Characterisation of resistance spot welding Electrodes with annular recess design

Abstract

This study investigated the characteristics of resistance welding electrode with annular recess design for supplying an electric current to metallic workpieces for welding two workpieces together. It comprises cylindrical body of an electrically conductive material (copper) having an annular recess tip for contacting workpiece. The tip is formed with a recess which gave the tip annular workpiece contacting surface. The recess was filled with an electrical and heat resistant material containing ceramic cement known as kaolin mixed with clay. The novel idea of the tool design is to enable formation of molten material pool which forms weld nuggets with even strength around the joints. The beginning facts about the recess design tool is that the annular recess minimizes current and voltage concentration in the middle of the tool tips, thereby; causing molten materials to flow both to the centre and onto the perimeter of the spot welded joint where the welding pressure is applied. Studies have shown that conventional spot welding tools produce joints which are stronger on the edges but weaker in the centre of the nuggets which presumably experience high voltage concentration. The data collection was done through RSW welding experiments where welding of sheet metal materials with same thickness was carried out; alloy steel materials cut from a salvage vehicle were used to make the sample strips measuring 50mm by 175mm each. The design and manufacture of annular recess was carried. The welded joints were subjected to different tests to characterize the weldability and integrity of the welded joints. The tests involved destructive testing and non-destructive tests of the joints. Magnetic Particle Inspection (MPI) was used to test the weld joint integrity beside the visual inspection. In this research study, effect(s) of electrode tip geometry on the tensile-shear strength in resistance spot welded joints of metal steel sheet (body panel) cut out from an old salvage vehicle of similar thickness (1mm) was studied. The outcome of this study showed remarkable improvement on the tensile-shear strength of RSW joint with a nugget diameter about 8.1mm by approximately 11.4KN using the annular recess electrodes compared to 5.7KN at nugget diameter of 7.8mm using the conventional electrode. The annular recess electrode provided an enhanced joint strength by approximately 2 times than the conventional electrode. Therefore application of this new technique creates a new tool that can enhance RSW joints strength in metal fabrications and also improvement on the crash worthiness of the vehicle. Keywords: Current, Electrode, Nuggets, Resistance, Spot-welding,