Study On The Fabrication And The Solidification Behavior Of Sicp/al-mg Composites Prepared By Mechanical Stirring

In order to further understand the phenomenon pertaining to particle-reinforced metal matrix composites prepared by stir casting, including the control of reinforcement distribution, interface bonding and reaction, as well as the solidification of matrix alloy under the influence of reinforcement, and to provide theoretical guidance to the preparation technique, in the present work, SiCp/Al-Mg composites with homogenous microstructure and excellent performances were successfully fabricated by mechanical stirring method, and the stirring parameters were optimized. Through the experiment of resolidifying the composites under pressure , the nucleation and growth mechanism of the matrix alloy was analyzed, the solidification model was established, and the solidification kinetics was calculated. Through the directional solidification study, the model for the interaction between different crystal growing conditions and SiC particles was developed. Via theoretical analysis of the interaction between reinforcing particles and interfaces, the effects of SiCp on the solidification interface were discussed. By microstructural observation and performance testing of the composites prepared under different solidification conditions, the interaction among solidification parameters, microstructure and behavior were studied.SiCp/Al-Mg composites with various Mg contents of the matrix alloy and reinforcement volume fractions were developed with optimized stirring parameters. The reinforcements were pretreated in this experiment. The microstructure of the composites was observed using SEM and TEM. Tensile tests of the materials were performed at room temperature. The result shows that after calcination, a layer of SiO2 film formed at the surface of the reinforcing particles. This film reacted with Al and Mg provided by the matrix alloy, yielding a layer of MgAl2O4, which reduced the surface tension of the melt and improved the wettability between SiC particles and the matrix. By pretreating the particles and optimizing the stirring parameters, the uniform distribution of the reinforcements was obtained, and the mechanical properties of the composites were enhanced. It is confirmed, through comparing the microstructure and performances of the composites, that 10vol.% SiCp/Al-6.8Mg prepared with optimized parameters, namely 612℃/500rpm/30min, had better microstructural homogeneity and comprehensive mechanical behaviors.Solidification behavior of SiCp/Al-Mg composites was studied using various experimental methods including resolidification under pressure, unidirectional solidification and differential thermal analysis (DTA), and analyzing methods including optical microscopy, SEM, TEM and XRD. The result shows that SiC particles reduced theα(Al) phase nucleation temperature and increased nucleation undercooling in the composites. SiC particles could not serve as nucleation sites forα(Al) phase. During solidification, they were pushed to the last solidified zone, namely the crystal boundary, and formed a necklace-liked structure which impeded the growth ofα(Al) crystal. In the later stage of solidification, eutectic reaction took place between Al and element Si which was generated through the wetting between the reinforcement and the matrix. Al-Si eutectic phases were also produced during this process. It is indicated by solidification kinetics calculation that SiC particles increased the nucleation temperature and the amount of Si eutectic phase. Under unidirectional solidification conditions, by accelerating the solidification at the interface, the crystal morphology of the matrix alloy was changed, the interspace of the dendrites declined, and the uniformity of reinforcement distribution in SiCp/Al-Mg composites was effectively improved. Raising solidification pressure and reinforcement volume fraction, the critical solidification rate for the particles to be captured by solidifying interfaces dropped.The microstructure of the composites prepared under different solidification pressure was analyzed, their density was measured, and compression and hardness tests were conducted at room temperature. The result indicates that solidification conditions (reinforcement content, solidification pressure, cooling rate during solidification, etc.) had little effects on the nucleation mechanism and microstructural evolution of the composites. Increasing reinforcement content could decrease the nucleation temperature ofα(Al) phase, yet had no obvious effect on Al-Si eutectic reaction. The crystal size of the solidified materials was notably decreased with the increase of the reinforcement content. During solidification, the applied pressure enhanced the wettability between the reinforcement and the matrix, improved the nucleation undercooling level, raised the nucleation rate and the cooling rate of the composites, as well as contributed in the decline of crystal size of the matrix alloy. The applied pressure also caused the formation of preferred crystal growth orientation during solidification. With the applied pressure rising, the porosity in the composites was reduced and the density increased, and the mechanical performances of the composites were notably enhanced. Accelerating the cooling rate, the heat released during crystallization process increased, so did the nucleation undercooling level of the primaryα(Al) phase and Al-Si eutectic phase. The nucleation rate was also enhanced, while the average crystal size dropped. Since increasing the cooling rate can promote the capture of the particles by the solid-liquid interfaces during solidification, it was beneficial for the uniform distribution of the particles. With the crystal size being fined and particle distribution improved, with increased cooling rate, the properties of the composites were enhanced.Through the study in preparation techniques and solidification behavior of the composites, the factors that influence the uniformity of reinforcement distribution in particle reinforced metal matrix composites made by stirring casting were analyzed. Several ways to improve the reinforcement distribution were proposed, which may provide some theoretical guides to the fabrication of particle reinforced Al matrix composites by stirring casting method.