High-strain-rate superplasticity of an AL2009-SICw composite

Abstract

Discontinuously reinforced aluminium composites are considered to be excellent candidates for aerospace and automobile industries. However, their formability and machinability needs to be understood first before they can be widely used in these industries. Therefore, a lot of research effort has been spent in the study of the superplasticity of such composites over the past decade. The superplastic capability of many discontinuously reinforced aluminium matrix composites [1–7] has been discovered. Al 2009=SiCw composite is attractive for its high strength, stiffness and thermal stability and an ideal substitute for titanium used at service temperatures betwen 366 and 450 K. Also this material can be used in the sporting goods industry. Up to the present day, the superplastic deformation of Al 2009–20 vol % SiCw composite has not been investigated. In the present study, the superplastic behaviour of a SiC whisker reinforced 2009 aluminium matrix composite has been investigated. A 20 vol % SiC whisker-reinforced 2009 aluminium alloy composite sheet supplied by Advanced Composite Materials Corporation was investigated in this paper. The sheet thickness was 2.286 mm and the size of the â-SiC reinforcement is 0.45–0.65 μm in diameter and 5–80 μm in length. The composite was fabricated by powder metallurgy followed by extrusion and rolling. A typical scanning electron micrograph of the composite is shown in Fig. 1. The distribution of whiskers was found to be uniform and approximately aligned in the direction of the rolling direction. The solidus temperature of the composite was determined by a Du Pont 9900 differential scanning calorimeter (DSC). The solidus temperature of the composite was found to be 812 K when the DSC test started at room temperature and ended at about 893 K with a constant heating rate of 10 K miny1. Superplastic tensile samples with a gauge length of 5 mm and a width of 4 mm were machined from the sheet in the rolling direction. Superplastic tests at temperatures ranging from 723 to 813 K were conducted in an Instron test machine with a ceramic heater furnace. Prior to testing, the specimen was held in the furnace at the specified test temperature for about 20 min to establish thermal equilibrium and all testing was carried out in air at different constant crosshead velocities. The material showed the superplastic ability under certain conditions. The relationship between the total elongation-to-failure and the initial strain rate of the composite at different test temperatures is shown in Fig. 2. A maximum elongation of 190% was obtained at a strain rate of 6:7 3 10y1 sy1 and at 773 K which is far below the solidus temperature, 812 K. A similar phenomenon was also observed for Al2009–15 vol % SiCw composite [8]. That the optimum superplastic temperature was far below the solidus temperature was anomalous because the optimum temperatures of several other superplastic aluminium composites were found to be slightly higher than their solidus temperatures [1–7]. The optimum superplastic strain rate of the superplastic composites was obviously higher than the conventional superplastic strain rate of 10y5 –10y3 s [9] and this was considered to be highly beneficial in the superplastic forming of aluminium composites. The optimum superplastic temperature of several super-