Welding involves heating two pieces of metal (or plastic) so that they melt and fuse together, resulting in a very strong seam. This requires scorching temperatures of around 5,500 degrees Celsius (9,900 degrees Fahrenheit), which are most commonly generated with an electric arc.
An arc is a discharge of electrical current – just like a tiny lightning bolt. In arc welding, the parts to be welded are connected to a grounded wire, and an electrode (made of filler metal) is connected to the power supply. When the electrode is put into contact with the welding materials, then moved away, the air in between the two is ionised and electrons leap across the gap. This generates bright light and intense heat. As the arc is drawn along the join, both the tip of the electrode and the working materials become liquid and fuse together. At such high temperatures, the molten metal bubbles and spits, expelling a shower of incandescent droplets: these are the sparks you see. They can be as hot as 1,300 degrees Celsius (2,500 degrees Fahrenheit) so be sure to stand clear!
Answered by Alexandra Cheung.