Casting cracks can be divided into hot cracks and cold cracks. So how to repair the gravity casting crack defect?

1. Gravity casting fracture

Thermal cracking occurs.

Usually, at the end of the aggregation, the metal forms at high temperatures close to the solid phase line. Its shape is characterized by short and wide defects, curved shape and oxidation in joints. Unreasonable casting scheme, serious shrinkage of alloy, shrinkage of core sand and unreasonable casting process may lead to hot cracking. Sulfur and phosphorus in steel will damage the heat resistance of steel, resulting in an increased tendency to hot crack.

Hot crack prevention

Reasonable adjustment of alloying elements (strictly controlling sulfur and phosphorus contents in steel), reasonable planning of casting scheme, adoption of synthetic method and innovation of sand making method are all effective measures to prevent hot cracking.

2. Gravity casting cold treatment

Cold cracks appear.

Cold cracking refers to the alloy being in an elastic state when cooled to a low temperature. If the strength of the alloy exceeds this temperature, cold cracking, or cold cracking, occurs. The defects of cold cracks formed at low temperature are small, continuous and straight, clean in the crack, and occasionally mild oxidation. Special-shaped parts, especially the castings with large thickness difference, especially the large thin castings, are easy to produce cold cracks.

Cold crack prevention

In general, the steps to reduce the internal stress of castings or the brittleness of alloys are to protect the formation of cold cracks. For example, iron and phosphorus in iron will significantly reduce the impact resistance of the alloy, increase brittleness, easy to lead to cold cracking, so smelting metal to strictly limit.

Supplementary situation and equipment analysis.

Casting glue: simple and rough casting. In general, the repair site does not require follow-up treatment and there are no special hardness requirements. The added value of this casting is low. Welding materials: more than 90% of the casting enterprises choose welding materials to solve the casting defects encountered in production. Due to the use of metal filling materials (welding materials are generally matched with casting materials), the welding performance can basically reach the standard of the base material, simple operation, high welding efficiency, has been recognized and trusted by many manufacturers. At present, there are a wide variety of welding equipment on the market, and the equipment used to repair defects is roughly as follows:

Welding machine: cast iron and steel castings are mostly used for welding. Liquid aluminum extrusion, aluminum alloy gravity casting, aluminum alloy casting.

Benefits: Maintenance, high efficiency.

Defect: welding spot hardness is too high, internal stress, easy to crack. Annealing is usually required to meet the processing requirements. Moreover, due to the limitation of welding conditions, it is easy to produce secondary defects such as porosity and slag inclusion. Argon arc welding: investment casting (alloy steel, stainless steel investment casting) and aluminum alloy die casting parts mostly adopt argon arc welding. Some mold manufacturers and repair manufacturers also use this welding machine to repair mold defects. Advantages: high welding efficiency, high accuracy than electric welding machine. There are many kinds of welding wire, including stainless steel and aluminum alloy products. Suitable for high strength welding.

Disadvantages: Used to repair defects and small defects (blowhole and trachoma). There are marks on the edge of the molten pool due to excessive impact. Welded steel parts have hard spots. Color castings or thin wall parts are easy to produce thermal deformation due to the influence of heat during welding. The technical requirements for operation are very high.

Cold welding machine: is a restoration technology just born in the beginning of the 21st century. Because the workpiece welding process produces very little heat, so it becomes a cold welder. After several years of development, welding machine application direction and technology has made great progress in the repair market has occupied a considerable share. According to the classification of repair products, there are: edM surfacing machine patch and repair machine.

Repair welding machine: the welding part (0.05-0.20mm) is pasted on the workpiece surface using the principle of instantaneous high frequency discharge, and the thickness of the paste is consistent with the thickness of the welding part. The welding quality depends on whether the discharge is uniform. It is suitable for the wear and repair of workpiece in out-of-tolerance machining, which has a certain impact on the mold market. The machine can also fill defects (such as trachoma) with welding powder (or processing waste) and repair them by discharging electricity. The color difference of the workpiece after repair is small. The disadvantage is that patching is slow.

The common disadvantage of cold welding machine is that its welding efficiency is higher than that of electric welding machine and argon arc welding machine, but its outstanding advantages in repairing millimeter level defects and machining defects make people choose cold welding.