Multi-material design drives joining technology

The automotive industry is a case in point. Rather than shifting from one material to another, it is opting for a multi-materials approach to satisfy the increasing constraints from weight to safety. The result is that materials with radically different properties need to be joined together. Management of thermal expansion, electrical conductivity and corrosion are just a few issues that need to be considered when joining exotic materials with the more traditional ones. It is not just about lighter materials, though, but smarter design and using the optimum methods for specific interfaces and materials. At the recent Global Automotive Lightweight Materials Conference in London, there was much discussion around the various different ways of joining the materials being used to take weight out of a structure. Speaking at the event was Professor Frank Henning from the Composites Technologies Centre at the Fraunhofer Institute based in Canada. He says: "We are strong believers in a multi-material approach and think each material has its own benefits, so should be used where it makes most sense. And that can change all the time as new technologies are developed. "If you reduce weight just by replacing the material, then the designer did a bad job as you over engineered it in the first place. There is a lot that can be done in terms of shape optimisation, and the more expensive a material becomes the more we need to integrate functions to make it cost competitive." Professor Henning is researching methods of joining composite materials with steels and alloys, but in a single, automated, production process. He outlined two fundamental joining techniques to produce what he refers to as 'hybrid' parts: extrinsic and intrinsic. Extrinsic joints take two distinct parts and assemble them using rivets, adhesives or fasteners. However, new methods are being explored that could act as an alternative such as loop wound joints. "The loop system is a development that is coming from filament winding," says Professor Henning. "It is connected to a robot and the robot joins tubes with specially designed clenches by winding around the structures, joining them continuously." Intrinsic 'hybridisation' looks to integrate various different materials in to a single process. The idea, by Fraunhofer, is to integrate metal at the cure or moulding stages of production. "Intrinsic hybridisation in mould assembly might have a metal part mounted in a mould and the mould is then over injected or over pressure moulded," says Henning. Also at the event was Jaguar Land Rover (JLR) which gave an insight into its material choices on various parts of its vehicles. JLR has been a forerunner in adopting aluminium, and as a result has had to develop, and often pioneer, many new joining methods to ensure joints provide the strength and stiffness needed. The new Range Rover Evoque features an aluminium roof structure riveted and bonded to the main steel monocoque chassis. There is a further operation in the paint shop which seals the outside of the joint preventing any water getting in and risk of corrosion. "This saves mass in the area we really want, which is higher up," says Simon Black, senior manager body structures, Jaguar Land Rover. "This lowers the centre of gravity significantly and makes a contribution to improving the dynamic performance." The materials market is continually evolving and OEMs are placing more pressure on materials suppliers to deliver joining methods, as well as the raw materials, needed for 21st Century engineering challenges. Material and adhesive innovator Huntsman is focusing a great deal of its effort on producing high quality, complex, lightweight parts. It continues to develop technologies and processes that strike a balance between increasing throughput, low return on investment and quality. The company has helped UK-based NRG produce a composite with a magnesium hub. During the resin transfer moulding process, specially-coated titanium fasteners working within the specially-bonded bushes fasten the hub to the epoxy carbon fibre rim, so no additional auxiliary component bonding or finishing is required. The company is well versed in providing adhesives for different substrates. Last year it supported German based Erndtebrücker Eisenwerk (EEW) produce its compound liner clad (CLC) steel pipe. The company created a composite plate compounded out of a carbon-manganese plate and a corrosion resistance alloy (CRA) sheet, which needs to be bonded together with a high performance epoxy. The challenge lay in identifying an epoxy adhesive that could enable the cladding of steel plates with all commonly used grades of CRA sheets, prior to forming and welding the heavy composite clad plates into pipes. After assembling the composites for testing, the adhesives were hardened in a hot press at 180°C and differences in the resistance to shear and tensile forces were observed. For most of the products, disbonding occurred with some of the composite samples showing significant signs of breakage. However, one of the adhesives - Araldite AV 4600 – emerged as a clear winner, forming a strong and durable joint without any signs of disbonding and displaying the highest shear strength values of all products on test. Markus Bockelmann, leader of research and development at EEW, says: "The stress in compound cladding often reaches maximum levels due to bending or shear force, so a bonding solution that helps enhance performance and prevents issues such as inner buckling of the pipe is critical. "Araldite AV 4600 proved to be ideal for this application in providing the strength needed to withstand both bending in production and the shear and tensile stresses that the pipe would be subject to in application offshore." The CLC pipes produced using this system show outstanding mechanical and physical properties, making them ideal for high-stress environments such as offshore marine applications. To produce the composite clad plate that forms the CLC pipe, EEW used Araldite AV 4600 to bond the carbon-manganese steel plate and CRA sheet together in combination with additional resistance spot welding. The bond has been proven to be strong enough to form pipe diameters even less than 180mm. EEW's CLC pipes fulfil the specified requirements for liner pipes, providing more safety against inner buckling and collapse due to the bonding of Araldite AV4600 and the grid of spot welds. They can also be produced in much larger diameters than liner pipes. Indeed, the production process of CLC pipes does not set any limitation to the pipe diameter. Master Bond is also answering the call for multi-material bonding approaches and has developed a fibreglass adhesive, EP33, which facilitates the reliable bonding of fibreglass to a variety of different substrates. It has been specially designed to overcome thermal expansion mismatch complications. Curing at room temperature, it produces durable, high strength, and tough bonds between fibreglass, wood, metals, vulcanised rubbers and many plastics. It maintains strength of 220kg/cm² in shear even after exposure to 235°C.