That TO-220 would probably handle 50A but only at 1.5V. The TO-220 is limited to around 75W, so the amps must meet the volts for that wattage to hold.

What are you trying to do with your welder? There are better mosfet types one called an Iso-Top. like this image -


They are used in some welders and plasma cutters.

 

The detail not discussed adequately is the specific conditions for that 50 amp current to not cause destruction.
Certainly the heat sink matters a whole lot. But the pulse width matters a lot also. In a "class E" RF amplifier where the pulse duration is a microsecond or so it might be OK, but that is not a normal application.
It does take a fair amount of digging to discover the exact conditions in which that specification applies. And certainly it will not be a class "B" audio amplifier or servo amp.

 

The detail not discussed adequately is the specific conditions for that 50 amp current to not cause destruction.
Certainly the heat sink matters a whole lot. But the pulse width matters a lot also. In a "class E" RF amplifier where the pulse duration is a microsecond or so it might be OK, but that is not a normal application.
It does take a fair amount of digging to discover the exact conditions in which that specification applies. And certainly it will not be a class "B" audio amplifier or servo amp.

Where did you come up with this being a RFamp, class b audio amp or a servo amp out of this thread? The first post talked about welding cables, as he did in the other posts.

 

If you think the TO-220 leads are small for 50A you should see inside the transistor. (they are small, look at TO-247)