Influence of Prior Solidification Conditions on the Structure and Rheological Behavior of Partially Remelted Al-Si Alloys

Abstract

The aim of this paper is to study the influence of solidification conditions on the structure of semi-solid alloys achieved during partial remelting for various holding times and to determine the corresponding rheological behaviour during compression tests. A357 Al-alloys either solidified under pressure, or cast with inoculation, or electromagnetically stirred were used for this investigation. Compression tests were performed both at constant strain rate and with strain rate jumps to characterize the thixotropic behaviour of these alloys. Depending on the holding time in the semi-solid state before compression two main rheological regimes were observed after strain rate jumps: a first one typical of a pseudo-plastic behaviour (only observed for the pressurized material for small holding times) and a second one at long holding time for which the stress evolves with strain after the jump. Extensive study of the microstructure before and after deformation using image analysis coupled with some rheological study of composite materials was performed to allow correlation between microstructure and mechanical bbehaviour.