Materials for engineering, 3rd Edition - (Malestrom)

Metals and alloys 121
structure and strength of metal parts. This method of joining provides a more
uniform distribution of stress and a larger stress-bearing area than conventional
mechanical fasteners such as rivets or bolts. The adhesive must wet the
substrate to ensure a good bond and its presence minimizes or prevents
electrochemical corrosion between dissimilar metals (see Section 3.4.2).
There is also evidence that adhesive bonded joints show an improved resistance
to fatigue loading compared with welded joints. The upper limit of service
temperature of such joints is usually 175°C, although materials are available
that permit limited use up to 370°C.
Surface preparation
Chemical surface preparations that enhance adhesive bonding have been
available for many years. In the case of aluminium and titanium, these
preparations not only clean the surfaces, but also produce porous oxide
structures with distinctive morphologies, which allow the adhesive to penetrate
into the metal/oxide interface. This ‘mechanical interlocking’ of hardened
polymer with the substrate increases the bond strength and durability.
Iron does not form coherent adhesive oxides, so steel is usually only
chemically cleaned. The application of a surface coating such as zinc phosphate
or metallic zinc (‘galvanizing’) has been found to enhance the properties of
an adhesive bond.
Types of adhesives
Epoxy and acrylic adhesives are the most widely used structural adhesives.
These are thermosetting polymers, but for many steel applications the size of
the parts preclude the prolonged heating and high-pressure cure cycles that
are required by high-performance epoxies.
In the automotive industry, for example, adhesive bonding has been
employed principally in ‘non-structural’, i.e. non-load bearing applications.
These include the bonding of friction linings to braking components with
phenolic-based adhesives because of the high-temperature requirement.
Polyurethanes are employed to bond the fixed glazing to the body shell; this
confers a significant increase in body stiffness compared with the use of
rubber gaskets. Adhesives are now also widely used in the bonding of the
inner to the outer panels of doors.
Most of the adhesives intended for bonding metal bodyshell assemblies
require heat curing and this is most effectively carried out during the paint
curing cycle. Reactive hot melt adhesives have been developed that are
applied at elevated temperature and, on cooling, solidify to give some initial
strength in order to survive the early stages of the paint process. These
adhesives, based on epoxy, polyurethane or synthetic rubber (e.g.