Casting alloys

The metallic materials used to produce castings are called casting alloys, and almost all alloys can be used for casting production, except for those that are particularly difficult to melt. The common melting methods for casting alloys are: cupola melting, reflection furnace melting, electric arc furnace melting, medium frequency and industrial frequency induction furnace melting, crucible furnace melting, etc.

Casting alloys (including cast iron, cast steel and casting non-ferrous alloys) are important engineering materials, occupying a considerable position in industrial and agricultural production, national defense construction and people's daily life, especially in the machine manufacturing industry, accounting for a larger proportion.
The rapid development of modern casting alloy types and melting technology is mainly in the 1950s, such as breeding cast iron, ductile iron, peristaltic cast iron, malleable cast iron annealing cycle is significantly shortened, the development of electric furnace melting cast iron technology. With a variety of casting alloys, casting process methods and casting alloy melting with a wide range of in-depth research, many new technologies, new achievements are constantly applied to the production, especially the application of computers in the casting production, achieved satisfactory results.

Commonly used materials
Gray cast iron
The carbon content of gray cast iron is close to the eutectic composition, with small crystallization temperature interval and low melting point, graphite precipitation during crystallization, volume expansion, which can offset part of the shrinkage of the casting, so the total shrinkage is small and the fluidity is good. If the pouring temperature of gray cast iron is 1200-1280℃, the length of the spiral line indicating good or bad fluidity is 600-1200mm, and the total volume shrinkage is between 6.9% and 7.8%.
Carbon cast steel
Commonly used in the manufacture of machine parts cast steel carbon content of 0.25% to 0.45%. Cast steel casting temperature is high (about 1500 ℃, poor mobility, spiral length of 100mm, shrinkage, the total volume shrinkage rate of 12.4%, easy to absorb gas and oxidation in the melting process, so the casting performance of cast steel is poor, prone to sand sticking, under-pouring, cold compartment, shrinkage, cracking, porosity and other defects.
Non-ferrous metals
Copper alloy has low melting point and good fluidity. Tin bronze pouring temperature is 1040 ℃, the spiral length is 420mm, silicon brass pouring temperature is 1100 ℃, the spiral length is 1000mm. copper alloy melting easy to oxidation, some copper alloys (such as lead bronze) also easy to produce density segregation, melting to prevent alloy oxidation, burn, segregation.
Aluminum alloy pouring temperature is lower, generally at 680 ℃ spiral length of 700 ~ 800 mm. aluminum alloy at high temperature is easy to absorb gas and oxidation, affecting its mechanical properties, so the melting should pay attention to isolate the alloy liquid and furnace gas contact, and the use of some purification measures.

Performance
The process properties presented by the alloy in the casting production are called casting properties. It is an important factor to ensure the quality of the casting. The main casting properties of alloys are fluidity, shrinkage, tendency to segregation, etc.
1.Fluidity The ability of molten metal to flow is called fluidity. A liquid metal with good fluidity has a strong ability to fill the casting mold, and it is easy to obtain a casting with a complete shape, accurate size, clear outline or thin and complex walls.
Many factors affect the fluidity of the metal, such as high pouring temperature, can make the metal in the liquid state to maintain a longer time, but too high pouring temperature will lead to an increase in the total shrinkage of metal and absorb too much gas, resulting in shrinkage and porosity and other defects. In short, the pouring temperature should not be too high or too low. Alloy composition also has a big influence on the fluidity, such as eutectic alloy with low melting point and good fluidity.
2. Shrinkage The performance of a metal that shrinks in volume during cooling is called shrinkage. The shrinkage of metals can be divided into three parts: liquid shrinkage, solidification shrinkage and solid shrinkage. Among them, liquid shrinkage is produced in the high temperature state, only caused by the casting riser part of the metal liquid surface reduction; solidification shrinkage will cause shrinkage, shrinkage and other phenomena; solid shrinkage is obstructed, the casting of internal stress. In order to prevent shrinkage or shrinkage hole, should be expanded within the sprue, using the sprue to directly fill the shrinkage, or set up a riser at the wall thickness, the metal liquid from the riser to supplement the wall thickness of the solidification shrinkage.
3. bias tendency metal or alloy in the solidification process of the formation of the chemical composition of the uneven phenomenon is called bias. It is related to the liquid phase line and solid phase line temperature interval of the alloy, the larger the solidification temperature interval, the more significant the segregation. In addition, gray cast iron containing sulfur, phosphorus and carbon in the higher amount, easy to produce segregation. High pouring temperature and slow cooling rate can also cause segregation. When the segregation of castings is not too serious, the phenomenon of segregation can be eliminated by annealing treatment.