Analysis On Thermodynamics And Dynamics Of The Microstructure Solidfied Under Centrifugal Pressure

As integral pressure, centrifugal pressure also changes the thermodynamic and dynamic conditions of solidification process. But it is a gradient pressure. Different centrifugal radius and rotation speed have different centrifugal pressure, so it has specific characteristics for the microstructure evolution.For the contractive metal and alloy during solidification, centrifugal pressure leads to the metal and alloy melting point increase and that will benefits to solidification. With the increase of centrifugal radius and rotation speed, the melting point and undercooling are increased. When rotation speed increase to 8000 rpm and centrifugal radius is 2 meters, the undercooling of Al,Ti,Mg,Cd,Cu,Sn and Zn are 227.984,187.386,178.756,847.96,368.004,326.132,401.872K respectively. All of them exceed the undercooling for homogeneous nucleation, except Ti and Mg. The more increase of rotation speed and centrifugal radius, the more decrease of critical nucleation work and nucleation radius. This is benefit to the solidification of the outside alloy and metal. For example, when rotation speed increase to 6000 rpm and centrifugal radius is 2 meter, the critical nucleation radius of the outside Sn is 22% of the inside Sn, and it's critical nucleation work becomes one fourth; the critical nucleation radius of the outside Cu is 19.4% of the inside Cu, and it's critical nucleation work becomes one fiftieth. When rotation speed and centrifugal radius are small, the effect of gravity can not be omitted.With the increase of rotation speed and centrifugal radius, Centrifugal pressure has special effect on the solidification dynamics: Diffusion coefficient is decreased, when rotation speed is 8000rpm and centrifugal radius is 2m, the outside diffusion coefficient of Ti and Al castings are 1.75% and 0.76% of the inside Ti and Al casting respectively; Solid solubility and the degree of pressure undercooling are increased correspondingly; Crystal activation energy is increased, but thermodynamic barrier energy is decreased, and because of combined action, there is a critical rotation speed and centrifugal radius, which will leads nucleation rate to maximum; Grow rate is decreased, and because of crystal grow rate and rate of nucleation, there is another critical centrifugal...