Direct thermal method pouring temperature and holding time effect on aluminium alloy 6061 microstructure

Abstract

The microstructure variances of aluminium 6061 billet produce via direct thermal method with different pouring temperature and holding time are presented in this paper. The direct thermal method is one of the methods to create globular microstructure feedstock billet, which gives the material a thixotropic behaviour during semisolid metal processing. In this experimental work, a molten aluminium 6061 was poured into cylindrical copper mould before quenched in water at room temperature. The effect of pouring temperature of 660°C and 700°C and holding time of 20s and 60s were observed from the microstructure formation. The result shows that the combination of a pouring temperature of 660°C with a 20s holding time produces a finer near globular microstructure. These fines near globular microstructures gives a thixotropic behaviour improvement in better fluidity for a better flow during shaping. The pouring temperature just slightly above the liquidus temperature provides slower cooling rates from above to below the liquidus temperature. This process causes less superheat to be extracted by the cylindrical copper mould and gives a slow cooling rate action during the solidification stage that promotes the formation of further nuclei, which results in smaller grain size. The result also shows the combination of the lowest pouring temperature of 660°C with the lowest holding time of 20s produced the smallest grain size measured in area. However, the circularity and aspect ratio that indicates globular shape grain has a slight change in result which indicates that every feedstock billet has a near globular grain size. In conclusion, this work has shown that specific combination of pouring temperature and holding time has an effect on the microstructure formation of the feedstock billet produced by using direct thermal method.