FORMING FACTORS OF INDENTATION AND ITS SUPPRESSION.

In the spot welding process using high-current density (nearly 2. 0 kA/mm**2) and short current duration (4. 0 ms), the indentation on sheet is smaller than one in the conventional spot welding process (heat conduction type). But, in case of welding using R type or CF type electrode tips, its indentation is clear in visual inspection at welds showing the tear rupture at the tensile shear testing. In this report, it is cleared that the indentation is formed mainly by following three factors; the softening in sheet surface caused by heating by the contact resistance between electrode and sheet, the current concentration in the electrode edge, and electrode force. Furthermore, it is shown experimentally that the joint of thin steel sheets or Al alloy plates with enough tensile shear strength and extremely small indentation (about 0. 01 mm) are obtained easily by using the combination of R type and F type electrodes.