Taking AZ91D magnesium alloy as experimental material,the rheo-diecasting process was implemented by combining the self-developed taper barrel rheomoulding(TBR) machine with high pressure die casting(HPDC) machine.Microstructura characteristics of the rheo-diecasting components were investigated in different processing parameters.Microstructural evolution and solidification behavior of the semisolid slurry during the rheo-diecasting process were discussed,and tensile properties of the components were studied as well.The results show that,with the rotation speed of the internal taper barrel increasing,the microstructure of the components becomes fine with solid particles nearly spherical and uniformly distributed on the matrix.When the rotation speed is 700 r/min,the primary α-Mg particles have an average size of about 45 μm and a shape factor of about 0.81;the primary α-Mg particles become round and homogeneous with shearing temperature increasing,but the average size is slightly larger The solidification of the alloy melt during the rheo-diecasting process is composed of two distinct stages:the primary solidification and the secondary solidification.Compared with the conventional die-casting process,the rheo-diecasting process would improve the tensile properties of the components,especially the elongation by 80%.
To investigate the effects of rotation speed and shearing time on morphology of semisolid AZ91D alloy,experimental work was undertaken using a twin-screw slurry maker.The results show that increasing the rotation speed and reasonable time can give rise to substantial grain refinement during continuous shearing stage,which can be attributed to the increasing of effective nucleation rate caused by the extremely uniform temperature due to high shear rate and high degree of turbulence.Comparing with low rotation speed at the same thermal condition,the analysis indicates that the microstructures obtained at high rotation speed are homogenous spherical and fine grains instead of dendritic or rosette and exhibits uniform distribution in the eutectic matrix.
A computational model coupling an electromagnetic model with a macroscopic heat and fluid flow model in semi-solid aluminum alloy slurry preparation by annular electromagnetic stirring(A-EMS) was developed.Effects of A-EMS processing parameters,such as stirring current,stirring frequency and stirring gap width,on macroscopic transport phenomena during the solidification were analyzed by commercial software ANSYS 10.0 with corresponding experimental verification.The results show that the magnetic flux density and the melt velocity increase and the temperature difference decreases as stirring gap width and stirring frequency decrease or the stirring current increases.The slurry with the fine and uniform globular grain structure can be gained by adjusting gap width,electromagnetic frequency and current,such as under the conditions of 10 mm of gap width,10 Hz of electromagnetic frequency and 50 A of current.The calculated results are in reasonably good agreement with the measured ones.
Semi-solid A356 aluminum alloy slurry was prepared by using serpentine channel pouring process, and the influences of the channel diameters and pouring temperatures on the semi-solid A356 aluminum alloy slurry were investigated. The experimental results show that when the channel diameter is 20 and 25 mm, respectively, and the pouring temperature is 640-680 ℃, the average diameter of primary α（Al） grains in the prepared A356 aluminum alloy slurry is 50-75 and 55-78 μm, respectively, and the average shape factor of primary α（Al） grains is 0.89-0.76 and 0.86-0.72, respectively. With the decline in the pouring temperature, the microstructure of semi-solid A356 aluminum alloy slurry is more desirable and a serpentine channel with smaller diameter is also advantageous to the microstructure imProvement. During the preparation of semi-solid A356 aluminum alloy slurry, a large number of nuclei can be produced by the chilling effect of the serpentine channel, and owing to the combined effect of the chilled nuclei separation and melt self-stirring, primary α（Al） nuclei can be multiplied and spheroidized finally.
