| Summary: | 博士 === 國立中央大學 === 機械工程研究所 === 91 === An ultrasonic-vibration treatment is a new technique that can be used to diagnose oxide films on an aluminum alloy matrix. This technique combined with SEM observations can offer information about the shape and distribution of oxide films on aluminum alloys. The key principle of the method is dependent on the nucleation, the growth and collapse of acoustic cavitation bubbles in the liquid. This method has been proven to be applicable to the diagnosis of oxide films that form on aluminum alloys. The study can be divided into three main areas: acoustic cavitation damage behaviors; a fundamental study of the diagnosis of oxide films on aluminum castings; and an applied study on the diagnosis of oxide films due to cavitation damage on Al-XSi alloys. The study on damage patterns and strain energy produced by the impact of collapsed acoustic cavitation can helped us to realize the damaging behavior of acoustic cavitation bubbles that collapse near/on the Al-foil surface, and strain energy produced from micro-jet impacts on the Al-foil. The study on the diagnosis of oxide films by cavitation micro-jet impact not only helped to understand the behavior of acoustic cavitation that occurs near/or on the oxide film surface, but also shows how water micro-jet impacts can cause fractures in the oxide film, which erodes the surface of the treated sample. This eroded surface will show as a foggy mark in visual or optical observations. The study on the diagnosis of oxide films in Al-XSi alloys by cavitation damage helped us visualize fractured surface morphologies of an eroded surface and variations in the oxide films difference in varies with the silicon content. The individual can be summarized as follows:
Part 1: This study discusses the nucleation of cavitation bubbles during ultrasonic-vibration treatment. Different liquids were used to investigate the cavitation intensities of based on the damage marks displayed on aluminum foil samples. These damage marks resulted from the high impulsive pressure developed by micro-jet impacts associated with the action of shock waves. A micro-jet was initiated by the collapse of bubble clouds. The intensity of the cavitations depends on the physico-chemical properties and ultrasonic characteristics of the various liquids, such as the density and viscosity, the ultrasound frequencies and the traveling velocity. The effects of these factors on the intensity of the cavitation bubbles are discussed and the kinetic energies of the micro-jet impacts, along with the strain energy of deformed aluminum foil, are calculated and compared.
Part 2: This is a fundamental study on the diagnosis of oxide films caused by cavitation damage. Oxide films form readily when aluminum alloy castings are melted and/or poured. There are both primary and secondary types of oxide films. The former is inherited from the ingot and has been known to exist in aluminum alloy casting for a long period of time. During the filling of the mold cavity, the free unstable surface of the molten metal causes a secondary oxide film to form on the aluminum alloy castings. These oxide films are usually rich in oxygen, but are difficult to observe by optical microscope. This paper presents a simple but powerful method for observing the shape and size of oxide films on the aluminum matrix. During an ultrasonic-vibration treatment, cavitation bubbles could nucleate, grow and collapse, generating micro-jets on the surface of sample. These water micro-jets then had an impact on the oxide film initiating micro-cracks. The cracks grew or became linked together, which caused fractures in the oxide film. Tiny oxide particles became detached from the oxide film to erode the surface of the treated sample. This eroded surface would show as a foggy mark in visual or optical observations. A series of photographs were made and are shown to illustrate the cavitation erosion process of oxide film on the surface of an aluminum sample. In addition, the presented method is shown to be useful in the diagnosis of oxide films that form on aluminum and magnesium alloys, including in ingots, castings or wrought products.
Part 3: In this study we propose an ultrasonic-vibration method for the diagnosis of oxide films entrapped in Al-XSi alloys. These oxide film fractured and particles became detached from oxide film during the ultrasonic-vibration treatment. The polished surface became partly eroded after ultrasonic-vibration treatment. The eroded area was reflected as differently shaped visible foggy marks, including lumps, flakes, strips or spots. This paper presents summarized sequential illustration of the formation of eroded areas, and foggy marks on pure aluminum and Al-XSi alloys during ultrasonic-vibration treatment. In order to confirm that oxide films truly existed on the aluminum matrix, and to realize the mechanism where by oxide particles became detached from oxide films during ultrasonic-vibration treatment, the morphologies of fractured surface were observed by using scanning electronic microscopy (SEM), and the constituents of the oxide films were also analyzed by electron probe microanalysis (EPMA). For various types of Al-XSi alloys, the fractured surface morphologies of the oxide film were also compared and analyzed.
|
|---|
