Influence of microgravity on the morphology of the directionally solidified front in an alSi alloy

One aim of the experiments carried out in the GFQ during the German Spacelab Mission D1 was to study the influence of convection on the coarsening of secondary dendrite arm spacing in an AlSi 7.0 alloy during normal crystallization at constant velocitiesv _SF and the temperature gradientG _SF with quenching of the residual melt. When, under μg and 1 g conditions, the same temperature gradientG _SF ≈ 16 K/mm and two different velocities (5 and 8 mm/min) were used, dendrite arm coarsening was shown to be lower than in the 1 g reference experiments atv _SF ≈ 5 mm/min and nearly identical with the reference results atv _SF ≈ 8 mm/min. A separation of the different kinds of convection, gravity-driven convection and convection driven by the volume jump, was tried using the coarsening factorM. The influence of gravity convection on the dendrite spacings seems to be high, if the velocity of crystallization is low. This influence fades away, if the velocity is high(e.g., >8 mm/min). This paper is based on a presentation made in the symposium “Experimental Methods for Microgravity Materials Science Research” presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25–29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee.