Performance of Al-5Ti-1B and Al-3B grain refiners in investment casting of AlSi7Mg0.3 alloy with preheated ceramic moulds

The performances of Al-5Ti-1B and Al-3B grain refiners were investigated when casting AlSi7Mg0.3 alloy into preheated ceramic shell moulds. The former is a poor grain refiner when casting into shell moulds preheated to 300 degrees C. The grains of the Ti free AlSi7Mg0.3 inoculated with the Al-3B alloy, on the other hand, are very fine, uniform and nearly globular, implying a remarkable grain refining efficiency. The average grain size 2 min after grain refiner addition is 190 +/- 16 mm, evidencing over a 10-fold refinement with respect to the unrefined sample. However, grain refining with the Al-3B alloy requires precise control of the Ti concentration in the melt. The Ti level must be kept below 70 ppm, preferably below 50 ppm, to ensure an adequate refinement effect. The cooling rate correlates with the dendrite arm spacing but fails to predict the grain size. An order of magnitude difference in grain size is noted in two samples solidified under exactly the same cooling rate and thus with nearly identical Secondary Dendrite Arm Spacing (SDAS) values, suggesting that alloy composition, i.e. constitutional undercooling, is a more critical parameter for the grain size.