Influence of solidification variables on microstructure and microhardness of Al-Cu alloys using full factorial design

In this work a hypereutectic Al-40wt%Cu alloy was used to verify the influence of the input variables cooling rate (degrees C/min) and superheating (degrees C), on the output variable microhardness (HV). The samples were poured in ceramic and copper crucible and a thermocouple coupled to a data acquisition system measured the cooling rates that were 20 degrees C/min and 24 degrees C/min, respectively. The influence of these cooling rates and overheating of 100 degrees C and 200 degrees C above the melting temperature of the alloy on microhardness was evaluated. Vickers hardness measurements were performed on the sample cross section with an area of 0.8 cm2. The samples were characterized microscopically to evaluate the microstructural formation. The potential of the variables involved in the study was evaluated by a 22 full factorial design. The results of the factorial design indicated that the samples solidified using overheating to 200 degrees C above the melting temperature and cooled at a rate of 24 degrees C/min obtained higher microhardness values. The alloys superheated to higher temperatures showed more refined microstructures with elongated grains, disordered growth directions and short dendritic spacing, causing a transition in columnar to equiaxial microstructure.