materials or alloy additions that have not completely dissolved in the melt.
•Molten metal containing excess flux or foreign oxides.
•During solidification, the formation and segregation of insoluble intermetallic compounds concentrating in the residual liquid.
•Contaminants in wax pattern.
Remedies:
•Assuring that charge materials are clean and eliminating foreign metals.
•Using small pieces of alloying material and master alloys in making up the charge.
•Being sure that the bath is hot enough when making the additions.
•Not making additions too near to the time of pouring.
•For nonferrous alloys, protecting cast iron crucibles with a suitable wash coating.
Examples of seven basic categories of defects are shown in Fig. 1.8.
h) Porosity
Porosity is the presence of holes, spaces, or gaps inside a solid. Two main sources of porosity during casting are shrinkage and gas porosity. The first of these occurs due to the volume contraction between solid and liquid during solidification; if additional liquid is not supplied to compensate, then porosity will appear in the casting.
Fig. 1.8 Schematic illustration of seven basic categories of casting defects: a) joint flash, b) blowholes, c) hot-cracks, d) flow marks, e) poured shot, f) distorted casting, g) metallic inclusion.
The most obvious porosity defects are caused by the entrapment of 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
