Design Of Twin-screw Semi-solid Slurry Mixer And FEM Analysis Of The Flow And Temperature Field Of Al-Si Alloy

With a series of advantages such as low mass, good abrasion and corrosion resistance and low thermal expansivity, hypereutectic Al-Si alloy is a type of high quality material used in manufacturing precision wear parts like pistons, engine cylinder liners and automobile brake discs. Hypereutectic Al-Si alloy prepared by traditional melting and casting process has a lot of dendrites, which results in enlarging grain size, severely reducing the mechanical properties and cutting characteristics and limiting its development and application. Some studies show that semi-solid slurry with solid phase content between30%and40%can obtain fine metallographic structure of lower grain size and higher sphericity after solidification. So if we can design a device, pour the high temperature liquid Al-Si alloy into the inlet, by adjusting parameters of the device, make the Al-Si alloy melt outflowing from the outlet contains between30%and40%solid phase, then we can obtain hypereutectic Al-Si alloy with fine metallographic structure.According to the idea above, we designed twin-screw semi-solid slurry mixer, and selected Al-20%Si (wt.%) as research object. Then used FEM to simulate the temperature and flow field of Al-Si alloy melt in cooling process in the mixing tank, and studied how the three parameters (rotating speed of twin-screw, cold water flow and incline angle of mixing tank) affect the temperature and flow field of Al-Si alloy melt. Through orthogonal experiment we analyzed the influence of the three parameters above on grain size, in order to determine the optimal experimental parameter, and prepare Al-Si alloy samples with grain size lower than50μm.The Simulation results show that the larger of the cold water flow, either the less of the angle that the mixing tank inclines, the greater of Al-Si melt’s temperature drop. Rotating speed of twin-screw affects the temperature field of Al-Si melt a little. Temperature of Al-Si melt decreases from inlet to outlet gradually. Melt at bottom of mixing tank has a higher temperature than that at top of the tank. Isotherm bulges towards the direction which Al-Si melt flows to. Velocity of Al-Si melt from inlet to outlet decreases at first, and then increases, and reaches the highest temperature at outlet. When the solid phase content at outlet is between30%and40%, the grain size of samples we gained is also lower, which indicates that the simulation results is in good accordance with the experimental results.The analysis of orthogonal experiment shows that cold water flow is the main factor influencing grain size, then the rotating speed of twin-screw. Incline angle of mixing tank has the least influence on grain size. When cold water flow and incline angle of mixing tank increase gradually, both of their grain size decrease first, and then rise up. When rotating speed of twin-screw increases, grain size decreases and sphericity increases gradually, which indicates that twin-screw with a higher rotating speed can effectively smash dendrites in Al-Si alloy melt.By analyzing the orthogonal experiment, we found the optimal experimental parameter as follow:rotating speed of twin-screw at1400r/min, cold water flow at0.2kg/s and incline angle of mixing tank at15°. Under this parameter we got an Al-Si alloy sample with a grain size of48pm, which met our expectation nicely.