Steels_ Metallurgy and Applications, Third Edition

102 Steels: Metallurgy and Applications

cycle is completed in a fraction of a second. Spot welding is generally carded

out using AC current, but DC is also used.

The integrity of a spot weld is judged by means of a peel test or a chisel

test, which separate the sheet materials after welding. In some cases, separation

occurs through the weld and along the original interface of the materials (interface

failure), or else the material tears around the weld nugget (plug failure). Tensile

tests, using tensile-shear and cross-tension configurations, are also employed for

evaluating the static strength of spot welds.

Weldability lobes

The heat generated during resistance heating can be expressed by the following

equation:

H = 12RT

where H = the heat in joules,

I = the current in amps,

R = the resistance in ohms, and

T--- the time in seconds.

For a fixed welding time T l, therefore, the diameter of the weld nugget will

increase with current according to the relationship shown in Figure 1.102(a). At

a particular level of current at, the weld diameter will reach what is regarded

as the minimum acceptable size, namely a diameter of 3.5~/t, where t is the

sheet thickness. As the current is increased beyond this critical level, the size of

the nugget will increase until the stage is reached when the electrodes can no

longer contain the molten metal, giving a condition which is termed expulsion

or splash. This is achieved at current a2 and this is regarded as unacceptable,

irrespective of weld diameter. For a given welding time, therefore, there exists

a range of current that will produce acceptable welds, i.e. from that which just

meets the minimum acceptable size criterion to that which just avoids the splash

condition. This type of exercise can be repeated for different weld times and an

acceptable range of current can be defined for each particular time. These combi-

nations of current and time that produce acceptable welds are then expressed in

the form of weldability lobes, as illustrated in Figure 1.102(b). Thus currents

or times below the lower bound of the lobe produce welds that are below

the minimum acceptable diameter and which generally exhibit interface failure.

Conversely, the combination of currents and times above the lobe will lead to

the splash condition.

The width of the lobe can be taken as a measure of the weldability of a material,

wider lobes indicating a greater tolerance to changes in production conditions.

Resistance spot welding of high-strength steels
Experience has shown that the weldability lobes of higher-strength steels are
only slightly narrower than those of low-strength steels. 15z'153 The acceptable
range of current (available current) can be improved by using larger electrode
forces and the optimum results are obtained by increasing these forces by about