Both mechanical fastening and adhesive bonding require an overlapping joint configuration to
achieve the necessary joint strength, which then increases the weight, thickness, and stress concentration of the structure. This limitation restricts the use
of these joining techniques.
This has led to the pursuit of a joining technique
for dissimilar materials that has greater design flexibility and fabrication rates than adhesive joining
and mechanical fastening—and to the development
of welding approaches.
Welding. Conventional welding processes such
as shielded metal arc welding (SMAW), gas tungsten
arc welding (GTAW), gas metal arc welding (GMAW),
and submerged arc welding (SAW) have been used
to weld dissimilar materials in metal-to-metal
joints. However, the high energy inputs of these fusion welding processes result in material metallurgical distortion that hinders this application as well
as metal-to-polymer joints.
The melting temperature of metals is extremely
high compared to the melting temperature of polymers. Hence, polymers tend to degrade before metals melt.
Polymer welding can be done only on thermoplastics. This limitation exists because the processing of
thermosets and chemically cross-linked elastomers
is characterized by an irreversible cross-linking reaction that results in degradation; therefore, they
cannot be reshaped by heating. Thermoplastics and
thermoplastic elastomers can be melted and softened by heat because of the weakening of the secondary Van der Waals and hydrogen bonding forces
among interlocking polymer chains. This makes it
possible for thermoplastics and thermoplastic elastomers to be remolded upon application of heat,
and they can, consequently, be fusion welded.
Emerging Welding Techniques
for Joining Dissimilar Materials
Promising welding techniques and new applications for existing welding approaches for joining
dissimilar materials have been developed—
ultrasonic welding, laser welding, friction stir welding,
and friction spot welding—as a way to solve problems related to traditional joining techniques. The
effective application of these welding processes
necessitates an understanding of their capabilities and limitations and the behavior of metals and
1. Ultrasonic Welding. Ultrasonic welding is a
solid-state joining technique that initiates coalescence via the simultaneous application of localized
high-frequency vibration energy with a moderate
clamping force (see Figure 3).
This welding technique is characterized by low
energy input and requires the clamping and positioning of the workpieces between the welding tool
(sonotrode) and an anvil by static force. No micro-structural changes occur in the metal.
The workpieces can be two thin sheets or a thick
and a thin sheet in a simple lap joint or a butt joint,
depending on the direction of the energy supply of
elastic oscillations to the welding zone.
Ultrasonic vibration can be applied to welding
both metals and plastics, but the welding process
differs for each. The actual weld achieved depends
on how the ultrasonic energy (vibration) is delivered
to the weld.
In ultrasonic metal welding, the direction of ultrasonic oscillation is parallel to the weld area. When
the ultrasonic metal welding is realized, the frictional action of the workpiece surfaces initiates a
solid-state bond without any melting action of the
workpiece. In plastic welding, the case is reversed.
For ultrasonic plastic welding, the direction of ultrasonic oscillation is perpendicular to the weld area
(see Figure 4).
Researchers experimented with ultrasonic welding of aluminum sheets to carbon fiber-reinforced
thermoplastic composites. Their experiment studied the weldability of aluminum alloy 5754 to carbon fiber-reinforced polymer with thicknesses of 1
mm and 2 mm, respectively. They observed that a
sound weld occurred at amplitudes around 40 µm
as a result of displacement of the carbon fiber-reinforced thermoplastic matrix, thus leading to a better contact between the sheet metal and the fiber.
They also observed that intermolecular reactions
in the weld zone formed when oxide layers on the
sheet metal peeled off during the welding process.
The polymer matrix was actually displaced from the
welding zone, which allowed the ductile aluminum
to adapt the carbon fibers. This enabled mechanical
interlocking between the joining layers and conse-
quently increased the joint strength.
Finally, it was observed that the carbon fibers
surrounded the aluminum alloy as a result of the
plastic deformation of the aluminum sheet, thus
creating a successful weld between the metal and
2. Laser Welding. Laser welding offers unique
manufacturing opportunities. It complements the
fabrication and processing of joints that previously
had been difficult or impossible to achieve by
other welding methods. Laser welding of metals to
polymers can be used to achieve stable metallic,
chemical, and covalent bonds between metal and
polymer hybrid components (see Figure 5).
Ultrasonic welding is being used to join the components of a vehicle bumper.
This schematic comparison between ultrasonic plastic
and metal welding shows that the welding process differs, and the weld depends on how the ultrasonic energy is delivered to the weld.
A carbon fiber-reinforced thermoplastic brace has
been joined to a metal car door via laser-assisted metal
and plastic joining. Photo courtesy of Fraunhofer.
This schematic presentation of a laser-assisted metal-to-plastic joining process shows that plastic bubbles are
formed during the welding process that enable physical and chemical bonding between the metal and plastic
components. The metal does not melt in this process.