Metallic alloys used for load-bearing applications as in gears, bearings, bushes, washers, etc. are required to have high mechanical strength and good tribological properties for durability. For example, die-cast zinc alloy tends to creep during service and in order to improve the resistance to creep the size of such part is increased to help withstand the service load as in gearboxes. If the mechanical strength, of such parts is increased, the undesirable increase of size can be avoided. The conventional materials have been monolithic alloys such as white metal, copper-lead, or aluminum-tin alloys. The new approach described in this invention relates to the composites technology and produces composites comprised of conventional matrix alloy and reinforcing shots or aggregates dispersed in the matrix phase. Reinforcing shots or aggregates stay inside the composite product due to their complete mixing within the matrix phase to yield new composite alloys stronger than the conventional monolithic alloys while maintaining the surface characteristics same as monolithic unreinforced alloys. New strong composite alloys can be applied to gearboxes or big bearings and furthermore, the size miniaturization of mechanical devices becomes possible in automation, appliance, and automobile industries. These alloys can also be used in static structural parts such as brackets, door knobs, etc.

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Summary of the Invention

In the present invention new alloys are described in which conventional metallic alloys are mixed with metal shots or aggregates to increase the mechanical strength while mainatining the surface characteristics same as conventional alloys. All the shots or aggregates remain in the core region and the surface skin region contains only the monolithic alloy phase, thus maintaining the good tribological properties on the surface while increasing the overall mechanical strength of the product. The mixing of steel shots or aggregates with the alloys is done by using a special flux of ammonium chloride as a cleaning agent. The final product fabricated by the die casting technique is characterized by a strong central part with well-supported wear-resistant thin surface layer.

Brief Description of the Drawing

The foregoing as well as other aspects of the present invention will become clear from the following detailed description when taken in conjunction with the appended figure in which

Fig.1 shows a pictorial view of microstructure of new composite alloys comprised of steel shots and alloy matrix phase.

Detailed Description of the Preferred Embodiments

There are various kinds of metallic alloys used for bearings, bushes, washers, gears, or such load-bearing applications including structural parts as exemplified in white metal, copper-lead, aluminum-tin, and zinc-based alloys. In the present invention, the reinforcement of such monolithic alloys is done with steel shots or with aggregates of random geometry, the density of aggregates being preferably close to the matrix alloy for a uniform dispersion. Furthermore steel shots or aggregates must be bondable to the matrix phase and this is achieved by melting the alloy and then adding the shots or aggregates together with ammonium chloride under continuous stirring at a temperature greater than the melting point of the matrix phase but less than the fusing temperature of steel shots or aggregates. The molten metallic alloy dispersed with shots or aggregates are then die-cast into a mold. The die-cast products are stronger than the unreinforced alloys and the surface finish is as smooth as conventional alloys. The amount of reinforcing shots or aggregates must be greater than a minimum to enhance the mechanical strength appreciably and less than a maximum to maintain a good melt flow behavior. The mode of reinforcement is either monolithic,i.e., shots only or aggregates only, or hybrid,i.e., mixture of shots and aggregates. The geometry

$ of aggregates is either isotropic or anisotropic and aggregates often have a surface coating to improve the bondability to the matrix phase when an appropriate flux cannot be found.

Die-cast composite alloys do not exhibit cold flow behavior due to the high mechanical strength of core while preserving the good tribological behavior of monolithic alloys on the surface. Molten composite alloys are also spray-coated onto a substrate and the final parts are sometimes sintered using the powder metallurgical technique. Strengthened new composite alloys will be useful for dynamic and static structural parts and thus the size miniaturization in automation, appliance, and automobile industries has become possible.

The structure of an object according to the present invention is best seen in Fig.1, and as shown in this figure the object includes a plurality of shots 1 or aggregates 2 dispersed in a desired alloy matrix material 3,e.g., conventional metallic alloy. In a product the reinforcement is comprised of shots only, aggregates only, or mixture of both.

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Example 1. Zinc Alloys

The matrix alloy comprised of 73 wt. % zinc and 27 wt.% aluminum was prepared by melting and to this molten alloy steel shots were added using the ammonium chloride as a cleaning agent. The weight fraction of steel shots is less than about 50 wt.% and the size of steel shots is less than about 0.5 inch in diameter. The actual size of shot is about 0.5 mm in diameter ^, other matrix alloys tried were 88 wt.% zinc and 12 wt.% aluminum, and 97.5 wt.% zinc and 2.5 wt.% aluminum. They are basically zinc-aluminum alloys.

Example 2. Aluminum Alloys

The matrix alloy comprised of 80 wt.% aluminum and 20 wt« % tin was prepared by melting and to this molten alloy phase steel shots were added with ammonium chloride as a flux. The weight fraction of steel shots is less than about 50 wt.% and the size of shots is less than about 0.5 inch in diameter. The actual size of shot is about 0.5 mm in diameter.

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Example 3. Copper Alloys

The matrix alloy comprised of 80 wt.% copper, 10 wt.% tin, and 10 wt.% lead was prepared by melting and to this molten alloy steel shots were added with ammonium chloride as a flux agent. The weight fraction of steel shots is less than about 50 wt.% and the size of steel shots is less than about 0.5 inch in diameter. The actual size of shot is about 0.5 mm in diameter.

Example 4. Lead Alloys

The matrix alloy comprised of 83 wt.% lead, 15 wt.% antimony, and 1 wt.% tin was prepared by melting and to this molten alloy steel shots were added with ammonium chloride as a flux. The weight fraction of steel shots is less than about 50 wt.% and the size of steel shots is less than about 0.5 inch in diameter. The actual size of shot is about 0.5 mm in diameter.

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Example 5. Tin Alloys

The matrix alloy comprised of 89 wt.% tin, 7.5 wt.% antimony, and 3.5 wt.% copper was prepared by melting and to this molten alloy steel shots were added with ammonium chloride as a flux. The weight fraction of steel shots is less than about 50 wt.% and the size of shots is less than about 0.5 inch in diameter. The actual size of shot is about 0.5 mm in diameter.

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The compositions shown in the preceding five examples represent the actual try and compositional details can be changed as long as handling permits, i.e., flowable after mixing. In other words, any compositional variations must satisfy the following requirements.

(1 ) Mixable

(2) Flowable once mixed.

As a reinforcement, any strong bondaole aggregates or particles can replace steel shots to improve the creep resistance „as long as they are mixable with the matrix alloy phase.

From the requirement of flowability, shot is an ideal reinforcing geometry while from the viewpoint of strength, fiber shape is preferred. However the fiber geometry presents a flowability problem in the die casting process, producing defects in the cast product. The present technology of producing strong composite alloys can be applied to any size of load-bearing parts except for thin walls or shells. New alloys can be applied for both dynamic and static parts.