This paper forms the basis of an article entitled 'Adhesive metal bonding no longer stuck with past prejudices', which appeared in Welding & Metal Fabrication, 1999, Vol. 67, No. 9, October, pp20, 22-23 Welding & Metal Fabrication is published by dmg Business Media Ltd , Redhill, Surrey, UK.
Adhesive bonding is an important joining technique, used in almost all industrial sectors. The reliability of the joint is critical and an important aspect of manufacture is the surface condition, especially when bonding metallic substrates, which are covered by an oxide layer. Metals and metal oxides are high-energy solids. Hence, it would seem at first sight that such a surface would be readily wetted by organic adhesives having low surface energies, once any machine oil, protective greases or other contaminants had been removed. However, even in the absence of these impurities, a high-energy surface absorbs water vapour from the atmosphere and other contaminants (e.g. hydrocarbons), which lower the surface free energy of the substrate and may prevent spreading of the adhesive. Therefore, it is important that contamination is kept to a minimum. However, several adhesives, such as certain epoxies, are able to dissolve some organic contamination.
The clean appearance of an aluminium surface is very deceptive; it is actually composed of a thin layer of oxidation products, invisible to the unaided eye, which forms a weak link between the aluminium surface and the adhesive. Although the oxide film on some alloys is reasonably stable and gives robust joints with some adhesives without preparation, this must not be assumed and should be checked and evaluated. Failure to remove a weak oxide layer will result in a weak joint. The oxidation of aluminium in atmospheric oxygen is extremely rapid; commencing immediately a fresh surface is formed by abrasion. Such surfaces must therefore be bonded as soon as practicably possible. Even the strong, cohesive and firmly bonded deposits left by acid etching degrade and, if assembly cannot take place immediately, the surface must be protected by some form of coating.
There are many examples of the adhesive bonding of aluminium. A thixotropic anaerobic sealing compound has been applied to the lower face of a 2.2 litre aluminium engine block  . The liquid sealant cures to form a flexible, robust seal between the block and the steel baffle plate, which separates it from the aluminium sump. The sealant is also used on the face between the baffle plate and the sump. Being anaerobic in nature the liquid sealant cures spontaneously when the parts are assembled and access to atmospheric oxygen is denied. The flexible seal, which is produced, is not only resistant to hot engine oil but also accommodates high frequency vibration and the differential expansion of the various components.
Adhesives are used in the aerospace industry, the main reasons being:
- Smoother surfaces provide better aerodynamics
- The reduction or elimination of conventional riveting techniques which create stress concentrations and limit fatigue life
- The ability to use composite materials
- The significant reduction in weight achieved
- The ability to provide fully sealed joints. This last benefit allows wing cavities to be used as fuel tanks without the need for a plastic bag liner.
Aluminium alloys are the dominant materials bonded in the aerospace industry. Because of the inherent variability of the surface condition of aluminium, complex chemical surface preparations are generally used to stabilise the oxide state and ensure good adhesion durability. Many of the evaluation tests focus on the durability of the joint and the resistance of the adhesive to crack propagation, with tests such as the Boeing Wedge Test.
The British Aerospace 146, a 500mph, four-fan jet, 93-seat aeroplane for short haul (200 - 700 miles) feeder operations to and from small airports, relies heavily on adhesive bonding for the primary structure. Although mechanical fasteners are used in areas of high structural loads, extensive use of aluminium to aluminium bonds between stringers and skins throughout the wing, fuselage and empennage has eliminated the need for numerous detail parts  . Besides the weight saving, the bonded construction also provide long term durability. Uniform stress distribution due to minimised use of fasteners is conducive to longer fatigue lives and, with fewer holes, there are fewer places to be sealed for corrosion protection. Minimum use of fasteners also provides an airtight, fuel tight construction. Nevertheless, the design approach was conservative. The fail safe bonded joints have a safety factor of at least five, higher in critical areas.
The adhesive used for this construction is vinyl, which dates back to the first applications of metal to metal bonding by De Havilland (Hatfield, England) and later by Fokker (Amsterdam, The Netherlands)  . The adhesive, therefore has a long history of success: the De Havilland Dove (1946) and Comet (1949), the Fokker F27 and F227 (1950), the VC-10 (1962), BAC 111 (1964) and Trident 121 (1964). In addition to the BAe 146, other aircraft using vinyl phenolic adhesives include the BAe HS 125 executive jet (1962), the Short Bros Skyvan 330 (1963) and the Fokker F-28 (1967) and the latest Fokker F-29 in the mid 80s. The reliability requires essential pretreatments; the machined skin panels and other wing assembly details are degreased, etched and anodised to prepare the surfaces for bonding.
Toughened epoxies have been used on several experimental concept vehicles, where the structural integrity relies entirely on the adhesive joints. The British Leyland ECV3 used adhesives to bond together is aluminium monocoque body. A single part epoxy adhesive was also used to bond together sections of a novel aluminium chassis, for use on Leyland Truck's TX450 concept vehicle  . The combination of adhesive and aluminium allows a much lighter chassis while improving torsional stiffness.
