Microstructure refinement with forced convection in aluminium and superalloys

The cooling and average local solidification times were determined for slow solidifiation of Al-4.4 wt% Cu alloy under natural convection and under electromagnetically forced axisymmetric rotation during liquid cooling and solidification in graphite moulds. Cooling rates were measured within situ thermocouples. The conditions needed to stabilize the radial temperature gradient with rotation were established. The microstructure size decreased with increasing rotation, as did the local solidification times. The average grain and dendrite size without imposed rotation is coarser near the mould wall compared with the centre of the casting. This trend is reversed with imposed rotation. Rotation also led to a smaller spread of grain and dendrite size at any chosen height of the casting. These results are discussed in relation to existing theories, and several reasons for an improved heat transfer coefficient with rotation are presented. Forced convective solidification was then carried out for various shapes of integral investment cast Nimonic-90 alloy solidifying under modified conditions that prevented columnar grain formation. Similar results to those recorded for the aluminium case were obtained and are presented here. The major conclusion is that observations indicating a reduction of microstructure spacing during forced convection should also consider improved heat extraction at the mould-metal interface.List of symbols Gr Grashof number =gTZ ^{3 3}/ ³ - g _r acceleration in radial direction - g acceleration in direction - g _z acceleration inZ direction (gravity) - h heat transfer coefficient - k _l thermal conductivity of liquid - Nu _z Nusselt number =hZ/k _l - Pr Prandtl number =/ - Ra Rayleigh numberGr Pr - R radius of mould - Re _r Reynolds number =V ₀ R/ - T temperature - T temperature difference in radial direction - Ta Taylor number = ²4H ⁴ W ²/ ² - V velocity - W r.p.m. - thermal diffusivity - coefficient of volume expansion - viscosity - density Mr G. S. Reddy is also a post graduate student registered at the Banaras Hindu University, Varanasi, India.