When I think of welding, I generally consider machinery that is quite large, and not controlled via an Arduino. That may be the case most of the time, but this kit can actually make spot welds using a 12 volt car battery with an Arduino Nano for control. You’ll need to add a few details to complete a build, including a foot switch and an appropriate battery or other power supply.
Though the little welder doesn’t seem suited to joining large plates of steel together, it does a very good job of joining 18650 batteries together, as seen in the video below around 5:00. Shown a little later, it’s hard to remove these welds even when using pliers.
According to the Tindie page linked earlier, some improvements have been made since the video to help facilitate the assembly process. Updated information is available on the project’s instructables page. If you’d like to see source code for the Arduino welder and other information you might need if you want to make your own, check out KaeptnBalu’s GitHub page.