I have a pretty good guess as to what caused it to come off. The top piece appears to have the threaded hole go completely through. So I will bet, the screw turned and pushed out the bottom against the cell terminal and tore the factory laser weld. If you have cells like this, you need to be very careful to not use screws that are too long, and even if using studs and nuts, to be sure the stud does not turn and push out the bottom. This is not a good design. Of course, this does not help the OP on a fix, but hopefully it helps prevent others from suffering this type of failure.
As to a way to fix it.... that is a tougher question. How much current do you plan to charge and discharge on this cell? If it was mine... I would probably drill it only 1/4 inch deep with a 7/64th inch bit and tap it for a 6-32 stud. (3 mm for the metric folks). 1/4 inch deep should give a solid 4 threads. That should hold enough tension to clamp a buss bar on for a 50 amp connection. Drilling into the terminal is risky, but a small hole only 1/4 inch in should be safe. I would have two half full 5 gallon buckets of sand nearby. If it starts leaking, gets hot, or anything, drop it on one bucket, and pour the other over it.
I have tried using "Aluma Weld" type products to "solder" aluminum, and I did get it to work to fix the upper blade guide of my bandsaw, but it was tough to work with, and you still need to get the surface heated to 400F (205C) for it to stick. I think that is too risky for the cell.
When these cells with ‘welded’ studs first came out, I was kind of jealous and wished I had held off another 6 months before purchasing my cells.
After these reports, I’m feeling lucky.
The cells with tapped threads are a PITA. They are exceedingly fragile, strip super-easily, and are easily mud-tapped (either tapped too shallow or tapped off-square).
But at least once you get a stainless grubscrew locked into enough aluminum threads, you’ve got the ability to form a reliable connection with a lug or busbar.
And having tapped my own threads into 5 terminals, it’s manageable with the right tools.
I even salvaged a severely stripped terminal by filling the hole with JB Weld, allowing it to fully, fully cure, tapping the cured JB Weld, and then successfully JB Welding a grubscrew into those newly-tapped threads. That terminal takes as much torque as any of my unstripped aluminum terminals.
Again, a huge PITA but at least a reliable connection once you’ve got a grubscrew in place.
These welded connections, if they are not properly done, seem like a much greater reliability concern and potential safety hazard.
These resellers are just looking for ways to provide terminal threads without any guidance or specifications from the cell manufacturer.
BYD clearly knows what they are doing because they use these cells in EVs and battery packs but their welded connections are very different from the welds being delivered by resellers:
I’d have no concern with welded connections that completely surrounded the outer 1-2mm circumference of the terminal but it sounds like the welds you guys are getting are well below this.
EVE has apparently started supplying cells with welded terminals, but they look very different than the welded studs that many resellers are offering:
Those appear to be thick aluminum ‘posts’ with a large ‘doughnut’ where the post is welded onto the aluminum terminal surface.
That looks like a design ready for prime time and any new cells I purchase will be of that type (regardless of price premium).
So my advice is that if you are going to attempt to get your own welded connections: stick to aluminum-on-aluminum with a thick-enough doughnut weld (either around the outside edge of the cell terminal or a suitably large circumference within the terminal surface) to both provide sufficient mechanical strength as well as to provide sufficiently-large weld-area-only conductance.
This is what both EVE and BYD are doing and these are the manufacturers that know what connections these cells were designed to support.
The welded stainless studs on aluminum terminals design was arrived at by resellers who have little to no idea what they are doing and I now view them as even more dangerous than the lousy threaded terminals (possibility of low-conductance connections and/or failing connections during use).
As far as ways to salvage cells that have poorly-welded studs or no tapped threads, either a proper aluminum-on-aluminum weld job as I’ve outlined above or a properly-tapped M6 thread would be my suggestion (followed by a properly Loctited stainless grubscrew).