Defects of shrinkage porosity in ductile iron castings

Nodular graphite cast iron castings due to its characteristics of the paste solidification of the production of cast iron due to poor shrinkage often produce shrinkage, shrinkage and other defects. In order to predict these defects in the casting process, the numerical simulation of casting process was carried out at home and abroad in the early years. The casting process numerical analogy using numerical simulation technology, the actual casting analogy formation process in the virtual environment, including the liquid metal filling process, solidification process, stress forming process, the main factors to determine the degree of influence in the forming process, organization, performance prediction and possible defects, provide the basis for reducing optimization of process waste.

In 1962, Denmark's Forsund was the first one to adopt the solidification process of electronic computer analog castings, and then the United States, Britain, Germany, Japan, France and so on have carried out the research in this area. China started late in 70s, the Dalian University of Technology, Shenyang Research Institute of Foundry, the first in China to carry out research on the technology, and in 1980 published a research report (Guo Keren et al., digital analogy, solidification process of large castings of Journal of Dalian Institute of technology, 1980 (2) 1 - 16; Shenyang Research Institute of Foundry, casting solidification thermal field of electronic computer analogy, 1980 (1) 14 - 22, then in Colleges and universities in China to invest a lot of manpower to carry out this research.

The numerical analogy consists of three parts: preprocessing, computing and postprocessing, including the establishment of geometric model, grid division, boundary condition (initial and boundary conditions) determined by numerical calculation, the calculation results show that the processing and graphics. The basic methods of numerical analogy are finite difference method, finite element method and boundary element method.

Numerical simulation of solidification process, heat transfer analysis of the main casting process. Including the selection of numerical calculation method, latent heat treatment, the prediction of shrinkage and shrinkage prediction, the heat transfer of the casting and the mold surface.

Numerical simulation of flow field involves the transfer of momentum, energy and mass, which is more difficult. The use of numerical solution techniques are MAC, SAMC, AOF - SOLA and SOLA - MAC method.

Casting stress analogy, this research carried out late, mainly to carry out the stress analysis of the elastic plastic state, at present there are Heyn model, elastic plastic model, Perzyna model, uniform internal variable model, etc..

Organizational analogy, is still in its infancy. Macroscopic, meso and micro points of analogy. Can calculate the number of nuclei, the analysis of the type of initial crystal, dendrite growth rate, the analogy of the organizational change, the prediction of mechanical properties. At present, there are deterministic models, such as Monte, Cellular, Automaton and other statistical methods, such as phase field model, etc..

Computers and their applications are rapidly developing in the field of technology, casting as one of the important industrial areas, should strengthen investment. Research and development of computer application in Foundry Research and production fields, completely change the past "open type, closed pouring" state, the application of computer will also promote the development and application of ductile cast iron.