Additive manufacturing crosses the finishing line

A place for additive manufacturing

Swansea University Race Engineering is one of the Formula Student teams that has been at the forefront of the competition since 2001, experimenting with new designs and technologies. The team is made up of engineering students from across different disciplines and levels of study, who work alongside experts from the Materials Advanced Characterisation Centre (MACH1) and the ASTUTE project based at Swansea University, and funded by the Welsh Government and Welsh European Funding Office, respectively.

The Swansea University team focuses on developing different aspects of the car, including suspension, engine, chassis and heat transfer. The students' approach to innovation is pragmatic: if you take a car apart, improve each component and then put all the parts back together, the whole will be greater than the sum of the parts.
One of the components that was given a radical make-over this year was the car's intercooler. The aim was to redesign the component to make it lighter and more compact than its old counterpart.

A combination of these factors was expected to improve the car's speed. To achieve the objective, the team looked towards additive manufacturing, more specifically, metal powder bed fusion.

Metal powder bed fusion uses a powerful high precision laser to fuse fine metallic powders in order to form highly complex functional components. The design of the parts is optimised using 3D computer-aided design (CAD) and the components themselves can be built from a wide range of metal powders melted in a tightly controlled inert atmosphere, in layers with a thickness ranging from 20 μm to 100 μm.

Part requirements

Clearly, the location of the intercooler is essential because the part requires flowing ambient air to cool the boost air into the engine. Commonly, intercoolers are located behind the front grill of a vehicle, but due to their weight, it's not always the best option for race cars. In fact, in the Swansea University formula student car, location was one of the challenges. The intercooler is situated behind the driver, above the engine – not an optimum position in terms of heat transfer.

The new high performing heat exchanger needed to have excellent heat transfer and low pressure drop through the system. The wider range of geometries made available by additive manufacturing meant the team could experiment with more innovative designs that allowed better airflow. Interestingly, although additive manufacturing has previously been used in Formula 1, the current project is unique due to the large scale and cross flow nature of a car intercooler.

Overcoming design challenges

The main hurdles of the project came from design, material and software challenges. The team's objectives were to ensure a high density of boost air for the engine, while still maintaining a good mass flow rate of air. This intercooler also needed to be as light as possible, to improve acceleration.

“Design had a massive impact on the end product, even more so than the material, which is something we didn't originally expect,” explained Dr. Nicholas Lavery, director of the Materials Advanced Characterisation Centre (MACH1) at Swansea University. “Although our students had to go back to the drawing board many times, they were very pleased with the end product. It's also interesting when results are unexpected; it either means you've muddled up your calculations or you're on to something good. Luckily, in this case, it was the latter.”

To identify the best alternative, students tested four intercooler core designs. The different iterations reflected the design principles and capabilities of different manufacturing methods. The first core design, used as a benchmark, was a conventionally manufactured aluminium alloy, AlSi10Mg core with a simple vertical design on the ambient side and a sine wave shape on the boost side.

The second core was designed from 316L stainless steel using metal powder bed fusion. The design was identical to the first one, but was used to test how the added surface roughness available with additive manufacturing impacted heat transfer and pressure drop.

The real benefit of additive manufacturing came when the students started experimenting with the design freedom of the technology. The third intercooler core had a lattice of three dimensional star shapes to form the mesh – a design only made possible by additive manufacturing machines. The mesh was generated using additive manufacturing software and although it took several iterations, the team knew that they were on to a winner.

The fourth and final core used the same three dimensional star lattice structure used in the mesh core, but also made the strut cross-section finer, which meant a higher surface area and density.

Although superior to casting, the surface finish was not as smooth as a manufactured surface. The tests found that the higher surface roughness of additively manufactured intercoolers outperformed conventional counterparts.

This counter intuitive result could be due to better air mixing, altered flow conditions or a combination of both. A good example is shark skin which is rough but very efficient. Recently, biomimicry, the study and emulation of nature in engineering, has become topical. Additive manufacturing could play a role in creating and researching novel shapes.