Effect of brass-alloy machining-swarf additive on the microstructure, hardness and toughness of gray cast iron

In this study, the effects of brass machining swarf with different weight percentages on cast iron were investigated. The addition of 1 wt% swarf resulted in a refinement of Type A and E graphite morphologies and an increase in the volume fraction of pearlite. As the swarf content was increased to 3 wt% and 5 wt%, further refinement of the graphite was observed, owing to accelerated solidification rates. In addition, a transitional pearlite interface became evident at these higher concentrations. Increasing the swarf amount was found to decrease elemental diffusion rates and reduce the extent of copper and zinc concentration gradients (or segregation zones) from approximately 5 to 2 μm. These localized, alloy-element-rich regions facilitated the formation of an interface featuring a fine pearlite matrix. At low weight percentages (1 wt%), the swarf dissolved completely into the matrix, whereas at higher percentages (3 wt% and 5 wt%), it remained dispersed as fine particles. Correspondingly, the average graphite size decreased progressively: measuring 123 μm in the baseline sample, 109 μm in the GCI 1 wt% sample, 72 μm in the 3 wt% sample, and 66 μm in the 5 wt% sample. The addition of 1 wt% swarf to the cast iron led to an increase in hardness from 200 to 212 HB. However, as the swarf content increased, a reduction in hardness was observed, with the composites containing 3 wt% and 5 wt% swarf achieving hardness values of 197 HB and 185 HB, respectively. This phenomenon corresponds to the presence of the softer brass phase in the composite structure. The microhardness of these swarf was measured at approximately 99 Vickers, which was the minimum value. A brittle fracture was observed in the sample with GCI and the 1 wt% sample, in contrast to the samples with 3 wt% and 5 wt%, which showed a combination of ductile and brittle failure mechanisms that impeded crack growth. The measured impact energies for gray cast iron and the composites containing 1 wt%, 3 wt%, and 5 wt% were found to be 3, 4.2, 5.7, and 10.6 J, respectively.