Aluminium-based Metal Matrix Composites: Stepping into the middle-ground between unreinforced metals and carbon composites
For lightness, stiffness and strength, aluminium matrix composites (AMCs) are an advanced composites material reinforced with fibres or particulates, which provide sustainability and enhanced capability.
Advanced composite materials combine the properties of high strength, high stiffness, low weight, enhanced damage tolerance and corrosion resistance, and in some cases also have special thermal and electrical properties. These composites are classified according to the material being reinforced, with reinforcements typically made with a long fibre, short fibre, or particulates. Hence Metal Matrix Composites (MMCs) are metallic materials reinforced with a secondary high-performance material. Within the category of MMCs, Alvant specialises in Aluminium Matrix Composites (AMCs).
AMCs first became known, primarily for their use in automotive components, in the 1980s. At this time AMCs were in their infancy, their properties largely unproven and sometimes over-sold. As a consequence, the reputation of AMCs suffered, and as carbon composites became more widely adopted AMCs were largely forgotten. In the three decades since, however, research and development into the manufacturing of AMCs has resulted in game-changing progress. AMCs’ properties are now precisely manufacturable, dependable, and more affordable and repairable than carbon composites. These qualities are affirmed by the high calibre of companies Alvant is working with and supplying to.
AMCs are not a single material but a family of materials. This means their stiffness, strength, density, thermal, and electrical properties can be tailored with particle-reinforcement or continuous-fibre-reinforcement (CFR) for applications where higher performance is needed.
To manufacture CFR-AMCs, Alvant uses a preform, a three-dimensional fabric form designed to precisely conform to a specific shape and to meet exact mechanical and structural requirements.
Unique, patented manufacturing process
Elements of Alvant’s unique manufacturing process, Advanced Liquid Pressure Forming (ALPF), are the subject of patents. In addition to using continuous-fibre materials for reinforcement, to achieve certain physical and mechanical properties ALVANT can also use its Intellectual Property to develop particle or fibre ‘whisker’ reinforced AMCs.
Components are not necessarily manufactured entirely from aluminium matrix composite if they have regions of low stress where enhanced mechanical properties are not required. In such cases, components can be reinforced locally in a method known as hybrid-AMC. Reinforcements are provided precisely where needed by using aluminium matrix composites as inserts inside the larger cast for the aluminium component. This limits the fibre content, simplifies the AMC insert geometry, and reduces costs.
An alternative to conventional metals and more expensive composites
AMCs are suitable for applications where conventional metals are expected to approach or exceed their performance limits. Advantages of using AMCs rather than unreinforced metals include greater strength, higher stiffness, reduced weight, better wear resistance, and a lower coefficient of thermal and electrical conductivity. These characteristics mean that AMCs provide the longitudinal strength of steel at one-third of the weight. AMCs also offer significant weight and performance benefits over other conventional unreinforced materials such as common alloys.
Advantages of using AMCs rather than polymer fibre reinforced materials, such as carbon composite, include higher transverse strength, and stiffness, better temperature capabilities, no moisture absorption, fire resistance, improved damage tolerance, and easier repairability. AMCs’ strengths in the x-, y- and z-axes, as well as giving performance advantages, also allow greater freedom in design. AMCs are suitable where they can meet the engineered component’s performance requirements, or address concerns about the likely need for repairs.
AMCs’ product benefits mean they have potential uses in a wide range of engineering applications. It is in defence, aerospace and other forms of transport where AMCs will likely be applied in the largest quantities, but AMCs are equally suitable for high-end consumer products which need to be light, strong and capable of sustaining damage – to give just some examples, products such as mobile devices, biomechanical prosthetics, wheelchairs, prams, bicycles, and luggage.
Where safety and reliability are essential, AMCs are well-suited to products such as high-pressure seals, aircraft landing gear, and seats. Where performance and precision are vital, AMCs can benefit products such as robotics, metrology machines, electric motors, automotive suspensions, and sports equipment. And because AMCs are capable of withstanding extreme temperatures, they are suitable for components in high-voltage battery systems, unmanned aerial vehicles which fly at high altitudes, and vehicle powertrains.
The advantageous material properties of AMCs
AMCs have a number of advantages over alternative materials which might be considered for the same types of application. The table below quantifies the key advantages of continuous-fibre-reinforced AMCs (CFR-AMCs)
|Density||3.4 g/cm³||AMCs have much greater strength than steel at less than half the weight.|
|250 GPa||This measure of stiffness indicates the material’s ability to resist deformation under load. AMCs perform better than most grades of steel (200 GPa).|
|Ultimate tensile strength||1230 MPa||The amount of force AMCs can take before failure is double that of structural steel (approximately 600 MPa).|
|Compressive modulus||250 GPa||With greater elasticity under compression than unidirectional high-modulus carbon fibre (135 GPa), AMCs have superior impact tolerance.|
|Compressive strength||1530 MPa||The compressive strength of AMCs is greater than that of carbon fibre (850-1200 MPa depending on grade of carbon). AMCs are stronger in compression than tension – because the fibres construct complex strengthening mechanisms in the aluminium.|
|Youngs modulus at 250°C||230 GPa||The stiffness of AMCs is reduced only fractionally at high temperature, whereas the stiffness of most carbon composites greatly diminishes at about 125°C.|
|Ultimate tensile strength at 250°C||1150 MPa||The strength of AMCs is reduced only fractionally at extreme high temperature, and doesn’t drop significantly until about 350-400°C, whereas carbon fibre loses much of it strength at about 125°C.|
|Youngs modulus||160 GPa||Stiffness in this direction is far greater than that of carbon fibre (10-15 GPa).|
|Ultimate tensile strength||150 MPa||Strength in this direction is also far greater than in carbon fibre (40-50 MPa).|
One Pascal equates to one Newton per square metre of pressure.
One megapascal is equivalent to one million Pascals and one gigapascal to one billion Pascals.
AMCs are a good 50 per cent stiffer than carbon fibre (unidirectional carbon-epoxy composite) in the longitudinal direction, and close to three times as strong in the transverse direction.
AMCs retain their properties at high temperatures, unlike carbon fibre, making them better-suited to high service temperature components in applications such as internal combustion engines.
AMCs have much higher damage tolerance than high strength aluminium alloys and similar fatigue strength to steel.
AMCs also have similar fatigue response to steel. This means AMC components could replace high stress, high cycle steel components for a significant weight saving.
If AMC components do get damaged while in use, they are more impact-tolerant than carbon fibre, meaning that they retain more performance after damage.
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