Why are dendrites no issue in Rheocasting?

Dendrites form in aluminium alloys during solidification due to the thermodynamics and kinetics of crystallization. Alloys release latent heat during solidification as they transition from the liquid to the solid state. The equilibrium solidification temperature represents the temperature at which this phase change occurs without undercooling.

However, in practical situations, undercooling can lead to a difference between the actual solidification and equilibrium temperatures. As the alloy cools and transitions from liquid to solid, nucleation occurs, creating small, stable clusters of atoms called nuclei. These nuclei start forming at the tool steel’s contact surface. The rapid solidification results in a smooth surface layer with good mechanical properties. The temperature flow is from the surface layer towards the cooling system of the die.

The latent heat of the solidification heats the surface layer while the melt cools down. With an increasing thickness of the outer layer, the temperature flow reverses. When the melt is cooler than the surface layer, the temperature flow reverses into the melt, and dendrites form. Anisotropy in the material influences the growth rate along different crystallographic directions, leading to preferential growth along specific paths. The diffusion of atoms from the liquid phase to the solid phase near the solidification front causes dendritic arms to extend. The presence of undercooling enhances diffusion rates, resulting in faster growth of crystals at the solidification front. The dendrites harm the mechanical properties and porosity levels.

Using Rheocasting helps to prevent the dendrites from forming.

The material is processed in its semi-solid state in Rheocasting, where the solid phase coexists with a liquid phase. The semi-solid slurry has a globular microstructure consisting of spherical particles of the solid phase surrounded by a liquid phase. This unique microstructure is far from the dendritic structure seen in conventional castings. The semi-solid slurry is formed by partially solidifying the material just below its liquidus temperature. The temperature level depends on the chemical composition of the alloy. The slurry temperature, therefore, varies during the production process while keeping the solid fraction constant. This process significantly reduces the undercooling, typically leading to dendritic growth during solidification. The semi-solid slurry in rheocasting is relatively homogeneous and isotropic, which results in improved mechanical properties and reduced anisotropy compared to conventional castings. Improved Flow Characteristics: The semi-solid slurry exhibits pseudoplastic behaviour, which flows more readily under mechanical stress. This property allows for better filling of complex moulds and reduces the likelihood of defects in the cast component.

Due to these benefits, rheocasting is particularly advantageous for producing high-quality castings with improved mechanical properties, reduced porosity, and enhanced performance in various industries. In the Rheocasting Workshop, you will learn more about Rheocasting and its implementation to acquire new profitable castings. Schedule a free Consultation Call down below now.