RESISTANCE WELDING
In resistance welding, coalescence is produced by the heat obtained
from the electric resistance of the workpiece to the flow of electric
current in a circuit of which the workpiece is a part, and by the application
of pressure. The specific processes include resistance spot welding,
resistance seam welding, and projection welding. Figure 13.3.11 shows
diagrammatic outlines of the processes.
The resistance of the welding circuit should be a maximum at the
interface of the parts to be joined, and the heat generated there must
reach a value high enough to cause localized fusion under pressure.
Electrodes are of copper alloyed with such metals as molybdenum and
tungsten, with high electrical conductivity, good thermal conductivity,
and sufficient mechanical strength to withstand the high pressures to
which they are subjected. The electrodes are water-cooled. The resistance
at the surfaces of contact between the work and the electrodes
must be kept low. This may be accomplished by using smooth, clean
work surfaces and a high electrode pressure.
In resistance spot welding (Fig. 13.3.11), the parts are lapped and held
in place under pressure. The size and shape of the electrodes control the
size and shape of the welds, which are usually circular.
Designing for spot welding involves six elements: tip size, edge distance,
contacting overlap, spot spacing, spot weld shear strength, and electrode clearance. For mild steel, the diameter of the tip face, in terms
of sheet thickness t, may be taken as 0.1 1 2t for thin material, and as Öt
for thicker material; all dimensions in inches. Edge distance should be
sufficient to provide enough metal around the weld to retain it when in
the molten condition. Contacting overlap is generally taken as the diameter
of the weld nugget plus twice the minimum edge distance. Spot
spacing must be sufficient to ensure that the welding current will not
shunt through the previously made weld.
Resistance spot welding
(a)
Resistance seam welding
(b)
Workpiece
Workpiece
Fig. 13.3.11 (a) Resistance spot; (b) resistance seam; (c) projection welding.
Resistance spot welding machines vary from small, manually operated
units to large, elaborately instrumented units designed to produce highquality
welds, as on aircraft parts. Portable gun-type machines are
available for use where the assemblies are too large to be transported to
a fixed machine. Spot welds may be made singly or in multiples, the
latter generally made on special purpose machines. Spacing of electrodes
is important to avoid excessive shunting of welding current.
The resistance seam welding process (Fig. 13.3.11) produces a series of
spot welds made by circular or wheel type electrodes. The weld may be
a series of closely spaced individual spot welds, overlapping spot welds,
or a continuous weld nugget. The weld shape for individual welds is
rectangular, continuous welds are about 80 percent of the width of the
roll electrode face.
A mash weld is a seam weld in which the finished weld is only slightly
thicker than the sheets, and the lap disappears. It is limited to thicknesses
of about 16 gage and an overlap of 11⁄2 times the sheet thickness.
Operating the machine at reduced speed, with increased pressure and
noninterrupted current, a strong quality weld may be secured that will
be 10 to 25 percent thicker than the sheets. The process is applicable to
mild steel but has limited use on stainless steel; it cannot be used on
nonferrous metals. A modification of this technique employs a straight
butt joint. This produces a slight depression at the weld, but the strength
is satisfactory on some applications, e.g., for the production of some
electric-welded pipe and tubing.
Cleanliness of sheets is of even more importance in seam welding
than in spot welding. Best results are secured with cold-rolled steel,
wiped clean of oil; the next best with pickled hot-rolled steel. Grinding
or polishing is sometimes performed, but not sand- or shot-blasting.
In projection welding (Fig. 13.3.11), the heat for welding is derived
from the localization of resistance at predetermined points by means of
projections, embossments, or the intersections of elements of the assembly.
The projections may be made by stamping or machining. The
process is essentially the same as spot welding, and the projections seem
to concentrate the current and pressure. Welds may be made singly or in
multiple with somewhat less difficulty than is encountered in spot
welding. When made in multiple, all welds may be made simultaneously.
The advantages of projection welding are (1) the heat balance
for difficult assemblies is readily secured, (2) the results are generally
more uniform, (3) a closer spacing of welds is possible, and (4) electrode
life is increased. Sometimes it is possible to projection-weld joints
that could not be welded by other means.

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