Bismuth appears to have significant potential as a nontoxic alternative to lead for enhancing the machinability of alpha + beta brass because of similarity of features between lead and bismuth, e.g. low solubility for copper alloy and low melting point. In this paper, morphological influence of bismuth substituted for lead in free cutting brass on machinability was investigated. It was assumed that an increase in number of particles of bismuth, that was a decrease in mean spacing of bismuth particles, was more effective than an increase in content of bismuth on making cutting chips fine. This is achieved by refinement of matrix alpha grains, that is reason why bismuth particles grow following to grain growth of matrix alpha grains because of the high wettability of bismuth at grain boundaries of alpha copper. From SEM observation of a cross section of a cutting chip and a cut surface of a material with bismuth, existences of melting bismuth seemed to play an important part of tearing the cutting chip of this material. Consequently it was deduced that the morphology of bismuth, i.e. the distribution has a remarkable effect on machinability of alpha + beta brass.
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