Aluminium cladding has been bonded to the outside of buildings with formaldehyde reactive adhesives. These are typically used for bonding timber components and manufacturing plywood. 'Glulam' or glued wood laminates are actively promoted as an alternative to steel and concrete for some light structural applications  . Historically, polyvinyl acetate, (PVAC), adhesives dominated in the 1960s construction boom. However, it was noted that such adhesives had limited durability in wet, outdoor conditions and this fact restricted their use to indoor applications. Later developments in acrylic and ethylene-PVAC co-polymers overcame some of these limitations. In the application to aluminium cladding very large bond areas are normally involved thus providing adequate safety margins against failure after long service lives.
Figure 1 shows an example of adhesively bonded aluminium in the marine industry.
The variety of structural steel alloys available may usually be bonded readily, and this has resulted in many applications and potential applications of adhesively bonded joints in steel structures. Many of the most demanding of such applications are in the automotive industry.
Adhesives are used extensively in automotive body production for sealing, reinforcing and vibration resistance. In this way better corrosion resistance, greater body strength and reduced noise can be achieved, giving considerable add on value.
Exterior metal body plates in the bonnet, boot lid and doors are typically joined to the interior plates to creating a hem and using spot welding. These hems now incorporate a single part epoxy which reinforces the welds and reduces the number of weld points required, as in Ford car bonnets and Austin Rover Maestro and Acclaim car bodies [6,7] . Adhesives have been applied to overlap sections of door panels, thus improving corrosion resistance. Similar seam sealants are applied robotically on the inner and outer door panels for the Ford Transit  . Many of these adhesives are heat curing and take advantage of the baking cycle in the body painting process to achieve full cure.
Much investigation has been conducted into used adhesives as a primary means of providing structural joints in vehicles, rather than simply as a back up to welding. Adhesive requirements for such structural applications and the variety of opportunities available have been examined by several car companies [9,10] .
David Brown Tractors use a rubber-toughened epoxy to bond the steel bonnet and various stiffeners on the model shown in Fig. 2. Anaerobic adhesives are used extensively in a variety of engine threadlocking, sealing and retaining applications. However, caution has to be exercised about application to certain engine parts, where temperatures exceed adhesive capabilities, e.g. cylinder head gaskets where temperature may rise beyond 200°C.
The increased use of adhesive technology for automotive construction is encouraged by the increased resistance to fatigue loads. The fatigue properties of bonded top hat box structures are influenced by adhesive type, sheet thickness, overlap length and flange width. High strength/modulus adhesives confer high rigidity to the structure and therefore enhance the fatigue properties. Similarly, increasing overlap length/flange width also results in improvements, which can be attributed to a redistribution of stress in the joint. The fatigue properties of weldbonded structures are similar to adhesively bonded structures. Whilst higher fatigue strengths are possibly at thickness of 0.7mm, this advantage can be lost if the sheet increases above 1.2mm  .
The first choice of many design and production engineers for fabricating and repairing components is often one of the many fusion-welding techniques available. However, with the exception of decarburised malleable irons, cast irons are difficult to join using fusion welding. This can preclude the selection and use of iron castings, although they may be less expensive than carbon and low-alloy steel castings and forging.
There are considerable benefits in the use of non-fusion joining processes such as brazing and adhesive bonding. Cast iron can be successfully brazed using silver and copper base alloys and correct joint design enables the joint to be stronger than the base material. Adhesive bonding is slowly gaining acceptance in general engineering assembly operations and recent work has demonstration that the technique can be successfully applied to iron castings if consideration is given to joint design to exploit the strong compressive shear properties of adhesives.
There is little published work on practices to be used with the adhesive bonding of iron casting. However, French work  was concerned with the effects of the surface roughness and the elastic modulus of the adhesive on the strength and the microcrack propagation in a pure shear joint.
In spite of the lack of quantitative information on the properties of adhesively bonded cast iron joints, there are several established applications. For example, brass valve-seats have been bonded into the cast iron housing of a valve for liquid fuels: the cast iron allowed good pressure tightness to be achieved at low cost and the brass valve-seats ensured good sealing  . A second example concerned bonded cast iron gate valve bodies where, it was claimed, the use of adhesive bonding reduced weight and improved the appearance and corrosion resistance  . A further example is the bonding of end plates to a hollow lawn mower roller, to avoid the problems of core removal when the component had been produced as a one piece casting  .
Further examples include the bonding of Ni-hard® tiles to the inside of coal shutes in a South Yorkshire power station  and gearbox assemblies in Renault cars  . In the latter case, a crown wheel was traditionally bolted to a cast iron differential casing, but a more compact assembly is now produced by the combination of shrink fitting and adhesive bonding. The shrink fitting is used to ensure that the joint line is subjected to compressive forces.
The resistance of traditional engineering practices to the use of adhesives is being broken down. Adhesives have now been developed which clearly demonstrate the contribution they can make to the work of both the structural and mechanical engineer. Perhaps of even greater significance is the growing understanding of adhesives as materials in their own right and the need to consider joint design based on the properties of adhesive selected as an integral part of product development.
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