热加工作业考研题目答案分享——metal casting 1

Metal casting homework one

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• 1.Explain why casting is an important manufacturing process.
• 10.Explain the reasons for hot tearing in casting.
• 11.Why is it important to remove dross or slag during the pouring of molten metal into the mold? What methods are used to remove them?
• 21.Why does porosity have detrimental effects on mechanical properties of castings? Would physical properties, such as thermal and electrical conductivity, also be adversely affected by porosity? Explain.
• 31.It has long been observed that (a) low pouring temperatures promote the formation of equiaxed grains over columnar grains and (b) equiaxed grain become finer as the pouring temperature decreases. Explain these two phenomena.
• 34.How can you tell whether cavities in a casting are due to shrinkage or entrained air bubbles?

1.Explain why casting is an important manufacturing process.

First, casting can be used for manufacturing products with complex shape that is difficult to be obtained by other method.
Second, the method can be utilized in mass production which facilitates the efficient production of a large number of similar products.
Third, casting is an effective way to produce porous materials which have lots of outstanding properties.

10.Explain the reasons for hot tearing in casting.

Hot tearing, also known as hot cracking, is a defect in casting operation. There are some reasons cause the defect in the following.
First, residual stresses in the material.
Second, the temperature of the flow is low relative to high freezing temperature range.

11.Why is it important to remove dross or slag during the pouring of molten metal into the mold? What methods are used to remove them?

Because dross or slag has already solidified, on the one hand, maybe it would block the Runner or Gate so that it would cause misrun; on the other hand, it can be considered as one type of impurities (inclusions) that have a significant adverse effect on fluidity. Thus, it is extremely essential to remove them.
There are some ideas to remove them. For example, keep the molten temperature of the metal thoroughly in order to prevent dross or slag are created. Or utilize a specific designed equipment to filter the molten metal.

21.Why does porosity have detrimental effects on mechanical properties of castings? Would physical properties, such as thermal and electrical conductivity, also be adversely affected by porosity? Explain.

Because porosity can lower the area of cross-section which may cause the hardness of the metal decreases. Moreover, stress risers is usually situated in porosity, which is harmful to metal’s physical properties.
Higher the porosity, lower the thermal and electrical conductivity [1]. For thermal conductivity of the metal, porosity carries air which is more difficult to transfer heat than the material, metal. Considering electrical conductivity, the affection is also the same [2]. Because porosity will higher electrical resistant, the electrical conductivity of the metal would be reduced.

31.It has long been observed that (a) low pouring temperatures promote the formation of equiaxed grains over columnar grains and (b) equiaxed grain become finer as the pouring temperature decreases. Explain these two phenomena.

(a) With the lower pouring temperatures, the abbreviation of temperature between molten metal and mold walls would be reduced. Therefore, much more equiaxed grains will be produced. Because equiaxed grains grow up in the low driving force of heat transfer.
(b) With the lower pouring temperatures, the grains grow up much more difficult. Because the speed of nucleation and crystal growth will decrease in lower temperature. So that equiaxed grain become finer.

34.How can you tell whether cavities in a casting are due to shrinkage or entrained air bubbles?

If cavities are formed by bubbles, the surface of them would be fluent. Instead, the surface of cavities would be relatively rough if they are due to shrinkage.

Reference
[1] W. Woodside and J. H. Messmer, “Thermal conductivity of porous media. I. Unconsolidated sands,” J. Appl. Phys., vol. 32, no. 9, pp. 1688–1699, 1961.
[2] J. N. Roberts and L. M. Schwartz, “Grain consolidation and electrical conductivity in porous media,” Phys. Rev. B, vol. 31, no. 9, pp. 5990–5997, 1985.