| dc.description.abstract | This dissertation mainly contains three related issues as following; (1) Analysis of the oxide film in Al-XSi-YMg alloys, (2) Effect of degassing on the oxide film and inclusion of Al-Si alloys, (3) Effect of oxide film and inclusion on the relative porosity of Al-(0-13%)Si and A356 alloys.
The details of each part were addressed behind:
(1) It’s experimentally difficult to directly observe and differentiate the entrapped oxide films in aluminum matrix of a casting. A few researchers have focused on the morphology of entrapped oxide films in aluminum castings and fully discussed there. Their morphology and size of oxide film was analyzed mostly based upon scanning electron microscopy (SEM) equipped with EDAX analysis. Using the ultrasonic-vibration treatment (UVT) to reveal the exact position of an entrapped oxide film on the polished surface of a specimen is the main technique used before diagnosis and analysis. Therefore, the entrapped oxide film of a specimen in the morphology, the size and its distribution were investigated and measured here.
The polished samples were placed on the bottom of the container in ultrasonic vibrator. Foggy marks (as reflective areas) would gradually show on the polished surface of chilled sample during ultrasonic vibration treatment. These marks were distinguishable and had been identified as oxide film. The shining marks shown on the chilled samples of pure Al are usually lengthy and feather-like strips. Increasing silicon content in the Al-Si alloys decreases the extent of shining cloud and/ or reduces the length of shining strips. But the counts of shining spots are increased. Function of Mg in the Al-XSi-YMg (X:6-7%, Y: 0-1%) alloy tended to reduces the size but greatly increased the counts of shining spots. Mg in the Al-Si-Mg melt causes a reaction to form a spinel on the interface of the oxide film and the melt. This improved the bonding between the matrix of the aluminum alloy casting and the entrapped oxide film. The oxide film broke into particles during ultrasonic vibration treatment. These particles were collected by filtration from the water in the container of the ultrasonic vibrator. The constituents of oxide particles were also analyzed by EDAX and/or EPMA mappings. Summarily, the oxide film entrapped in the aluminum casting may be in the form of cloud- or strip-type of clustering particles. The imposed ultrasonic energy breaks the oxide film into pieces. The morphology of oxide particle may take various shapes depending upon its constituents. If the oxide film is rich in alumina, the shape of the particle is angular. Increasing silicon content or the amounts of silica, oxide particle turns out to be sub-rounded or rounded depending on the amount of silica existed in the oxide particle. Increasing the magnesium content increases the amount of magnesia or spinel. The oxide particle becomes irregular in shape or flake-like, depending on the relative contents of alumina, silica and magnesia. If alumina has been significantly replaced by magnesia, the particle becomes lump.
The area from where the oxide particles detached, appeared as an eroded or flat zone under SEM observation, but was identified as a shining mark by optical observation. The oxide film entrapped in different aluminum alloy castings varies and depends on its constituents.
(2) Degassing treatment introduced in producing an aluminum casting dominantly eliminates the inclusions and then upgrades the quality of the melts. Effect of degassing treatment on inclusion particles of aluminum alloy chilled samples were then fully assessed and discussed here. Experimental observations showed that inclusion particles entrapped in chilled samples were mostly below 10 μm. A thin slice cut from chilled block (50 mm in diameter, 10 mm in thickness) was cooled by liquid nitrogen and then fractured into two pieces revealing the inclusion particles existed on fractured surface. In melting process, inclusion particles suspended in Al-melts can be effectively reduced by the floatation and/or sedimentation. However, experiment results indicated that degassing treatment by diffuser resulted in increasing inclusion particle counts in the Al-7Si melts. Such increase results from breakage of free surface of melt and bubbles collapsed on free surface. The particles entrapped into melt following the movement of convection loop near surface layer of melt during degassing treatment.
(3) The oxide, mostly alumina, in pure aluminum indicates a sound interface. This interface between matrix and the oxide film hardly shows any visible cleavage or gap. Reasonably, relative porosity for pure aluminum demonstrates the lowest value in this study. Increasing silicon content in the Al melt increases the particle counts and its oxide films are mostly containing silicon; or mullite. Silicon in the Al-melt reduces the solubility of hydrogen but it displays a great affinity to attract hydrogen. Hydrogen is much more difficult to diffuse out of solidified Al-6%Si melt than that of pure aluminum. Additionally, the greater extent of mushy zone in Al-6%Si alloy results in increasing the possibility for pore nucleation during solidification. Therefore, Al-6%Si alloy exponentially increases the relative porosity at high level of hydrogen. Although, Al-13%Si alloy suggests the same trend following the Al-6%Si alloy in relative porosity, it obtains the lower value due mainly to the light reduction of inclusion content and higher latent heat release of silicon.
Increasing magnesium (＞0.5 wt%) in Al-7%Si-0.51%Mg alloy increases the amount of oxide films suspended in the melt. The oxide film have potentially provided the more pre-existing nucleation sites and probably accumulated the gas from the creeping flow of the melt for bubble formation. With the high level of hydrogen, the bubble initially accelerated to grow into a big pore resulting in the high relative porosity obtained in reduced pressure test sample. Adding Sr in A356 melt modified the eutectic silicon and increased the total amount of surface area of Si to retard the diffusion of hydrogen out of the melt during condensation. With the assistance of Sr addition, bubble favorably nucleated and grew in A356 melt. This resulted in pores uniformly distributed but smaller in size than that of Al-6%Si alloy over the specimen. However, A356 alloy still obtained the higher relative porosity (＞5%) than that of Al-6%Si owing to its uniform distribution of pores.
In A356 without Sr, the high fraction of spinel of oxides (including inclusion and oxide film) existed in matrix resulted from the interaction of oxides and melt during smelting. The high interfacial energy between spinel and melt alleviated the nucleation rate of embryonic bubble. The relative porosity, therefore, is totally lower in A356 without Sr comparing with that of Al-6%Si and A356 alloys at any levels of hydrogen measured in the melt. | en_US |