Effects Of Melt Filling Conditions And Mould Cooling Condition On Microstructure Of Micro-Solid Solidification

Micro-solid solidification forming is a low pressure filling casting method of metal or alloy slurries with a low solid fraction of 3%～10% in the melt which are prepared by controlling the temperature of alloy melt and applying electromagnetic stirring or other vibration methods.In this paper,the effects of the melt filling conditions(the melt temperature,intensity of electromagnetic stirring and filling rate)and cooling condition in sand mold on the microstructure of ZL101 alloy of are studied.For the problem that the grain size in the micro-solid solidification microstructure is difficult to quantify and statistics,based on the Image-Pro Plus software,a new and more accurate method was put forward to quantitatively statistic-treat the number,area and perimeter of the non-dendrite grains of primary α-Al phase in the micro-solid solidification microstructure.The micro-solid solidification forming technology was first discussed in sand mold of test bar.Results show that,when the electromagnetic stirring parameters are 35Hz/180 V and the gate temperature of melt is 618℃,the refinement and non-dendrite effects of primary α-Al phase in ZL101 alloy are the best.Compared with the test bars of ZL101 alloy in sand mold by gravity casting,the strength of the bars by micro-solid solidification filling(with a delayed activation of 4s for electromagnetic stirring and filter mesh in the gate)is slightly increased from 164 MPa to 168 MPa,however,the elongation is significantly improved from 3.8% to 5.3%.The effects of melt filling conditions and cooling condition in mould on micro-solid solidification microstructure were investigated in sand step mould,and the formation mechanism of non-dendritic microstructure was also discussed.Lower melt temperature is one of the prerequisites to obtain non-dendrite microstructure in micro-solid solidification.The cooling rate of melt in the mould(corresponding to the wall thickness of a casting)is another key factor for the non-dendritic microstructure,which cannot be obtained at a higher cooling rate.Electromagnetic stirring is an important dynamic factor for the refinement and nondendrite of grains,depending on the superheat degree of melt.Solidification pressure can significantly refine grain,but promote the dendritic growth of grains.Two key conditions of non-dendritic microstructure by micro-solid solidification are obtained in this study: relatively low melt temperature that leads to an abundant nucleation during the filling of melt and slow cooling rate of melt in the mold that guarantees the uniform growth in form of spheres of the existed small crystals in the melt during solidification.