To improve the mechanical and physical properties of castings, it is well known in the art to use inserts which can locally improve the strength, wear resistance or other characteristics of the casting. The inserts are placed within the mold prior to pouring the molten casting material and, upon cooling of the molten casting material, the inserts form an integral part of the finished cast product. A common example of such an application is in the engine block of an internal combustion engine. In particular, aluminum alloy engine blocks, typically an aluminum-silicon alloy, often make use of cast iron inserts, or liners, which are more durable than the cast aluminum walls of the engine block, particularly when aluminum pistons are used. A process well known to those skilled in the art is referred to as the "Al-fin" process, which entails dipping a cast iron insert in molten aluminum prior to placing the insert in a mold and casting a molten aluminum around the insert.
However, a disadvantage with the use of cast iron liners is the significant additional weight which is incurred. In addition, cast iron does not conduct heat away from the cylinder as well as aluminum, which raises the temperature of the cylinder and imposes higher temperature-related stresses and wear on the engine's internal components. Another disadvantage with using iron is that there is a mismatch between coefficients of thermal expansion between iron and aluminum and its alloys, which can cause debonding of the insert.
As a result, it is generally preferable to provide inserts which are lower in weight while also providing better heat transfer capability and a more closely matched coefficient of thermal expansion. Naturally, aluminum-base alloys and composites are generally suitable in terms of weight, heat transfer and thermal expansion, for use with aluminum castings. Unfortunately, aluminum-base inserts do not metallurgically bond well to casting materials because the insert forms an aluminum oxide layer at the insert's surface. The presence of oxides produces a weak bond because of the inability of the molten casting material to wet the insert's surface.
To overcome this problem, one approach known in the art has been to form an insert which can be penetrated by the casting material under high pressure to form a mechanical/metallurgical bond between the casting material and the insert. One such approach uses alumina and carbon fibers which have been highly compressed to form a cylindrical insert. An aluminum casting material is then pressurized sufficiently during the casting process to penetrate the fiber inserts without structurally damaging them. While durability is improved, manufacturing costs are significantly higher than that of iron liners in aluminum blocks.
An approach for promoting a metallurgical bond between the insert and the casting material is taught by U.S. Pat. No. 4,687,043 to Weiss et al. Weiss et al. provide an aluminum composite insert whose outer surface is covered with an aluminum alloy. The insert is then coated with a molten solder alloy. The molten solder alloy is selected to have a melting temperature which is below the melting temperature of the insert's aluminum alloy cover layer. The insert is dipped into the molten solder alloy to separate from the cover layer the oxides already present and to prevent the formation of new oxides thereon. In addition, Weiss et al. teach that the casting material must be at a temperature which is higher than the melting temperatures of the insert's cover layer and the solder alloy. This enables the casting material to flush the molten solder alloy from the cover layer during the casting process to expose the cover layer to the casting material, allowing the casting material and the cover layer to form an oxide-free metallurgical bond. The molten solder alloy is intended to be mixed with the casting material and not remain on the surface of the insert.
In addition, Weiss et al. teach a zinc solder alloy which contains about 10 to 30 weight percent tin and about 5 to 25 weight percent cadmium to reduce the melting temperature of the solder alloy below the temperature of the casting mold. A disadvantage to the use of tin for the purposes taught by Weiss et al. is that tin embrittles the solder alloy and the interface between the insert and the casting, while also reducing their corrosion resistance. The presence of tin also slows down the age hardening by reducing the Guinier-Preston (GP) zones formation rate, potentially causing a weak interface between the insert and the casting. Similar to tin, cadmium reduces corrosion resistance. But most importantly, cadmium poses an environmental concern in that it is highly toxic. As a result, the use of cadmium is always avoided where possible, and sometimes prohibited.
Thus, it would be desirable to provide a method of promoting a metallurgical bond between a strong, wear-resistant insert and an aluminum alloy casting material which would be economical for use in mass production, while also avoiding the concerns for embrittlement and toxicity.