Typically used to produce leak-tight joints in sheet
assemblies such as automotive gasoline tanks, resistance
seam welding (RSEW) utilizes the fundamentals of
resistance welding to produce a seam along the length
of the joint. The seam is created with copper wheel-type
electrodes that make a continuous weld by generating
overlapping spot welds, as shown in Figure 1.18, or one
continuous weld (see below).
Because the overlapping of welds causes a shunting
effect (that is, decreased electrical resistance due to weld
overlap), the current and duty cycle required for resistance
seam welding are somewhat higher than those
used in resistance spot welding.
Resistance seam welding machines designed to weld
at high speeds apply a continuous flow of current to the
workpieces. This flow forms a continuously fused weld
(as opposed to overlapping welds) between the lapped
faying surfaces. Another type of resistance spot welding
utilizes current interruptions that are long enough to
form a series of separate spot welds. This type of operation
is known as roll spot welding.
Projection Welding
Used primarily to join one stamped, forged, or
machined part (e.g., a fastener such as a nut or a bolt)
to another part (e.g., sheet metal), projection welding
(RPW) is a resistance welding process that produces
welds at predetermined points, indicated by projections
made on the parts during their manufacture. The electrodes
used in this process are similar to those described
for resistance spot welding. The specially prepared electrode
tips precisely locate the projections on workpieces
to be joined. Figure 1.19 illustrates this concept, which
is used to produce projection welds in various carbon
and alloy steels and some nickel alloys.
In this process, one of the two workpieces to be
joined is designed to have one or more protrusions or
projections. These projections become weld nuggets
when the welding current and electrode force are
applied. When more than one projection is formed on
the part, the height of every projection must be even to
permit it to be in contact with the second member. This
allows each joining member to be heated properly when
the current and force are applied. When the thicknesses
of the workpieces are unequal, the projection should be
located on the thicker piece. This ensures that the heating
of both members is balanced, which is conducive to
the production of strong welds.
An advantage offered by resistance projection welding
is its short weld time cycle, which leads to high productivity.
The ability to produce numerous welds
simultaneously with a single set of electrodes is highly
desirable compared to processes that are limited to