Defect characteristics and detection methods
1.External shrinkage cavity
It is a depression exposed on the casting. The rough formation of the whole concave part of the surface is characterized by coarse crystallization, which occurs under the riser or at the transition position of the thick and thin section of the casting, or at the sharp corner of the intersection of the two walls. It can be found after external inspection, sandblasting or oxidation treatment
2.Internal shrinkage cavity
It is a common internal hole in the thick part of casting. It is found by X-ray and machining that the hole shape is irregular, but the internal surface is generally smooth.
It is found by X-ray fluoroscopy and machining that the area near the shrinkage cavity of the thick part of the casting has coarse grains and loose and uncompacted structure.
When the casting is close to the location of shrinkage cavity, its grain is coarse.
Causes of formation
The main reason for the formation of internal and external shrinkage cavity, porosity and micro porosity is that the volume shrinkage of the alloy can not be supplemented during the transformation from liquid to solid
- Too high pouring temperature or small thermal conductivity and small heat capacity of the mold make the cooling of the casting too slow
- The design structure of the casting is unreasonable, the wall thickness difference at the joint of thick and thin sections is too large, forming hot spot, slow cooling and no alloy feeding
- The position of alloy liquid entering the cavity is not correct
- There is no feeding riser installed in the thick part, or the location is improper, or the amount of feeding liquid is not enough
- The poor exhaust performance of the mold makes the air cushion in the mold cavity have large back pressure, which blocks the flow direction of the alloy liquid and then solidifies the parts to be supplemented
Prevention methods and remedial measures
- If possible, improve the design structure and shape of the casting, such as reducing the wall thickness difference between the thick wall and the thin wall, thickening or thickening the stiffener, changing the thick part into the hollow structure, etc. to prevent the hot spot, try to make all parts of the casting cool evenly and solidify at the same time
- A feeding riser shall be installed at the thick and large part of the casting. The location and size of the riser shall ensure that the alloy liquid in the riser is higher than the location of the feeding part, the feeding amount is more, and there is enough feeding pressure head (usually ≥ 60mm)
- In order to achieve the solidification after the parts with large riser thickness, the insulating riser or heating riser can be used, or the riser can be heated
- Pour the alloy liquid from the riser or the thick part with riser
- Properly reduce pouring temperature and pouring speed
- After pouring, when the alloy liquid is full of the upper plane of the riser, the alloy liquid is poured from the riser again
- Place chill iron or quench materials (such as zircon and magnesia) with large heat capacity and good thermal conductivity in places where shrinkage cavity and looseness are easy to occur in the thick and large parts of the casting, so as to cool the parts first and obtain the feeding of alloy liquid first