Uneven chemical composition
Composition segregation: During the casting process, due to the complexity of the solidification process, chemical composition segregation is prone to occur in the casting. The phase change temperature and phase change products of alloys in different chemical composition areas are different, and the volume change during phase change is also different. This difference causes phase change stress between different parts.
Influence of impurity elements: The presence of impurity elements will change the phase change characteristics of the alloy. Impurities may hinder or promote the phase change process, causing different phase change behaviors in different parts of the alloy, thereby generating phase change stress.
Influence of casting phase change stress on casting quality
Leading to reduced dimensional accuracy
Phase change stress will cause uneven volume changes in the casting during the phase change process, resulting in changes in the overall or local dimensions of the casting and dimensional deviations. For example, in some precision cast parts, the dimensional changes caused by phase change stress may make the parts unable to meet assembly requirements and increase the scrap rate.
Causes cracking of castings
When the phase change stress exceeds the strength limit of the casting material at the corresponding temperature, cracks will appear in the casting. The generation of cracks will not only cause defects in the appearance of the casting, but more seriously, it will greatly reduce the mechanical properties and service life of the casting, making it easy for the casting to break from the crack when it is under load, resulting in failure of the entire component.
Causes uneven organization
Phase change stress affects the organizational transformation inside the casting, making the metallographic organization of various parts of the casting uneven. For example, in some alloy steel castings, phase change stress may cause coarse grains to form in some areas, while fine grains are formed in other areas. Different grain sizes and organizational morphologies will make the mechanical properties of the casting anisotropic, reducing the comprehensive performance of the casting.
Generates residual stress
If the phase change stress is not completely released after the casting is cooled, it will exist in the casting in the form of residual stress. Residual stress will cause the casting to be dimensionally unstable due to stress redistribution during subsequent processing and use, affecting the accuracy retention of parts. In addition, residual stress will also be superimposed on working stress, reducing the fatigue strength and stress corrosion resistance of the casting, making the casting more prone to fatigue cracks and stress corrosion cracking during use.
Casting shrinkage stress
Causes of shrinkage stress in castings
Liquid metal shrinkage
Volume change: During the cooling process from pouring temperature to solidification temperature, liquid metal will shrink in liquid form, and the volume will gradually decrease. When liquid metal flows and fills in the mold, if it is restricted by the mold, core, etc. and cannot shrink freely, stress will be generated inside the liquid metal.
Filling process: During the filling process, there is friction between the liquid metal and the mold wall, as well as the flow resistance of the liquid metal in the mold cavity, which will hinder the free shrinkage of the liquid metal, thereby generating shrinkage stress.
Solidification shrinkage
Phase change shrinkage: When metal solidifies, it changes from liquid to solid, and volume shrinkage will occur. For example, when pure metal solidifies, the formation of crystal structure will make the atoms more closely arranged, resulting in a decrease in volume. When alloys solidify, the shrinkage is more complicated due to the transition of multiple phases. If the shrinkage is hindered, stress will be generated.
Solidification order: For castings with complex shapes and uneven wall thickness, the solidification order of different parts is different. The part that solidifies first will hinder the shrinkage of the part that solidifies later, and the part that solidifies later will generate tensile stress on the part that solidifies first when shrinking, thereby forming shrinkage stress.
Solid-state shrinkage
Thermal expansion and contraction: After the casting solidifies, as it continues to cool to room temperature, the solid metal will shrink due to the principle of thermal expansion and contraction. At this point, the casting already has a certain strength and rigidity. If the shrinkage is restricted by the mold, core, the structure of the casting itself or other external factors, shrinkage stress will be generated.
Structural constraints: The structural design of the casting will affect its solid-state shrinkage. For example, castings with complex structures, such as those with reinforcing ribs, bosses, etc., are mutually constrained and cannot shrink freely during solid-state contraction, which will produce large shrinkage stress.
Influence of molds and cores
Poor yieldability: The yieldability of molds and cores refers to their ability to deform as the casting shrinks, thereby reducing the ability to hinder the shrinkage of the casting. If the yieldability of the mold and core is not good, such as using a material with greater rigidity or too high a compactness, it will seriously hinder the shrinkage of the casting, resulting in increased shrinkage stress.
Residual resistance: During the cooling process of the casting, adhesion or friction may occur between the mold and the core and the casting, which will increase the resistance to the shrinkage of the casting and generate shrinkage stress.
Impact of shrinkage stress of castings on casting quality:
Cause dimensional deviation
Shrinkage stress will cause uneven shrinkage of the casting during the cooling process, resulting in deviation between the size of the casting and the design size. For castings with high precision requirements, such as aircraft engine blades, dimensional deviation may make the parts unable to meet the assembly requirements and affect the performance of the entire equipment.
Castings with complex shapes have different shrinkage stresses in different parts, which will cause the casting to twist and deform, further increasing the dimensional deviation.
Cause casting cracking
When the shrinkage stress exceeds the strength limit of the casting material, the casting will crack. Microcracks will become a source of stress concentration, gradually expand in subsequent use, and reduce the bearing capacity of the casting.
Surface cracks affect the appearance quality of the casting, while internal cracks are difficult to detect and will cause sudden breakage during use, resulting in serious consequences. For example, cracks in the cylinder block of a car engine may cause engine failure.
