Composites joined directly to metals

A new joining technique will significantly reduce weight and costs in aerospace and high end automotive and marine construction. Tom Shelley reports

By sculpting protuberances on the surfaces of metal tongues and other shapes, it is possible to form metal to composite joints that require no mechanical fasteners or in some cases, adhesive, and may be stronger than the parent composite. Surfaces are completely smooth and flush, ideal for both aerodynamic performance and aesthetic appearance. Failure mode is energy absorbing and the technique can be expected to find favour with designers of many kinds of engineering product, whether for use in the air, on the road, in the water, under the sea or even in the human body. Given the name "Comeld", the new methodology is the patented invention of Bruce Dance and Ewen Kellar at TWI, formerly The Welding Institute, headquartered near Cambridge. Project leader Dr Faye Smith explained its virtues to Eureka on a recent visit there. Key is what has been named, the "Surfi-Sculpt" (The trade mark apparently requires putting it in italics) profiling on the surface of the metal surfaces produced using an electron beam. The first joints to be assessed are inspired by traditional wooden tongue and groove joints but with an added serrated grip. Failure can involve deformation of the protuberances, which have been given the name, "proggles", followed by delamination in the adjacent composite. That keyed to the proggles stays attached. In one case of carbon fibre reinforced composite joined to titanium, failure took place in the titanium, due to the dimensions of the joint. Tests to failure were undertaken in tension at 1mm/minute. In the other failures, which occurred through shear failure within the composite, energy absorption was considerable, more than double that required to pull a conventional step joint apart. The joints should thus find considerable favour with automotive and other designers engaged in enhancing crash energy absorption. Initial development work has so far been with titanium, aluminium and stainless steel. The composite has mostly been woven glass, vacuum infused with polyester resin. The proggles can be made any shape or profile: vertical, spiked, rounded or leaning away from the load. While a 25mm x 30mm area takes eight seconds to sculpt, the technology is not overly expensive. TWI expects that most users would use a jobbing sub contractor to sculpt the surfaces rather than invest in electron beam machines themselves. Metal surfaces may need to be surface treated for optimum bonding, although not in any very exotic manner. All specimens made by TWI were first grit blasted and degreased. Stainless steel specimens were etched with oxalic/concentrated sulphuric acid. Titanium specimens were etched with alkaline peroxide. Aluminium specimens were etched with chromic acid and primed with BR127. The vacuum infusion of resin into the stack of fibre was then undertaken with the metal part in place. The joints shown to Eureka mostly had steps within the tongue, making it a tongue extending from a tongue, but other shape configurations are possible. Joints can be single or double tongued, the tongues can be inclined, or the joints can be step joints, planar or inclined. The optimum shapes and densities of proggles have yet to be determined, and will probably be found to depend on the combination of materials and joint design used. Tests have to be undertaken of different kinds of failure mode, such as bend testing, fatigue testing, impact loads and combinations of cyclic mechanical loading with environmental conditions liable to degrade the composite and/or metal used. For these reasons, further research and development is to be undertaken by TWI within a group sponsored project. TWI runs quite a number of these, in which funds are drawn from a number of companies in order to finance research and development whose results are shared among the participants. The most obviously interested participants are likely to be engaged in aerospace manufacture. The joints are flush, making them aerodynamically attractive, and much lighter than joints requiring use of mechanical fasteners. They also avoid stress concentrations around holes for mechanical fasteners. As well as being aerodynamically favourable, they are also hydrodynamically and aesthetically favourable, making them potentially very attractive to boat builders, especially constructors of pleasure craft. Boat builders already use composite hulls almost universally except for canal craft, and considerable cost saving should result from more easily being able to attach aluminium alloy superstructures, rather than making the whole construction out of composite, which is expensive. It even offers a good way to attach composite boat hulls to wooden superstructures, favoured by some owners. This could be accomplished with a metal intervening strip bonded into the composite. Wooden hulls are a major nuisance because they rot, but wooden superstructures are favoured because they are easy to work with and have a pleasing, traditional boat appearance. Following the same logic, it is now possible to insert a strip of metal into the edge of a composite sheet and weld a metal construction to it. Having a direct composite to metal bond possibly without need for adhesive is also potentially of great interest to both the food and pharmaceutical machinery designers and to makers of surgical implants. The other major group of designers expected to be interested are those in the automotive industries. The Foresight Vehicle programme made strong predictions that composites would find increasing application in car manufacture because of their ability to save weight. Saving weight improves acceleration and reduces fuel consumption, likely to be an increasingly sought after goal in the future. Performance cars already often use composite bodywork but this is expected to become mainstream as manufacturing costs drop. Motor cars are, in any case, one area where purchasers tend not to go for the cheapest option, but for the best they can afford. Purchasers of trucks and buses also tend not to go for the cheapest option but for that which will give them the greatest operating profit over the life of the vehicle. A tonne taken out of the weight of the construction allows a tonne more cargo to be carried as well as reducing fuel costs per tonne mile. TWI Faye Smith at TWI Eureka says: A new way of joining metals to composites which is cleaner, stronger and lighter than anything previously contemplated has to be a major breakthrough in engineering design. Pointers * Sculpted 'proggles' on the surfaces of metal tongues and other shapes allow metal to be joined to composite sheets without need for mechanical fasteners or adhesive. * Joints can be stronger than the parent composite. * Surfaces are completely smooth and flush, ideal for both aerodynamic performance and aesthetic appearance. * Failure mode is energy absorbing