With the application of ductile iron in the global industrial field more and more widely, the quality requirements of ductile iron castings are more and more strict. Some safety related internal requirements of castings are even zero defects. Shrinkage cavity and shrinkage porosity, as the most common casting defects of ductile iron, not only do great harm to product performance, but also are difficult to carry out 100% non-destructive detection brings great difficulties to mass production. A lot of efforts of foundry workers have been devoted to exploring the causes of shrinkage porosity and finding solutions.
Although there is no consistent conclusion on the microscopic theoretical research on the cause of shrinkage porosity of ductile iron, macroscopically, as mentioned earlier in this chapter, the main factors affecting shrinkage porosity of ductile iron include casting modulus, metallurgical quality, mold strength and casting technology. All the measures to eliminate shrinkage porosity of ductile iron are aimed at one or more of the above aspects.
Among them, the modulus of casting is the design attribute of the product, which can not be changed in many cases; the metallurgical quality, including spheroidizing and inoculation, is greatly affected by the grade of raw materials, actual operation specifications and equipment capacity, and the metallurgical quality is derived from theoretical reasoning, which can not be quantified, most of which rely on experience and fluctuate greatly; the influencing factors of mold strength are relatively large It is easy to understand, but for sand mold foundry, this parameter is greatly limited by the production mode and equipment capacity, and can not be effectively improved in a short time; as the most stable link in the process, the casting process has become the main direction of everyone’s research, and the core idea is to eliminate shrinkage porosity defects by intervening the solidification sequence of castings by changing the process. At present, the method widely used in the world is to set cold iron in the thick part with high shrinkage porosity to speed up the solidification of the area, or to increase the number of risers, or even to use the heating riser with higher feeding efficiency to feed the hot spot of the casting. It can be seen that no matter which way, the casting process becomes too complex, because it is necessary to place cold iron and cold iron for each mold The heating riser greatly reduces the production efficiency and yield, and increases the production cost.
In this paper, according to the casting process development requirements of a brake caliper casting for automobile disc brake, combined with the production conditions of disa molding line in Denmark, through the research of mold technology, the scheme of how to avoid shrinkage porosity in the casting was explored, and the scheme was verified by the simulation of magma software and the actual pouring test. It is expected that the temperature gradient of the casting can be effectively intervened by changing the position of parting surface, the shape of riser and the distance between the casting and the heat preservation block, without introducing additional process (arranging cold iron and heating riser), and not affecting the production efficiency and cost. Under the existing conditions, only by changing the casting parting surface position, riser shape and the distance from the casting, can effectively intervene the temperature gradient of the casting, thus affecting the solidification sequence of the casting and avoiding the internal shrinkage cavity and porosity Defects.