Diagnosis of Oxide Films by Cavitation Micro-Jet Impact

Abstract

Oxide films form readily when aluminum alloy castings are melted and/or poured. They could be primary or secondary types of oxide films. The former type is inherited from the ingot and has existed 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 exist on the aluminum alloy castings. The oxide films are usually rich in oxygen and are difficult to observe by optical micrographs. This paper presents a simple but powerful method to observe the shape and size of oxide films on the aluminum matrix. During ultrasonic-vibration treatment, cavitation bubbles nucleated, grew and collapsed, generating micro-jets on the surface of sample. The 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. Small or tiny oxide particles detached from the oxide film to erode the surface of the treated sample. This eroded surface would show as a foggy mark via visual or optical observation. 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 was shown to be useful in the diagnosis of oxide films that formed on aluminum and magnesium alloys, including the ingots, casting or wrought products.

Diagnosis and Analysis of Oxide Films in Cast Magnesium Alloys by Ultrasonic-Vibration Treatment

“…Comparison of diagnosing oxide films in cast aluminum and magnesium alloys The description of diagnosing oxide films in cast aluminum alloys proposed by Huang 8) and Chen et al 9,10) that associated with previous results are summarized in Table 2. Cavitation bubbles are readily at the sites neighboring with oxide film, such as the interface of different oxides where the bubbles readily collapse.…”

“…Oxide films are relatively difficult to identify based on optical-microscope observations. Huang 8) and Chen 9,10) et al proposed an ultrasonic-vibration method for the diagnosis of oxide films entrapped in cast aluminum alloys. The principles of the method are based on the phenomena of acoustic cavitation.…”

Marking Oxide Films on the Section of Al-XSi Alloys by Ultrasonic-vibration Treatment

“…In tap water, the acoustic pressure measured near oxide film showed a distinct intensity from that near aluminum matrix. 14) Tap water often contains some tiny particles, which can provide inhomogeneous nucleation sites for the formation of cavitation bubbles. These particles thus initiate and encourage the nucleation of cavitation bubbles in water during the propagation of ultrasound.…”

Effects of Degassing and Fluxing on the Quality of Al-7%Si and A356.2 Alloys

“…4) Shih and coworkers developed an ultrasonic-vibration treatment which could reveal foggy marks on the polished surface of chilled samples. [5][6][7] After this ultrasonic vibration treatment, the oxide film entrapped in the Al-Si-Mg alloys would be revealed as a foggy area caused by cavitation micro-jet impacts, as shown in Fig. 1.…”