Pulsing of the welding current is one approach for refining the fusion zone microstructure in materials joined by fusion welding. The effect of current pulse frequency on weld bead microstructure, tensile strength, and hardness in joining of powder metallurgical steel preform sheets to wrought copper was studied. Considering weld strength as the quality characteristic in the selection of process parameters, the Taguchi method is used to analyse the effect of each process parameter individually and of their interaction on weld strength, and subsequently to determine the process parameters leading to optimum weld strength. The application of pulsed current causes iron dendrites to become more equiaxed and uniformly distributed owing to dendrite fragmentation. Further, there was an optimum frequency range over which the microstructural refinement was maximal. The same optimum frequency range corresponded to maximum tensile strength. Enhanced fluid flow and reduced thermal gradients are thought to be responsible for refining the solidification structure and the resulting stronger welds.STWJ/407a
The present paper reports the feasibility studies on joining low density steel powder metallurgical (PM) preforms with wrought copper. In joining of these materials by tungsten inert gas (TIG) welding, the welding current, pulse frequency and the density of the PM preforms were found to have a strong influence in controlling the weld strength. When welded with lower welding currents (continuous current), copper infiltrated into the PM steel and with higher currents, thermal effects dominated in controlling the weld strength. Through the application of a pulsed current, grains became more equiaxed due to dendrite fragmentation. Microstructural refinement was found to be at a maximum at an optimum pulse frequency, which resulted in maximum tensile strength.
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