Joining forces for multiple properties

Advances in technology are allowing weld bonded laminates and constructs – which combine stainless steel and aluminium – to advance into greater commercial use. Tom Shelley reports

Solid and plasticised state bonding developments are allowing the improved and lower cost fabrication of laminates and welded constructions of steel and aluminium and other hard-to-join metal combinations. US-based companies seem to favour improved roll bonding, while concerns in the UK and Russia seem to favour friction welding. Both approaches have their merits and demerits, but achieve the same objectives: high temperature enduring fabrications that combine properties – especially good heat transfer through aluminium with the strength of carbon steel or the corrosion resistance of stainless steel. Potential beneficiaries of these processes range from nuclear reactor components to domestic cookware. Eureka's interest arose from two separate sources. The first was coming across cookware, made by US company Calphalon, which purported to be made from laminated stainless steel/aluminium sheet. The second was a meeting with Russian scientist Dr Yuri Kostin, of the St Petersburg-based company Prometey. Both parties flatly refused to give details of their processes, except that Dr Kostin said that while it is relatively easy to produce thick aluminium-steel sections, the real trick is to be able to produce thin laminates with 3 to 5mm thickness of steel and 4 to 7mm of aluminium. Prometey's aim, in the latter case, is primarily to facilitate the welding of aluminium superstructures to steel hulls in ship building. He said his company could also clad austenitic stainless steel to carbon steel without cracks, and make various other fabrications from combinations of dissimilar metals by friction welding. The problem in all cases, is to avoid the formation of the brittle intermetallic alloy FeAl3. This requires working at temperatures well below any melting points. Since successful manufacturing processes all seem to involve fairly secret proprietary techniques, we went to the horse's mouths on welding techniques, to ask how it is done. Dr Amir Shirzadi, of the Department of Materials Science and Metallurgy at Cambridge told us that he had seen a stainless steel saucepan with an aluminium plate bonded to its base at the Federal Armed Forces University in Hamburg. He said he had been told that it had been made by a high pressure diffusion bonding process close to forge welding. John Weston, of TWI, just down the road from Cambridge said: "Such joints are normally made by rolling, explosive bonding or by friction techniques. We are not sure of the details of the rolling techniques, but in all likelihood the sheets of stainless steel and aluminium are welded within a container (possibly evacuated) and then hot rolled. You might try Spur Industries of Spokane, Washington, for more information." Rick James, the president of Spur Industries told us: "Our process is roll bonding. We are small enough that every part has several inspections, so we compete with makers of large clad plates on cost, quality and yield. We specialise in aluminium on steel, aluminium on stainless steel and aluminium on copper. We also do aluminium on titanium, aluminium on brass and other speciality items for specific customers. "Roll bonding or bonding by rolling together two metals is done in a variety of ways. Certain process parameters are advocated depending on the size of end parts, the finish gauge determining whether it is clad plate, bar or sheet or foil. Our process is geared to produce smaller parts ready to weld or machine, cost effectively. The specifics of our actual process steps and our unique preparation technologies would be inappropriate in this response." Spur Industries appears to specialise not in making bulk products, but in manufacturing transition inserts. These include products to allow the welding of aluminium and stainless steel pipe flanges to the appropriate sides of a aluminium/stainless pipe insert. Welding an aluminium/copper electric conductor insert to an aluminium busbar, on the other hand, allows a copper cable lead to be bolted on without resistance problems arising from the oxide layer on the aluminium. The company also offers structural aluminium/steel inserts similar to those made by Prometey. The company claims the inserts are suitable for use over the temperature range, -200°C to 450°C with bond strengths 20% greater than those of the clad metal. The other very suitable process, friction welding, where a component made of one metal is pressed and rubbed hard against another, was originally developed in the old Soviet Union. However, many of the advances in it have been achieved at TWI in the UK under the direction of Dave Nicholas. The basic idea is to generate sufficient heat to allow the different materials to plastically deform and mix with each other, forming a very strong bond, but without generating enough heat to cause melting. Components can be rotated against each other or rubbed against each other. By rotating and pressing a rod against a plate, and moving it across the surface of the plate as it is consumed, it is possible to coat one metal with another. It tends to be easier to join thick sections than thin, but thin sections can easily be produced from thick by rolling. Variants of friction welding include processes such as friction hydro pillar processing (FHPP) and friction stir welding (FSW). FHPP involves rotating a consumable rod in a circular hole and FSW involves rotating a hard, non-consumable probe between the abutting faces of a work piece. The most noted practitioner of friction coating in the UK is Portsmouth-based Frictec. Currently, this company uses its technology to manufacture hard coated cutting knives. Friction coating and joining undoubtedly produces the highest integrity bonds, but roll bonding would appear to be the most economic way of laminating and joining larger areas. Any of our readers who have more precise details of good techniques to produce stainless steel/aluminium laminates and fabrications, or who perhaps already produce products, are invited to share their knowledge by writing to us at the usual address on page one or by sending us an email. Pointers Stainless steel can be laminated and weld bonded to aluminium The favoured processes seem to be roll bonding, which is the most economic, and friction welding, which produces joints of the highest integrity It is now possible to produce relatively thin laminated sheet combinations as well as thick ones TWI John Weston at TWI Spur Industries Rick James at Spur Industries Frictec