ABSTRACT
Investigations of Solidification of Some Low Melting Point Binary Multiphase Alloys in Drop Tube
Liang Chen （Materials Processing Engineering）
Supervised by Prof. Xinghong Luo
Drop tube is one of the effective ways to simulate the microgravity conditions on the ground. Besides, it also supplies a special environment for the investigation of melt solidification under containerless condition at the same time. The number of short drop tube was far more than that of long drops tube due to some technic and cost factors, which made the investigation by drop tubes focus much more on containerless and high undercooling effects than on microgravity effects. Even with long drop tubes because the microgravity durations were short and, moreover, the effects of microgravity on solidification were often mixed with those of high undercooling, it was also very hard to discover the effects of gravity on solidification merely. To solve this problem, on the basis of calculation and analysis of the thermal history of some metals during their free fall in drop tube under different initial and boundary conditions, a new investigation method, i.e., comparing the quenched microstructure of the molten droplets before and after their free fall in drop tube, was suggested to study the effect of microgravity on solidification. Applying this method, some binary multiphase alloys with low melting points, Al-Al3Ni, Al-Al2Cu and Ag-Cu eutectic alloy, Al-5wt.%Pb hyper-monotectic alloy and Cu-10wt.%Co hyper-peritectic alloy were investigated. Some new phenomena were found under the drop tube containerless conditions. The main works were as follows:
1. Design of electromagnetic levitation coil and the principle of the experiment. The factors (power frequency, coil shape and voltage, etc.) which control the state of levitation were analysed, and some insights gained were applied in the design of the levitation coil. The thermal history of several metallic droplets during their free fall in drop tube were calculated and the result showed that the temperature of the droplet would change just a little bit during their dropping in evacuated drop tube if the alloy possessed low melting point, low emmisivity and high heat capacity. Based on this principle silicone oil quenching in drop tube was proposed for the investigation on how the microgravity environment affected the solidification behavior.
2. The solidification of eutectic alloys (Al-Al3Ni, Al-Al2Cu, Ag-Cu) in drop tube. A novel regular particulate eutectic microstructure was observed in the Al-Al3Ni alloy, which was not reported in the published literatures. About this phenomenon analysis and discussion were made according to the characteristics of the specific eutectic alloy and the solidification environment. The Ag dendrites formed in the Ag-Cu eutectic only when solidified in the drop tube filled with Ar gas. While, the Al dendrites of the Al-Al3Ni eutectic appeared under any solidification conditions. The reason why the dendrites emerged in the different types of eutectic alloys was elucidated and the growth velocity of the Al dendrite was calculated applying the LKT model. By comparing the solidification samples before and after dropping, the effect of convection on the solidification behavior was clarified. The results showed that the diffusion coefficient, the thermal field in the droplet and the lamellar spacing were changed by the convection. And the difference of the microstructure solidified in different environments can be explained by different states of convection.
3. The solidification of Al-5wt.%Pb hyper-montectic in drop tube. The distributions of Pb particles and the microstructure of the Al-5wt.%Pb hyper-monotectic alloy were investigated. It was found that the convection droven by the gravity or the electromagnetic stirring had a great effect on the microstructure of the samples solidified under different conditions. Meanwhile, in the gas environment the effect of Marangoni convection on the formation of microstructure of monotectic alloy cannot be neglected.
4. The solidification of hyper-peritectic alloy (Cu-10wt.%Co) in drop tube. The effect of convection on the pattern of primary Co dendrite was studied. The result showed that convection could improve the ability of nucleation in the melt and strong convection could break up the primary dendrites, which made the remained primary phase appear as particulate patterns. Shell-like distribution of dendrites was observed on the surface of the quenched sample before free fall. The anaysis indicated that it might be related with the temperature gradient caused by queching in the silicone oil.
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