Analysis of casting cracking on cast copper parts
1. Cracking on cast copper parts
Hot cracking of alloys refers to the tendency of alloys to form cracks at high temperature. It is one of the common casting defects of some copper alloy castings. Usually, the shape of hot cracking is tortuous and irregular, and it occurs along the grain boundary. The cracked surface is often strongly oxidized and has no metallic luster.
According to its position on the casting, hot cracks are often divided into external and internal cracks. External cracks often begin at irregular places on the surface of castings, sharpedge, changes in section thickness, and other similar places where stress concentration can occur. It gradually extends to the interior of the casting, with wide surface and narrow interior, and sometimes runs through the entire casting section. The internal crack occurs at the last solidification place inside the casting. Generally, it will not extend to the surface of the casting. The surface of the crack is very uneven, often has many forks,with lighter oxidation degree than that of the outer crack.
It is generallyacknowledged that the hot cracking of alloys occurs during solidification, that is, when most alloys have been solidified, but there is a small amount of liquid between dendrites. At this time, the linear shrinkage of the alloy is large, and the strength of the alloy is low. If the casting mold hinders its shrinkage, the casting will produce a large shrinkage stress acting on the hot spot. When the strain at the hot spot is greater than the allowable strain of the alloy at this temperature, hot cracking will occur.
However, for the same alloy, whether hot cracking occurs in castings often depends on factors such as mold resistance, casting structure, pouring process and so on. In the process of casting condensation, any way that can reduce its shrinkage stress and improve the high temperature strength of the alloy will help to prevent the occurrence of hot cracks. Therefore, in actual production, effective measures such as increasing the yield of the mold, improving the mold structure,optimizing the position where the alloy is introduced into the mold, and reasonably setting reinforcing ribs and cooling are usually taken to avoid hot cracking. Thetendency of the hot cracking occurrence of the alloy depends on the properties of the alloy itself. Generally speaking, the greater the difference between the temperature at which the complete dendrite skeleton begins to form during the solidification process and the temperature at the end of solidification, and thebiggershrinkage rate of the alloy during this period, thehigherthe hot cracking tendency of the alloy.
2. Pressure resistance
Poor pressure resistance of castings refers to the leakage of pressure from the inside or outside of the castings when pressure is applied to the castings, which may be manifested as oil leakage, air leakage, water leakage, etc. it is one of the most difficult problems to solve in the casting defects, and the cause may be the defects caused by the combination of various defects.
When the casting is immersed in water, the inner cavity of the casting is filled with compressed air. The compressed air passes through the path formed by the inner surface defects, inner defects and outer surface defects of the casting to the water surface, and bubbles appear, showing poor pressureresistance.
In copper alloy castings, the requirements for alloy compactness are high, so improving the hydraulic resistance of copper alloy castings and avoiding leakage become the key in production. After production verification, as long as a certain thickness of columnar crystal or fine equiaxed crystal structure is obtained in the casting section, the hydraulic resistance performance can meet the use requirements. However, columnar crystals are easy to obtain under rapid cooling conditions and difficult to obtain under general sand casting conditions. The columnar crystal structure obtained by increasing the pouring temperature for specific castings does have a very obvious effect. The pouring temperature of copper castings is low, the alloy structure is loose, the casting leakage problem issevere, and "tin sweat" and hot cracks occur from time to time.
With increasing the pouring temperature, the structure of the casting is dense columnar crystal,thus qualified casting obtained.