Microstructure evolution of semi-solid billet fabricated by semi-solid isothermal heat treatment of wrought AlSi7Mg aluminum alloy

Abstract: In this paper, the as-cast AlSi7Mg aluminum alloy billet was upsetted above recrystallization temperature to obtain wrought aluminum alloy with 50% deformation degree, and semi-solid isothermal treatment method was employed to achieve semi-solid billet of the wrought AlSi7Mg aluminum alloy. The microstructure observation and evolution law were investigated via optical microscope and scanning electron microscope. The results show that the primary α-Al phase can be changed from dendrite to spherical structure by semi-solid isothermal treatment of the wrought alloy (SSITWA). The complete transformation of the primary α-Al phase from dendrite to spherical structure can be achieved by increasing the isothermal temperature and prolongation of holding time. However, when the holding time is too long, the spheroidized grains will grow abnormally and eventually become irregularly shaped grains. High isothermal temperature can increase the primary α-Al size and reduce the time of dendrite to spheroidal grains. The average grain size of AlSi7Mg aluminum alloy semi-solid billet fabricated by SSITWA varies in a range of from 15 μm to 65 μm when the temperature and holding time change. The content of Al and Si elements in primary α-Al phase is obviously higher than that in liquid eutectic phase consisting of Al and Si elements, and the distribution of Mg element is uniform in the microstructure of semi-solid billet fabricated by SSITWA.