Almost more interesting are the microstructures that occur under the different processing conditions. For example, the figure below shows the microstructures of two specimens collected at the same fraction solid. The bottom one was cooled more slowly, thus stirred longer, than the top one. The lower cooling rate and longer stirring time produce larger particles. Yet the fluidity tests show that longer stirring increases fluidity.
Several factors explain that seeming contradiction.
For one thing, the larger particles are spherical, while the smaller ones are irregular in shape, thus less able to slide by one another. In addition, the smaller particles have a higher surface area and are more likely to interact and stick together than the larger particles are. The result is irregular agglomerates that move less easily than the large spherical particles do. In those experiments, the researchers completely melted the aluminum alloy and then worked with the semisolid as it formed during cooling. But that approach replicates only one procedure used by industry.