2. Mechanical Integrity of Welded Joints and
Methods In Characterising Material Constitutive Parameters
2.1. Spot Welding Process
In a spot welding process Fig. 2.1, two or three overlapped or stacked components are welded together as a result of the heat created by the electrical resistance, which is
provided by the work- pieces as they are held together under pressure between two electrodes. As shown in the figure, the secondary circuit of a resistance welding machine
and the work- piece being welded constitute a series of resistances Aslanlar S., 2006. The overall resistance also varies with material and the sheet thickness. The force applied
to the electrodes assures its continuity. As illustrated in Fig. 2.2, the sequence of operation must first develop sufficient heat to raise a confined volume of metal to the molten state.
This metal is then allowed to cool while under pressure until it has adequate strength to hold the parts together. The current density and pressure must be such that a full nugget is
formed, but not so high causing molten metal being expelled from the weld zone. In addition, the duration of weld current must be sufficiently short to prevent excessive
heating of the electrode faces The welding process is a complex thermal mechanical process and the finished assembly consists of regions with significantly different
microstructures and properties, including the base metal, heat affected zone HAZ and
weld nugget Fan X.,2007.
2.2. Effects of welding parameters on weld quality
Weld quality depends on process and material variables that affect the heat generation or the electrode pressure, among which, the current, force and time are the three main
parameters which determine the strength of the welded joint of a material. The selection of these three parameters is normally determined by generating a weld lobe as shown in Fig.
2.3. The weld lobe curve is a graphical representation of range of welding variables over which acceptable spot welds are formed on a specific material welded with a pre-selected
electrode force Aravinthan, 2003. It is determined by making spot welds using different combinations of weld time and welding current. Only welds made with currents and weld
times lying within the lobe area are acceptable. Welds made with currents and times exceeding the upper curve experience expulsion; while welds made with currents and
times below the lower curve are of insufficient size, or no weld is formed. In both cases,
the weld is not acceptable Aslanlar, et al, 2008.
2.3. Typical applications and materials systems in resistance spot welding
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