By infusing different materials into metals, scientists and engineers are creating stronger lightweight metals that are being used in a multitude of industries to increase efficiency and performance.
Sometimes, the whole really is greater than the sum of its parts. A perfect example is metal matrix composites (MMCs). MMCs are metals or alloys that incorporate particles, fibers, or hollow micro-balloons made of a different material.
Scientists and engineers have created these superior materials by metals with other alloys, ceramic, and other organic compounds in order to improve the properties of standard materials. For instance, aluminum can be reinforced with boron, carbon, silicon carbide, alumina, or graphite to create a composite that is 30% to 40% stronger and more rigid than barebones aluminum. Metal matrix composites break down into four different categories: dispersion hardened and particles; layer composites; fiber composites; and infiltration composites. Some of the advantages of using MMCs are:
- Higher temperature capability
- Fire resistance
- Higher transverse stiffness and strength
- No moisture absorption
- Higher electrical and thermal conductivities
- Better radiation resistance
Manufacturers then utilize these composites to create better, more stable, and lighter weight products for various industries (i.e., automotive, aerospace, and defense).
According to Mahitha Mallishetty, a metals and minerals analysts from Technavio Research, “The global metal matrix composites market size is projected to reach 10.8 kilotons by 2021, growing at a CAGR of over 6% during the forecast period.” A market report from Grand View Research [4] in 2014 says that the largest product segment was the aluminum-based metal matrix.
It accounted for 30% of the demand. The need for lightweight and high tensile strength parts are the driving factors for the demand of aluminum to soar. The report predicts that aluminum will continue to grow and be the leader of MMCs until 2022.
Refractory matrix metals, metals that contain ceramic material, are poised to be the second-largest MMC market by 2022. Their demand will grow due to is multifunctional properties.
They can be used for tools, nuclear radiation control rods, solar panels, spacecraft exteriors, and catalysts in chemical reactions. They have a high tensile strength, malleability, and ductility.
The bar graph above shows the growth trend of different metals used for MMCs until 2022.
The North America market accounted for over 34.8% of the total MMC demand in 2013 and is expected to remain the largest market for 2022. The continued growth in the Asia Pacific will drive the increase of MMC, making it the second-largest marketplace in the world.
Metal Matrix Composites for the Auto Industry
Automakers in particular are increasingly turning to MMCs in order to meet strict fuel economy requirements. The challenge has been to reduce the weight of vehicles without sacrificing consumer demands for comfort and safety. To overcome those obstacles, automakers take advantage of MMCs’ unique characteristics, which provide the ability to meet specific and rigorous design requirements.
Nanotechnology
In recent years, manufacturers have started incorporating nanometer-sized additives into industrial fabrication and metal products. These additives have nano (one-billionth of a meter) fillers dispersed within the metal matrix akin to the threads that help to strengthen fiberglass patches.
This technique has been used to make cutting-edge engineering materials for automakers. The new materials will not only reduce mass, but also improve reliability and efficiency.
According to the Technology Information Forecasting and Assessment Council, with an “optimized fabrication process and controlled nano-sized second phase dispersion, thermal stability and mechanical properties such as adhesion resistance, flexural strength, toughness and hardness can be enhanced, which can result into improved nano-dispersion.” Interestingly, these characteristics can be added without changing the materials’ chemical compositions.
An instance of these exemplary strength improvements can be seen in aluminum nano-composites wherein the components’ rigidity is increased significantly. However, a very minimal amount of material is added, which ultimately produces a lighter product.
The Center for Composite Materials, the Center for Advanced Materials Manufacturing (CAMM), and the University of Wisconsin Milwaukee (UWM) are leading innovation in this field. By working and manipulating MMCs, the UWM researchers have tailored lightweight materials to possess a multitude of beneficial properties that the automotive industry has found especially helpful, including:
● Strength and stiffness
● Hardness and wear resistance
● Thermal conductivity
● Energy absorption
The newest MMCs that are being developed have self-repairing, self-cleaning, and self-lubricating properties. These composites are expected to enhance automotive energy efficiency and reliability.
