Design And Application Of Sand Mold With Cavity Structures

The quality of castings depends on their cooling speed,so it is of great significance to study the cooling control of castings.There are some traditional methods to control their cooling rate,but rare studies are about the applicatiton of cavity structure in the field of casting.Air cavity structure is of good insulation effect,and the cooling medium can be pumped into it to improve the cooling capability.With the development of 3D sand printing technologies,the molding of sand mold with air cavity structure has become a reality.The effect of the air cavity structure in sand mold on the control of cooling rate of castings is studied in this thesis,which is of great engineering significance for cortrolling the quality of castings.The cavity structure is designed for the sand mold for controlling cooling rate of castings.Because of the low thermal conductivity of the air in the cavity structure,the cavity structure has the effect of thermal insulation,and can be designed for risers or thinwalled areas of a casting.When cooling medium is circulated into the air cavity structure,the casting performance can be improved and the residual stress of different areas of the casting can be achieved.Considering the structural strength of sand mold and avoiding air flow in the air cavity structure,baffle or honeycomb support structures were put forward.The designed sand mold with air cavity structure can be directly used for 3D printing.The influence of the thickness of the air cavity,the distance between the air cavity and the riser,the layer number of air cavities and the pouring temperature on the thermal insulation effect of the air cavity are studied by numerical simulation.The results show that the air cavity can effectively reduce the heat flow and delay the solidification of the molten metal in the riser compared to the dense mold.The radiant heat flux is sensitive to the temperature,indicating that the structure is suitable for the metals with low melting temperature.In order to achieve good insulation effect of air cavity on riser for aluminum alloy castings,the optimum layers of cavity structures are 3 ~ 4 layers,the thickness of the air cavity is 10~15mm,and the optimum distance between the air cavity and the riser is 5~10mm.The sand molds with air cavity structure for actual castings are designed and printed by 3D printing technology.For stress frame and the bar shaped castings,the air cavity structure is designed surrounding the riser to verify the insulation effect.The experimental results show that the three layers of air cavities can prolong the solidification of the molten metal in the riser by 30%,and the shrinkage area in the riser is moving towards the top of the riser.Open air cavity structure is used for air blowing during the solidification process to increase the cooling rate of one thin rod of the stress frame casting,the experiment results show that the the secondary dendrite arm spacing of the microstructure of the thin rod is reduced and the hardness of the thin rod is slightly improved.After solidification of the casting,the air cavity structure is sealed,the experimental results show that the temperature difference between the thin rod and the thick rod were decreased and the residual stress of the thin rod surrounded with three layers of air cavities was reduced by 30%.