Abstract
The achievement of hypereutectic Al-Si alloys with completely nodular eutectic Si has been studied. The procedure contained two steps: firstly, achieving hypereutectic direct electrolytic Al-Si alloys (HDEASA) with completely eutectic structure and secondly, spheroidizing eutectic Si upon soaking. HDEASAs with Si level in the range from 13.2 wt% to 17.6 wt% were made from direct electrolytic eutectic Al-Si alloy ingot and Al-50 wt% Si hardener. As cast microstructure of HDEASA was composed of primary-free quasi-eutectic cells. In comparison with commercial alloys, the lower heating temperature of 505°C -515°C for 4-8 hrs was required to fully spheroidize Si crystal, either fine fibrous or even flaky in casting. Most of the spheres ranged in size from 1.0μm to 4.0μm. Many measurements were focused on the variety of Si phase size in eutectic cells against the distance from the center of the cells. The origin of granulation of primary-free quasi-eutectic Si crystal is associated with its thermodynamic structural instability, accompanied by crystallographic defects, related to the electrolytic process.
Highlights
Hypereutectic Al-Si alloy offering excellent wear resistance, lower density and thermal expansion coefficient, and superior dimensional stability at high temperature is selected as the material for the fabrication of automotive components and other wear-resisting parts as the drive to reduce fuel consumption and CO2 emissions has recently become more intense
In 1953 Mascre reported that the primary Si in hypereutectic alloys could be made dendritically or roughly spherical in chill casting by adding Na in the range of 0.05wt% to 0.1wt% to the melt [2]
P-addition significantly causes a reduction in the size of primary Si particles, increasing the number evenly distributed through the eutectic matrix
Summary
Hypereutectic Al-Si alloy offering excellent wear resistance, lower density and thermal expansion coefficient, and superior dimensional stability at high temperature is selected as the material for the fabrication of automotive components and other wear-resisting parts as the drive to reduce fuel consumption and CO2 emissions has recently become more intense. The combination of Si+P [13], RE+P [14], B+P [15] or Al-Ti-C [16] strongly enhances the refinement effect on primary Si induced by P-addition with modified eutectic Si. Recently it was found that adding a small amount of γ-Al2O3 nanoparticles simultaneously refined and modified the Si particles, and reduced the porosity in cast Al–20Si–4.5Cu, resulting in unusual ductility enhancement [17, 18]. It was found that adding a small amount of γ-Al2O3 nanoparticles simultaneously refined and modified the Si particles, and reduced the porosity in cast Al–20Si–4.5Cu, resulting in unusual ductility enhancement [17, 18] These chemical additions, either individually or complex, don’t lead the morphological change from multi-faceted primary Si into spheres.
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