Rheological behavior and microstructural evolution of semi-solid hypereutectic Al-Si-Mg-Cu alloys using rheoforming process /

Abstract

The aim of the current study was to investigate the rheological behavior and microstructural evolution of hypereutectic Al-Si-Cu and Al-Si-Mg-Cu alloys using conventional and modified SEED process (Swirled Enthalpy Equilibration Device). In the first part; the feasibility of semi-solid processing of hypereutectic Al-Si-Cu A390 alloys using a novel rheoforming process was investigated. A combination of the SEED process, isothermal holding using insulation and addition of solid alloy during swirling was introduced as a novel method to improve the processability of semi-solid A390 slurries. The effects of isothermal holding and the addition of solid alloy on the temperature gradient between the centre and the wall and on the formation of _-Al particles were examined. In addition, phosphorus and strontium were added to the molten metal to refine the primary and eutectic silicon structure to facilitate semi-solid processing. It was found that the combination of the SEED process with two additional processing steps can produce semisolid 390 alloys that can be rheoformed. In the second part, the effects of Mg additions ranging from 6 to 15% on the solidification behaviour of hypereutectic Al-155i-xMg-4Cu alloys was investigated using thermodynamic calculations, thermal analysis and extensive microstructural examination. The Mg level strongly influenced the microstructural evolution of the primary Mg2Si phase as well as the solidification behaviour. Thermodynamic predictions using ThermoCalc software reported the occurrence of six reactions, comprising the formation of primary Mg2 Si, two pre-eutectic binary reactions, forming either Mg2Si + Si or Mg2Si + _-Al phases, the main ternary eutectic reaction forming Mg2Si + Si + _-Al, and two post-eutectic reactions resulting in the precipitation of the Q-Al5Mg8Cu 2Si6 and [theta]-Al2Cu phases, respectively. Microstructures of the four alloys studied confirmed the presence of these phases, in addition to that of the [pi]-Al8Mg3FeSi 6 phase. The presence of the [pi]-phase was also confirmed by thermal analysis. In the third part, the effects of P and Sr on the microstructure of hypereutectic Al-155i14Mg-4Cu alloy were studied. The microstructural examination and phase identification were carried out using optical microscopy and scanning electron microscopy (SEM). The effects of individual and combined additions of P and Sr on the eutectic arrest in Al-155i14Mg-4Cu alloy were examined using thermal analysis. The mean size of primary Mg2Si decreases from about 350 [mu]m to less than 60 [mu]m and the morphology changes from coarse dendritic type or equiaxed to polygonal type. In addition, the morphology of the eutectic Mg2Si phase changes from coarse Chinese script to fine fiber-like, while that of the eutectic Si phase changes from coarse acicular shape to a fine fibrous form. With Sr addition, the morphology of the [pi]-Fe phase evolved from Chinese script to a fine twin platelet form. Furthermore, the thermal analysis results reveal that the addition of Sr or Sr and P reduces the temperature of eutectic nucleation and growth. Finally, the rheological behaviour and microstructure of semi-solid hypereutectic A390, P-refined A390, Al-15Si-10.5Mg-4Cu and Al-15Si-13.5Mg-4Cu alloys were investigated by using parallel plate viscometry. The flow deformation of these alloys in the semi-solid state was characterized at different deformation rates and at variable solid fractions. The calculated viscosity for variable shear rate was deduced using the analytical method developed by Laxmanan and Flemings. Microstructures of the four alloys, after partial solidification, were examined in order to characterize the flow behaviour during deformation. (Abstract shortened by UMI.)