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
Because of spot welding processes requires relatively simple equipment; it can be easily converted into an automated operation. Once the welding parameters is established, it
should be possible to produce repeatable welds. The resistance spot welding is the most widely used joining process for sheet materials. The process is used in preference to
mechanical fasteners, such as rivets or screws, when disassembly for maintenance is not required Chou, 2003. The process is used extensively for joining low carbon steel
components for the bodies and chassis of automobiles, trucks, trailers, buses, mobile homes, motor homes and recreational vehicles and roils road passenger cars, as well as
cabinets, office furniture, and white-goods manufacturing industries Hasanbasoglu A., et al
, 2007. It is most widely used in joining process for the fabrication of sheet formed components, with or without requirement of special technique Vural, 2004. Many
metallic material such as mild steel, high strength steel, galvanized interstitial free steel, stainless steel, austenitic stainless steel, nickel, aluminium, titanium alloy and brass can be
welded by resistance spot welding Jou M., 2003; Vural M., et al, 2006; Kahraman N.,
2007. Small scale resistance spot welding SSRSW is also one of the main micro- joining processes being employed in the fabrication of electronic components and devices
to joint thin sheet metals of thickness less than 0.2-0.5 mm Chang B. H., et al, 2003.
2.4 Microstructure and phase transformations during welding As described in the sections above, spot welding involves thermal, metallurgical and
mechanical processes, which result in a structure of mixed phases and properties. Fig. 2.4 shows a typical cross-section of spot welded joint of a low carbon steel Bayraktar, 2004.
There are three main different regions – the base material, the nugget and the heat- affected-zone HAZ. The block in the centre is the nugget that consists of martensite and
bainitic phases Ni K, et al, 2004. The region around the nugget, the so-called heat- affected zone HAZ, has a mixed microstructure consisting of martensite, bainite, ferrite
and pearlite. The nugget is much harder than the base material due to the quenching effect, while the HAZ has a gradient mechanical property and a mixed microstructure with the
strength decreasing from the nugget to the base. In many cases, failures of spot welded joints tend to occur around this region, specifically around the heat-affected zone HAZ
Mukhopadhyay M., 2009.
Many research has been conducted to improve the understanding on spot welded joint as the interactions between electrical, thermal, metallurgical and mechanical phenomena.
Hou Z., et al, 2006. One active research field is on the prediction of the dimension of spot welded joints by simulating the welding process with the finite element modelling
Emmanuel H., et al, 2007; Rahman M. M., et al, 2008. Another active research field is
on the study of microstructure development Sun D.Q., et al, 2007; Bakavos D., et al,
2010. The microstructure models have to consider the thermo-physical properties of the
