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WASEDA UNIVERSITY
School/Graduate School of Creative Science & Engineering
Department of Modern Mechanical Engineering
Transporters & Energy Plants Materials Science and Engineering

Yoshida Makoto Laboratory





Aluminium Alloy Development with Ultrasonic Vibration Induction

Introduction

     Further reduction of greenhouse effect gas such as carbon dioxide is essential to solve the severing environmental problems. In order to achieve this goal, global automobile manufacturers are working aggressively to improve the combustion efficiency of the engine and to reduce the weight of the car body. Aluminum alloy as a light material with high strength is an important material that make up automobile body and chassis parts. In order to further enhance these characteristics, it is necessary to improve the microstructure of the material.

On improving the microstructure of aluminium alloy, following measures have been taken.

  1. Adding microstructure refinement material.
  2. Increasing the cooling speed during manufacturing process.

However, for each measure taken, there exists problem that are indispensable.

  1. Cost increase due to the additional refinement material.
  2. Cooling speed differs between thick and thin casting part.

     Therefore, in order to develop a lighter aluminium alloy with excellent workability, our group focuses on ultrasonic vibration to improve (to refine) the microstructure of the aluminium alloy.






Experimental


     It is generally known that by adding ultrasonic vibration to molten metal, the solidified structure will be improved. In particular, it has been reported that the structure remarkably improves upon addition of ultrasonic vibration across the liquidus temperature (TL) at which a solid begins to crystallize. However, the mechanism of microstructure improvement has not been elucidated yet. By clarifying the mechanism, it will be easy to improve the microstructure efficiently, so that the strength can also be improved. Table 1 lists the microstructure improvement mechanism theories that have been reported before in previous studies. Our group focuses on the super-cooling phenomenon in the vicinity of TL, in which molten metal continues to remain liquid even if it is cooled down to the temperature where liquid supposedly begin to solidify. We treat the molten metal with ultrasonic vibration at 4 different conditions centering at the vicinity of TL, and accordingly elucidate the mechanism of microstructure improvement. The conditions as shown in Fig. 1 are defined as: (1) at temperature higher than TL, (2) prior to recalescence, (3) during recalescence, and (4) after recalescence. A schematic diagram of an ultrasonic vibration-inducing device is shown in Fig. 2. The vertical vibration of the horn resulted in the addition of ultrasonic vibration to the molten metal.



Table 1 The mechanism for improving grain by ultrasonic vibration
Microstructure improvement mechanism theory Remarks Associated location
Dendrite segmentation Dendrite-arms in the solidified microstructure is segmented by ultrasonic vibration (4)
Cavitation Microstructure is improved by bubbles that appeared in molten metal by the addition of ultrasonic vibration (1), (2), (3), (4)
Non-equilibrium nucleation Above TL, nucleation is generated and with the addition of the ultrasonic vibration the nucleation is promoted (1)
Under-cooling nucleation promotion Improvement of microstructure by ultrasonic addition in super-cooling area (2), (3)





Fig. 1 Conditions of application of ultrasonic vibration on cooling curve near to TL: (1) before TL, (2) before recalescence, (3) on recalescence, and (4) after recalescence.





Fig. 2 Schematic diagram of ultrasonic treatment equipment





Fig. 3 Macrostructure treated (1) before TL, (2) before recalescence, (3) on recalescence, and (4) after recalescence.








Conclusion


     Fig. 3 shows the macrostructure of solidified aluminium alloys with the addition of ultrasonic vibration. The macrostructure is refined as can be seen at the lower part when ultrasonic vibration was induced at conditions (1), (2), and (3). However, at condition (4), the resulted macrostructure shows almost no difference as compared to that of without ultrasonic vibration. This implied that the dendrite segmentation theory did not contribute to the refinement of microstructure and it is also doubtful that cavitation theory might contribute to the improvement of the microstructure.

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