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.
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.
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.