Elimination Technology of Internal Stress in Precision Castings

Elimination Technology of Internal Stress in Precision Castings

During the solidification and cooling process of precision casting, internal stress is inevitably generated in the casting due to factors such as hindered shrinkage, different cooling rates in different parts, and volume changes caused by structural transformation. Internal stress in castings can cause cracks or deformations during storage, subsequent processing, and use, reducing the dimensional accuracy and performance of castings, and even leading to their scrapping. Therefore, for castings with significant casting residual stress, especially large castings with complex shapes, internal stress relief treatment should be carried out before mechanical processing. Today, precision casting manufacturer Yufeng Metal will discuss in detail the techniques for eliminating internal stresses in precision castings.

Vibration elimination of internal stress in steel castings

1、 Vibration aging parameters

1. Determination of vibration frequency: Excessive vibration frequency can generate unbearable noise on the production site. Generally, cantilever clamping is used, with a resonance frequency of 33.2Hz and low noise. In order to reduce noise, a ZSJ-1 vibration exciter with very low noise was designed to clamp on the workbench. Each shift can perform vibration aging on 14-18 castings on the workbench, with a vibration frequency of about 30Hz, a current of 2A, a voltage of 150V, and one end clamped for 15 minutes each.

2. Current monitoring and internal friction test of vibration aging: During vibration aging treatment, the motor current of the vibration source is 0.2-0.3A lower than before vibration. However, after the current drops to a stable value, the current no longer decreases when continuing vibration. At the same time, the resonance peak of the casting shifts and becomes steeper, and the shape becomes sharper. The steepening of the resonance peak is caused by a decrease in internal friction of the casting after vibration aging. For this purpose, the natural frequency and internal friction Q-1 of the casting before and after vibration aging treatment were tested using attenuation method.

2、 Vibration aging mechanism

Vibration causes the internal stress of the casting to tend towards equilibrium: The internal stress of the casting is the main cause of deformation, and only by eliminating the internal stress can it effectively prevent the deformation of the casting and limit it to a smaller range. However, experiments have shown that vibration aging cannot significantly reduce the internal stress of castings, let alone reduce the internal stress of castings to zero. Some castings only reduce their internal stress by 20% -30% after vibration aging, but their dimensional stability is good. A possible explanation is that the internal stress of the casting is uneven, and it is this uneven stress that causes the overall deformation of the casting. Vibration can cause the internal stress of castings to become more uniform, promoting the size of castings to become more stable.

Precision machining