MIG heat input vs. TIG

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Hey guys,    I have been watching the site for a long time and finally got the courage to post.    A while back I had a project where I needed to weld on two 5x5x3/4" thick sub plates to the side of a vehicle.  The vehicle was constructed of AR500 1/2" thick.  The plates were 3/4" just to give room for threads needed to attach a mounting bracket for storage of a steel boom/arm that could go on the front of the vehicle.  The plates only needed to hold 100lbs. each so the weld bead need not be huge.    The boom and receiver hitch I made for the vehicle was done via stick but when it came to welding to the side of the rig I knew that stick was so hot that I would have a high fire danger not being able to get to the backside of the plate since it was covered by interior panels.       The professional welder I had do the "high risk" welding to the vehicle used his MIG at 230amps and basically spot welded it over and over with about a one second pull to make a nickel bead, then a three second cool off before moving 3/8" to the next bead.  He did an inch or two on each of the four sides before going to another side and letting the heat dissipate.  After a few rounds the weld was complete around all the sides and the paint wasn't even disturbed just 1" away from the bead. I was amazed at how "little" the heat affected zone was with this technique.(attached photo).     Finally my question.........  I have since acquired a Dynasty 350 and have been able to start doing some cool projects.  If I tried to match the technique of the MIG with either pulse settings or foot control on the TIG would the heat be greater or the same as the MIG?   I know I would have to be on the TIG arc much longer to get a puddle to form with the rod but I am still a little new when it comes to the heat input from the MIG process.   Here's my lame example I'm trying to run over in my head;  If I set the TIG to 250amps and hammer the pedal for one second, and I do the same at 250amps on the MIG,  is the heat input the same when it comes to the back side of the 1/2" AR500 plate? I have the opportunity to do this project for some other companies and want to know if I should start practicing with the Dynasty 350 or just buy the MM252 and chalk it up to an investment.  Either way,  I can't afford to replace any of the $500,000 vehicles if it happens to burn...  Either way,  I'll double check my insurance coverage.... Attached Images
Reply:Id buy the mig and use that. It will be quicker and easier and to make the deal even better you would now own one of the best migs in that size. Definately a good investment.
Reply:Hello Kaiser26k3, the simple answer in my eyes is to go with a GMAW process over the GTAW one. You are speaking of welding to AR500(armor plate), I believe this is a quench and temper type of material and WILL be susceptible to heat input. The GMAW process will put a considerable amount less heat into the base material and if welded in a similar manner to the one you described should certainly address your requirements very adequately. GTAW is known for and provides very deep penetration characteristics which can further be altered by using different mixes of helium and argon over the generally applied straight argon. It also has a greater degree of weld/base metal dilution over any other process. This is probably not desirable for your purpose.      Your wire penetration characteristics can be similarly controlled/altered by gas selection. I don't have the specifics readily available to me right now, yet if you google "gases and their effect on the weld pool" you will likely end up with some information to explain this effectively. Hope this has helped some and I'm sure that you will receive additional comments to further educate you on what your challenges might be. Best regards, Allanaevald
Reply:Look at it this way first, if I were to make a 6" long 1/4" fillet weld with MIG and TIG, the MIG bead would have lower heat input because the travel speed is much faster. Using your example of 250 amps, the TIG bead would be lower because it is a less efficient process.  In MIG welding heat is actually added to the process when using a CO2 mix because of the dissociation property.Below is a formula for calculating heat input so as you can see amps and time are only part of the consideration.Q = heat input (kJ/mm), V = voltage (V), I = current (A), and S = welding speed (mm/min)But the way you described making the weld makes me nervous.  You essentially tack welded on a 1/2" piece of AR500 plate?  Thats asking for problems, and it should get redone.Have we all gone mad?
Reply:Originally Posted by sn0border88But the way you described making the weld makes me nervous.  You essentially tack welded on a 1/2" piece of AR500 plate?  Thats asking for problems, and it should get redone.
Reply:Thanks guys for the advice,  this is just the kind of discussion I was hoping for.  I watched patiently as it was done and the beads on the weld overlap and looked to have pretty good fusion between them.  Normally I would have hoped  for more of a formal weld but with the heat issue and the nominal weight (100lbs.) I wasn't concerned about the technique shown and trusted the welders choice.  Should I be?  The photo is from a rig in service for two years now. Is the better technique to weld a solid 1" bead at each trigger pull and then go from side to side all the way around until completely welded?  More than and inch at a time would start to scar me about the interior heat issue again.  Thnx again for the help.
Reply:But the way you described making the weld makes me nervous. You essentially tack welded on a 1/2" piece of AR500 plate? Thats asking for problems, and it should get redone.
Reply:Originally Posted by Kaiser26k3Thanks guys for the advice,  this is just the kind of discussion I was hoping for.  I watched patiently as it was done and the beads on the weld overlap and looked to have pretty good fusion between them.  Normally I would have hoped  for more of a formal weld but with the heat issue and the nominal weight (100lbs.) I wasn't concerned about the technique shown and trusted the welders choice.  Should I be?  The photo is from a rig in service for two years now. Is the better technique to weld a solid 1" bead at each trigger pull and then go from side to side all the way around until completely welded?  More than and inch at a time would start to scar me about the interior heat issue again.  Thnx again for the help.
