Answer:
Pre- heating allows the metal to not be shocked by the temp change and allows for better penetration as well as less undercut
Step-by-step explanation:
you can perform a butt joint with tig, mig, stick, oxy fuel, pulse, spool, etc. tig and mig I know for sure can spot and butt weld.
tig is used for a lot of spot welding because it's used on aluminum and aluminum can disintegrate if it incurs too much heat, making spot welds ideal for most situations