By Paul Kah, Raimo Suoranta,
Jukka Martikainen, and Carl Magnus
Editor’s Note: This article has been adapted from the
white paper “Techniques for joining dissimilar materials: metals and polymers,” Lappeenranta University
of Technology, Lappeenranta, Finland, 2013.
The growing prevalence of polymer materi- als in structural and automotive applica- tions because of their low weight, high specific strength, elasticity, and low cost has spurred
research into the combination of polymers and
metals in manufacturing. Parts made with metal-to-polymer joints now are in high demand in the
automotive and aerospace industries.
One of the goals for the use of dissimilar joints is
to enhance product design flexibility so that differing materials can be used in an efficient and functional manner based on their specific properties.
Metal-to-polymer assemblies combine the strength
and ductility of metal with the physical-chemical resistance and light weight of polymers. Metal is used
in sections where high stiffness and strength are
needed, whereas plastic provides unique chemical
Therefore, it is important to maximize the joint
contribution of each material to ensure optimal
operational performance while still maintaining a
weight- and cost-effective approach.
However, joining dissimilar materials often is difficult to achieve. The behavior of such joints is rarely
fully understood, particularly when using bonding
and heating techniques.
Several joining techniques are commonly used for
hybrid joints between metal and polymer workpieces. They are adhesive bonding, mechanical fastening, and welding.
Each joining technique has advantages and disadvantages. The most appropriate method will depend on application and service.
Mechanical Fastening. Originally used for metal-to-metal joining, mechanical fastening is now used
for metal-to-plastic joining too. It comprises using
clamping components such as screws and rivets for
joint formation without fusing the joint surfaces.
It requires mechanical operations such as drilling
holes and making screw threads.
Different types of mechanical joining techniques
exist for metal-to-plastic joints, but the emphasis
currently is on riveting, because it establishes a reliable joint. Some types of riveting need a heating
cycle during which the rivets are heated before the
fastening so that the rivets shrink upon cooling,
clamping the component tightly.
Test results showed that in rivet joining between
a metallic and polymeric material, the process depends on sheet thickness and the geometric parameters of the rivet, such as tool design and the
riveting force (see Figure 1). Because the bottom
material undergoes the greatest deformation, it is
important that the polymeric material be placed
under the metallic sheet.
Joint configuration often depends solely on service conditions, such as whether it must be leak-proof. In some cases, the joint may be designed to
tolerate a mismatch in the coefficient of thermal expansion during assembly. A joint also can be made
to allow freedom of movement in the plane perpendicular to the clamping member.
Mechanical fastening remains the most often
used joining method because of its simplicity. However, it has limitations, such as increased component weight and stress concentration around fastener holes, which degrade strength and eventually
Adhesive Bonding. Adhesive bonding is a solid-state joining technique that relies on the formation
of intermolecular forces between the workpieces
and the polymeric adhesive itself for joint formation
(see Figure 2). It involves the use of a polymeric
adhesive, which undergoes a chemical or physical
reaction, for joint formation.
The use of adhesive-metal joining has grown substantially in recent years because of the development of very strong and tough adhesives that can
withstand both static and alternating loads. Also,
they generally weigh less than mechanical fasteners,
affording considerable weight reduction. In addition,
stress distribution during loading is homogeneous.
However, adhesive bond joints can prove to be
problematic, because the bonded joints cannot be
disassembled without damage and can emit harmful environmental emissions. In addition, the joints
are prone to degradation from moisture, humidity, and temperature and have low resistance in a
chemically reactive environment.
Furthermore, bonding demands extensive surface preparation. The workpiece surface properties
in an adhesive bond play a vital role in the bonding
process, and bond strength and joint durability can
be significantly improved by surface treating the
workpieces before the bonding. During surface pretreatment, the workpiece’s surface tension increases, but the contact angle of water decreases. Typical
surface pretreatment techniques include solvent
cleaning, surface chemistry alteration, and abrasion
and other topographical changes.
An additional limitation is that bonded joints often fail instantaneously instead of progressively.
The most important limiting factor for bonding is
uncertainty in forecasting the long-term durability
of this kind of joint because of difficulties in carrying out reliable nondestructive testing.
Overview of techniques for joining
Ultrasonic welding, which has been used to join
dissimilar plastics, is now being used to join plastic to
How metals and
In a typical self-piercing riveting procedure, a small die
displaces material and a rivet inserts itself in its place.
Bonding’s use has increased in the auto industry to
join plastic components to metal, as shown in this
illustration of the 2014 Cadillac CTS. The lines represent
structural adhesives. About 120 meters of structural
adhesive are used to join frame rails, pillars, strut
towers, wheel housings and floor parts.