith op
Reply:Originally Posted by SundownIIISo Desert Rider,Does that mean that you do a lot of multi-pass welds on the "thick stuff--aluminum" with your TA 185?What kind of strength, relative to the base metal, do you think you achieve?
Reply:Good answers, very similar to my instructor. He basically answers a lot of questions saying the answer depends on too many variables. Thanks.Bob.Miller Dynasty 200 DXMillermatic 211Hypertherm PM 600In Tool acquisition Mode
Reply:Originally Posted by DesertRider33In a properly made multi pass weld, the root has proper penetration before any subsequent passes go in.  Subsequent passes are made to fill the joint to proper throat depth (for a fillet) or flush with the base metal (for a groove).   Trying to do one massive weave pass to fill a big joint is worse than putting in multiple stringers while maintaining acceptable interpass temps.   That is what I was always taught.
Reply:I have qualified several weld procedure specifications to both the ASME B & PV  and AWS D1.2 code using both single and multipass welds. A problem can occur if the welder decides to "pretty up " a poorly executed bead by "washing" it with no filler. I certainly can not speak to all aluminum alloys, but to the best of my knowledge, aluminum reverts to the anneal condition in the HAZ no matter the number of passes. the reater heat input will yield a larger and therefore a weaker (lower tensile strength) area. This can only be remedied by solution heat treating.  My experience has been that the designers take into account the lower load caring capabilities and increase the cross section.
Reply:In the tool and die world it is done all the time.
Reply:This is a follow up on a previous discussion.My original position was that every additional pass in welding aluminum further reduces the strength of the base material.After reading responses on both sides, some saying it's ok, and some saying it's not, I decided that "an internet forum" didn't really provide much of an answer.I sent an e-mail to Alcotec (buy quite a bit of material from them) and today I got my response.  I tried to post the response, but have been unable to upload a file in html format.  When I tried to upload it, it tells me invalid file.  If anyone's interested, and can explain how to upload a file I received as an email, I'll post it up.  If not, I'll copy pertinent parts when I get a chance to type it up.Thom Burns (Technical Services Director, ALCOTEC) brings up several interesting points, particularly regarding the shrinkage of aluminum, but, basically confirms my previous statement that multi-pass welding of aluminum (as opposed to steel) reduces the strength of the base material.Bottom line, I think it's safe to say, is those people trying to make up for an underpowered machine by multipass welding are fooling themselves into thinking it's the proper way.Syncro 250 DX Dynasty 200 DXMM 251 w/30A SG XMT 304 w/714 Feeder & Optima PulserHH187Dialarc 250 AC/DCHypertherm PM 1250Smith, Harris, Victor O/ASmith and Thermco Gas MixersAccess to a full fab shop with CNC Plasma, Water Jet, etc.
Reply:SundownIII,Dang it now I have to go out and  buy a DX700 (just to stay safe) on that one project that requires more umph then my 200dx will handle! (just kidding)  I have a lot to learn yet on the thinner material.  Right now I am saving money for the 2 visits my wife had to the ER for a goofy over speeding heart condition?  (I promised her that the HF starting on my Dynasty didn't cause it)  However when the the Cardiologist Consult suggested that they could cortorize the bad circuit, my thoughts immediately went to the tig process?? TimA rich man is not the one who has the most but instead one who needs theleast. Retired IUEC local 19AK Bush pilot,  Chaplain  CMA Chapt 26victor torchesMiller110v migDynasty 200dx (new in Dec 08)Hypertherm PM-45MM-252 (new 2/09)
Reply:Some random thoughts:I suspect that the primary consideration with multipass welding of aluminum would be interpass temperature.  So long as a guideline such as the one shown here:http://books.google.com/books?id=Lsk...um=7#PPA396,M1is observed (you need to scroll up a page to read the entire section), the HAZ should be no greater than that in a comparable weldment performed in a single pass.  This, of course, assumes that no post weld heat treatment is to occur, which I imagine is the case if large enough equipment is unavailable to perform a single pass.All of the above said, and considering that I have been welding not more than a year, I would expect that filler metal choice and quantity would have an effect on both internal stress and distortion.  I would imagine that fillers prone to hot cracking would be more so, and that it would be more difficult to keep distortion under control.  Some of this might be alleviated by opening the root on either side of the weldment and welding from both sides.
Reply:Sundown, this is probably more general than the info you received but may touch on some of the points...  http://www.aws.org/wj/feb03/feature1.html? May not be very legible if you have to resize it afterwoods but you could copy and paste the email into paint and save as a jpegMultipass Al welds are an everyday occurence- there are wires specifically for multiple passes on 6xxx series... http://www.alcotec.com/us/en/support/upload/a4643tds.pdf. Obviously more relevant regarding distortion (shrinkage) than HAZ softening as (per the pdf) it's designed for situations wher PWHT will be carried outDepending on the source multiple passes are typically recomended for sections above 6 - 8mm. Mass of the weldment is going to play a big role in what a given machine is capable of as is the shielding gas- an argon-helium mix over pure argon for example.Last edited by hotrodder; 05-24-2009 at 02:14 AM.