Liquidus Line

The liquidus curve in the phase diagram shows you when the melt starts to solidify during cooling. In welding, this is important because the structure in the weld forms from it. If you cool too fast, the structure becomes hard and brittle; if you cool too slowly, the grain becomes coarse and the weld loses toughness. So you have to control the cooling to get a reasonable, tough structure – that depends on the alloy and what the component has to withstand later.