IRF1404 MOSFET fake?
The challenge is to determine the Energy needed to make a reliable weld. (Joules)  Often these are made with Ultrasonic welds to avoid sparks with hydrogen gas.







simulate this circuit &ndash; Schematic created using CircuitLab

One can use a low side switch that does not exceed the Safe Operating Area (SOA) of a single device.  Putting ganged devices does not assume they will share current equally as there are many factors to consider.

Using a battery only with a timed pulse is possibly exceeding the maximum current of the FET's  with the battery rated for say 700 CCA at 0 degrees and possibly 900A at room temp.

A better way might use a very low ESR Super Cap rated for >=15V.

If we assume from the geometry that 1 weld was successful with 0.5 Joules shared in each x 10 that >5 Joules is needed for the thickness of material and area of the weld.

Using some x Farad Supercap may be a better solution with a halogen lamp that charges up the Cap between welds with some cold resistance that minimizes the current at the end of the discharge but provides a somewhat constant current flow to the cap to charge up in a reasonable time e.g. &lt;= 5 seconds or as desired.

Assuming constant V for unknown time to get 5 Joules at 12V , what is C if 10 switches, cap and wire ESR = 1 mΩ ?
$$ E = ½CV²    $$ 
Imax = 12V/1mΩ = 12,000 Amps
C= 5J *2 /12²(V+) = 10/144 = 0.069 F
T=RC= 1mΩ * 0.069F =~ 0.07 ms  ( 70 us )
With 0.1uH of wire loop inductance ω²=1/(LC), f=ω/2π
 or  f=1/√(LC) = 12 kHz  so it is an overdamped pulse.    

E = ½CV² 

Below is just a switch shown where the Nfet can be connected. A mechanical switch would have to be rated for these surges would otherwise wear out fast if you plan on doing a lot. The tungsten contact requires a certain area and pressure to ensure a low resistance to improve power transfer efficiency where the resistance of the contact must be close to the switch loop resistance. (Max power transfer theorem)

Here is the SOA limits for that FET.  There is a high probability they were legit parts and you exceeded the SOA safe limit with 100ms. I would estimate it is be safe for 10A * 12V @ 100mS if they are shared equally.