Metal Shaping Processes. Vukota Boljanovic
gases within the molten solution. Typically, hydrogen precipitates into melt by contact with the atmosphere or when there is too much moisture in the flux. Since hydrogen is highly soluble in molten metal, it is best to avoid superheating metals beyond their melting temperature and to avoid holding the material in a molten state any longer than is required. Gases can be scavenged from the molten metal by introducing an inert gas such as argon or nitrogen and bubbling it through the metal. To reduce the absorption of gases from the atmosphere, leaving any slag or dross, cover the molten metal until just prior to pouring it into the mold. Porosity is detrimental to the ductility of a casting and its surface finish, making it permeable and thus affecting the pressure tightness of a cast pressure vessel.
The loss in casting properties measured by a tensile test may reflect the amount of porosity in a casting. Because imperfections become areas of higher stress concentration, the percentage of property loss becomes greater when the strength requirement is higher. A metallographic examination can determine whether porosity exists in a casting. X-ray techniques are also used for nondestructive evaluations of porosity in castings.
REVIEW QUESTIONS
1.1Identify some of the important advantages of shape casting processes.
1.2What are some limitations and disadvantages of casting?
1.3Name the two basic mold types that distinguish casting processes.
1.4How can heat energy be expressed?
1.5What does “heat of fusion” mean in casting?
1.6Explain the phase diagram of heating metal to melting temperature.
1.7What is gravity sand casting?
1.8What is the difference between a pattern and a core in a sand casting?
1.9What is the law of mass continuity?
1.10Why should turbulent flow of molten metal into a mold be avoided?
1.11Identify factors that affect molten metal fluidity.
1.12How is solidification of pure metals different from solidification of alloys?
1.13What is “chill” in casting?
1.14What is Chvorinov’s rule in casting, and how can it be mathematically expressed?
1.15Why does shrinkage occur, and how can it be compensated for?
1.16Identify the most common defects in casting.
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METAL CASTING PROCESSES
2.3 Other Expendable Mold Casting Processes
2.4 Permanent Mold Casting Processes
2.5 Melting Practices and Furnaces
Metal casting processes are among the oldest methods for manufacturing metal goods. In most early casting processes the mold or form used had to be destroyed in order to remove the product after solidification. This type of mold is called an expendable mold. Since a new mold is required for each new casting, production rates in expendable mold processes are often limited by the time required to make the mold, rather than by the time needed to make the casting itself. The second type of mold is the permanent mold; permanent molds are used to produce components in endless quantities.
Casting has significant advantages compared with other methods of component manufacture. Castings are generally cheaper than components made in other ways. The casting process in one or another of its forms provides the designer with an unrestricted choice of shape that can be made in a single stage. A casting can usually be made much closer to the chosen design, which provides savings in both material and finishing processes compared with other methods of manufacture. In addition, the cast structure has the highest resistance to deformation at elevated temperatures, so that castings have higher creep strengths than wrought and fabricated components. Cast metal may also have superior wear resistance than the equivalent forged metal. These advantages combine to ensure that casting has become the most important process for the manufacture of components in metals (and in some other materials). In 2004 the total worldwide casting production was 75 million tons/year. Major applications of casting include the following:
•Transport: automobile, airspace, railways, shipping
•Heavy equipment: machining
•Plant machinery: chemical, petroleum, paper, sugar, textile, steel and thermoplastic
•Defense: vehicles, artillery, munitions, storage and supporting equipment
•Electrical machines: motors, generators, pumps, compressors
•Household: appliances, kitchen and gardening equipment, furniture, and fitting
•Art objects: sculptures, idols, furniture, lamp stands, and decorative items.
The advantages of casting have, in the past, been offset by significant disadvantages compared with wrought products. Castings are considered to be less ductile than the equivalent wrought product, and they have a less consistent performance in fatigue; they also have inferior integrity. The difference in ductility may be more apparent than real, however. A forged or rolled component may have a higher ductility than a casting in the direction of forging or rolling but a significantly lower transverse ductility. This is a distinct advantage if the longitudinal direction has to resist the principal stress, but it is not necessarily a sign of inferiority of the casting process.
Metal casting processes may be classified in several different ways:
•According to the mold type: (1) expendable mold (destroyed after each casting) and (2) permanent mold (reused many times);
•According to the type of pattern used for making a sand mold: (1) expendable pattern (melted for each mold), the pattern material being wax; and (2) permanent pattern (reused for many molds), the pattern material being wood or metal.
•According to the type of core used for producing a hole in casting: (1) expendable core (used in both sand and metal molds), the core material being sand; and (2) a permanent core (used with a permanent mold only), the core material being metal.
•According to the method by which the mold is filled: (1) gravity (sand casting, gravity die casting); (2) pressure (low and high pressure die casting); and (3) vacuum (vacuum investment casting).
Sand casting is a metal-forming process in which a molten metal compound is poured into a sand mold to produce a workpiece’s desired shape.
Sand casting has historically been the most popular casting method, producing by far the greatest tonnage of castings used in any country. Today, however, with the widespread conversion of automotive components from ferrous metals to aluminum, sand casting’s position as the dominant molding method is threatened. It is usually the least expensive way of making a component; its inherent cost advantage over other methods continues to make it an attractive molding method.
Basically, sand casting consists of six production steps:
Pattern. Preparing and placing a pattern having the shape of the desired workpiece.
Molding. Making