Join together

Tom Shelley reports on a technique similar to adhesive bonding that will join dissimilar materials to one another Metals and alloys that cannot normally be joined can be securely attached to each other – or to metal matrix composites, ceramics or carbon – using a technique called active soldering. Bonding temperatures tend to be fairly low, so the technique is mainly being applied to the production of electronic heat spreaders and sensor assemblies. But since it has shown it can bond combinations such as graphite and aluminium, or stainless steel and zirconia, it could allow the manufacture of totally novel products. The technique was revealed by Norman Stockham of TWI, addressing one of the last of the DTI Global Watch mission report seminars in London. Dr Stockham said that while the basic idea has been around for a while, the current master of the technique – which has done much to push it forward – is US company S-Bond. “Bonding is initiated by spreading a layer of solder on the surface to be bonded with a stainless steel brush, which also abrades the surface – similar to the way a brass brush is used for aluminium soldering,” said Dr Stockham. When Eureka put this to Randall Redd, president and CEO of S-Bond, he commented: “Unlike conventional fluxless aluminium joining – where the brush is used to create a hole in the oxide layer on the aluminium – the agitation allows the solder to wet the aluminium, whereby the active ingredients in the solder penetrate the oxide layer. This results in a substantial difference in the amount of scrubbing and the force needed to achieve a bond.” When this is done, the titanium in the solder migrates to the interface with the material to be bonded and reacts with the opposing surface to remove oxides and nitrides and transport them into the bulk of the solder as inert material. The process can be conducted in an air environment but requires a low level mechanical shearing action to break a thin tin-titanium oxide layer on the molten solder and initiate the reaction with the component interfaces. Redd adds: “The agitation to open the oxide skin on our products can be achieved by a variety of methods – brushing, scraping, moving the parts relative to each other or ultrasound. The particular technique is chosen based on the requirements of the application.” The bonding method is apparently to begin by heating the substrate surface to a temperature that depends on the type of solder to be used, and establish an active layer of solder on the surface either by brushing or by one of the other methods described. Enough solder is applied to fill any gap, and the two surfaces are brought together and loaded to 2psi (14kPa) after which the combination is cooled. Typically, the process is conducted at 10-20ºC above the melting point of the solder. S-bond has focussed its attentions on lead free solders based on compositions that range from indium-tin, maximum service temperature about 100ºC, fully molten at 120ºC, to zinc aluminium, maximum service temperature 390ºC, fully molten at 415ºC. Dr Stockham said that active solders can be made using other compositions. “Aluminium will do it and vanadium will do it,” he says. “As far as we know, most alternative compositions have to be used at much higher temperatures – often around 600ºC.” Also we know of nobody else who has developed processes to bond such a wide range of materials. As well as metals, such as copper, aluminium, titanium, Kovar and nickel, it is possible to bond silicon, silicon carbide, aluminium nitride, titanium carbide, alumina, sapphire, zirconia and tungsten carbide. Metal matrix composites bonded include aluminium-silicon carbide, alumiunium-graphite and nickel-titanium carbide. It is also possible to bond most of these materials to graphite, carbon-carbon composites, graphite – dense or foamed – and diamond. As regards applications, Redd comments: “In addition to thermal management applications, our products are used commercially to create assemblies in F1 racing cars, portable laser systems, oil drilling instrumentation, and the environmental control system of the International Space Station.” The techniques could open up a range of other possibilities such as soldering a zirconia knife blades into stainless steel handles, or gemstones into novel jewellery fabrications, or hard wear resistant ceramics into metal matrix composite supports for cutting tools. Pointers * Active soldering can bond combinations of materials that cannot be bonded by any other method * Combinations can include metals, ceramics, metal matrix composites and various forms of carbon * Bonding temperatures are normally about 10-20ºC above the melting point of the solder, only around 100ºC in the case of indium tin solders