MMCs are a class of materials having many applications where mechanical properties such as strength, abrasion resistance, thermal resistance, or lightness are sought. MMCs are composed of a metal matrix and reinforcement. Herein the reinforcements include, and are preferably composed principally of, ceramics or cermets. There are many fabrication routes for generating MMCs, but typically a lowest cost route involves melting the metal, adding powdered ceramics or cermets, stirring, and then cooling the mixture to solidify. This production route is often called ‘stir casting’. The cooling may be performed by casting the mixture, by injection molding or by extrusion using a variety of techniques known in the art.
There are problems in the art with choosing reinforcement and metal materials. Some candidates react with each other. For example, it was natural to try carbon fibers in aluminum, as both are used in the aerospace industry for their lightness and strength. However, aluminum reacts with carbon to form Al4C3, which is brittle, moisture sensitive, and therefore problematic. Therefore carbon fibers are typically coated to prevent this reaction. Such coatings add cost and difficulties to the production of MMCs, and introduce other problems. The coating has to reliably passivate the carbon, on one side and present a non-reactive surface to the metal on the other.
If the reinforcement is selected (or coated) so that it does not react with the molten metal, there is still an important hurdle to producing useful MMCs: integration. The interfaces between the reinforcement and the liquid metal, when there is low affinity between the metal and reinforcement, are crucial to the strength of the material. Liquid metals and particularly aluminum typically exhibit poor wetting with reinforcement particles. In many cases this is attributable to the formation of a matrix oxide layer at the interface with the particles that hinders intimate contact. If the interfaces are not wetted, even with good mixing, and equal net forces on the reinforcements and metal, separation of the reinforcements and metal are likely, leading to a generally unwanted bulk mixture that is heterogeneous. This heterogeneity may be exacerbated by thermal contraction during solidification, which typically affects the metal much more than the reinforcements.
The more ceramic in the mixture, the more wetting is required to produce a MMC solid that is free of voids to form monolithic, integrated materials. Generally, the smaller the sizes of the surfaces of the reinforcement, the more wetting is required for integration. This is unfortunate because it is desired to retain small reinforcement particle sizes for some applications, and a range of reinforcement to matrix ratios are frequently desired.
Thus it is known in the art to use wetting agents in liquid metal and ceramic mixtures to promote intimate contact between the powders and metal. Magnesium seems to be the preferred wetting agent. For example, [1] Chaudhury teaches a stir casting method of producing a MMC with Al as the metal, and rutile TiO2 powders as the reinforcement. It is noted that using finer rutile particles led to a high rejection rate, and limited amounts of the powder could be retained in the melt. About 2 wt. % of magnesium was plunged into the melt to increase wettability. Even with the Mg, only 11 wt. % of TiO2 was successfully incorporated into the melt, and a greater degree of segregation of the TiO2 from the Al was observed at the top in comparison with the bottom of the castings, which indicates a lack of uniformity. Furthermore microvoids were observed in the particle rich zones.
According to [2] Hashim et al., addition of alloying elements can help. Excellent bonding between ceramic and molten matrix can be achieved when reactive elements are added to induce wettability. For example, addition of magnesium, calcium, titanium, or zirconium to the melt may promote wetting by reducing the surface tension of the melt, decreasing the solid-liquid interfacial energy of the melt, or inducing wettability by chemical reaction. According to [2], it has been found that magnesium has a greater effect in incorporating reinforcement particles into aluminum based melts than others that were tried, including cerium, lanthanum, zirconium, titanium, bismuth, lead, zinc, and copper. Mg successfully promotes wetting of alumina, and is thought to be suitable in aluminum with most reinforcements.
[3] Rohatgi reviews cast Al MMCs for automotive applications. It mentions that stir casting and pressure infiltration are two solidification techniques that both require mixing and wetting between the molten alloys and reinforcements. According to [3]: “High-strength, high-stiffness polycrystalline α-alumina (Al2O3)/Al composites have been prepared by a pressure-infiltration process. For nonwetting metals, the α-Al2O3 is coated with a metal by vapor deposition or by electroless plating before infiltration. Titanium-boron coatings have also been used for graphite (Gr)/Al and Al2O3/Al composites. However, in terms fabricability and cost, modification of the matrix by adding small amounts of reactive elements (e.g., Mg, Ca, Li or Na) is preferred. Alumina-reinforced aluminum composites, as well as several particle-filled MMCs, have been synthesized by adding reactive agents to the melts.”
Typically MMCs produced by stir casting (as opposed to the infiltration techniques that can incorporate very large amounts of reinforcements but require a costly and time-consuming ceramic pre-form to be fabricated beforehand) are substantially limited in the amount of reinforcement they can include. So the table III of Al MMCs in [3] shows that all of the MMCs have 5-20 wt. % of reinforcements, except Lanxide, which used the pressure infiltration process, which is more expensive than the preferred stir casting technique (as expressly noted therein). It should also be noted that the very high concentrations of reinforcements in these applications are associated with significantly greater strength and modulus than the 5-20 wt. % MMCs. All of the reinforcements used were ceramic powders (except for short fibres used by Honda).
Some information can be gleaned about the effect of calcium on surface tension from work on metal foams, and the distribution of calcium oxide within foamed metal, for example from [4] Hui, and [5] Banhart. While it is not exactly clear in these two references what the effect is, it does appear to have a notable effect on the viscosity and surface tension of a foaming metal. Per [4], the surface tension of commercially pure Al, drops rapidly with the addition of 2 wt. % of Ca.
While calcium may be included in foamed metal compositions in order to control frothing, calcium is not a particularly inviting element to include in Al melts. According to [6] calcium, lithium, and sodium are elements that are regarded as impurities in many aluminum alloys. The impurities contribute to the rejection rate of aluminum sheet and bar products. Rejected products must be remelted and recast. During this process, a portion of the aluminum is lost to oxidation (melt loss). Removal of calcium, lithium, and sodium increase overall melt loss of aluminum alloys. These impurities increase the hydrogen solubility in the melt and promote the formation of porosity in aluminum castings. According to Aluminum Alloys Castings Properties, Processes and Applications Chapter 2/15, Section 2.5.6: Calcium is a weak aluminum-silicon eutectic modifier. It increases hydrogen solubility and is often responsible for casting porosity at trace concentration levels. Calcium greater than approximately 0.005% also adversely affects ductility in aluminium-magnesium alloys.
Accordingly there is a need for a technique for improving integration of ceramic powders into molten metal to produce MMCs that can be stir cast, for example, especially techniques that allow for the integration of a greater amount of the ceramic powders.
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