Study On Measurement Technology About Solid-phase Volume Fraction Of Semi-solid Magnesium Alloy

Thixomolding involves three very important technology:semi-solid slurry preparation, reheating, and Thixoforming. In reheating process, solid-phase volume fraction directly impacts on the thixotropy of the semi-solid slurry and also on the conveying of the slurry,and different technology needs different solid-phase volume fraction. Therefore, the monitoring and control of the solid-phase volume fraction is a very important process in the thixotropic casting process, which has a direct impact on the shaping performance of semi-solid billet. The purpose of this paper provides new methods for monitoring and controlling the solid-phase volume fraction in the production.Here the principles, characteristics and scope of application of resistance method, enthalpy method, expansion method and metallography method are analyzed,and new methods are put forward such as retraction rate method,"softening point"method and temperature-time method.In this paper, the concept of retraction rate and"softening point" are defined and the principle of the solid-phase volume fraction determination is described in detail.The factors and laws of the solid-phase volume fraction and the qualitative relationship between them are studied . In addition, the heating voltage on the "softening point" are also studied Retraction rate method and "softening point" method is designed based on the effect of the solid-phase volume fraction on Thixotropy.The solid-phase volume fraction is smaller, thixotropy is better and retraction rate is greater. In Thixomolding thixotropy directly impacts on the shaping performance of semi-solid billet, and retraction rate and "softening point" can be a direct response to thixotropy.Therefore, the two methods are suitable for monitoring and controlling solid volume fraction in Thixomolding.When the temperature of the alloy unchanges in the semi-solid temperature range, solid-phase volume fraction minishes with time postpones. Time and solid phase volume fraction curve can be got by metallography method at certain temperature. In the process of practical application, solid-phase volume fraction can be got by checking time and solid phase volume fraction curve. This method is suitable for semi-solid slurry preparation monitoring and controlling the solid-phase volume fraction.The relation between linear expansion coefficient and temperature of AZ91D magnesium alloy is studied by experimental data of the temperature and expansion.