Liquidus line in phase diagrams

The liquidus line is the temperature above which the material is completely liquid. Below it, it's still mushy or has solid parts. For welding, this means: if you stay too far below it, the material doesn't melt through properly and you get a weak weld with pores. If you go well above it, everything becomes too runny and the pool runs away from you. So make sure you stay in the right window and keep the weld pool clean and calm.