Reply:If this is just as you say for storage, as opposed to use, I can see you getting away with not needing a ton of weld if it's only 100 lbs. I'd also want to know exactly what is behind that plate on the inside. Is it lined with carpet for some reason? Is it just dead air space behind the panel? Are there electric wires run against the hull? I'd want to do a few "tests" and see whats going on. Get yourself a few chunks of 1/2" plate, for this it probably doesn't have to be AR, but it would be nice. Something big enough to have some mass and area to soak up heat like the final product will. Fab up a few mock pieces that will attach to it. Doesn't have to be complete, just the base where the welds will be made is all that probably matters. You are more interested in replicating the amount of bead area for heat input, and extra mass on the actuall bracket will just help reduce the heat. Then do a few welds and measure the actual temps on the back of the 1/2" to see how hot it actually gets. When you check the back directly behind the beads, also check the front say 1" away from the bead so you get an idea what's going on up front as well. That way you can use the IR thermometer to help you know when it's good to go to the next bead with out having the ability to check the back.I'd probably start out by runing one conventionally just to see how hot that plate in the back actually gets. When you measure the temps, I'd be willing to bet you can run one bead with no issues even with the volts cranked up if you keep the bead size reasonable, say 1/4" fillet. ( I'm guessing that top bead length isn't more than say 3" in length from the picts)The question then is how long you will need to let it cool before you can run the next bead. I'd also probably try a 3 pass bead alternating sides and wait a bit between beads. I'd keep the voltage way down, almost like running beads with a 110v mig near max. In this case you wouldn't be that concerned with penetration, just having sufficient bead area to support the weight. Measure how hot the back is getting between beads so you have an idea when you might need to stop or let it cool more.Then try one where you run short interupted beads but not spots and see what happens there. I'm even betting you could probably run these with 3/32" 7018 if you played with the test a bit. A sn0 said all those "dots" that are very apparant in the vertical section of that weld are places for cracks just waiting to happen. Something banging up and down in use, even in transport, will try to get those cracks started. You want to cut down all the start/stops to eliminate these as much as possible..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:Originally Posted by DSWThen try one where you run short interupted beads but not spots and see what happens there. I'm even betting you could probably run these with 3/32" 7018 if you played with the test a bit.
Reply:I was working on rebuilding the A frame for my plow this morning and did a few quick tests. 2"x 3/8" flat wleded to 3x2" 1/4" wall square tube. MM185, 75/25, .030 soild wire (what was in the machine and I wanted to use up what little was left) voltage tap 6 wire speed 70. (I'd guess this was roughly about 22-23v 400-450on the wire if I'd been using the XMT )Base steel 60 deg. at start. Weld lengths 4-6" long at a time. Temp 1" from the bead when done 250-300 deg. F After running several jumping around, Base temp of steel 125 deg, 1" from bead 350-380 deg, 2" from weld 180 or less. Attached Images.No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:I guess the funny part is that I could have sticked the plates on myself and been better off..!  Live and learn I guess.I'm also cringing a bit on just how much stick I used for the construction of the receiver for the boom and head attachment.  Since it was done on a "good guy" basis I just went stick because it was what I had at the time.  Now that I have a better perspective and a little more experience.  I think I'll invest in a MM252 and get the follow-up jobs done in a much easier/faster fashion.I think I was scarred as a child slag hammering behind my dad's stick projects and wondering why I didn't get a helmet to wear like he had. "Hey dad, why do my eyes itch after a while?"..    All the memories came back to me on the first scratch start of this project.  Too funny.Thank you guys so much for the help and advice.  The next two jobs I have in line are gonna need a lot of 1/2" plate joining and I think the 252 will be more than justified.  The Dynasty 350 is my pride and joy but speed and efficiency are the name of the game as I have found out the hard way.  I've TIG'd a bunch of stuff that really didn't need it and now that I have some "heavier" projects coming I think the MIG is a no-brainer.I hope I don't piss off any of the stick pro's here because I know it's possible to lay down some pretty beads with that set-up.   Live and learn.
Reply:I did some one sided welds on some high value cars and backfilled the non access side with Nitrogen.  I stuck a hose in the area between the panels and flooded the area.  No O2, no fire. Between proper sized stitch welds I cooled the area to room temp before making the stitch next to an earlier one.
Reply:Now that's thinking ahead.
Reply:Originally Posted by Kaiser26k3Now that's thinking ahead.
Reply:MIG welding (GMAW) has a higher initial heat input.TIG welding (GMAW) has a lower heat input but excellent penetration.What you need is a high deposition rate which is better suited to MIG.As far as armor plate be careful about matching filler metals.Do not forget to disconnect the battery on a vehicle before doing any welding on it.Those vertical spot welds with craters in them make me worried as well.Last edited by Donald Branscom; 02-14-2011 at 07:19 PM.AWS certified welding inspectorAWS certified welder
Reply:I think the MM252 will be a much needed asset in the future projects and now with the help found here I will definitely go back over the pictured weld and re-do.
Reply:A special thanks to DSW for all the input and information regarding my post.  This kind of interaction is invaluable for someone like me.