1. The hot cracking of copper casting alloys is the tendency of the alloys to crack at high temperature. It is one of the common casting defects of some copper alloy castings. The shape of the hot crack is usually tortuous and irregular, occurring along the grain boundary. The surface of the crack is often strongly oxidized and has no metallic luster. According to its position on the casting, the hot crack can be divided into outer crack and inner crack. The outer crack usually begins at the surface irregularities, sharp corners, changes in section thickness, and other similar places where stress concentration can occur. It gradually extends to the inside of the casting, the surface is wider and the inside is narrower, and sometimes it penetrates the whole casting section. Usually does not extend to the surface of the casting. The surface of the crack is not smooth, often have a lot of bifurcation, less oxidation than the outer crack.

It is generally believed that the hot cracking of the alloy occurs during solidification, i. e. when most of the alloy has been solidified but there is a small amount of liquid between the DENDRITES. At this point, the linear shrinkage of the alloy is large, and the strength of the alloy is low. If the mold blocks its shrinkage, the casting will have a greater shrinkage stress on the hot spot, when the strain at the hot spot is greater than the alloy’s allowable strain at that temperature, the hot crack will occur. However, for the same alloy, whether the casting produces hot crack or not often depends on mold resistance, casting structure, pouring process and other factors. In the process of casting condensation, any way that can reduce the shrinkage stress and improve the high temperature strength of the alloy will help to prevent the occurrence of hot cracking. Therefore, in practical production, effective measures such as increasing the yield of casting mold, improving the structure of casting mold, improving the place where the alloy is introduced into casting mold, and reasonably setting reinforcing ribs and cooling are usually taken to avoid hot cracking. The tendency of hot cracking depends on the properties of the alloy. In general, the greater the difference between the temperature at which the dendrite skeleton begins to form during solidification and the temperature at the end of solidification, and the greater the shrinkage of the alloy during this period, the greater the tendency of the alloy to crack.

2. The poor pressure resistance of pressure-resistant casting is when the pressure is applied to the die casting, the pressure leaks out from the inside or outside of the casting, which may appear as oil leakage, gas leakage, water leakage, etc. , the reason may be the combination of various defects resulting in bad. The casting is immersed in water, the cavity of the casting is filled with Compressed air, and the Compressed air passes through the channels formed by the defects on the inner surface, the inner surface and the outer surface of the casting to the water surface.

In the copper alloy casting, the density of the alloy is required to be high, so improving the hydraulic resistance of the copper alloy casting to avoid leakage is the key to production. It has been proved by production that the hydraulic pressure resistance of the casting can meet the requirement as long as the columnar or fine equiaxed grain is obtained in the section. However, columnar crystals are easy to be obtained under the condition of Rapid Cooling and difficult to be obtained under the condition of general sand mold casting. It is very obvious effect to obtain columnar crystal structure by raising pouring temperature for concrete casting. The casting temperature of copper castings is low, the alloy structure is loose, the casting leakage is serious, “Tin sweat”, hot crack often occurs. When the pouring temperature was raised, the structure of the casting was compact columnar crystal, and the qualified casting was obtained.

Source: Copper Alloy Casting

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