Clad-extrusion for producing SiCw/6061Al composite tubes

Abstract

Plastic forming is one of the most important techniques of discontinuously reinforced aluminum matrix composite (DRA) applications [1–4]. Hot-extrusion is one of the important technologies of plastic forming of DRA. Many attempts have been put on extrusion technologies and their influence on microstructures and properties of DRA for the plastic purpose. A semisolid extrusion process of aluminum alloys with alumina short fibers (FRM) has been developed to produce the composite bars by using a cladding method at the temperature above the solidus of the matrix [1]. However, it should be mentioned here that deformation at the temperature above the solidus of the matrix may deteriorate the composite properties because of the superburning of the matrix alloy [5]. Thus, most of extrusion processes of DRA have been performed at the temperatures below the solidus of the matrix. At the temperature below the solidus of the matrix, if the extrusion parameters are improper, then the surface quality of the extrudate is rather poor. SiCw/Al composite tubes would be made by using a forward extrusion method. Even if SiCw/Al composite are extruded at the temperature below the solidus of the matrix, the surface quality of SiCw/Al composite tubes made using the forward-extrusion method is difficult to be controlled and there often is tearing on the surfaces of the composite tubes. To suppress this, in this work, a clad-extrusion method is used for producing SiCw/6061Al composite tubes at the temperature below the solidus of the matrix 6061 alloy. In this way a solid extrusion could be performed without tearing. The SiCw/6061 Al composites used here were fabricated by the squeeze casting technique using the β-SiC whisker (TWS-100) as a reinforcing element and the commercial 6061Alloy as a base metal. Mold and whisker perform temperature, pouring temperature and applied pressure were 450 ◦C, 750 ◦C, 60 MPa, respectively. The whisker volume fraction (Vf) was 20%. The solidus temperature of the matrix 6061 alloy in the SiCw/6061 Al composite was measured to be 580 ◦C by the DSC technique. The SiCw/6061 Al composite tubes were made by using a forward extrusion method. Fig. 1 shows a schematic illustration of the clad-extrusion for producing the SiCw/6061Al composite tubes. The extrusion temperature was 520 ◦C . The extrusion ratio was about 15. A flat die made of tempered steel was used. Matrix 6061 alloy was selected as the cladding layer. To decrease the friction between the die and the billet, all the extrusion tests were carried out under lubricating with the graphite-oil. Before extrusion, the composite billets were held for 20 min at the extrusion temperature of 520 ◦C, which was below the solidus temperature of the matrix 6061 alloy. Simple solid-state extrusion causes tearing on the surface of extruded SiCw/6061Al composite tubes. Fig. 2 shows the appearance of a SiCw/6061Al composite tube made by using a simple hot-extrusion method at 520 ◦C. It can be seen that, from this figure, there are fir-tree-like cracks on the surface layer of the composite tube. The tearing cracks on the surface of the composite tube are shown in detail in Fig. 3.