Pretreatment process of SiC particles and fabrication technology of SiC particulate reinforced Zn-Al alloy matrix composite

In the present paper, a novel pretreatment process for SiC particulate and a new mechanical-electromagnetic combination stirring process for fabricati ng Zn-Al(ZA27)/SiCp composites are described. The optimal pretreatment rout e and the most appropriate SiC particle parameters were experimentally dete rmined . The pretreated SiC particles were easily incorporated and disperse d in the ZA27 alloy melt and were not agglomerated before addition to the m elt. The surface status of the SiC particles before and after pretreatment was observed and analysed by scanning electron microscopy, energy dispersiv e spectroscopy, X-ray diffraction, and transmission electron microscopy. It was found that gas existing on the SiC particle surfaces by physical and c hemical absorption was a significant hindrance to the incorporation and dis persion of SiC particles in the alloy melt. The gas absorption was induced by ultrafine SiC powders, fracture steps, and ions existing on the SiC part icle surfaces. The carbon, silicon, and oxygen contents on the SiC surface were varied with different pretreatment techniques. Moreover, a dense layer of amorphous SiO2, which improves wetting of SiC particles in the ZA27 mel t, was formed owing to calcination of SiC particles in air. The new combine d stirring process exploits the advantages of both mechanical and electroma gnetic stirring of the melt at the different processing stages during fabri cation. The microstructural characteristics of the resulting composites are : homogeneously distributed SiC particles, fewer macro gas blows and inclus ions, and little shrinkage porosity in comparison to composites fabricated by a mechanical stirring process. Finally, the mechanisms of degassing and reducing the porosity and the number of oxide inclusions are discussed. (C) 2001 IoM Communications Ltd.