Study On Directional Solidification Process Of Aluminum Alloy Based On Water Cooling Multi-layer Shell Sand Mold

The solidification process of the water metal determines the microstructure and properties of a casting,while its solidification is mainly affected by the cooling mold.Therefore,controlling the cooling of the mold is one of the keys to improve the casting quality,and it is also the focus of research in casting aspect.The 3D printing technology has got rid of the complexity limitation of traditional sand mold.In this paper,a kind of multi-layer shell sand mold was designed by using sand mold 3D printing,and the watercooling directional solidification process by using this mold structure was proposed,aiming at obtaining better microstructure and properties compared with traditional sand mold casting.The heat transfer of multi-layer shell sand mold was systematically analyzed.The air layer improves the thermal insulation performance of the multi-layer shell sand mold,which is conducive to the thermal insulation of the upper part.And combined with the water cooling at the bottom,it promotes the directional solidification of the casting.1)The surface of the innermost sand mold shell is overheated in the rising stage of the cooling water.Then the part in contact with the cooling water will undergo boiling,and the lower part of the casting will undergo intense cooling.The sand mold shell will be heated by steam in the hollow layer to keep heat on the upper part of the casting.The combination greatly increases the temperature gradient in the height,which is conducive to the formation of directional solidification microstructure.2)After water is completely immersed in the sand mold and the boiling heat transfer stops,the casting sand mold will be in the stage of no phase change convection cooling.This will greatly improve the cooling efficiency after solidification and help to reduce residual stress and deformation.Numerical simulation of the cylindrical casting was carried out based on four-layer shell sand mold.It was found that the thermal insulation of the multi-layer shell sand mold was 1.8 times that of the single-layer shell sand mold,2.6 times that of the dense sand mold,and the temperature uniformity was the best.Higher water level rising rate led to delayed solidification interface,which leads to the sag rate of solidification front.And the sag rate increased with the increase of the water level overpassed the sodification front.By a varied rising rate of the water level,the level difference of the water and the solidifiction front can be kept constant,and the solidification front will be flat.This process is an ideal directional solidification status.A four-layer shell sand mold was designed for a cylindrical casting and printed by 3DP method.A water cooling directional solidification equipment platform was developed,and a water-cooled directional solidification experiment for the casting was conducted.The results show that: 1)The air layer owned better heat insulation effect than the dense sand mold,and the vapor generated by boiling heated the sand mold above the melt level,thereby delaying the solidification of the casting at the corresponding position.2)Under the watercooling directional solidification process,the solidification cooling efficiency of the casting was greatly improved.The cooling speed of the casting with diameter of 40 mm was increased by 5.8 times of natural cooling,and the casting with diameter of 20 mm was increased by 2.7 times.The average temperature gradient in the height direction reached 3.0?/mm,which realized directional solidification of castings.3)The casting with water-cooled directional solidification process owned refined and more round ?-Al phase grains,and the secondary dendritic arm spacing was smaller.The eutectic silicon particles were evenly distributed among the ?-Al dendrites.And their sizes were between 2?8 ?m,which was only one fifth of that under natural cooling.4)Under the water-cooled directional solidification process,the tensile strength of casting with diameter of 40 mm were increased by 56%,yield strength by 20%,elongation by 185%,and hardness by 62.6%;and casting with diameter of 20 mm were increased by 18%,3%,29% and 17.5% respectively.In summary,the water-cooled directional solidification process based on the multi-layer shell sand mold has brought more excellent microstructure and mechanical properties of the test castings.