Study On Apparent Viscosity Of Semi-solid Alloy Melt

Apparent viscosity is an important expression parameter to describe the rheological behavior of semi-solid alloy melt. At present, rheological behavior of semi-solid alloys are mainly described by phenomenon rheological models via experiments. However, few researches on the rheological behavior during rheological forming of semi-solid alloys are investigated, especially the evolution of microstructure and apparent viscosity in the processes of preparing, delivering and cavity filling, etc. So, rheological forming process have not been simulated and predicted accurately yet. Apparent viscosity model of semi-solid melt, microstructure evolution during rheological forming, and the evolution of apparent viscosity with the time and forming were studied systemically in this paper.Semi-solid alloys rheometer apparatus has been developed in our lab via extensive studies and analyses of present testing methods of rheological behavior of semi-solid alloys. It can simulate mainly rheological process such as semi-solid melt preparation, delivering and cavity filling and measure apparent viscosity of semi-solid melt. It shows that the measuring error of the rheometer is less than 5% by experiments and theoretical calculation. Thus, it can provide reliable and accurate experimental data for apparent viscosity of semi-solid alloys. Metallographic quantitative analysis software was developed by VB and Matlab software based on the metallographic principle. It can smooth the boundary of particles according to the boundary later theory, break the connective particles each other by the actual shear condition, and pick out and remove little particles by threshold. In addition, the microstructure parameters of the effective solid fraction, particle size and shape, etc can be measured by this software.According to the rheology, incompressible fluid rheological and energy dissipation theory, there are three energy dissipation types for semi-solid alloys in pipe: the viscous friction between semi-solid metal and pipe wall, the collisions among the solid particles, and the liquid flow around particles. In addition the corresponding calculation model and energy dissipation equation are gained respectively, then the calculating model for apparent viscosity of semi-solid alloys melt was built up. The apparent viscosity of A356 aluminum was measured by coaxial double-bucket rheometer, the results indicated that the maximum relative error between theoretical value and measured value is 9.25%, and most error is from 2%-7%.The evolution with time of the microstructure parameter such as the effective solid fraction, particle size and shape in the processes of semi-solid melt preparing, delivering and cavity filling, etc was studied by rheological helical line extrusion forming and simulation forming by rheometer. It discovered the effect of rheological condition on the microstructure of semi-solid melt, and set up the calculating model for the evolution of apparent viscosity with time combined semi-solid melt rheological rule with the forming process. So, it provides the theoretical and actual production guide for simulation, forecast, control and defect prevention in semi-solid forming process.