High-tensile steel plates and lightweight aluminum are being employed as materials in order to achieve weight savings in automotive subframe. Closed-section structures are also in general use today in order to efficiently increase parts stiffness in comparison to open sections. Aluminum hollow-cast subframe have also been brought into practical use. Hollow-cast subframe are manufactured using sand cores in gravity die casting (GDC) or low-pressure die casting (LPDC) processes. Using these manufacturing methods, it is difficult to reduce product thickness, and the limitations of the methods therefore make the achievement of weight reductions a challenge. The research discussed in this paper developed a lightweight, hollow subframe technology employing high-pressure die casting (HPDC), a method well-suited to reducing wall thickness, as the manufacturing method. Hollow-casting using HPDC was developed as a method of forming water jackets for water-cooled automotive engines. Because the volume of the sand cores used in the method is low despite the complexity of their shape, the hollow-casting of large parts such as subframe necessitated the molding of larger sand cores than are conventionally employed. In addition, it was necessary to develop a sand core baking technology that produced a good strength balance, making it possible for the cores to resist casting pressures but collapse easily in the sand removal process following casting. A sand core technology balancing pressure resistance with collapsibility, which had previously represented an issue, was developed in order to make it possible to hollow-cast large parts, and a non-heat-treated Al-Mg-Si alloy was employed in order to reduce costs. The developed hollow aluminum subframe is approximately 40% lighter than a conventional subframe manufactured from welded steel plates (Fig. 1).
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