Polymer Stir Welding opens up new possibilities in the production of hybrid components

Similar to friction stir welding or ultrasonic welding, PSW is not a conventional fusion welding process. Polymer Stir Welding, patented by royos joining solutions GmbH, Lieboch, Austria, is a modern welding process in the broader sense, developed primarily for joining thermoplastic polymers with metals. In contrast to conventional welding processes, in which metals or polymers are usually melted, the metal in the PSW process remains solid, while only the polymer is melted.

A heated, rotating tool pin is introduced into the joining zone between polymer and metal (Fig. 1). Local melting of the polymer occurs, while at the same time the pin generates mechanical surface structuring and undercuts in the metal. The molten polymer is pressed into these structures and solidifies there, creating a positive mechanical interlock between the two materials, which ensures a strong and durable joint (Fig. 2).

New opportunities for sustainable hybrid composites

When we think of lightweight construction, lightweight metals or polymers usually come to mind – but rarely the possibility of combining the advantages of both material worlds. Industry too is increasingly relying on a balanced material mix in order to achieve cost benefits, weight reduction, and, above all, to meet the targets or regulatory requirements for CO₂ reduction. In raw material production, polymers emit significantly less CO₂ than metals, especially for primary alloys – high-grade alloys as used in the automotive sector or aerospace. Their extraction is also less resource-intensive (keyword: bauxite mining for aluminium). Research is also focusing intensively on the production and industrial application of bio-based polymers made from plants and plant fibres, which are considered particularly environmentally friendly. Until now, the production of metal-polymer hybrid components was regarded as cost-intensive and labour-intensive. Injection moulding, adhesive bonding or screwing were the established methods.

For decades, the arguments have remained the same when it comes to creating durable joints between polymers and metals: Screwing increases weight and cycle time; adhesive bonding requires surface pretreatment, curing time, generates VOC emissions (volatile organic compounds) and significantly impairs recyclability. Designers accepted this Gordian knot of extra cost, process risk and ecological footprint because there was simply no alternative.

With PSW, for example, an injection-moulded PA66-GF30 component can be permanently joined in line takt with an aluminium tray – without pretreatment, adhesive or additional seals.

How PSW works – briefly explained

The PSW process combines three process steps that are necessary for achieving an optimum joint between metal and polymer. First, the metal is conditioned, i.e. cleaned, structured and preheated. At the same time, the polymer is melted and can penetrate into the newly created surface structure of the metal. Subsequently, the joining zone cools down, causing the polymer to solidify and form a particularly strong joint: the metal is only locally and slightly heated, so that no significant thermal stresses occur. Furthermore, no chips are released, and the fresh surface is directly sealed.

Since PSW operates with very low process forces of less than 2 kN, even very thin sheets or foils can be joined without visible distortion. The process force is required on the one hand to guide the stirring pin through the polymer onto the metal surface, and on the other hand to press the shoe against the polymer surface to prevent leakage of the molten polymer. By comparison, friction stir welding for the production of a cooling channel of a battery tray requires up to 10 kN of contact force. The lower forces make it possible to design components even lighter, as they only have to withstand low loads.

In combination with fibre-reinforced materials, another advantage of PSW comes into play: the fibre orientation in thermoplastic glass- or carbon-fibre reinforced polymers is largely preserved, so that the strength values are significantly higher than those of the joining methods used so far.

Weight down, strength up

Especially in the field of electromobility, next-generation batteries have achieved progress in terms of fire behaviour and operating temperature range, making polymers increasingly suitable as housing materials. The idea: the metal takes on the load-bearing functions, while the polymer realises the component functions such as protective casing, ribs, clips or media guides.

As a calculation based on a battery tray for a PHEV (plug-in hybrid electric vehicle) shows, this approach using PSW joints can save up to 50 % in weight and reduce the CO₂ footprint by up to 80 %. For comparison: polyamide saves about 10 kg of CO₂ per 1 kg in direct comparison with aluminium [1].

The same applies to the strengths of aluminium–polymer hybrid joints. Fig. 3 shows a direct comparison of the strength values of composites produced using the PSW process and those produced by adhesive bonding.