Evaluation of the Rheological Behavior of a Semi-Solid Al�CSiC Composite using a Parallel-Plate Drop-Forge Viscometer

Abstract

This paper presents of studies performed to assess the effect of rheological behavior on the near-net shape forming of an Al–20 vol% SiC composite of Duralcan F3A.20S and of the mother aluminum alloy A356 for comparison. Isothermal experiments were conducted using results of a parallel-plate drop-forge viscometer in a temperature range from 849 K (576 ºC) to 862 K (590 ºC). Each experiment indicated that the viscosity decreased in the early increasing shear rate stage and subsequently increased with decreasing shear rate. The overall relationship between the viscosity,μ[Pa.s], and the shear rate,γ[s-1], can be described by a power-law model of μ= 3.2 × 107γ-1.5 for Duralcan and μ = 1.6 × 107γ-1.5 for A356. The power-law index was the same for both materials, whereas the power-law constant of Duralcan was two times greater than that of the A356 mother alloy because of the distribution of 20 vol% SiC particles. The decrease in the viscosity that accompanied an increase in the shear rate depended on both the temperature and the applied force. The viscosities of 32 kPa.s for both Duralcan and A356 at the maximum effective duration of deformation, which obtained from the plot as a function of the viscosity, appeared to be the points at which the dominant factor governing the visco-plastic flow process transitions from plastic forming to casting. The viscosity must also represent optimum semi-solid forming conditions, as indicated by the achievement of both a moderate working time and an adequate deformation. The optimum operating temperature for both materials can be ranged from 855 K (582 ˚C) to 857 K (584 ˚C), which is just above the melting point.