On the solidification of Al alloys during microgravity

Abstract: Thermal analyses have been performed on Al, Al-Cu alloys,Al-Sn alloys, Pb, Sn, Pb-Sn alloys and Al-based amorphousalloys were also investigated. The cooling rates varied fromless than a Kelvin per minute to more than 10,000 Kelvin persecond. Solidification was made under normal gravity and undermicrogravity conditions. In the last case parabolic flight wereused to reduce the gravity.All the samples, except the Al-based alloys containing Ysolidified by forming dendrite crystals. It was found that thedendrite structure was coarser in the samples that solidifiedunder microgravity conditions compared to referenceexperiments. For pure metals and Al-Cu alloys the latent heatof solidification is decreasing with increasing cooling, whilefor Al-Sn alloys the latent heat was independent of the coolingrate. It was also found that in the samples immediately heatedafter solidification the melting temperature were below thealloy liquidus temperature.The experimental results were explained by the possibilityof excess vacancies formed at the solidification front. Anexcess of vacancies would decrease the melting temperature andthe latent heat for Al, Pb or Sn. A thermodynamic analysis wasperformed to explain the result and to find if excess vacanciesare depending on the alloy system.Key-words: Thermal analyses, solidification, rapid cooling,microgravity, parabolic flight, latent heat, undercooling,melting, dendrite, dendrite arm spacing, Al, Al-Cu, Al-Sn,Pb-Sn, amorphous metals, vacancy