The microstructural changes of step-cooled AL 6061 alloy during hot compression testing

Abstract

A microstructural evolution of wrought aluminum alloys during hot forming processes, such as extrusion, rolling and forging, has long been studied. A unique step-cooled treatment was practice by some aluminum alloy suppliers to produce Al 6063 billets with higher extrudability. The microstructure changes from this process were examined, and the softening mechanism resulted from the formation of meta-stable ß'-Mg2Si phase during the precipitation process. In the present work, the consequential microstructural development of the step-cooled Al 6061 alloys was investigated due to its composition similarity to Al 6063. The specimens were homogenized at 585°C and step-cooled at temperatures from 340°C to 370°C with holding time from 1 hour to 4 hours. Hot compression tests were performed at 220°C and at two ram speeds 2.25 inch/minute and 11.25 inch/minute to reach average strain rate of 0.1 and 0.5 respectively. Comparative Al 6063 data from hot compression test were made to evaluate the formability efficiency. The mechanical test results showed 9% improvement over the formability of Al 6061, while Al 6063 gained 21% formability increase over the original billet sample. Metallographic analysis over the compressed specimens showed localized deformation softening due to the chilling effect of the press platen. The precipitation phases of Mg2Si were examined under SEM using the relief etching technique. The effect of ß'-Mg2Si precipitation on the formability of Al 6061 alloy is discussed.