Mechanical Response of Al-1.09Mg2Si Alloy under Varying Mould and Thermal Ageing Conditions

Samples of the 6063 (Al-1.09Mg2Si) alloy ingot were melted in a crucible furnace and cast in metal and sand moulds, respectively. Standard tensile, hardness, and microstructural test specimens were prepared from cast samples, solution treated at 520°C, soaked for 6？hrs, and immediately quenched at ambient temperature in a trough containing water to assume a supersaturated structure. The quenched specimens were then thermally aged at 175°C for 3–7？hrs. Results show that at different ageing time, varied fractions of precipitates and intermetallics evolved in the specimens’ matrices which affect the resulting mechanical properties. The metal mould specimens aged for four hours (MTA-4) exhibited superior ultimate tensile strength of 247.8？MPa; microhardness, 68.5？HV; elongation, 28.2% . It is concluded that the extent of improvement in mechanical properties depends on the fractions, coherence, and distribution of precipitates along with the type of intermetallics developed in the alloy during ageing process. 1. Introduction Casting is one of the most versatile methods of producing structural aluminium alloy components. However, the rather large preponderance of defects in cast aluminium components often limits their performance and adversely impacts their commercial values. Hence there is the need for a novel processing approach to improve the cast microstructure for enhanced performance. Generally, the poor mechanical properties of cast aluminium alloys can be improved through either alloy addition or various forms of heat treatment [1]. During the heat treatment of cast aluminium alloys, an advantage is made of the characteristic decrease in solubility at low temperature of magnesium (Mg) and silicon (Si) which are the main alloying elements in Al-Mg-Si alloy. Further, Keist [2] confirms that the appreciable decrease in concentration of the alloying elements at room temperature is the fundamental phenomenon that provides the basis for increasing substantially the hardness and strength of aluminium alloys through isothermal treatment. Similarly, Siddiqui et al. [3] have shown that improved ductility can be achieved by process annealing at 415°C, soaked between two and three hours coupled with a cooling rate 30°C per hour. Generally, strength improvement of most 6063 aluminium alloys can be effected in a three-pronged approach comprising solution heat treatment, quenching, and precipitation of solute atoms [4]. However, the greatest challenge usually encountered with this approach is effective control of the second-phase precipitates. Lumley et al. [5]