No matter investment casting, sand casting or die casting, it is avoidless to meet casting defects. In last article, we introduced the causes & prevention methods of common investment casting defects, such as blowholes, shrinkage, slag holes, crack, cold shut, ect, which are caused by improper casting operations or inadequate experience. In this situation, to reach usage standard and reduce waste & loss, investment casting foundries will do repairing measures for investment castings. So what are the repairing methods?
Caster Glue Repairing
Caster glue is a kind of room temperature cured type adhesive, packed in metal hose. It is made from epoxy resin, curing agent, toughening agent, and active diluent and various high quality fillers. It features good heat resistance, corrosion resistance and low shrinkage. So it can be used for repairing of steel castings, iron casting, aluminum castings and other castings with defects such as gas holes, sand holes, pits, as well as the defects found during precision machining.
After repairing, purchasers may be concerned if the color of requiring area will be consistent with the casting itself, because it is an important symbol to reflect repairing quality of casting appearance. Despite the castings after repairing can meet the application requirements, if the appearance looks different, even if the casting defect repairing is perfect, people’ heart “defect” can not be eliminated and can’t trust to accept this kind of castings. In order to eliminate the psychological and make defective castings perfect, we select a kind of caster glue whose color is the same as grey iron castings, and combine the its physical and chemical properties, so that it is unable to identify after repairing.
Strength is a very important indicator of immanent quality, for domestic excellent caster glue, peel strength under normal temperature is 1000 N/cm2, the shear strength is above 3200 N/cm2, which is enough to meet the casting defect repairing. Besides, strength is also related to temperature, it also need to strictly follow the technology to maximize its sticky relay.
Many investment castings need to work at high temperature for a long time. For castings after repairing with caster glue, will them get deformed or can still maintain the original performance? Test proves that adhesive strength of caster glue will keep unchanged within 200 ℃ temperature. But such repairing methods is mostly used for simple investment castings with no machining and strength & hardness requirements.
More than 90% of investment casting manufacturers choose welding repairing to solve investment casting defects that encountered in the production process. Weld repairing is the working process that weld metal filling material to investment casting body directly. Welding material used will match the casting material, so the performance of welding section will basically reach the standard of casting body. For the simple operation and high efficiency. Weld repairing for investment casting defects is widely recognized and trusted by many manufacturers.
Generally used welding machines are:
Advantages: Can be used to repair large defects with high efficiency.
Disadvantages: After welding, the hardness of solder joints will be higher, and the inside section will exist internal stress, which is easy to produce cracks. So investment castings will generally need annealing heat treatment before further machining. And due to welding limitation, internal of investment castings after welding repairing will be easy to produce the secondary defects such as porosity, slag.
2. Argon arc welding machines
Argon arc welding machine is widely adopted for welding repairing of precision castings (alloy steel, stainless steel casting), and aluminum die castings . Some mold factories also use this welder to fix the mould defects.
Advantages: high efficiency and high accuracy. There are many kinds of welding wire, mostly used in stainless steel castings and aluminum alloy castings. Can be used for high welding strength.
Disadvantages: When it is used for repairing of minor defect (gas holes, sand holes), due to the high impact, the edge of the melting pool has traces. And because of high temperature, deformationcan be easily produced when welding. So it requires high operation technique.
After repairing of investment casting defects, we need to confirm that the defects have been removed effectively. To check the effect, it is better to do NDT inspection on the welding surface, such as PT or UT. As the hardness or other properties might be changed during welding, so if there is any heat treatment requirement, after repairing, the investment castings should go back to do heat treatment again for desired properties.