Page 276 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Quantitative Problems 255

I0.6. Define shrinkage and porosity. How can you tell l0.l0. Explain the reasons for hot tearing in castings.
whether cavities in a casting are due to porosity or to
l0.l I. Why is it important to remove dross or slag during
shrinkage? the pouring of molten metal into the mold? What methods
I0.7. What is the function of chills? What are they made of? are used to remove them?
I0.8. Why is the Reynolds number important in casting? I0.l2. Why is Bernoulli’s equation important in casting?
l0.9. How is fluidity defined? Why is it important? I0. 3. Describe thixocasting and rheocasting.
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QUALITATIVE PROBLEMS

I0.l4. Describe the stages involved in the contraction of |0.28. Note the shape of the two risers shown in Fig. 10.8,
metals during casting. and discuss your observations with respect to Eq. (10.7).
l0.I5. Explain the effects of mold materials on fluid flow I0.29. Is there any difference in the tendency for shrinkage
and heat transfer in casting operations. void formation in metals with short and long freezing ranges,
I0.I6. It is known that pouring metal at a high rate into a respectively? Explain.
mold can have certain disadvantages. Are there any disadvan- I0.30. What is the influence of the cross sectional area of
tages to pouring it very slowly? the spiral channel shown in Fig. 10.9 on fluidity test results?
I0.I7. Describe the events depicted in Fig. 10.5. What is the effect of sprue height? If this test is run with the
I0. 8. Would you be concerned about the fact that portions entire test setup heated to elevated temperatures, would the
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of internal chills are left within the casting? Explain. results be more useful? Explain.
I0.3l. It has long been observed that (a) low pouring tem-
IO. 9. What practical demonstrations can you suggest to in-
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dicate the relationship of the solidification time to the volume peratures (i.e., low superheat) promote the formation of
and surface area of a casting? equiaxed grains over columnar grains and (b) equiaxed
grains become finer as the pouring temperature decreases.
l0.20. Explain why a casting may have to be subjected to
Explain these two phenomena.
various heat treatments.
I0.2I. Why does porosity have detrimental effects on the I0.32. In casting metal alloys, what would you expect to
mechanical properties of castings? Would physical properties, occur if the mold were agitated (vibrated) aggressively after
such as thermal and electrical conductivity, also be adversely the molten metal had been in the mold for a sufficient
amount of time to form a skin?
affected by porosity? Explain.
l0.22. A spoked handwheel is to be cast in gray iron. In l0.33. If you inspect a typical cube of ice, you are likely to see
order to prevent hot tearing of the spokes, would you insulate air pockets and cracks in the cube. Some ice cubes, however,
are tubular in shape and do not have noticeable air pockets or
the spokes or chill them? Explain.
l0.23. Which of the following considerations are important cracks in their structure. Explain this phenomenon.
for a riser to function properly? Must it: (a) have a surface |0.34. How can you tell whether cavities in a casting are
area larger than the part being cast, (b) be kept open to at- due to shrinkage or entrained air bubbles?
mospheric pressure, and/or (c) solidify first? Explain. l0.35. Describe the drawbacks to having a riser that is (a)
l0.24. Explain why the constant C in Eq. (10.7) depends on too large and (b) too small.
mold material, metal properties, and temperature. I0.36. Reproduce Fig. 10.2 for a casting that is spherical in
I0.25. Are external chills as effective as internal chills? shape.
Explain. I0.37. List the process variables that affect the fluidity
|0.26. Explain why, as shown in Table 10.1, gray cast index as shown in Fig. 10.9.
iron undergoes expansion rather than contraction during |0.38. Assume that you have a method of measuring poros-
solidification. ity in a casting. Could you use this information to accurately
|0.27. Referring to Fig. 10.11, explain why internal cor- predict the strength of the casting? Explain.
ners, such as A, develop a thinner skin than external corners,
such as B, during solidification.

QUANTITATIVE PROBLEMS

|0.39. Sketch a plot of specific volume versus temperature ll |0.40. A round casting is 0.2 m in diameter and 1 m in
for a metal that shrinks as it cools from the liquid state to length. Another casting of the same metal is elliptical in cross
room temperature. On the graph, mark the area where section, with a major-to-minor axis ratio of Z, and has the
shrinkage is compensated for by risers. same length and cross sectional area as the round casting. Both