Doctoral Dissertations

Date of Award

12-2005

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Thomas T. Meek

Committee Members

Philip Rack, Peter Liaw, Qingyou Han, James R. Thompson

Abstract

This exposition presents the novel thermodynamical and microstructural modification to light alloys, such as aluminum alloys and magnesium alloys, by ultrasonic vibrations during their solidification processes. Ultrasonic vibration has proven to be effective in controlling columnar dendritic structure, reducing the size of equiaxed grains, and under some conditions, producing globular non-dendritic grains. Despite this, the solidification process under the effect of ultrasonic vibration was not clear. Not only was there no such research on how ultrasonic vibration affected its solidification thermodynamically, but also its effects on the as-cast microstructure, including the primary fcc phase, the eutectics, and the secondary phases, were not systematically studied. In addition, most studies had been empirical and phenomenological rather than quantitative.

Prior to the experiments, thermodynamic simulations were carried out using the Scheil model to determine the temperature versus solid fraction curve of the alloys. The starting temperature for ultrasonic processing and the casting temperature were predetermined according to the simulation result. An experimental apparatus which supplied a powerful 1500 Watts at 20 KHz of ultrasonic power was designed and built.

Thermal analysis experiments were performed. The result shows that, with ultrasonic vibration, the steady growth temperature and the minimum supercooling temperature have been elevated; while the recalescence time decreased, which indicates a much slower growth rate of primary fcc aluminum grains. The difference between dendrites nucleation/growth and thickening is not significant in the casting with ultrasonic vibration, which might suggest dendrites formation might not present in this solidification process.

The mechanisms for ultrasonic influence on solidification have been discussed. Two types of ultrasonic processing techniques were developed and attempted. The first one related to introducing the vibration into the solidifying specimen through the liquid, while the second through the formally solidified part. For the first ultrasonic processing technique, the treatment was employed isothermally, intermittently, and continuously. In contrast to the fully developed dendrites up to several millimeters in length in untreated A356 alloy, fine globular primary fcc Al grains sized less than 200 mm were obtained in the specimen treated with 5 second intermittent ultrasonic vibrations. However, dendrites were not completely broken down into fine grains in the isothermally or continuously processed specimens. It may imply that there is limited effect of dendrite fragmentation on the formation of globular/non-dendrite microstructure in the acoustically processed melt, and acoustically induced heterogeneous nucleation seems to be the dominant mechanism for the formation of a globular microstructure. For the second approach, ultrasonic treatment was performed continuously. During the treatment, grain refinement reached an unprecedented level. The average grains were globular with size ranges from 20 to 40 mm. Superfine globular grains of size less than 20 mm were obtained in the area near the ultrasonic radiator. Similar grain refinement could only be reached by using a quenching method with a much faster cooling rate.

The main parameters of ultrasonic processing, such as casting temperature, ultrasonic intensity, and the distance from the radiator, have been investigated. It is concluded that high acoustic amplitude/intensity favors the formation of small, spherical primary aluminum grains. The casting temperature of 630°C brings about best grain refinement result. The primary aluminum grain size in a casting increases with the increasing distance from the acoustic radiator.

In order to examine the feasibility of ultrasonic vibration for SSM processing, high intensity ultrasonic vibration has been applied during the casting of A356 alloy at high volume. Non-dendritic/globular grains have been obtained. Grain refiner can further refine A356 alloy structure, with the combination of ultrasonic vibration.

Experiments on the grain refinement of other aluminum alloys have been carried out. Fine globular grains were obtained in various aluminum alloys, including A354, 319, 6063, 6061, 2618 alloys. It was found that 670 °C is the optimum casting temperature for grain refinement of 2618 with the aid of ultrasonic vibration.

The effect of ultrasonic vibration on the modification of eutectic silicon in aluminum-silicon alloys has been studied. The introduction of ultrasonic vibration into A356 alloy modified the morphology of eutectic silicon from a coarse acicular plate-like form to a finely dispersed rosette-like form. The length, width, and aspect ratio of eutectic silicon all reduced significantly. This modification is beneficial to the mechanical properties.

Ultrasonic grain refinement and secondary phases modification to magnesium AM60B alloy have been examined. With ultrasonic vibration, alloy experienced a reduction in size of primary α-Mg grains from 760 µm to about 25~48 µm in diameter, which is much better than other traditional grain refinement methods. The morphology of eutectic phases was modified from a mainly fully divorced blocky morphology dispersed among dendrite arms, to a mainly lamellar/script morphology across the grain boundaries. Furthermore, the volume fraction of the eutectic morphology is less.

Ultrasonic processing of solidifying metals can have a number of applications. Incorporating ultrasonic vibration into a die casting machine would dramatically increase the integrity and properties of die castings. Ultrasonic vibration may be used for producing semisolid feedstock directly from molten metal. Ultrasonic techniques can also find applications in forging industries for processing alloys that are difficult to cast.

Ultrasonic treatment has the advantages of being environmentally favorable, cost effective, and ready to be combined with other known physical processing technologies for liquid and solidifying metal. It is expected that the results of this study will impact a wide range of alloy processing including DC casting, continuous casting, vacuum arc remelting, and foundry processing in the areas of grain refinement, semi-solid metalcasting (SSM), and the production of new and novel microstructures.

It is highly recommended to continue both the research reported in this study and the application and commercialization of this technology.

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