Aluminium matrix composites

Materials World magazine
30 May 2019

Liquid pressure forming is helping Alvant make aluminium composites lighter than traditional metals.

Reducing weight without compromising structural integrity is the biggest aim for most engineering projects. Aerospace, automotive, defence, marine and consumer goods manufacturers all look for ways to increase product capabilities and performance while meeting ambitious goals for fuel efficiency, carbon emissions and sustainability. Aluminium matrix composites (AMCs) are a group of composite materials in which the aluminium is reinforced with a secondary high-performance material, making them suitable for applications where conventional metals are expected to approach or exceed their performance limits.

Comparing material potential

AMCs come in multiple variants. Reinforcing the metallic material with a secondary high-performance material typically uses a long-fibre, short-fibre or particulate. Properties, such as stiffness, strength and density can be tailored through particle-reinforcement with continuous fibre reinforcement (CFR), which is preferred for applications where higher performance is needed.

These materials can be used to engineer durable lightweight components for harsh environments. Compared with unreinforced metals, AMCs can also have lower coefficients of thermal expansion and superior electrical conductivity. They offer multiple advantages over polymer fibre-reinforced materials such as carbon composites, including higher transverse strength and stiffness, a higher thermal operating range, easier reparability and more opportunity to recycle.

How AMCs are made

We have been exploring the potential of liquid pressure forming (LPF) to manufacture AMCs. The company has developed its own version of the process by which aluminium, which acts as the matrix, and high-strength reinforcement materials is formed to create four AMC materials families. These include:

  • - AlXal – a continuous fibre-reinforced AMC
  • - ParXal – a particle loaded AMC
  • - AerXal – an aluminium syntactic foam, and
  • - CorXa – a multi-phase AMC similar in concept to a sandwich
  • material but made in a single-shot process providing ultra-high stiffness and low density (~1.5g/cm3).

To manage cost and complexity, components are not necessarily manufactured entirely from AMC, for example, 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. In these applications, performance is provided where needed by using AMC inserts applied to the larger aluminium component. This limits the fibre content, simplifies the AMC insert geometry and reduces costs while increasing the performance and capability of the component.

Advantages over legacy materials

AMCs can offer superior longitudinal strength to that of steel, at one-third of the weight. Advantages of using AMCs over polymer fibre-reinforced materials include higher transverse strength, stiffness, better temperature and fire resistance, no moisture absorption, improved damage tolerance and easier repairability.

AMCs are 50% stiffer than carbon-fibre in the longitudinal direction, and close to three times as strong in the transverse direction. By retaining their properties at high temperatures, unlike carbon-fibre, AMCs are also suited to high service temperature components in applications such as engines, electric motors and defence products. For harsh environments, they can have superior fatigue strength to steels. If AMC components are damaged while in use, they are more impact-tolerant than carbon-fibre, so they retain more performance after damage.

Continuous fibre-base

AlXal is a continuous fibre-based AMC. The company received a £513,000 grant from Innovate UK to design, manufacture and test an AlXal brake rod with an expected 30% weight reduction over an equivalent titanium component, while maintaining a comparable strength to steel, reducing fuel burn and noise, improving reliability and lowering costs.

In aerospace, current landing gear systems are typically stronger and heavier than necessary and account for about 3% of aircraft weight, with a corresponding effect on fuel consumption. Alvant partnered with technology group Safran Landing Systems on a two-year aerospace project worth £28mln, titled large landing gear of the future, which aimed to reduce landing gear component weight
by up to 30%.

AMCs can have superior strength and less than half the weight of steel, meaning highly loaded components made from traditional metals, such as steel, titanium and aluminium, can be replaced by lightweight, low-inertia parts without any increase in package size. The Safran-Alvant project will look at how AMCs can challenge traditional materials in the design and manufacture of landing gear assemblies.

Multi-phase material

Alvant has developed a metal matrix composite (MMC) multi-phase material, named CorXal, which may offer an alternative to titanium and carbon sandwich composite offerings. Traditional sandwich materials are typically assembled from carbon composite or unreinforced metallic panes with a variety of honeycomb cores. The metallic pane sandwiches are typically flat, 2D panels, while carbon composite varieties can take 3D forms. CorXal is produced via a one-shot manufacturing process and has the potential to increase a component’s strength and stiffness-to-weight ratios.

The multi-phase material method ensures the product is more resistant to shrinkage, damage and high temperatures, making it suitable for harsher environments. It is also possible to create more complex shapes while reducing the post-processing and assembly times usually associated with sandwich panel construction by as much as 30%.

Alvant also claims that different material selections for both the skin and core are possible so that the MMC multi-phase material properties can be tailored to suit specific requirements at the lowest weight. For comparison, typical Alvant multi-phase material density is 1.9g/cm3, whereas aluminium is 2.7g/cm3, steel is 7.9g/cm3, titanium is 4.5g/cm3 and carbon-fibre composite is 1.6g/cm3.

Alvant is working with aerospace and defence companies in using CorXal in the leading edge of aircraft wings, as well as various other brackets and structures. The wing is subject to huge changes in temperature and is susceptible to bird and lightning strikes. The existing materials used have been found to be susceptible to bond degradation between the skin and core elements. This can be caused by environmental factors such as temperature, moisture ingress and contamination, which can negatively impact on the integrity of the bond. The result has been a tendency to design components with excessive safety factors to compensate for this potential degradation.

Due to the one-shot production of CorXal, there is no bond interface between the skin and core elements – it has a continuous aluminium matrix throughout providing a closed system. This means parts made from CorXal will not succumb to the environmental conditions, maintaining the necessary structural properties needed, increasing stiffness by up to four times over some grades of aluminium, and reducing weight by a target of 40% compared with some steels.

Blue sky engineering

The potential benefits of AMCs open opportunities for the materials to be used in future areas of engineering development such as electrified transport, renewable energy and healthcare. Also in high-end consumer products that need to be light, strong and capable of sustaining damage such as mobile devices, biomechanical prosthetics, sports equipment and personal mobility products including wheelchairs and folding bicycles.

AMCs could find applications in high-pressure seals, aircraft landing gear and seats. Where performance, efficiency and precision are vital, the materials could offer a significant opportunity for robotics, electric motors and automotive suspensions. Due to their capability to withstand extreme temperatures, AMCs may also be suitable for components in high-voltage battery systems, unmanned aerial vehicles that fly at high altitudes and vehicle powertrains.

Richard Thompson is Alvant Commercial Director.