Casting defects are defects in castings that occur in the casting process. There are various defect phenomena such as cracks in castings, rough surfaces, and casting nests with porosities inside. Casting defects are classified into several types, each with different causes. In addition, since there are several pre-casting processes in the casting process, and even defects in the previous process lead to casting defects, there are complex cases where casting defects are not limited to one, and there are actually many complex causes.
Typical defects fall into six categories. This time, we will look at the causes and countermeasures of "casting cracking".
What is Casting Cracking?
During solidification, cracks occur in the final coagulation part of the casting body and at the tip of the molten metal.
In the case of jewelry such as rings with a high copper content as a discount (such as pink gold or red gold), cracks are not seen immediately after casting, but cracks may occur when the ring is resized and the ring is tapped with a ring bar, or when the metal is heated again with a burner.
Why Cracks in Castings occur, and What are the countermeasures?
There are three main causes of casting cracking. We will introduce the causes and countermeasures, respectively.
１ Cracking due to Cold Shut
At the point where molten metal flowing from multiple directions meets during casting, a cold shut occurs at the collision confluence surface due to the lack of temperature when the molten metal tip merges from the front of each other. It is characterized by a rounded tip at one end and a round shape at the other.
Increase the casting temperature of the casting mold or the casting temperature of the bullion to increase the solidification time during casting.
２ Intergranular Cracking ( including brittleness)
Polycrystalline metals (alloys) contain gases and foreign dioxide (intermetallic compounds and oxides) between crystal grains. Due to the low adhesion between crystals, cracks occur in places where intrinsic gases and impurities tend to accumulate, such as places where shrinkage stress is concentrated during solidification or where molten metal merges.
Change the melting point by installing a vent without gas, changing the thickness, mounting location, and angle of the sprue.
In alloys containing copper, copper is oxidized and deposition during crystallization of solidification and extruded into the surface layer of the cast. When copper content such as pink gold or red gold, segregation occurs during solidification, and brittleness may increase due to alloys containing intermetallic compounds and their crystal structure, and brittle fracture may occur due to external forces. Care must be paid to the alloy of gold and copper because it creates a regular lattice during solidification. In addition, it is necessary to be careful in the same way because reverse segregation is often caused even in bronze.
In the case of these alloys, the mold is immediately cooled after casting to reduce crystal growth by reducing the time to solidification. This prevents intergranular cracking during secondary heating.
Directional solidification is not secured, and the solidification part is broken so that it is torn by the stress when it solidifies and contracts due to a time difference.
Summary of Casting Cracking
We have introduced the causes and countermeasures, but we will summarize what to do when casting cracks occur.
- Increase the casting temperature of casting molds and bullion to increase the solidification time
- Install a vent for degassing
- Change the thickness, mounting location, and angle of the sprue
- For pink gold or red gold, cool the casting mold immediately after casting to reduce solidification time
There is not always one cause. Review all the items again to see how they work.