Bearing Up Under Pressure
UWM has also developed lead-free aluminum and copper matrix composites containing graphite particles. These MMCs replace standard copper-lead bearings often used in crankshaft main-bearing caps. Similar to other MMCs, UWM’s new development is also far lighter weight and more wear-resistant than traditional materials. In addition, the graphite particles provide a continuous film for self-lubrication of the component. Lastly, unlike lead, graphite is nontoxic.
Pistons, Cylinders, and Brakes
By embedding graphite particles in aluminum, the UWM composites group also prepared a lightweight and self-lubricating material that can be used to avoid a potentially debilitating problem with pistons and cylinders seizing up. In conventional engines, aluminum pistons and cylinder liners can stick together during cold start-up or when engine oil is low. However, engine components made from the aluminum-graphite composite equip the engine with built-in anti-seizing protection.
Automotive disk brakes and brake calipers (typically made of cast iron) are another area where MMCs can be utilized to significantly reduce weight of said components. The UWM team has developed aluminum-based MMCs and alloys that use reduced silicon carbide to help decrease potential cost and overcome machinability barriers. Furthermore, aluminum-fly ash composites developed at UWM have been explored to make prototype brake rotors.
The turbofan above was constructed using MMC materials providing a lightweight but still strong design with higher efficiency for the whole aircraft.
Aerospace Applications
Some 50 years ago, aircraft designers began taking advantage of the high strength-to-weight ratio associated with composites. Engineers started swapping out aluminum parts for MMC-based components. The design change reduced both the aircraft weight and increased fuel efficiency significantly. The composites were also attractive to aircraft manufacturers because of their resistance to corrosion and wear.
The extent to which composite materials have been used in aircraft has changed considerably over the years. For example, in today’s F-22 fighters, carbon fiber composites and related materials compose nearly one-third of the jet's structure. The F-16 Fighting Falcon also uses monofilament silicon carbide fibers in a titanium matrix for a structural component of the jet’s landing gear.
Many of the sophisticated capabilities of modern military aircrafts would not even be possible without today’s advanced composites. For instance, the V-22 (Osprey) tilt-rotor craft, is able to take off, land, and hover like a helicopter, as well as re-orient its rotors in midair and fly like a turboprop airplane.
That kind of aeronautical split personality is due in part to the graphite-fiberglass rotors as well as other lightweight composite-based structures in the rotor system. The new structures are strong enough to tolerate high centrifugal forces yet remain slightly flexible. Similarly, the extreme aerial maneuverability of F-18 fighter jets is partly due to composites used in the aircraft’s wings, flaps, stabilizers, and other critical parts.
Self-Healing Properties?
One of the most fascinating developments by the UWM team is the creation of materials with self-repairing capabilities, which are surprisingly similar to the healing properties in biological systems. These self-healing materials actually repair damage such as cracks and voids, either by outside influence or autonomously. Several techniques are used to impart a self-healing characteristic to metals. Initially, specialized heat treatments and incorporation of alloy wires are applied that can “remember” their shape.
When stretched across a crack, these wires are heated to pull the edges of the fissure together as it returns to its original shape. For the automotive industry, self-healing materials are elevating vehicle longevity, reducing maintenance, and increasing reliability. UWM is also developing MMCs that have self-cleaning components, bolstering safety and reducing auto maintenance.
The Cost Factor
Most processes used to prepare metal composites, such as vapor-phase or mixing and pressing composites, tend to be expensive. However, the UWM research group hopes to fix that.
The team has developed low-cost, liquid-based methods in which fibers or particulates are added to the metal in a molten state. The hot slurry is then poured into a mold where it cools and solidifies. This solidification method is conducive to forming large and complex parts.
As an additional cost reduction, the team is also developing synthesis methods that make use of waste materials. By blending micrometer-sized, hollow fly ash particles (a coal byproduct) with aluminum or lead, the group makes inexpensive, lightweight composites known as syntactic foams. Low density and high impact resistance characterize these foams.
What’s Next?
It looks like the future is here. MMCs are going to change industrial fabrication for years to come. That being said, researchers admit that they have only just begun to explore the possibilities of these new materials. However, it is clear the use of custom-designed metal matrix composites will continue to expand into an enormous number of applications.
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