Effects of Copper and Magnesium on Phase Formation Modeling and Mechanical Behavior in AL-CU-MG Alloys

Abstract

The current work emphasizes the establishment of a relationship between microstructure, and copper and magnesium addition in aluminum based alloys. Aluminum alloys containing 4 and 6% copper and 0.50 and 1% magnesium were cast from commercially pure aluminum ingots and homogenized at 400oC. Images of the microstructures of the as-cast and homogenized alloys were acquired, and image analysis of the phases was performed on the acquired images to obtain area fractions of the phases present in the microstructure. Using the CALPHAD modeling method, thermodynamic modeling of the alloys was carried out in the equilibrium cooling condition since after homogenization for a prolonged time the alloys should reach a close to equilibrium cooling condition. Phase fractions predicted in modeling were matched closely with image analysis data, given that phases present in the homogenized alloys can be interpreted through modeling in the equilibrium cooling condition. EDX analysis of the samples identified the phases present in the alloys as Al2Cu, Mg2Si and Al7Cu2Mg. With increasing copper content, it was found both in modeling and experimentally that the amount of Al2Cu phase increased, which improved the hardness values of the alloys. However, in the homogenized condition, the hardness values slightly decreased compared to those of the as-cast condition due to retention of a lower fraction of Al2Cu phase in the homogenized condition. This was ascertained in the modeling of the alloys. In contrast, Mg2Si and Al7Cu2Mg phases were formed when magnesium was added in the predefined amount; however, these phases were not found to be effective at improving the hardness of the alloys, and the hardness values were barely modified. Apparently, the hardness enhancement after magnesium addition occurred due to a solute effect. To observe the phase effect of magnesium in aluminum alloys, binary alloys of aluminum-magnesium should be cast.