Laser direct joining methods between metals
and polymers, typically known as laser-assisted
metal and plastic joining, have been proposed. The
metal-to-polymer joint interface is heated up by the
incident laser beam, and melting temperature is attained in the plastic material in a narrow region adjacent to the interface. The resulting high temperature initiates the formation of bubbles in the melted
plastic close to the interface, which spread and
diffuse into the molten phase and consequently increase seam dimension between the plastic and the
metal (see Figure 6).
The bonding mechanism occurs as a result of the
combined influences of chemical bonding between
the metal oxide film and the carbon atoms of polymers, and the physical bonding phenomenon resulting from the Van der Waals force and mechanical bonding.
The advantages are very fast welding times, small
heat input, and the process’s high adaptability.
High joint strength can be achieved in laser direct
metal-to-polymer joining, and this method is ap-
plicable to several metals, such as steel, titanium,
aluminum, and iron.
It should be noted, however, that the metal does
not melt in this joining process. The limitations are
the many parameters, such as travel speed and
welding power, that influence the quality and reliability of the eventual joint. Laser welding also has
limited design flexibility and is suitable mainly for
lap joints because of the need for effective absorption of the laser beam.
Due to the low thermal conductivity of plastics,
this means that heat remains concentrated in the
material interaction zone. Furthermore, the behavior of the heat depends on the optical properties of
the plastics, which are a function of its molecular
composition, such as the plastic’s color and the incident beam’s wavelength.
3. Friction Stir Welding. This welding process incorporates three distinct phases: plunging, stirring,
and retracting. During welding, a high-speed rotating tool with a probe pin is plunged at a specific rate
into the overlapping weld spot until the shoulder of
the tool contacts the upper workpiece. This causes
plastic deformation around the pin.
Conventional friction stir welding encounters
problems when welding plastics. These problems
are related to the poor thermal conductivity and diffusion from the macromolecular structure of thermoplastics.
4. Friction Spot Joining. Friction spot joining is a
variant of linear friction stir welding except that there
is no linear movement of the tool during joining.
During friction spot joining, the friction between
the pin and the workpiece generates most of the
heat energy for the joint. Friction spot joining of
metals to polymers incorporates two distinct pro-
cesses: the sleeve plunge and the pin plunge.
In sleeve plunging, the workpieces are initially
overlapped and clamped between a backing plate
and a clamping ring with the metal part on top of
the polymer workpiece (see Figure 7). The sleeve
and pin rotational motion is then initiated, with
both pieces rotating in the same direction.
At some point, the sleeve touches down on the
upper metal workpiece, bringing about frictional
heating. Simultaneously, the sleeve is inserted
into the metal workpiece, thus plasticizing the
metal, and the pin is retracted, which consequently results in the formation of an annular space,
or reservoir. The plasticized metal is then squeezed
into the created reservoir as a result of the sleeve
Upon completion of the joining process, the
sleeve is retracted from the metallic workpiece surface and the pin extrudes the entrapped plasticized
material back into the weld. The keyhole is consequently refilled.
Tool plunging is set in such a way that plunging
takes place only in the metallic workpiece. This is
done to avoid damage to the fiber reinforcement
of the polymeric workpiece, which can reduce joint
strength. The plasticized metallic workpiece is further deformed by the sleeve plunging, resulting in
the formation of a metallic nub on the surface of the
Friction spot welding has been feasible in welding
dissimilar metals such as aluminum alloy and magnesium alloy.
This schematic shows how friction spot joining works.
A sleeve plunges and plasticizes the metal; the spot refills and joint consolidation occurs.