Haolong Shangguan

The hollow sand mold can affect the cooling of the casting during the solidification process. A chimney structure of a hollow sand mold that mainly consists of a two-layer shell with through channels was proposed. The design method was proposed for the chimney structure. Due to the chimney effect, cool air can enter the chimney from the bottom entrance, and hot air flows out from its top opening. Air transfers heat from the sand mold and forms a temperature gradient from bottom to top. A hollow sand mold with a chimney structure was applied to the wedge plate casting. During the solidification process, natural cooling and forced air cooling through open channels were applied. The effects of the chimney on the microstructure, mechanical properties, and residual stress of the castings under these two different cooling conditions were analyzed and compared with those of the casting in a dense mold. During the solidification stage of the castings, the chimney structure prolonged their solidification time. After solidification, the chimney structure under forced air condition increased the cooling rate of the castings by 61% and the temperature gradient along the vertical direction by a factor of three. The Si content in the Al matrix increased by 20.4%, and the tensile strength increased by 32.66%.

The author-proposed skeletal sand mold, which mainly includes a shell, air cavities and a truss support structure, has been experimentally proven to be very useful in controlling the cooling of casting at local areas and at different periods of the casting process. The modeling and simulation of the casting process using a skeletal sand mold were systemically analyzed. Complicated casting/mold and mold/air boundaries, and the thermal and mechanical behavior of the skeletal sand mold during the casting process were highlighted. A numerical simulation of the casting process of a stress frame specimen using a skeletal sand mold was performed. The temperature, stress and displacement fields of the casting and skeletal sand mold were obtained and compared with those using a traditional sand mold. The simulated results were validated with experiments. Using the skeletal sand mold, the cooling rate of the casting can be greatly improved due to the significant heat release from mold surface to environment. The residual stress and deformation of the casting can be reduced because of the decreased stiffness of this kind of mold. Although the skeletal sand mold is susceptible to cracking, it can be avoided by filleting in the conjunctions and increasing the shell thickness.

3D printing technologies have been used gradually for the fabrication of sand molds and cores for castings, even though these molds and cores are dense structures. In this paper, a generation method for lattice-reinforced thickness-varying shell molds is proposed and presented. The first step is the discretization of the STL (Stereo Lithography) model of a casting into finite difference meshes. After this, a shell is formed by surrounding the casting with varying thickness, which is roughly proportional to the surface temperature distribution of the casting that is acquired by virtually cooling it in the environment. A regular lattice is subsequently constructed to support the shell. The outside surface of the shell and lattice in the cubic mesh format is then converted to STL format to serve as the external surface of the new shell mold. The internal surface of the new mold is the casting’s surface with the normals of all of the triangles in STL format reversed. Experimental verification was performed on an Al alloy wheel hub casting. Its lattice-reinforced thickness-varying shell mold was generated by the proposed method and fabricated by the binder jetting 3D printing. The poured wheel hub casting was sound and of good surface smoothness. The cooling rate of the wheel hub casting was greatly increased due to the shell mold structure. This lattice-reinforced thickness-varying shell mold generation method is of great significance for mold design for castings to achieve cooling control.