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Metal Casting Technologies : June 2007
18 www.metals.rala.com.au TECHNICAL FEATURE treatment and ageing variables. Work  is also underway to characterize the fine scale microstructures produced during precipitation treatments and to examine variations in microstructure and properties within certain wheel designs, notably at rim-rib junctions. Some of the characterization studies are being done at Chiang Mai University which has a well equipped scanning and transmission electron microscopy centre (Figure 1). The effects of solution treatment and ageing of Al Alloys on the matrix cannot be studied by conventional metallography (Figure 2) since the "precipitates" formed in the matrix during ageing are much too fine to be resolved. The nature and crystallography of the precipitate can only be directly studied by TEM as seen in Figure 3 which shows diffraction contrast in a bright field image from very small needle-like zones of β´Mg2Si that were identified by electron diffraction. This TEM study is aimed at relating heat treated mechanical properties of A356 castings to the type, morphology and orientation of precipitate zones obtained via different processing conditions. Similar characterization research work at Chiang Mai also includes microstructural studies of semi-solid cast metals, including High Cr Irons and Al Alloys, in cooperation with KMUTT and MTEC. A PhD student is examining the primary phase and eutectic constituents obtained at various parameters of semi-solid processing and subsequent heat treatment. At KMUTT a study  of minor additions on secondary dendrite arm spacing (SDAS) and mechanical properties has revealed that, in contrast to work elsewhere, Antimony (Sb) additions did not have a clear refining effect on SDAS in permanent mould castings. Moreover, in the solution treated and aged condition, increasing the Sb level from 0 to 0.28% decreased tensile strength and ductility. This deterioration is believed to be due to the formation of Mg3Sb2. SEMI-SOLID PROCESSING At present two university groups are working on semi-solid processing of Al Alloys. At KMUTT a cooling plate technique is being used to develop semi-solid Al matrix composite material. The influence of particle size and volume % of reinforcement and process parameters such as stirring, pouring temperature, inclination angle of cooling plate, etc are being studied in relation to the mechanical properties and wear resistance of Al matrix composites . At Prince of Songkla University a technique based on using very fine Argon gas bubbles is being developed to prepare semi-solid material. The vigorous convention from these bubbles introduced via a graphite distributor breaks up the first formed dendrites producing almost globular primary Al solid solution. A comparison between the microstructures of conventionally cast and semi- solid processed A356 is shown in Figure 4. The current system is capable of preparing 2kg of Al Alloy and a larger system is to be constructed for industrial trial [19-21]. The original work was carried out at MIT in the USA and is the subject of a US Provisional Patent and a Thai Patent Application. Funding for continuation work in Thailand is from the Reverse Brain Drain Project of the National Science & Development Technology Agency. Figure 3: Bright field TEM view of Al matrix in A356 after solution treatment plus aging at 160oC for 12 hours. The thin needle-like precipitates of Mg2Si (seen aligned as dark/light contrast in the matrix) are around 20nm or less in length. After Imurai . Figure 2: Microstructures (x200) of aluminium alloy A356 (a) as cast and (b) solution treated at 540˚C and aging at 160°C for 8 hours. After Imurai . A B Figure 4: Microstructures (x50) of conventional and semi-solid cast A356. After Wannasin . (a) Conventional casting. (b) Semi-solid casting. A B