Magnesium

Magnesium is a light metal that immediately forms an oxide layer in air. That's the main problem when welding, because you have to brush it off thoroughly beforehand, otherwise you get porosity and inclusions. When you weld the stuff, it melts fast and burns violently with a bright flame if handled wrong – chips and dust are especially tricky. Best to weld it with AC and argon. If it does catch fire: never use water, that makes everything worse, use dry extinguishing powder instead.