HiETA was founded in 2011 to develop metal additive-manufacturing (AM) methods for the production of complex light-weight structures for various types of heat-management applications.

The parts manufactured include recuperators, turbo-machinery and combustion components for micro gas-turbines, phase-change heat exchangers for fuel cells and integrated waste-heat-recovery systems — plus components for highly efficient internal-combustion engines (including turbo-machinery and sections for handling exhaust gases).

Technology from Renishaw (www.renishaw.com) — one of the world’s leading engineering and scientific technology companies — is helping HiETA to move metal AM from prototype manufacture into the commercial production of its specialist range of heat exchangers.

In particular, the recent installation of a Renishaw RenAM 500M system at the company’s Bristol facilities has allowed manufacturing times — and therefore production costs — to be reduced dramatically.

The RenAM 500M is a laser powder-bed fusion additive-manufacturing system designed specifically for the production of metal components on the factory floor.

As well as incorporating a 500W laser that gives faster processing than earlier models, the new equipment features an automated powder-handling system that enables more-consistent process quality and reduced operator time on the machine.

Stephen Mellor, HiETA’s leading project engineer, became the company’s first employee after an approach by the directors, who had registered some patents on using additive technologies to produce heat exchangers.

He had first become involved with AM while studying engineering at Exeter University, and he subsequently specialised in research on the technology for his PhD.

HiETA now has more than 25 staff and a range of facilities that cover the whole AM product development process.

This can start with a review of the customer’s requirements, then progress from an initial design through computational fluid dynamics (CFD) and finite-element analysis (FEA) to manufacturing with the Renishaw equipment, followed by testing and validation.

The challenge

Traditionally, heat exchangers of the type HiETA is concerned with are made from thin sheets of material that are welded together. The complexity of their design makes production both challenging and time-consuming, while the material used for the welding process adds to the overall weight of the part.

Prior to the work at HiETA, little research had been undertaken into the use of AM for the manufacture of heat exchangers, so the initial challenge was to confirm that AM could successfully generate sufficiently thin walls of
the required quality and then produce a complete component with the complexity of a typical heat exchanger.

The next challenge was to use the knowledge and experience developed to move the process from the manufacture of samples and prototypes into low-volume production.

Having already worked closely with Renishaw to develop specific parameter sets for the production of leak-free thin walls (down to thicknesses of 150µm) in Inconel, HiETA chose to partner with Renishaw and to use its AM250 system across a range of projects.

To start with, both companies produced samples using a variety of settings on AM250s at Renishaw’s facility in Stone (Staffordshire) and at HiETA’s base on the Bristol and Bath Science Park.

The resulting samples were heat-treated and then ‘characterised’ at HiETA and Renishaw.

The test results enabled the companies to confirm the optimum parameters on the machines for thin-walled structures and also allowed HiETA to develop a design guidebook with parameters for heat transfer in heat exchangers manufactured using laser powder-bed fusion technology.

Staged development

Having achieved a leak-free integral wall, the next stage was to move to a complete full-size unit that could be completed in a reasonable build time.

Two projects were undertaken, both with the participation of the Silverstone-based vehicle integrator Delta Motorsports. The first was a cuboid heat exchanger (recuperator) to be used as a range extender for electric vehicles.

The second aimed to take the design of the components to higher levels of complexity beyond the traditional cuboid shape, as more-complex shapes can improve product performance and cycle efficiency, provide ‘packaging’ benefits and reduce costs.

The design chosen for this stage was a recuperator of annular form that could be wrapped around other components and contain integrated manifolds to give a more compact overall system. HiETA also used these projects to develop an extraction process for removing excess powder material from the cores of the heat exchangers.

The first result of the partnership between HiETA and Renishaw was the basic data needed to set up the AM equipment to successfully produce thin-walled structures — and to provide the parameters needed to predict the performance of heat exchangers manufactured with the Renishaw equipment.

The thermal-transfer and fluid-flow data that resulted has been incorporated into the CFD and FEA programs used by HiETA. These can be used for an initial assessment of the likely performance of new component designs, to confirm that they have the potential to meet the customer’s requirements.

Renishaw has also added software improvements that facilitate the processing of the large amounts of data when the complete recuperator is sliced into thin layers, and creating the build instructions needed for the complete part.

Although the first attempt at making a complete product on the AM250 system produced a successful component, the build time was 17 days. Improvements to the hardware and software, together with optimisation of the process parameters, saw this time reduced to 80hr.

Detailed testing showed that the component would deliver the pressure drop and heat transfer required; however, this performance was achieved with a weight and volume reduction of about 30% compared to an equivalent part made by conventional methods.

Mr Mellor said: “With almost all of our projects, we are trying to light-weight components and solve thermal management issues. Through our partnership with Renishaw, we have produced components that are typically around 40% lighter and smaller than anything equivalent that is available on the market.

“This is possible because, with Renishaw’s AM technology, we can design and manufacture many novel and high-performing surfaces integrated into a single component. This would be very difficult to do with conventional methods.”

Following its success with the Renishaw AM250, HiETA invested in the more powerful RenAM 500M system, to enable more cost-effective production of commercial components in low volumes.

“We are now producing parts for engines with real commercial applications and for customers with very demanding requirements. We use Renishaw’s AM technology to produce very complex parts that give high performance at competitive prices.”