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The directional columnar solidification of Al-Cu alloy dendrites was studied in-situ by synchrotron X-ray radiography to investigate the spatial and temporal distribution of dendrite fragmentation, without and with an applied pulsed electromagnetic field (PEMF) and in different solidification directions. The differing arrangements had a strong and reproducible influence on fragmentation behaviour that was rationalised using a model of solute-driven remelting of dendrite arms in microstructural regions of high vulnerability. Although absolute rates of fragmentation varied, all the fragmentation distributions had a characteristic peak in fragmentation rate at a distance of 500-800 µm into the mushy zone. Consistent with the need for solute-rich liquid flow for dendrite root re-melting, this peak was explained to occur under conditions where there was both comparatively steep liquid solute concentration gradients, measured directly from the radiographs, and relatively high permeability of the dendrite network to liquid flow.

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In this article, the effect of temperature and load on the dry sliding wear behavior of an Al-7Si alloy reinforced with in-situ TiB 2 particles is reported. The coefficient of friction and wear rate of Al-7Si alloy and in-situ composites were measured using a high-temperature pin-on-disc apparatus. The wear tests were performed in the temperature range of 100°C to 300°C, at an applied load of 40 to 120 N. With increasing temperature, the wear rate of Al-7Si alloy increased drastically and it decreased with the increase in weight percent of TiB 2 particles. The transition from mild to severe wear for Al-7Si alloy was observed at 80, 60, and 40 N at temperatures of 100°C, 150°C, and 200°C, respectively. The alloy reinforced with 5 wt pct of TiB 2 showed the transition at 120 N and 200°C, and the transition load further decreased to 80 N at 300°C. However, within the present experimental range of load, the addition of 10 wt pct of TiB 2 showed the transition only at 100 N and 300°C. The addition of TiB 2 particles increases the wear resistance properties of Al-7Si alloy even at high temperature.

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