1. Field of the Invention
The invention relates to precision casting processes and, more particularly, relates to a process of casting a semi-solid thixotropic metal alloy material about a core at a temperature above the melting point of the core material and of subsequently melting the core from the casting. The semi-solid alloy may be cast, e.g., by thixocasting, rheocasting, or sub liquidus casting.
2. Discussion of the Related Art
The typical cast metal part is formed in coreless dies or in dies with cores that must be mechanically removed from the part after casting. Of course, the mechanical removal requirement severely limits the range of core uses. The core cannot be formed with protrusions or other complex shapes that would form undercuts, threads, bores, etc. in the casting because the protrusions on the core would prohibit its subsequent mechanical withdrawal from the casting. As a result, threads, bores, undercuts, etc. must be machined into the cast part after casting and core removal at considerable expense to the manufacturer. In fact, post-casting machining costs often represents 50% to 75% of the cost of a finished precision-cast part having complex internal shapes.
Some of these problems could be alleviated if a suitable dissolvable core were to be used in a casting process. Currently, the investment casting process, also known as the “lost wax” process, comes close to meeting this goal. However, parts formed by this process can have complex external shapes, but not complex internal shapes. They also usually require grinding, polishing, or other secondary machining operations for fine features such as threads, bores, and seal grooves. Other processes, which cast a metal shot about a sand or salt core and subsequently remove the core by flushing it from the resultant casting, also come close to meeting this goal, but also require secondary finishing operations to meet tolerances for their finer features. Parts formed from these other processes also tend to have high internal porosity. This porosity is a problem in applications such as brake calipers in which the part needs to be precise and also hold a hydraulic pressure. It also prevents heat treatment because the trapped gases in the pores blister the casting during heat treatment. It is also quite expensive.
Melt-away core casting processes have been proposed in which a metal part is cast about a core formed from a metal having a lower melting point than the melting point of the metal casting and in which the core is subsequently melted away. See, e.g., U.S. Pat. No. 1,544,930 to Pack; U.S. Pat. No. 3,258,816 to Rearwin; U.S. Pat. No. 5,263,531 to Drury et al.; and U.S. Pat. No 5,355,933 to Voss. In each of those processes, a fully-molten aluminum-alloy metal is cast about a zinc-alloy core, and the zinc-alloy core is removed from the part, e.g., by subsequent heat treatment of the aluminum-alloy part. Drury et al. and Voss additionally disclose that their processes are applicable to complex cores so as to produce parts having complex internal shapes. However, all of these processes exhibit disadvantages severely limiting their range of practical applications.
Most notably, in all of the melt-away core casting processes described above, great care must be taken to avoid melting the core during the casting process. This is understandable because a great deal of heat is available for transfer to the core from the molten metal of the shot, and extreme measures must be taken to insulate the core from this heat or to prevent this heat transfer from melting the core. For instance, Pack's process appears to be limited to castings having simple undercuts and hence not requiring complex cores. Rearwin and Voss require the application of a layer of insulating material such as Vermiculite to at least those parts of the core that are relatively thin when compared to the cast metal part in order to prevent the core from melting during the casting process. Drury et al. discloses chilling its core to approximately −300° F. prior to casting in order to prevent over-heating of the core during casting. Moreover, it is believed that all of these melt-away core casting processes are limited to applications in which 1) the core is relatively massive when compared to the casting, and 2) liquid metal injection takes place at relatively low pressures and at relatively low shot flow velocities.
The need therefore remains for a versatile melt-away core casting process that can form precision castings economically and with high repeatability.