An adhesive-free way of joining aluminium and composite parts – already proven in an industry project – is now becoming a commercial reality. Lou Reade reports.
Hipercar is a collaborative research project – led by car manufacturer Ariel Motor Company – that has created a prototype low-emission sports car using a variety of new technologies and lightweight materials.
While the tangible end result is a model sports car – which will be built at Ariel’s factory in Somerset – the project has also thrown up a new manufacturing technique that will have wide use in the automotive industry and beyond.
The technique, Powderbond, joins aluminium and composite parts together – without the need for adhesives – and was developed by project partner, Powdertech Surface Science.
“Most joining techniques focus on surface preparation of the composite, but we come at it from a different angle,” says Powdertech’s sales director, James Grant.
Instead of treating the surface of the composite part – as is usual for adhesive bonding – Powderbond applies a coating system to the aluminium surface. The coating can be restricted to the sections that need to be joined, but can be applied to the entire part, as it offers protection against corrosion.
“Once the coating has cured, it has a long shelf life,” states Grant.
This means that, once treated, the aluminium is ready to be joined – but this need not be done immediately. This gives it a distinct advantage over adhesive bonding, which must be carried out quickly once the surface of the composite part has been activated.
In fact, the rationale behind developing Powderbond was to give manufacturers an alternative to the ‘pain’ of adhesive bonding: it avoids the need for surface treatment, and overcomes problems such as long cure times and a messy work environment.
Nick Welton, Powdertech’s technical lead, explains: “Powderbond’s main advantage is that it gets rid of adhesives – and all the disadvantages they bring in terms of time and cost.”
A powder of good
Powdertech is currently applying for a patent on the technique, so is reluctant to reveal exact details of how Powderbond works. However, Welton explains that the coating has very high bond strength with the aluminium. Applying this coating is where Powdertech’s expertise lies. To make hybrid parts, the aluminium and composite parts are brought together using standard processes, such as injection moulding or fabric lay-up. Heat at the material interface creates the bond.
“This fuses the parts together,” says Welton. “You then get some melt mixing between the coating, and the binder in the composite.”
In the Hipercar project, it was used mainly in the passenger compartment, or ‘tub’ of the car. The tub is a mixture of aluminium extrusions, aluminium sheet, woven glass fibre and polypropylene (PP). Powderbond was used to join these elements together.
“Powderbond was used mainly to bond the aluminium extrusions – for the side impact crash beams – to the PP,” Welton explains. “This was done directly in the mould.”
An early aim of Hipercar was to use an all-aluminium chassis, with a ‘mixed material’ design as a second option. A critical factor behind this second design was the bonding of the dissimilar materials – and early attempts produced low bond strengths.
“This is when we decided to road-test Powderbond,” Welton adds.
Ariel assessed the technique – very thoroughly, says Welton – before it could be applied to any parts. The new design, underpinned by Powderbond, helped to cut overall part count by around 75%.
Another change during the project was to move away from making the tub as a single component – as the tooling cost proved to be too expensive.
“Instead, we used simple low-cost tooling, and made it in five parts,” says Welton.
Strong bond benefits
Powdertech says the first obvious benefit of Powderbond is improved bond strength – claiming it is higher than one created through traditional plasma-treatment of composites, followed by adhesive bonding. There is also greater repeatability of joint performance – a factor mentioned repeatedly in ‘voice of the customer’ analyses.
“Unless you’re talking about very high volumes, adhesive is normally applied manually,” says Welton. “This increases the chance of variability through factors such as how thickly it is applied.”
According to Welton the surface treatment process used in Powderbond is automated, which leads to better reproducibility and fewer rejects.
“Composites manufacturers have told us that, for bonded joints, a reject rate of 10-25% is common. Powderbond can help to cut that.”
So, why polypropylene?
When high performance sports cars use composite materials, they tend to copy race engineering by choosing epoxy resins and other thermoset composites. However, Hipercar focused on a thermoplastic composite based on PP – in part, because it was a lower cost material.
Powdertech says there were two main reasons for developing a technique that works with glass-filled PP composite. Firstly, it wanted to work with a formable composite that had immediate commercial appeal.
“Thermoplastic composite is the volume option for automotive OEMs,” says Welton. “As well as its recyclability, there are other advantages – such as ease of use and shorter cycle times. It fits with what the OEMs want.”
And secondly, it was a challenge: PP is a notoriously difficult material to join, and this set the bar very high.
“If we can prove it works with PP, it should work with other materials,” he notes.
The company has already started work on this, and expects to have a version of Powderbond for polyamide ready within a few months. It also intends to tailor the process to work with other materials, depending on demand from manufacturers.
While the company’s current focus is on the industry it knows best – automotive – it says Powderbond could be used everywhere from white goods to aerospace components. It has already made some progress in this latter case: it was one of 32 companies invited to present its technology at a recent SME Instrument event in Hamburg – in front of aerospace industry researchers.
For now, though, the focus remains on automotive. And, there is also a follow-up to Hipercar: Ariel has won funding from the Advanced Propulsion Centre (APC) to take the prototype through to full commercial production – with the first models scheduled to hit the road in 2020.
“They’ve already told us that they’re looking to use Powderbond in this next part of the project,” Welton concludes.