Bonding of Metals

The adhesives used for bonding metals are preferably reactive adhesives, predom­inantly epoxies, phenolics, (meth)acrylates, polyurethanes, although poly(vinyl chlo­ride) plastisols, MS polymers, and rubber adhesives also are used for elastic bonds [81] — [85]. The range of bond strengths obtainable extends from high-strength, struc­tural bonds (tensile shear strength ca. 40N/mm2) to highly elastic adhesive sealing compounds (tensile shear strength ca. 1 N/mm2). However, the strength of a bond depends to a large extent upon the nature and direction of the forces acting upon it and on the temperature. Because the intrinsic strength of the cured adhesive resins is not comparable with that of metals, provision must be made for sufficiently large joint areas. Accordingly, the design of adhesive joints in metals must be appropriate for
bonding. Resistance to aging and corrosion depends on the nature of the surface pretreatment and on the type of adhesive used.

Подпись:Epoxies, polyurethanes, and (meth)acrylates represent the principal types of adhesive for structural bonding of plastics and metals. These technologies compete for various applications in the transportation and marine industries because of their strength, good environmental durability and cost effectiveness. Historically, urethanes were chosen for low-temperature applications because their hard segment/soft segment structure typ­ically allows them to retain flexibility in cold environments. Conversely, epoxies were preferred for high-temperature environments because of their strength and high cross­link density. Methacrylates fill the gap between these two technologies. They offer good resistance to high and low temperatures, and they provide the distinct advantages of rapid curing at room temperature, less sensitivity to mixing ratio and higher tolerance of surface contamination. They are proven performers in the transportation and marine industries. Typical applications include bonding fiber-reinforced plastics to metal.

The aeroplane industry was the first to incorporate metal bonding in assembly procedures on a significant scale. The increasing use of high-strength lightweight metals which cannot be satisfactorily welded and the reduction in overall weight through the introduction of light-gauge constructions were other important influencing factors. Light-gauge sheets (skins for fuselage and wings, control surfaces, etc.) could be stiff­ened more effectively by cementing onto corresponding profiles than by riveting. The problem of reinforced openings (windows, door frames) and of distributing stresses over large cross-sectional areas (rotor blades) was satisfactorily solved by multilayer construction, especially supporting-core and sandwich constructions, the widespread application of which actually was made possible by bonding. In this method, high-shear cores of various structures (e. g., honeycomb), are bonded to thin cover layers or skins of aluminum and other materials. This gives a lightweight, nondenting, torsion-resistant construction suitable for aircraft floors and wings. Suitable adhesives are phenolic — poly(vinyl formal) and epoxy adhesives and also epoxy-nylon adhesive films.

Vehicle manufacture (automobiles, railroad vehicles) is another significant application for metal-to-metal bonding.

In lightweight metal constructions, tubes and hollow profiles of any cross section can be designed and bonded as socket joints. Door and window frames are made from steel and aluminum profiles with angles bonded in place (epoxy adhesives), a heat-insulating intermediate layer being applied by bonding or made by casting with an adhesive (polyurethane, epoxy adhesives).

In bridge building, adhesive joints (epoxies, polyesters) are used in combination with high-strength, on-torque threaded fasteners (to absorb peel forces) for load-bearing steel constructions.

In the electrical industry, it is above all the bonding of sheet packs (dynamo sheets, transformers, motors) and the fixing of small parts (ferrite cores) that are increasing in significance (epoxy resins, cyanoacrylates). Adhesives (epoxy resins) containing con­ductive additives also are used, for example, for printed circuits.

In machine construction and instrument and tool manufacture, plain bearings, axle bearing guides, bushings, etc., are made by bonding, and punches are cemented into stamping tools with epoxy resins, cyanoacrylates, and acrylate adhesives. Screws, gear wheels, shafts, etc., are secured with anaerobic adhesives.

Подпись: AdhesivesAdhesive pastes are used as repair kits, often in combination with woven glass — filament cloth for reinforcement. In this way, bodywork is repaired, worn surfaces are renewed, and pipes are sealed (epoxies, unsaturated polyester resins).

The sandwich construction method mentioned above involves joining metals to other materials by bonding. Further examples include the bonding of brake linings (phenolic adhesives) and compound materials in ski manufacture, where aluminum is bonded to plastics, wood, etc. (phenolic and epoxy adhesives). Highly alloyed steels, beryllium and titanium alloys, and other special metals can be bonded with adhesives (e. g., polyi — mides, polybenzimidazoles) that have comparable high-temperature resistance.

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