CHARACTERISTICS OF GrA-Ni® METAL MATRIX COMPOSITES

THE MATERIAL

INCO Limited has developed a new class of particulate reinforced aluminum composites made with nickel-coated graphite in addition to either SiC or Al₂O₃ particles. These graphitic aluminum composites have been named GrA-Ni® alloys. The addition of graphite to aluminum metal matrix composites provides three unique benefits:

1. Greater stability of particle distribution during casting process;
   
2. Improved machinability;
   
3. Increased resistance to wear.
   

I. Stability of particle distribution

Graphitic aluminum (GrA-Ni®) alloys use the phenomenon of hindered rising and settling to achieve a uniform dispersion of particles in the final casting. The specific gravity of SiC and Al₂O₃, 3.2 and 3.9 respectively, is higher than aluminum casting alloys, nominally 2.7 g/cc. During casting, the more dense particles form clusters with graphite and prevent the graphite from floating, while the graphite hinders the silicon carbide or alumina from sinking.

Alloy compositions have been selected on the basis of stability of the mixture of particles coupled with mechanical performance. Both SiC and Al₂O₃ based alloys are currently made with an aluminum alloy A359 (Al - 9% Si) matrix and can be die cast, sand cast, or permanent mold cast.

I-1 SiC based alloys

GrA-Ni® alloy 10S.4G, containing10 vol% silicon carbide and 4 vol% graphite, has been developed for applications including brake drums, clutch plates, bearings and pump components where relatively high strength and resistance to abrasive wear are important. Thermal conductivity of SiC reinforced alloys is higher than unreinforced aluminum. As SiC is easier than Al₂O₃ to stabilize with graphite during casting, this alloy has also been targeted for thick section castings having longer solidification times. During sand casting trials, the 10S.4G alloy has been left unstirred in the molten state for up 20 minutes without segregation of the particles. GrA-Ni® alloy 10S.4G offers an excellent combination of wear resistance and machinability, having essentially similar wear performance to A359-20 vol% SiC composites with almost twice the tool life for a given volume of material removal.

I-2 Al₂O₃ based alloys

Due to the relatively high density of alumina, MMCs reinforced with Al₂O₃ have typically been available only in wrought DC cast billet form. Graphite helps to stabilize alumina and allows the alloy to be produced in foundry ingot form. Alumina reinforced alloys have been made with 3 to 5 vol% graphite. As little as 5 vol% alumina is required to give good wear properties with the addition of graphite. The alloy containing 5 vol% alumina and 3vol% graphite, GrA-Ni® 5A.3G, has been developed for cost sensitive applications such as automotive cylinder liners which require low machining cost as well as low friction and wear of the counterface material. These alloys have lower thermal conductivity than unreinforced aluminum and are recommended for thinner wall sections having shorter solidification times.

II. Machinability

The addition of graphite makes these discontinuously reinforced aluminum alloys not only easier to machine, but also allows machining without lubricant, which improves the quality of machining chips for recycling. The 10S.4G alloys are best machined with diamond tooling. As shown below the alumina composites are much easier to machine and in fact approach aluminum A380 alloys in ease of machining. While the alumina based alloys can be machined with conventional carbide tooling, diamond tooling is recommended for high volume production. A machining cost comparison is shown in Figure 1 below, which takes into account tool wear, cutting speed and labour costs.

Figure 1. Comparison of Machining Costs for Discontinuous Particle Reinforced Aluminum

III. Wear Resistance

The dry sliding wear resistance of the GrA-Ni® 10S.4G and 5A.3G composites against SAE52100 tool steel is compared to a grey cast iron, A356 aluminum alloy and 20 vol% SiC composite in Figure 2 below. Graphite provides seizure resistance while the hard particulate provides abrasion resistance, resulting in a material with excellent dry sliding wear performance.

Figure 2. Block-on-Ring Dry Sliding Wear of Various MMCs, Cast Grey Iron and Aluminum Alloy A356 Against SAE52100 Tool Steel

PHYSICAL PROPERTIES

Physical and thermal properties are given for A356, Al-9% Si - 20 vol% SiC, GrA-Ni® 10S: 4G, Experimental GrA-Ni® 6S: 3G and GrA-Ni® 5A: 3G composites in Tables 1 & 2. All alloys were tested in T6 condition. Data for cast iron are also listed.

COST

GrA-Ni® alloy composite materials are available in large tonnage lots at prices ranging from approximately $2.00 to $3.00/lb. depending on the quantity ordered.