Mixing & solidification Processing of AI-AI2o3 Composite

Abstract

In the present investigation, mixing has been carried out by dispersing fine alumina particles (Avg. size- 25 µm) in molten Al-Mg alloy using a turbine blade stirrer with impeller diameter 'd'. The molten alloy has a depth of H when kept in a crucible of diameter D and the stirrer is placed at height 'h' from its bottom. The dimensionless variables. d/D and h/H, have been varied to observe the dependence of optimum values of these variables on stirrer geometry. These studies are expected to contribute to optimum processing route for stircasting of composites. In the current investigation, it has been observed that a turbine stirrer incorporates higher aunount of particles in the ingot cast out of a slurry mixed by it as compared to a flat blade stirrer in the range of experimental parameters used. But the pore-particle ratio is also higher in the cast ingot processed by a turbine stirrer. In the cast ingots processed by a turbine stirrer the particle content increases when d/D ratio increased from 0.33 to 0.48 but a further increase of d/D to 0.62 did not change significantly the particle content. A similar trend in pore-particle ratio is also observed and the lowest poreparticle ratio was observed for d/D=0.62. For mixing a slurry of molten alloy with poorly wetting particles it appears that the recommendation of chemical engineers to keep the stirrer at h/H=0.35 may not work as well from the point of view of particle content in the ingot cast with the slurry. By enhancing h/H to 0.5, the particle content increased significantly and a lower pore-particle ratio was also achieved.The microstructural evidence clearly indicates formation of bubbles with particles adhering to it and even their coalescence during processing. Some of these bubbles are eventually invaded by the molten alloy. When the manner of stirring is changed from continuous four minutes to an interrupted one of two minutes each intervened by a two minute period of cessation of tirring, it has been observed that the particle content at the top reduces but that at the bottom increases making particle content more uniform along the length of the ingot. This has been attributed to enhanced floatation of bubbles containing particles and reincorporation of the latter. During interrupted stirring the porosity did not reduce as unuch as the particles and the poreparticle ratio at the top increased. Enhanced processing time may have contributed to 10increased dissolution of gases contributing to porosity and so3 a reduced extent of bubble-particle combine did not reduce the overall porosity as much as it was expected. When the stirring was interrupted fora longer time of 10 minutes the uniformity of the ingot in respect of particle content improved to an extent but the overall porosity increased significantly. These facts possibly indicate that most of bubbleparticle combines form during mixing and not during solidification.