Castings with residual internal stresses are in an unstable state—thicker sections are subjected to tensile stresses, while thinner sections experience compressive stresses. In such a state, the casting will spontaneously deform to reduce internal stresses and achieve a more stable condition. As a result, the areas under tensile stress tend to shrink, while the areas under compressive stress tend to elongate, thereby reducing or eliminating the residual stress within the casting. Deformation often leads to reduced dimensional accuracy of the casting, and in severe cases, can render the casting unusable. Therefore, it should be prevented.
Types of Casting Deformation
Various types of deformation can occur in cast structures, and the modes of deformation differ depending on the nature of the casting. From a production standpoint, casting deformation can generally be classified into two main categories: bending deformation and flaring deformation. The deformation of castings in investment casting is mainly characterized by bending deformation. The deformation of castings in sand casting is mainly characterized by overall shrinkage or flaring deformation.
Factors Affecting Casting Deformation
For materials with a certain degree of plasticity (such as steel and non-ferrous metals), deformation can be corrected. However, for brittle materials like gray cast iron, deformation is difficult to correct. Once deformation occurs, internal stress can often only be reduced but not completely eliminated.
In summary, casting deformation is a complex issue involving numerous factors. It is not feasible to analyze deformation based on a specific part alone, so only some of the main influencing factors can be considered.
1. Thermophysical Properties of Metal Materials
The thermophysical properties of metal materials significantly influence deformation. Generally, the larger the linear expansion coefficient of the material, the greater the resulting plastic deformation and the more substantial the longitudinal and transverse shrinkage after cooling. For example, stainless steel has a higher linear expansion coefficient than low-carbon steel, and thus deforms more. Metals with high thermal conductivity, such as aluminum and its alloys, have high linear expansion coefficients and low yield strength at high temperatures, resulting in greater deformation.
2. Process Factors
In casting production, deformation can result from several process-related issues. For instance, wood used for making patterns may not have been dried properly or may have reabsorbed moisture, causing pattern deformation, which in turn leads to deformation of the sand mold and the final casting. Insufficient stiffness or strength of the pattern, uneven placement, or poor clamping between the upper and lower mold halves can also result in casting deformation.
Post time: Apr-29-2025