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For the purpose of improving the toughness of flash weld, the effect of upsetting conditions on the flash weld of 980MPa grade high strength steel bar has been studied. The main results obtained are as follows; (1) Characteristic features have been observed in the displacement profile when the upsetting condition is set to higher current and lower pressure during upsetting. In case of higher current conditions, the upsetting speed increases rapidly and then goes down in a short time, which phenomena is repeated several times until its upsetting process is finished. (2) According to high-speed camera observation, such rapid increase in the upsetting speed is attributed to the extrusion of molten metal layer formed at the weld interface due to excessive joule heating. The specimens showing the melting-extruding upsetting phenomena have revealed superior weld toughness rather than those produced by conventional upsetting conditions. (3) The reason of toughness improvement in the high current condition where the melting and extruding upsetting mode is observed is considered to be the extrusion of both oxides defects and base metal inclusions, and the achievement of small upset deformation due to the lower upsetting pressure.

Influence of chemical elements of base material has been investigated on interface quality of flash weld. Bend test of welds was carried out for 43 kinds of sheet steels which included solid solution strengthened, dual phase and precipitation hardened high strength steels up to 590 MPa.Fractography anlysis indicated that cracks at weld interface were caused by oxide inclusions. Aluminum, silicon and manganese were major elements of the inclusion, and whose influence were determined to be strong as 10:1:0.01 in the order. Carbon was also proved to affect cracking at interface by reducing ductility of weld. Equivalent formula was experimentally established for the influence of those elements on interface cracking.The ordinary bend test was proved to be used as a simple and reliable method in quantitative evaluation of the interface quality of flash weld, while notch bend test showed higher ability in the evaluation than that.

Initial stage of upset was investigated. The welding machine for experiment was designed so that platen movement for upsetting can be initiated at any desired voltage phase of AC power source. The voltage phase of AC power source at the instant of platen movement for upsetting, named "upsetting point", showed significant influence on weld interface quality. The quality was best at the phase round 90 degree and worst at round 0 (or 180) degree. Good relasionship was observed between the phase and the quality. The influence of upsetting point control on weld interface quality was proved to be greater than that of duration of upset current by conventional current density, i.e. less than 80 A per square millimeter.

Electrode life tests were conducted in resistance spot welding of organic composite coated steel sheets. The electrode lives obtained were almost equivalent to those of ordinal coated steels despite the fact that electrodes during the tests rapidly wore out. The long electrode lives can be attributed to a high contact resistance between the material coated with an organic film and an electrode. That is to say, although the high contact resistance leads to the rapid wear of electrodes caused by a frequent surface flash, this in turn effectively restricts a current path into a specific contact area between the material surface and the electrode and maintains a high current density sufficient to form a nugget even when the electrode wears out.