（1） Realize sequential solidification
“Sequential solidification” is the basic process principle to prevent shrinkage cavity and porosity of castings. The meaning of sequential solidification is to create conditions to make the part away from the riser solidify first, the casting solidifies towards the riser, and the alloy in the riser solidifies finally. The realization of sequential solidification can ensure the continuous supply of molten metal in the riser during solidification and make the casting compact. The shrinkage defects occurred in the solidification process of the casting are transferred to the riser.
The traditional sequential solidification theory requires three conditions: ① the solidification time of liquid metal in the riser should not be less than that of the casting; ② There shall be enough liquid metal in the riser to supply the shrinkage of the casting; ⑨ During the solidification process of the casting, there is always a feeding channel connected with the riser, and a certain expansion angle of the feeding channel is required Φ。
The advantages of sequential solidification are: strong adaptability. In principle, all kinds of alloy castings can be applied. The riser can effectively eliminate the shrinkage casting defects. Under the conditions of smooth solidification interface, shell solidification and rough interface solidification, the shrinkage casting defects can also be prevented. For most thick wall metal parts (except gray cast iron, nodular iron and vermicular iron), the principle of sequential solidification is often used to obtain sound castings.
The disadvantages are: there is a large temperature difference during solidification of the casting, the internal stress is large during cooling, and it is easy to warm and deform; Riser and metal subsidies consume metal and reduce the yield of casting process; The cost of cutting riser removal subsidy is high, which increases the casting cost; When the alloy solidifies in paste or sponge, the effect of eliminating shrinkage casting defects with riser is not good.
（2） Realize simultaneous solidification
All parts of the casting shall be solidified at the same time, without feeding channel and riser. The design purpose is to disperse shrinkage porosity (if any) on all sections to the greatest extent, so as to reduce the harm to the performance of castings and obtain qualified castings.
The advantages of simultaneous solidification are: small internal stress, small deformation and few cracks; Saving metal and high yield of casting process; The modeling is simplified, the workload of cleaning castings is reduced, and the production cost is reduced. The main disadvantage is that castings often have axis shrinkage and casting defects.
（3） Practical riser feeding method using eutectic expansion of cast iron
Eutectic graphitization expansion exists in nodular cast iron, gray cast iron and vermicular cast iron, which plays a certain role of self feeding in the solidification process of castings. Taking advantage of this feature, different feeding methods are used according to the metallurgical quality of cast iron, the size of casting modulus (cooling rate) and the stiffness of mold. The modulus of casting is above 2.5era, and high hardness mold and no riser casting are used; If the modulus of the casting is within the range of 0.5-2.5cm, the high hardness mold shall be used, and the pressure riser shall be used; If the modulus is in the range of 0.5-2.5era, green sand or shell mold shall be used. Because the mold is soft, the pressure control riser shall be used. Thinner castings can be fed by gating system. Iron castings do not have to follow the principle of sequential solidification or simultaneous solidification. According to different conditions, dense castings can be obtained by using eutectic expansion without riser or very small riser.
（4） Pressurized feeding
Placing the mold in the pressure chamber and closing the pressure chamber immediately after pouring can make the casting solidify under the action of pressure, which can eliminate or reduce the micro shrinkage casting defects. The earlier the pressure is built, the higher the pressure is, and the better the feeding effect is.
（5） Eliminating internal defects by hot isostatic pressing
Hot isostatic pressing (hip) is a process in which the pressure is transferred to the surface of the casting equivalently from all directions through inert gas medium under high temperature and high pressure, and the internal pores are closed and disappear through the process of creep diffusion combination of metal. Hot isostatic pressing process can eliminate casting defects such as shrinkage cavity, porosity, porosity, crack and air hole, improve the performance and reliability of castings. Especially for castings with high performance requirements, such as turbine blades for aircraft, the hidden danger of accidents can be reduced after hot isostatic pressing treatment.