How to Repair Defects in Precision Castings

Precision castings are metal parts manufactured through precision casting technology, with high dimensional accuracy and surface finish. However, during the production process, due to various factors, precision castings may exhibit various defects, such as porosity, shrinkage, cracks, deformation, etc. These defects not only affect the appearance quality of castings, but also affect their performance and even lead to the overall product being unqualified. Therefore, how to repair defects in precision castings has become an urgent problem to be solved.
1、 Types and causes of defects in precision castings
stoma
Porosity refers to the circular, elliptical, elongated and other shaped pores on the surface or inside of a casting. The generation of pores is mainly due to the incomplete discharge of gas in the metal liquid or the incomplete escape of gas in the mold, resulting in the metal liquid not fully filling the mold cavity.
Shrinkage cavity
Shrinkage refers to the irregularly shaped and unevenly sized pores on the surface or inside of a casting. The generation of shrinkage cavities is mainly due to the volume shrinkage of the molten metal during the solidification process, which cannot be supplemented, resulting in the formation of pores inside or on the surface of the casting.
crack
Cracks refer to the presence of fracture marks on the surface or inside of a casting. The occurrence of cracks is mainly due to factors such as unreasonable mold design, unsuitable casting process, and excessive shrinkage rate of metal materials, which cause the casting to crack under stress during the cooling process.
deformation
Deformation refers to the geometric shape of the casting that does not meet the design requirements. The occurrence of deformation is mainly due to factors such as unreasonable mold design, excessive thermal expansion coefficient of mold materials, improper gate speed and temperature control, which hinder the flow of metal liquid into the mold and form deformation.
2、 Methods for repairing defects in precision castings
For different types of precision casting defects, the following repair methods can be used:
Stomatal repair
For smaller and dispersed pores, the drilling method can be used to drill through the pores and remove internal residues. For larger or dense pores, repair welding can be used to fill the pores. During the repair process, appropriate welding materials and processes should be selected to ensure the appearance and quality of the castings.
Shrinkage repair
For defects such as shrinkage holes, repair welding, metal coating, mechanical processing, and other repair methods can be used. For larger shrinkage holes, repair welding can be used to fill them, and commonly used welding materials include iron based metals, copper based metals, nickel based metals, etc. For smaller shrinkage pores, metal coating method can be used for coating, and commonly used coating materials include epoxy resin, polyester resin, polyurethane, etc. For castings with high appearance requirements, mechanical processing can be used to remove shrinkage cavities and trim the contour.
Crack repair
For linear defects such as cracks, repair methods such as knocking out, top prying, gas cutting, and drilling can be used. Knockout method is suitable for shallow and short cracks, which are eliminated by knocking out local areas on the surface of the casting. The top prying method is to apply external pressure to open the metal on the other side of the crack, suitable for shallow and short cracks. The gas cutting method is used to melt and remove the metal on both sides of the crack through gas cutting, which is suitable for deeper cracks. The drilling method is to separate and remove the metal at both ends of the crack through drilling, suitable for shallow and short cracks. During the repair process, attention should be paid to protecting other parts of the casting from damage and deformation.
Deformation repair
For shape defects such as deformation, repair methods such as mechanical processing, pressure processing, and heating treatment can be used. The mechanical machining method is to trim the geometric shape and size of castings through mechanical cutting, and commonly used processing equipment includes CNC machine tools, machining centers, etc. The pressure processing method adjusts the geometric shape and size of castings by applying external pressure, and commonly used processing equipment includes correction machines, press machines, etc. The heating treatment method is to soften and redistribute stress on the material of the casting through heating, and commonly used heating equipment includes ovens, heat treatment furnaces, etc. During the repair process, attention should be paid to reducing the internal stress of the casting and preventing the generation of new internal stresses.
3、 Precautions for repairing defects in precision castings
When repairing defects in precision castings, the following precautions should be taken:
Before repairing, a comprehensive inspection and analysis should be conducted on the casting to determine the type and cause of defects.
During the repair process, appropriate repair techniques and tools should be used to ensure repair quality and safe use.
During the repair process, attention should be paid to protecting other parts of the casting from damage and deformation.
The repaired castings should undergo comprehensive quality inspection and performance testing to ensure that they meet the expected performance and quality requirements.
For some serious defects or irreparable castings, they should be scrapped to avoid affecting product quality and usage safety.
In summary, the repair of defects in precision castings is a complex and important process that requires different repair methods and processes for different types of defects. At the same time, attention should be paid to protecting other parts of the casting from damage and deformation during the repair process, ensuring that the repaired casting meets the expected performance and quality requirements.