Mechanical Stirring Technique Research Articles

Effects of whisker surface treatment on microstructure and properties of Al 18B 4O 33w/6061Al composites

6061Al matrix composites reinforced by ZnO-coated Al 18B 4O 33 whiskers were fabricated by a semi-solid mechanical stirring technique. The effects of ZnO coating on interfacial reaction between whiskers and matrix and the tensile properties of the composites were investigated. Tensile tests on composites were performed at room temperature, and microstructures were observed by scanning electron microscopy (SEM). The results show that the surface treatment of whiskers could reduce interfacial reaction effectively, improve the wettability between whiskers and matrix and enhance the tensile properties of the composites obviously. In addition, semi-solid stirring parameters were also under preliminary study. The stirring parameters were determined by the distribution of whiskers in the composites. The composites with homogeneously distributed whiskers were fabricated by semi-solid stirring at 610 °C for 30 min.

Thixotropic behavior and structure evolution of A356 alloy in the semisolid state

The study on the behavior of metals in the semisolid state was pioneered by David Spencer at MIT in 1971. He applied a shear strain on solidifying Sn-15 pct Pb alloy, and discovered a remarkable reduction in shear stress comparing with those that were not sheared during solidification. The grain structure of the sheared alloy was nondendritic, as opposed to the dendritic structure of not-sheared alloy. Since this early work, semisolid processing (SSP) has become a widely studied and accepted process. Comparing with conventional cast materials, SSP materials have less segregation and finer grain size. Practically, SSP can increase part-forming rate, reduce the thermal shock imposed on the mold, increase mold life, etc. A356 Al-Si alloy has the following characteristics, i.e. good castability and mechanical properties, light weight and a better oxidation resistance, and is widely used. The solidus and liquidus of the alloy are 830 K and 886 K, respectively, which provide a considerably wide semisolid regime that is suitable for the studies on the semisolid behaviors. The evolution of nondendritic structure and the thixotropic behavior of A356 alloy during semisolid processing were studied. During the process, semisolid slurry was produced using mechanical stirring technique.

Casting defects and properties of cast A356 aluminium alloy reinforced with SiC particles

This paper is concerned with casting defects and properties of test bars cast from an A356 aluminium alloy reinforced with SiC particles. The composite materials were produced by the molten aluminium mechanical stirring technique. Particular attention was given to the characteristics of the casting defects which appeared at the tensile fracture surfaces of the cast test bars by means of extensive analytical electron microscopy. These observations showed that the casting defects on the fracture surface were agglomerates of SiC particles, aluminium and silicon oxides, mixtures of SiC particles with aluminium and silicon oxides, and brittle chemical compounds containing aluminium and silicon elements. These defects either were present in the as-received ingots or were introduced in the casting process during which oxide films were easily incorporated into the liquid metal. The number and areas of the defects on the fracture surface have an obvious effect on the mechanical properties of the cast test bars. Appropriate methods of improving the quality of the composite ingots as well as of optimizing the casting technique must be used to minimize these defects.