See MELTING RANGE.

Pure metals that are elements have a definite melting point, i.e, solid and liquid are in equilibrium at a specific temperature. Alloys, because they are a mixture of two or more metals, melt or solidify within a range of temperatures and not at a single temperature.

For example, with an alloy, a liquidus is found, (liquidus is the temperature at which various compositions of the alloy system begin to freeze on cooling, or complete their melting on heating), and a solidus is found (solidus is the temperature at which freezing is complete on cooling, or at which melting begins in the solid alloy on heating). Noting the range of temperature between liquidus and solidus can yield important information about an alloy because it can be indicative of susceptibility of the particular alloy to cracking during solidification. A large difference between liquidus and solidus favors conditions that promote hot cracking. Reference: Linnert, George E. Welding Metallurgy, Volume 1, 4th Edition. American Welding