Alternative Types of Welding
Cold welding
Cold welding, the joining of materials without the use of heat, can be
accomplished simply by pressing them together. Surfaces have to be well
prepared, and pressure sufficient to produce 35 to 90 percent deformation at
the joint is necessary, depending on the material. Lapped jointsin sheets and
cold-buttwelding of wires constitute the major applications of this
technique. Pressure can be applied by punch presses, rolling stands, or
pneumatic tooling. Pressures of 1,400,000 to 2,800,000 kilopascals (200,000
to 400,000 pounds per square inch) are needed to produce a joint in
aluminum; almost all other metals need higher pressures.
Friction welding
In friction welding two work pieces are brought together under load
with one part rapidly revolving. Frictional heat is developed at the interface
until the material becomes plastic, at which time the rotation is stopped and
the load is increased to consolidate the joint. A strong joint results with the
plastic deformation, and in this sense the process may be considered a
variation of pressure welding. The process is self-regulating, for, as the
temperature at the joint rises, the friction coefficient is reduced and
overheating cannot occur. The machines are almost like lathes in appearance.
Speed, force, and time are the main variables. The process has been
automated for the production of axle casings in the automotive industry.
Laser welding
Laser welding is accomplished when the light energy emitted from a
laser source focused upon a work-piece to fuse materials together. The
limited availability of lasers of sufficient power for most welding purposes
has so far restricted its use in this area. Another difficulty is that the speed
and the thickness that can be welded are controlled not so much by power but
by the thermal conductivity of the metals and by the avoidance of metal
vaporization at the surface. Particular applications of the process with very
thin materials up to 0.5 mm (0.02 inch) have, however, been very successful.
The process is useful in the joining of miniaturized electrical circuitry.
Diffusion bonding
This type of bonding relies on the effect of applied pressure at an
elevated temperature for an appreciable period of time. Generally, the
pressure applied must be less than that necessary to cause 5 percent
deformation so that the process can be applied to finishedmachine parts. The process has been used most extensively in the aerospace industries for joining
materials and shapes that otherwise could not be made—for example,
multiple-finned channels and honeycombconstruction. Steel can be
diffusion bonded at above 1,000 ° C (1,800 ° F) in a few minutes.
Ultrasonic welding
Ultrasonic joining is achieved by clamping the two pieces to be welded
between an anviland a vibrating probe or sonotrode. The vibration raises the
temperature at the interface and produces the weld. The main variables are
the clamping force, power input, and welding time. A weld can be made in
0.005 second on thin wires and up to 1 second with material 1.3 mm (0.05
inch) thick. Spot welds and continuous seam welds are made with good
reliability. Applications include extensive use on lead bonding to integrated
circuitry, transistor canning, and aluminum can bodies.
Explosive welding
Explosive welding takes place when two plates are impacted together
under an explosive force at high velocity. The lower plate is laid on a firm
surface, such as a heavier steel plate. The upper plate is placed carefully at an
angle of approximately 5° to the lower plate with a sheet of explosive
material on top. The charge is detonated from the hinge of the two plates, and
a weld takes place in microseconds by very rapid plastic deformation of the
material at the interface. A completed weld has the appearance of waves at
the joint caused by a jetting action of metal between the plates.