Influence of Different Drooping Characteristics of Power Sources on Nugget Formation : Study on Projection Welding (1st Report)

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The influence of characteristics of the power sources on the nugget formation was investigated under the various conditions of the weld current and the electrode force which control the collapse of projection as basic approach how to supply the input power to the projection part in order to form efficiently the nugget and obtain a good weldment with stable mechanical strength. The power sources with no-load voltages V_<20> of 3.3V and of 6.3V were used in this experiment. The voltages between the electrodes were found to approach the value of 0.8±0.1V asymptotically at the weld time about 10 to 20 cycles enough to attain thermal equilibrium, independently of the electrode force and the characteristic curves of the power sources. When a power source with V_<20>=3.3V was operated under low electrode force where the projection was scarcely collapsed, for the weld current above the critical value the tensile shear strength was stable with small amount of scatter and the welds failed in tear fracture in spite of occurrence of the initial splash during collapse of projection and the splash in the latter half weld time, but when a power source with V_<20>=6.3V was operated, the tensile shear strength was unstable with a large amount of scatter and the welds failed in the shear or tear fracture at a rate of about 50% because of the initial intensive splash, so that the critical current could not be defined. Thus, the drooping characteristics with high no-load voltage were found to be able to supply more initial heat input into the projection part than those with low no-load voltage, but under low electrode force an enlargement of the area of the current path due to the initial extensive splash decreased the current density and the heat efficiency, and after that these deep drooping characteristics were found to be unable to supply the following heat input enough to compensate for this heat loss. Under high electrode force where the projection was almost collapsed, the critical current was defined even if power sources with different no-load voltages were operated. And the value of critical current in the case of using a power source with V_<20>=6.3V was found to be 1, 000A smaller than that in the case of using a power source with V_<20>=3.3V. These results were rearranged and discussed in terms of the heat input into the projection part.
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Influence of Different Drooping Characteristics of Power Sources on Nugget Formation : Study on Projection Welding (1st Report)

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