The design freedoms inherent in investment casting help keep aluminum metal matrix composites cost effective.
Paul Mikkola, Retired, Hitchiner Manufacturing, Milford, New Hampshire, and Bruce Willson, O’Fallon Casting, O’Fallon, Missouri
The particulate silicon carbide reinforcement in aluminum alloy metal matrix composite (MMC) enhances this lightweight material with improved mechanical property attributes for stiffness, vibration dampening, wear resistance, high thermal conductivity and low coefficient of thermal expansion. With its unique set of properties, MMC has been employed in diverse applications such as moving structures in high speed equipment for manufacturing, brake rotors for vehicles, heat sinks for electronics, and housings and mirrors for optics.
In the 1980s and 1990s, casting shapes from MMC ingot was envisioned to be a major enabling technology for manufacturing MMC metal components. This perception prompted the development of metalcasting processes and specialized techniques to overcome the natural tendency of the silicon carbide particles to clump or precipitate from the aluminum matrix. The inherent abrasiveness of the silicon carbide particulates in the alloy also gave MMC a reputation, perhaps unfairly, for being difficult and expensive to machine. For myriad reasons, a broad market for cast and other process shapes has not developed, and so, despite its many attributes, MMC remains an underutilized material option.
As with other materials that are difficult to machine, the near-net-shape capability of investment casting is an effective counterbalance to help mitigate the cost of machining aluminum MMC. With effective casting designs, this capability, combined with refinements in the alloy and in secondary machining, makes investment cast aluminum alloy/silicon carbide particulates a viable and affordable option for engineers to incorporate lightweight MMC shapes into their products.
Cast Metal Matrix Composites
Although increasing development activities have led to system solutions using metal composite materials, the use of especially innovative systems, particularly in the area of light metals, has not been realized. The reason for this is insufficient process stability and reliability. Combined with production and processing problems, it results in inadequate economic efficiency. Application areas are cost orientated and conservative. Often, the industry is not willing to face additional costs for the use of such materials. For all these reasons, metal matrix composites are only at the beginning of the evolution curve of modern materials. As lightweight, stiff, wear-resistant applications become of greater value to transportation and other industries, more high volume uses will evolve; brake rotors, pistons and drive yokes are examples.
MMC production melting metallurgy and pouring presently are of greater technical importance than using powder metallurgy composite material. Casting is more economical and has the advantage of being able to use well proven processes such as investment casting. Casting also offers the engineer more freedom than other processes.
An objective in the development of light metal composite materials may be to increase the modulus of elasticity (Young’s modulus). Using the universally accepted linear and inverse rule of mixtures, this potential increase can be estimated whereby the well-known border cases apply only to certain geometric alignments of the components in the composite materials. As the percentage of SiCp increases, the modulus moves linearly in the direction of the SiCp modulus and away from the aluminum alloy. This reaction is similar in other properties such as thermal conductivity and expansion.
Aluminum alloy/silicon carbide MMC castings can be produced in 20%, 30% and 40% concentrations of silicon carbide by volume and heat treatable, commonly being furnished to a T77P heat treatment.
This enables engineers to design components to their desired operating environment. For instance, the property of wear resistance can be understood easily because the aluminum alloys are relatively soft compared to the hard carbide particles, enhancing the wear properties with increased silicon carbide particulate (SiCp). Bonding between the SiCp particles and the aluminum matrix is an important consideration. Purchasing pre-alloyed ingot stock with the percent reinforcement desired and metallurgical bond between the parent metal and particles is recommended. The base alloy of currently produced MMC is Aluminum 359, which can be heat treated for specific applications. In addition, the particle size of the SiCp can be varied for more design freedom.
Although elongation and fracture toughness decrease with the SiCp in the MMC, the values often are better than other alternative methods to achieve stiffness and thermal properties. This is because as the higher particle contents are cast, the material takes on more of a ceramic character with lower fatigue life and brittle failure without plastic deformation.
The fluidity of aluminum MMC alloys decreases as the concentration of SiCp increases; the difficulty of successfully casting higher SiCp concentrations increases. The tendency of aluminum alloy MMC to form shrinkage and gas porosity also becomes a greater concern with higher densities of SiCp. It is essential to minimize turbulence within the melt and during mold filling. Hot isostatic pressing higher percent SiCp MMCs sometimes is necessary to eliminate subsurface gas voids.
The reinforcement of metals can have many different objectives and opens up the possibility for application of these materials in areas where weight reduction is the top priority. The precondition here is the improvement of the component properties. The objectives for light metal composite materials are:
- Increase in yield strength and tensile strength at room temperature and above while maintaining the minimum ductility or component toughness.
- Increase in creep resistance at higher temperatures compared to that of conventional alloys.
- Increase in fatigue strength, especially at higher temperatures.
- Improvement of thermal shock resistance.
- Improvement of corrosion resistance.
- Increase in Young’s modulus.
- Reduction of thermal elongation and engineering thermal conductivity.
Investment Casting Design Considerations
As it is with other hard alloys such as inconel or even stainless steel, the secondary machining of MMC castings is a significant cost consideration. The near-net-shape advantage and the greater freedom of design that is inherent with the investment casting process provide opportunities to greatly reduce or eliminate machining. The successful implementation of cast MMC can depend on possessing the necessary engineering skills to design products that take advantage of the investment casting process.
For engineers, the near-net-shape advantage means only critical surfaces need be machined. Raised pads and undercuts can be employed to relieve the surrounding area of the part. Only minimal amounts of machine stock must be added to these machined features to assure cleanup.
Investment casting also can take advantage of part count reduction. This capability is particularly beneficial when designing for MMC because the fewer assembly points of a unitized structure also correlates to a reduction in the cost for machining.
The special properties of aluminum/silicon carbide MMCs can be of benefit and are available to solve a myriad of design challenges for weight reduction, stiffness, vibration, heat transfer, wear and/or thermal expansion. The investment casting process offers a unique capability for the economical manufacture of MMCs.