I hope the anime will shed light on how the Tinkas do their smithing, cause it's bizarre that even though they don't have any Fire moves for Forging, their only Electric move is Thunder Wave which I doubt can be used for Arc Welding, also they don't learn Thunder Wave naturally so majority of them wouldn't have access to this move anyways, and they learn no Poison moves for Gas Welding?
So, the only solutions I can think of on how they weld the metal is Laser Welding by using moves like Steel Beam and Flash Cannon and Solid-State Welding by just pounding a rock/another piece of metal/brute strength to force the metal together, and with Thunder Wave they could be magnetizing the metal to stick together, but like I said earlier they don't learn this move naturally, so it's taken out of the question.

In my opinion it has to be through Solid-State Welding, but I don't know how durable that type of welding process is for metal used for crafting Humongo Weapons?