| Aluminum matrix composites have attracted more and more attention in aerospace, military, automobile and other industries because they have many advantages, such as light density, increased specific strength, improved specific stiffness, higher wear resitance and so on, over matrix materials. At present, the ceramic particle reinforcement of most majority of metal matrix composites(MMCs) is micron-sized. In general, the toughness of composites will reduce when the micron-sized particle reinforcement content is too high. However, adding small amount of nanometer ceramic particles(uaually less than 2wt.%) to base materials which is called metal matrix nano-composites(MMNCs) can obtain increased strength and high toughness at the same time. In recent years, MMNCs have become the research focus of MMCs.However, adding nano-particles to metal melt is very difficult due to poor wettability between them, and clusters of nanometer ceramic particles agglomerate easily. So the preparation technology of Al matrix composites reinforced by nano-SiCp was researched in this paper. 0.5wt.%, 1wt.% and 2wt.% nano-SiCp/A356 composites were prepared by combining dry high energy ball milling, ultrasonic vibration treatment and squeeze casting. First, the mixture of SiC powder with average size of 40 nm and micron-sized aluminum powder was milled without process control agent(PCA) to prepare millimeter-sized SiCp/Al compound particles. The sizes of SiCp/Al compound particles were among 0.5 to 2 millimeter. Then the Si Cp/Al compound particles were added to Al-Si alloy melt above 700? to fully melt and diffuse. At last, the composite melt was directly formed under pressure, or treated by ultrasonic vibration in liquid state and in semisolid state, then poured into the die and solidified under pressure. The effects of dry milling parameters on morphology and size of the SiCp/Al compound particles and nano-SiCp distribution, and the effects of different preparation processes,processe parameters on the microstructure and mechanical properties of the composites were researched.The results of dry milling for SiCp/Al compound particles showed that the rotation speed influenced the morphology and size of the SiCp/Al compound particles, while the ball-to-powder weight ratio mainly influenced the size. Milling for 14 hours, when rotation speed increasing from 180 rpm to 360 rpm, the compound particles transformed from small flakes to globular particles. When the ball-to-powder weight ratio decreasing from 10:1 to 3:1, the average size of the globular compound particles increased from 1mm to 1.5mm.When the nano-SiCp content in the mixed powders was no more than 4wt.%, nano-SiCp can uniformly distributed in millimeter-sized SiCp/Al compound particles. Besides, the formation processes of compound particles was divided into three stages: first, homogeneous mixing of the mixed powders on macroscopic leve while all nano-SiCp were adhered to or inseted into surfaces of Al powders; second, uniform distribution of the nano-SiCp on microscopic level while Al powders were cold welded into millimeter-sized and nano-SiCp were inseted into Al matrix; third, the morphology and size of SiCp/Al compound particles achieving a balance level when they tranformed into globosity.Millimeter-sized nano-SiCp/Al compound particles have greatly improved the wettability of the nano-SiCp and metal melt, and it is easy to add nano-SiCp to the liquid aluminum alloy. After adding nano-SiCp, the grain and eutectic silicon size of the nano-SiCp/A356 composites became much smaller than matrix A356 alloy. Ultrasonic vibration introduced into the composites melt can significantly improve the particle dispersion and refine grain size. The majority of nano-SiCp generally uniformly distributed in the eutectic structure, and a very small amount of nano-SiCp were uniformly distributed in the grains. When adding 2wt.% nano-SiCp, the SiC clusters with size of 1?m appeared.Nano-SiCp/A356 composites prepared by ultrasonic liquid process had higher mechanical properties than the composites prepared by liquid or ultrasonic semisolid process. Among these three processes, with the forming pressure increasing, the tensile strength and elongation of the composites increased gradually and not changed obviously when the forming pressure was over 200 MPa. With the nano-SiCp content increasing, the tensile strength of the composites decreased gradually when the composites solidified with no forming pressure, and it firstly increased and then reduced when the composites solidified with forming pressure and reached the maximum value at 1wt.% nano-SiCp addition. The as-cast and T6 heat treated tensile strength of 1wt.%nano-SiCp/A356 prepared by ultrasonic liquid process and under 200 MPa pressure reached 260 MPa and 344 MPa respectively, which were increased by 4.4% and 8.2% respectively, compared with the A356 alloy prepared by the same conditons. |
PDF Full Text Request