Casting Densification

Casting Densification Process

Reprinted from ALCOA TECHNOLOGY MARKETING DIVISION
Products/Processes/Services TMD Report N.5

Alcoa has developed a method designated the Alcoa 359 Process, which significantly improves the fatigue strength of aluminum sand and permanent mold castings. In achieving this improvement, the process substantially eliminates the microporosity normally encountered in aluminum castings as a result of internal gas and shrinkage. Thus, the method makes possible the salvaging of castings which have been scrapped for reasons of internal porosity, thereby achieving a significant improvement in recovery. This combination of factors results in a competitive advantage to users of the Alcoa 359 Process.

The technique involves the exposure of aluminum castings to a high isostatic pressure at selected combinations of pressure (15,000 psig), temperature and time. Such treatment eliminates internal gas shrinkage porosity. The resulting high density aluminum castings will meet Class A radiographic specifications, and exhibit higher and more uniform tensile properties. However, the most significant improvement is an increase in fatigue strength approaching that of forgings. This represents an improvement in fatigue strength of up to 300 percent over ordinary cast products, and laboratory tests indicate that these property improvements can be obtained consistently. There is no question about the ability of the process to enable castings to meet Class A x-ray qualifications provided the porosity is internally contained. Table I presents tensile properties of test bars from 7 x 7 x V4-in. permanent mold cast plates comparing untreated castings andcastings treated by this process. Table II shows results of tensile and fatigue tests on specimens cut from various production sand and permanent mold castings. Following these is a graph showing rotating beam fatigue curves comparing castings treated by the Alcoa 359 Process to standard permanent mold castings and forgings.

Alcoa's evaluation of the utilization of this process concluded that diesel pistons constituted a prime potential market for permanent mold castings treated by the Alcoa 359 Process. Obviously, this is not the only market. Any part which benefits from superior fatigue properties could justify the additional expense of this process.

This process is covered by U. S. Patent No. 3,496,624.

Table 1: Tensile properties of bars from 7 x 7 3/4-in. permanent mold plates

Table 2: Results of tensile and fatigue tests on specimens cut from various production sand and permanent mold castings