Steels_ Metallurgy and Applications, Third Edition

104 Steels: Metallurgy and Applications

Resistance spot welding of zinc-coated steels
In relation to hot dip products, a distinction must be made between plain zinc
and galvanneal coatings. Because of its lower contact resistance, higher currents
are required for plain zinc-coated strip such as G275, compared with equivalent
thicknesses of uncoated mild steel. On average, the currents have to be increased
by about 20%. On the other hand, galvanneal-coated strip has a higher surface
resistance than G275 and requires only a slight increase in current compared
with uncoated steel. The weld times employed for zinc-coated steels may be
50% longer than those used for uncoated steel and the electrode pressure is
generally increased.
The spot welding of zinc-coated steels is characterized by an electrode life
which is shorter than that obtained with uncoated steels. This results from
the alloying, mushrooming and erosion between the electrode material and the
coating. Work by Williams 154 showed, however, that electrode life is affected very
considerably by the type of coating. Thus for both the hot dip and electrolytically
deposited coatings, the longest lives are obtained with iron-zinc and nickel-zinc
coatings and values in excess of 500 welds can be achieved. Under non-optimum
shop floor conditions, however, the number of welds which can be produced can
be considerably less.

Resistance seam welding

The principle of resistance seam welding is similar to that of resistance spot
welding except that the current is supplied to the work using electrodes in the
form of wheels and the work moves continuously between the wheels to form
a continuous weld. With coated steels, the electrodes can become contaminated
and poor welds are produced. This problem is overcome in the Soudronic process
by using continuously moving copper wires between the wheels and the work
to act as intermediate electrodes, as illustrated in Figure 1.103. At each moment
during the weld, therefore, the work is contacted by an effectively new and clean
electrode.

Wire feed

Figure 1.103 Principle of the Soudronic weMing process l~s