I've been after a spot welder for a while now so I can use it to affordably rebuild battery packs - in particular for my DeWalt 18v tools. I had intended on building a Microwave Oven Transformer spot welder but I stumbled upon this quite affordable one from Malectrics which suits my needs.
ARDUINO SPOT WELDER QUICK START GUIDE V3
DIY ARDUINO SPOT WELDER SUPPORT