In the present invention, the alloy used to cast the inner layer may be a metal having a high coefficient of contraction, for example, a high-Mg or high-Zn aluminum alloy, e.g. an aluminum alloy containing at least 2.5 wt. % Mg, more preferably 2.5 to 15 wt. %, more preferably 2.5 to 9 wt. %, and even more preferably 2.5 to 7 wt. % Mg.