Aerodynamic Levitation Solidification Of Al-Ca Alloy And In Situ Research On Porosity

The aerodynamic levitation process, which possesses the advantages of containerlesssolidification, easy control and versatility to materials, is a kind of advanced materials processingtechnology. To accurately control the cooling rate during aerodynamic levitation is quite importantfor studying the processing-structure-properties relationship of metallic materials. In this paper,the characteristics of laser absorption, thermal radiation and heat convection of anaerodynamically levitated sample were analyzed, and a formula for the control of the cooling rateby means of tuning the laser power has been derived. By recording temperature-time curves withcooling rates of9,49,98and253℃/s for the aerodynamically levitated Al-7.7Ca (in mass pct.)eutectic alloy, the calculated values of cooling rates agree very well with the experimental data,verifying and validating the formula. Also, the microstructure of the aerodynamically levitatedAl-7.7Ca eutectic under different cooling rates was examined using optical microscopy (OM) andscanning electron microscopy (SEM). The metallographic observation shows that, under a lowcooling rate9℃/s, the solidification structure of Al-7.7Ca alloy during aerodynamic levitationexhibits lamellar regular eutectic. With increasing the cooling rate, the undercooling measuredfrom the recorded temperature-time curves increases, resulting in the refinement of the grain sizeand interlamellar spacing of regular eutectic. Moreover, there appears granular anomalous eutecticunder higher cooling rates of49,98and253℃/s. The volume fraction of anomalous eutectic isincreased with rising the cooling rate. The anomalous eutectic is attributed to the formation duringthe rapid solidification stage. By measuring the interlamellar spacing of regular eutectic fromAl-7.7Ca metallographs, the fitting of data of interlamellar spacing and undercooling is inaccordance with the classical JH model, showing that the regular eutectic is formed during slow solidification stage after recalescence.On the other hand, mechanical properties of alloys are always impaired by the presence ofporosity, but regular porosity can have their applications on porous alloys. The process conditions(e.g. cooling rate and overheating) have an obvious impact on the morphology and amount ofporosity in solidification structure. Thus, it is quite necessary and important to carry out moredeeply study on the formation of porosity. This paper reports in situ observation of gas dissolutionprocess in liquid Al-7.7Ca alloy and the porosity formation process in its solidification processfirstly by using Synchrotron X-ray Imaging technology. A serious of models was set up forstudying the gas dissolution and the porosity formation process. From the models, the equation ofcritical radius for nucleation and the equation of bubble growth rate were derived, and thecalculated results were in accord with experimental results. In experiments, two different ways ofpores growth were observed: columnar growth and sphere growth. Then the influencing factors ongrowth manner of pores were also studied. The results reveal that when the growth rate of pore isclose to the moving speed of solidification front, it will grow in the manner of columnar growth;when these two rates have a big difference, it will grow in the manner of sphere growth.