Fading mechanism of grain refinement of aluminum-silicon alloy with Al-Ti-B grain refiners

Al-Ti-B master alloys are well established for use as grain refiners in aluminum-silicon alloys. However, their effectiveness decreases after a prolonged contact time. This decreasing effectiveness is known as fading. The fading mechanism of grain refinement using Al-Ti-B grain refiner in aluminum-silicon alloy has been studied in this research. Experiments were conducted by adding either 2 or 4% Al-Ti-B master alloy to ASTM 356 held in steel containers with CO2 cured silicate sand linings at 720 degreesC. The melt was held for one of six different contact times (varying from 20 to 120 min) before allowing the molten aluminum to solidify. It was found that concentrations of Ti and B vary from a minimum at the upper part of the samples to a maximum at the lower part of the samples. The final grain size, accordingly, varies from the largest size at the top of samples to the finest size at the bottom of samples. In addition, eutectic silicon with duplex boride particles was found at the bottom of grain refined samples after the prolonged holding time. This research has found that the difference in density of both TiAl3 and TiB2 particles are the major factors affecting the fading mechanism. It also confirms the need to agitate melt before pouring especially in a large crucible of melt. However, the vigorous agitation of melt could partly bring the particles back into the melt and act as heterogeneous nucleation sites to a limited extent. (C) 2002 Elsevier Science B.V. All rights reserved.