CASTLE Rheo

The Rheo module simulates GISS technology which is an application of the process called Superheat Slurry Casting. In this process, instead of casting the alloy in a liquid state at a high temperature, GISS technology converts the liquid alloy into a semi-solid slurry at a temperature just above that of liquidus. The slurry maintains low viscosity and is able to fill the die at a significantly lower temperature, with undoubted benefits in terms of casting quality and savings in the process.

The solid fraction of the alloy, which can be controlled through the GISS unit, allows for greater control of the flow of metal into the die compared to the liquid alloy. A less turbulent flow allows to achieve a significant reduction of gas porosity. From a metallurgical point of view, the lower injection temperature reduces solidification shrinkage by limiting the formation of shrinkage porosity and generates higher solidification rates to the benefit of the crystalline structure of the casting.

Data input

The Rheo module is different from the HPDC standard since the user is required to enter the main parameters of use of the GISS technology:

  • dip time of the probe in the ladle
  • metal temperature when the probe enters the ladle
  • time between the GISS treatment in the ladle and the pouring into the shot sleeve

Once the alloy is selected, the interface displays the theoretical temperatures of liquidus and solidus, allowing the user to easily choose the optimal holding furnace temperature and metal temperature when the probe enters the ladle.

Solid Fraction Calculation

In GISS technology, the solid fraction is dynamic and increases after treatment in the ladle until the molten metal fills up the die. In fact, solid particles act as stable nuclei solidified within the liquid phase: the high density of fine solid particles will then grow non-dendritically during subsequent cooling in the die, yielding a nondendritic and globular microstructure. The software calculates the estimated value of the solid fraction (as a function of the chemical composition of the alloy, temperature, dip time, etc.) and provides the user with important information about the optimal process parameters.

Simulation Results

The Rheo module calculates filling and solidification considering the different metal solid fraction and viscosity conditions. Since the slurry has higher viscosity than liquid metal, the flow of the slurry into the die cavity is less turbulent and this results in less entrapment of gas.

Additionally, the difference between the metal temperature in the cavity and the solidification temperature is smaller, so the solidification time is shorter, and the shrinkage porosity is lower. Slurry has pre-existing fine solid particles, which effectively act as nuclei. So, after the slurry fills the die cavity, these solid particles will spontaneously grow larger to fill the die cavity so that shrinkage porosity can be significantly reduced. This results in less reject rate and higher part quality.