Tailor properties where needed using the CDC process.

Engineering components often call for different material properties at various locations of the part. For example, engine blocks require exceptional wear resistance in the cylinder bores and excellent castability and machinability in other areas. These varying needs can force engineers to compromise between the requirements or utilize expensive coatings or inserts, such as with cast iron cylinder liners used in engine blocks, to meet their needs.

A new process has been developed at the Univ. College Dublin in Ireland as an inexpensive technique that allows engineers to tailor material properties at different parts of a component. The CastDecantCast (CDO process allows metalcasters to produce functionally gradient materials in a single casting operation without the need for specialized equipment.

Current methods of coating a surface or casting around a solid insert generally are expensive and prone to difficulties associated with maintaining the joint between the surface coating or insert and the bulk casting. This is a particular problem with metals such as aluminum, which rapidly forms an oxide layer on the surface that leads to difficulties in bonding.

Functionally gradient materials avoid this interface with a gradual variation in the alloy composition. For instance, the CDC process can produce a component having a hardwearing surface layer which gradually changes to a tough interior (Fig. 1).

[FIGURE 1 OMITTED]

The CDC process can be applied to a number of gravity casting processes, including low pressure permanent mold casting (Fig. 2). In this method, two crucibles containing different alloys are placed beneath the casting mold. A heated manifold block containing two valves is placed between the crucibles and the mold. The first alloy is pressurized to fill the mold and held under pressure for a predetermined time until a thin layer has solidified on the mold walls. The pressure then is released, and the central, unsolidified part of the casting falls back into the crucible. Within a few seconds, the second alloy is pressurized to fill the remaining portion of the mold. The relatively short time between decanting the first alloy and the filling of the second alloy prevents the formation of an interfacial oxide layer. Some remelting of the first alloy will occur, resulting in a gradual change in composition between the surface and interior of the casting. Processing conditions can be varied to change the length of the interfacial region.

[FIGURE 2 OMITTED]

Development work has focused on aluminum-silicon alloys. Alloy A356 has many well-known advantages, such as excellent castability and toughness, but it has poor wear resistance. The standard method of providing wear resistance in parts such as an engine block is the utilization of cast iron or aluminum A390 inserts on the internal bores. This, however, significantly increases manufacturing costs, and the use of iron inserts increases recycling costs. The CDC process provides an inexpensive approach to manufacturing engine blocks by combining the wear and hot strength of alloy A390 on the bore surfaces with the advantages of castability and machinability of A356 in the main body of the component.

The CDC process also can be applied to iron and copper alloys and cast irons. Potential applications are outlined in Table 1.

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Table 1. Potential Property Combinations of the CDC process

       Surface Property                      Bulk Property

        Wear resistance           Machinability/toughness/castability
 Excellent anodized appearance                Castability
Electrical/thermal conductivity                Strength
    Metal matrix composite                 Monolithic alloy
        Bearing surface                    Strength/low cost
     Corrosion resistance                      Strength
       Bio compatibility              Strength/fatigue resistance