MIG vs. TIG again

[Carbaholic]

Most of the arguments I've heard that TIG is better than MIG say thing like: "It's just better" or "It looks better" or "Carrol Smith Says it's better" Does anyone have any numbers that quantify how much stronger a TIG weld is, how much lighter they are, and/or how much longer they take?

Thanks in advance

[Carbaholic]

Most of the arguments I've heard that TIG is better than MIG say thing like: "It's just better" or "It looks better" or "Carrol Smith Says it's better" Does anyone have any numbers that quantify how much stronger a TIG weld is, how much lighter they are, and/or how much longer they take?

Thanks in advance

[Garlic]

Have you read ALL OF THIS (http://fsae.com/eve/forums?a=search&reqWords=MIG+TIG&use_forum_scope=on&forum_scope=125607348)?

[SeanM]

it's not always about numbers being proof. Tig welding is more accurate then mig welding is with mig you have less control over the arc you simply touch the tip to the metal and pull the trigger tig you can stike an arch and then go from there. for an example of just how accurate tig welding is look at F1 a-arms the inserts or the rod-ends are tig welded in and that would not be possible to do with a mig welder.

[RiNaZ]

and i think MIG is faster and sloppier whereas TIG is slower and accurate (im talking about welding time).

Just started using TIG, been using MIG ... so i wouldnt know the difference yet, still learning on TIG. And i think i like MIG better ... more forgiving i guess ...

[erny]

i have just learnt both, as i see it mig just adds a bridge between the two metals, it adds rod contstantly . Tig seems to be more about getting the two metals to fuse then add a little rod when it gets thin, much more emphasis on fusing the two together.

I was told by a good teacher that with tig you aim for 2/3 penetration and when i look at my welds i see that the welds almost show on the inside of the tubes.

I have just started and my tig welds are much neater than any mig i have seen. the level of penetration must mean they are stronger

[Garlic]

Any weld done properly will be stronger than the material around it, so there's no way to compare that case.

TIG will always have less heat affected area. So the material aurrounding the weld will be stronger. If you MIG, you have to heat treat to regain strength. If you TIG, it's not always neccessary. Although it might be to have the best stiffness.

[Boston]

I thought only strength would be affected, not stiffness (steels have the same stiffness regardless of heat treat)

[DaveC]

First off, I'd only MIG mild steel, so having to heat treat a mig vs a tig weld isnt an issue. Stress relieving alloys is a good idea regardless of process, if you don't the HAZ will be a lot weaker, as we all probably know. I'm not sure how much this matters in an FSAE car, though. Hopefully you wont hit anything hard enough to find out what part of your frame failed first.


I owned a MIG machine for years before I bought a TIG, my mig welds are superior to my tig welds so far. You can get good at mig much faster, tig takes a while before you'll want to weld anything structural, or that requires some finesse. You can get good penetration with a mig, but it does operate like a caulk gun, making it hard to get a feel for whats happening.

Erny, I want to see pics of those tig welds youre proud of. A beginning tig welder making welds nicer than ANY mig weld? Thats great if you can, though... I'd be jealous http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Perry Harrington]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Garlic:
Any weld done properly will be stronger than the material around it, so there's no way to compare that case.

TIG will always have less heat affected area. So the material aurrounding the weld will be stronger. If you MIG, you have to heat treat to regain strength. If you TIG, it's not always neccessary. Although it might be to have the best stiffness. </div></BLOCKQUOTE>

You've got that backwards. MIG has a smaller heat affected zone than TIG, which is why it's preferred for some types of welding, such as autobody. TIG is the electric analog of gas welding, the operation is nearly identical. In gas welding a carburizing flame provides your "gas shield" to keep oxidation away from the weld zone. TIG uses argon for the shield.

With TIG, you can more easily see your penetration and have more confidence in a weld. MAny chassis are welded with MIG simply because it takes 1/3rd to 1/5th the time to weld. However, MIG welding a chassis is best done when you have experience with the voltage and amperage settings on your welder. MIG has the problem of not penetrating the surface and simply building up a fat bead that doesn't really do anything. Any weld is only as good as the tangenital line across the arc of the bead, convex or concave.

