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Usually when someone builds a custom frame they are typically Mig welded and fish plated but if a frame is Tig welded does that generate better penetration when welding pieces together so you don't have to fish plate? My understanding is limited in regards to Tig welding but what I've been taught Tig welding generates the best welds but I'm unsure if that better weld is enough penetration to circumvent fish plating.

  • 2
    For a vehicle's frame I would recommend MIG. Tig is generally used on thinner sheet metal, think motorcycle tanks, body panels. Again, it depends on the type of frame you are building. On a Ford Model A, you could in fact weld the "sub-rails" with tig, since it is a subframe built out of sheet metal. This is not the main frame, though. – Jonathan Musso Apr 21 '16 at 15:38
  • Huh? TIG welding is nearly always recommended over MIG welding as it adds less destructive heat to the system. Generally after a weld is complete, the weakest portion of any frame is the area of "untouched" metal just beyond the area being welded. If you add excessive heat to the system it creates a weak area. Additionally its pretty easy for a MIG weld to mask poor penetration (It looks good, but doesn't penetrate fully.) – zipzit Apr 21 '16 at 19:28
  • One thing more: What kind of frame are we talking about here? An OEM truck frame? Round tubes, square tubing? Motorcycle with tubular frame (like a Ducati?) Off road car with chromoly tubular frame? Custom car with uni-body construction (I wouldn't call that a frame at all, but others might?) Old cars pre-unibody days? Roll cage on street car? Where are you going here? – zipzit Apr 21 '16 at 19:33
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I would recommend checking the regulations of an appropriate race series, as anything that meets or exceeds these specifications should also be suitable for road use.

For example, the UK MSA regulations on Roll-over protection state:

K1.3.8. Guidance on Welding. All welding should be of
the highest possible quality with full penetration and
preferably using a gas shielded arc. Although good
external appearance of a weld does not necessarily
guarantee its quality, poor looking welds are never a sign
of good workmanship. When using heat-treated steel the
instructions of the manufacturer must be followed
(special electrodes, gas protected welding). It is to be
emphasised that the use of heat-treated or medium
carbon steels may cause problems and that bad
fabrication may result in a decrease in strength (caused
by brittle heat-affected zones) or inadequate ductility.

I've never seen a fishplate used on a roll-cage in a UK rally car. I'd also avoid end-on butt-joints like the one shown in your other question - far better to use a single length of metal for each member of the frame.

  • I was horrified by that picture in the other question. But not enough time to get into that right meow – Move More Comments Link To Top Apr 21 '16 at 15:36
  • I haven't seen any roll cage tube that is welded end to end though. It's always bent and terminates at a mounting plate. – DucatiKiller Apr 21 '16 at 18:55
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TIG (Tungsten Inert Gas) is used generally on thinner metals and or aluminum. It would have less penetration than MIG (Metal Inert Gas) Welding. I would not recommend TIG welding the Frame on any vehicle.

1

Tig welding should be fine as long as your machine is designed for welding at that thickness. For example if your frame rails are formed from 1/4" thick steel, you should be using something like a 300 amp tig setup.

1

TIG welds aren't inherently stronger than MIG welds, assuming that a similar filler metal is used.

The main advantage of TIG welding is that it gives very fine control of both current and metal deposition rate and as such better control of how much heat goes into the base metal. This becomes especially important when welding thin metal or high alloy steels (such as chrome-moly steels) which are very sensitive to overheating and consequent distortion and cracks in the heat affected zone.

TIG can also be advantageous in welding complex profiles as it gives the ability to control the welding current on the fly via a foot pedal. It also tends to give better visibility of the weld puddle which can make it easier to assess penetration and spot any defects during welding.

MIG can also be prone to 'cold starts' ie inadequate penetration at the start of a weld as the continuous wire feed makes it more difficult to initially establish a weld pool and the filler may just lie on the surface. Although more sophisticated machines may have features to compensate for this.

Overall there is no inherent reason why a good MIG weld should be any less strong than a good TIG weld, however in certain circumstances TIG makes achieving a good weld consistently easier.

In very general terms TIG tends to be preferred for :

  • Welding thin or delicate sections
  • Applications where the join needs to be 100% gas or liquid tight
  • High alloy steels, stainless steel and aluminium or magnesium alloys
  • Applications where the effective material thickness changes during a weld run.
  • Applications where the cosmetic appearance of the weld is important.

It is also worth noting that weld penetration is as much about proper joint preparation and setup as the welding process used, in particular ensuring the correct root gap and beveling profile (if applicable).

One of the advantages of MIG is that it tends to have higher productivity as it tends to have a faster deposition rate and is generally more convenient for the operator as is is essentially a one-handed operation.

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