Mechanical and Structural Properties of Microwave Sintered Tin-Copper-Antimony Alloys

Tin-copper-antimony (pewter) alloys have been traditionally produced by a casting process which consumes large amounts of energy, time and as a result is expensive. This research aims to investigate the possibilities of implementing powder metallurgy for pewter production through a sintering process using microwave energy. The optimum sintering conditions using microwave sintering are also of interest. Pewter alloys were examined to determine the effect of green density, sintering time, and sintering temperature on the mechanical and structural properties of the sintered compacts. Samples were prepared by hydraulic pressing of a well mixed and blended tin alloy powder with three different compositions; 97wt%Sn 2wt%Cu 1wt%Sb, 94wt%Sn 4wt%Cu 2wt%Sb and 91wt%Sn 6wt%Cu 3wt%Sb. Two compaction loads were used to produce the samples with different green densities. Pellets pressed at 96 MPa had an average relative density of 80.7%, while those pressed at 129 MPa had an average green density of 84.6%. Sixteen different time-temperature combinations were used for the heat treatments at 160 and 220 C for both conventional and microwave sintering. However, the sintering times had to be restricted to 15 and 30 minutes for microwave heating. Meanwhile, 60 and 120 minutes were used for conventional heating. It was found that for all three compositions, samples with a higher green density had a higher sintered density, compared to samples with lower green density, for the same sintering time and temperature. The relative density of sintered pewter alloys increased on average by 13% after conventional sintering and by about 14% after microwave sintering, when the sintering was carried out for the longer of the two sintering times and at the higher of the sintering temperatures selected, for all three compositions. Moreover, the hardness increased by 25.6%, 23.5% and 7.0% when microwave sintered relative to the conventional sintering for 97Sn2Cu1Sb, 94Sn4Cu2Sb and 91Sn6Cu3Sb alloys respectively. Nevertheless, the grain size remained similar for all three compositions under the same sintering conditions. The degree of grain growth in microwave sintered samples was marginally smaller (up to 23-24 μm) than in conventionally sintered samples which reached a grain size of 26-27 μm. In terms of strength, microwave sintering produced samples with similar properties to those conventionally sintered under the same sintering conditions for all three compositions. The tensile strengths obtained compared well with the strengths obtained from the casting process. Nonetheless, tensile strengths for both conventionally and microwave sintered material was higher in the transverse direction than in the longitudinal direction. In conventionally sintered material, there was an increase in transverse strength of about 6.9%, 5% and 4%, while for the microwave sintered material, the strength increase was 9.1%, 5.6% and 4.5% for 97Sn2Cu1Sb, 94Sn4Cu2Sb and 91Sn6Cu3Sb alloys respectively when compared to the longitudinal direction. The microwave sintered samples in general have improved hardness, better densification and a finer microstructure compared with the conventionally sintered samples and traditionally cast pewter. Increasing the Cu and Sb content increases the hardness and strength but in return, decreases its ductility. Hence, a pewter alloy with a moderate amount of Cu and Sb, i.e. 97Sn2Cu1Sb, microwave sintered at 220 C for 30 minutes would be the best choice for optimum mechanical properties