Effects of Cooling Rate and Grain Refining on Constituent Phase Particle Size in As-Cast 3004 Alloy

The directional solidification technique has been used to investigate the effects of cooling rate and grain refining on constituent phase particle size in as-cast 3004 alloy. Within the cooling rate range (0.10–28°C/s) obtained, dendrite arm spacing and particle size (area and length) in grain refined and nongrain refined castings are inversely proportional to cooling rate to some exponent, n, in accordance with the following empirical relationship:X = a T-nor directly proportional to the local solidification time to the same exponent, n, in accordance with a similar empirical relationship:X = b t_fnwhere X is the average secondary dendrite arm spacing or particle size in Jim, T is the cooling rate in °C/s, t_f is the local solidification time in seconds, a and b are constants that depend on system parameters such as the diffusion coefficient in liquid, solid-liquid interfacial energy and the equilibrium freezing range of the alloy.Particle surface area per unit volume and interparticle spacing decrease with an increase in cooling rate. At cooling rates of less than l°C/s, average constituent particle size in a nongrain refined casting is significantly larger than that in a grain refined casting. Particle size distribution becomes bimodal at cooling rates above l°C/s and 14.7°C/s in the nongrain refined and grain refined castings, respectively. Particle size in both castings increases linearly with an increase in the dendrite arm spacing. This is provisionally attributed to the fact that particle size in castings, like dendrite arm spacing, is controlled primarily by growth conditions during solidification of the melt.