A composite metal ingot, comprising at least two layers of differing alloy composition, wherein pairs of adjacent layers consisting of a first alloy and a second alloy are formed by applying the second alloy in a molten state to the surface of the first alloy while the surface of the first alloy is at a temperature between solidus and liquidus temperatures of the first alloy to form an interface there between, wherein the second alloy is a high or medium strength heat treatable aluminum alloy, and further wherein one or more alloy components from the second alloy are present within grain boundaries of the first alloy adjacent said interface.