If you don't know what you're doing, don't do this. Even if you know, probably don't. It works, and is a regular industrial process, sure, but you're trying to perform controlled melting of the protective housing of one of the many, tightly packed chemical energy bombs you're sticking together. Doesn't take too much of a mistake to go very wrong.
If you're building a battery pack in this day and age, use something like LiFePo prismatic cells and bolt-on busbars instead - way less dangerous chemistry, way less spicy process - but realistically speaking just buy the premade packs. For normal sizes, they're not more expensive (but don't buy the "too good to be true" ones) and means not having to deal with entirely unprotected battery terminals eager to give you a Very Bad Time.
Generally speaking the cells that are welded on are designed to be welded on in the areas were you do the welding. Doing something other then welding on them properly is going to be more unsafe then welding.
The proper tools to do this are not that expensive anymore in the greater scheme of things. It is just a question of whether or not it is worth to do it at the scale you are doing it or pay somebody else to do it.
Of course if you buy cells that are designed to be bolted together then bolt them together.
Of course the bolts, or whatever else provides the threads, on those cells are welded on.
> Generally speaking the cells that are welded on are designed to be welded on in the areas were you do the welding.
... by automated spot welder programmed to the specified timing and temperature control from the cell spec sheet, in a controlled environment with suitable protection and fire suppression for a battery manufacturing line. Not by a hobbyist's first try with a homemade spot welder and a safety squint.
I have made such spot welder and done such spot welding. Sure it's fun to do stupid things, but it remains stupid and unnecessary. For a homebrew battery bank, this is the wrong tool, wrong cell and wrong chemistry.
Buy premade, or if you must, buy boltable prismatic lifepo cells. They can dump a lot of power if your short them, but you can drill straight through them and they'll remain stable. The random 18650 li-ion cells... Not so much.
Seconding that advice to just use prismatic lifepo cells. Those have become really cheap, too: You can order brandnew 1kWh cells for $60ish + shipping even if you only need single digit quantities (those want to be squished a bit for longevity, so you might have to design a suitable enclosure).
Energy autarky has never been so affordable, progress on batteries and solar panels was awesome over the last decade.
If you're building a battery pack in this day and age, use something like LiFePo prismatic cells and bolt-on busbars instead - way less dangerous chemistry, way less spicy process - but realistically speaking just buy the premade packs. For normal sizes, they're not more expensive (but don't buy the "too good to be true" ones) and means not having to deal with entirely unprotected battery terminals eager to give you a Very Bad Time.