MIG welds are more brittle than TIG welds because the HAZ is so small. The area affected by the arc is so small that it has very great variations in stress in a very small area. TIG has the advantage that it's heat is more even, assuming constant current. The TIG weld's stresses trail off more gradually from the weld zone, leaving a less stressed piece. It's not possible to always heat treat a chassis, so living with an as welded stress is better with TIG. Coincidentally, there are a few rods that are useful for untreated, unrelieved CrMo welds, thos are 309 stainless and 3SMW "Super Missle Weld". SMW has a higher tensile strength than the base metal, so you are guaranteed that the base material will tear first. 309 is a compromise because it's cheaper (SMW is $27 per lb) and it relies on the chrome-nickel content to do the bond, but many saturday night racers use it. I only use SMW on CrMo because I don't like to gamble on my welds.

FWIW, 308L is also an excellent general purpose rod for chrome-nickel reliant welding. I've welded cast iron with the TIG using that rod and it's very hard when done. 316L is also a nice rod, mainly used for body jewelry by artisans.

--Perry

[Phil R]

if you compare a mig weld next to a tig weld i think you'll have all the proof you need.

for most applications tig is much cleaner and better. I'm not an expert and have very limited experence bit I would go towards tig for anything I can think of, ic pipes exhaust, cage, frame,

[DaveC]

^^^^^^^ ????http://fsae.com/groupee_common/emoticons/icon_confused.gif ???? ^^^^^^^^^

[PPAM20]

MIG and TIG welder here. Guys and gals, if you want to understand these two types of welding, I would read and re-read Mr Harrington's couple of paragraphs above. I have never read a better sumation of the pros and cons of the two types of welding. Kudos!!! and thanks for the SMW tip.

[Garlic]

Thanks for the correction Mr. Perry. Your explanation makes sense. I knew the MIG was usually more brittle than TIG, but I had my heat affected area theory backwards.

[JesseS]

Perry,
Do you recomend those same filler rods if we are planning to stress relieve the welds on the frame? Also, what experiences have people had with the filler rods recomended by Lincoln Electric, specifically, ER70S-2 and ER80S-D2.

[Perry Harrington]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by JesseS:
Perry,
Do you recomend those same filler rods if we are planning to stress relieve the welds on the frame? Also, what experiences have people had with the filler rods recomended by Lincoln Electric, specifically, ER70S-2 and ER80S-D2. </div></BLOCKQUOTE>

ER70S-2 is plain jane mild steel rod with a copper plate. It's tensile strength is 70,000psi and is ductile. The ER80 is 80,000psi and very ductile.

There are 2 schools of thought on 4130 welds, break at the weld, or tear at the tube. I personally think that it's dumb to make welds that break. 4130 is brittle and only used because you can drop 20% of your weight by substituting a thinner wall. If you screw up a 4130 frame, you have to replace the tweaked pieces anyway, so go with a strong weld.

Consider that roll cages are mostly made with mild steel. Mild steel bends, but doesn't break when subjected to a crash. Bending dissipates much more energy than breaking. If you take the 60% rule into account, ER70 or ER80 would be acceptable for a mild steel frame made from A36, because the weldment is much stronger than the base material. With the high potential tensile strength of CrMo materials, ER70 or ER80 don't really meet the 60% rule. That's why SMW is a better choice.

Stress relieving your joints on car is a very good thing to do, it helps ensure that you don't get fatigue cracking and it will take a bit more abuse. The main reason is to spread the stresses out further from the weldment. The smaller the HAZ, the more likely it will crack in that area. Using a strong rod will guarantee that the tubing will crack adjacent to the weld. If you stress relieve, at best you can expect the tubing to crimp, not tear.

Another thing to consider is do you need 4130? If you are using .125 wall tubing anyway, you probably don't need 4130. If you are meticulously substituting .095 or .083, then 4130 will benefit you weight wise. For suspension components I highly recommend 4130, for the chassis I suggest mild steel. Mild steel is very forgiving and will bend before breaking. You can also substitute DOM mild steel where ERW is a liability (bends).

These suggestions come from a practical standpoint, what *CAN* be done, based on FEA, is totally outside the realm of practical fabrication.

--Perry

[Carbaholic]

Thanks for the replies everyone. It turns out that I have already read the whole long Mig vs. Tig post and I wasn't satisfied with the information I got there.

After reading what people have said so for, I have two main questions.


First:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> Any weld done properly will be stronger than the material around it, so there's no way to compare that case. </div></BLOCKQUOTE>

I had heard the opposite was true. In fact, a friend of mine cut and welded some tubing, put them in a puller, and broke them. Both the MIG welded sample and the TIG welded sample broke at the weld. Of course we're not the best welders and it's entirely possible that the welds were bad.

Does anyone have a good answer to this puzzle?

Second:
what are the most important factors in the strenght of a weld? Tensile strength of the filler? HAZ size? Internal Stresses? Duct tape?

Thanks for all your posts
Carbo