Mp4f0711122

MP4F07

NANYANG TECHNOLOGICAL UNIVERSITY

SEMESTER 2 EXAMINATION 2011-2012

MP4F07- NET SHAPE MANUFACTURING

April/May 2012 Time Allowed: 2Yz hours

INSTRUCTIONS

I. This paper contains 3 questions and comprises 8 pages which include 1 page of Appendix.

2. Answer all 3 questions.

3. Marks for each question are as indicated.

4. This is a closed-book examination.

I (a) The data of two 316L austenitic stainless steel powders, A and B, of different particle shapes (spherical and irregular) were lost. The only data available was the repose angle: Powder A has a repose angle of 25° and Powder B has a repose angle of 35°. Explain which powder has a higher interparticle friction and which powder is fabricated by water atomization.

(3 marks)

(b) In some processes involving the use of powder, mixing with binders and lubricants is the first step in the preparation of a mixture for shaping into the required shape. Outline the objectives of mixing with binders and lubricants.

(3 marks)

(c) Suggest and describe a suitable process for producing large components from loose powder where full density and isotropic properties are required. List ONE ( 1) problem associated with the process and how the problem can be reduced.

(7 marks)

(d) The conventional powder metallurgy process is a well-established process. In die compaction, a set of punches and a die are used to compact the powder into the required shape. 316L austenitic stainless steel powder is used here because of its good corrosion resistance.

(i) The die compaction of the stainless steel powder into a circular compact of 20 mm thickness and green density of 5.0 g/cm3 requires a die fill of 45 mm. To increase the green density to 6.4 g/cm3

, a higher compaction pressure will be used. What will be the required die fill?

(4 marks)

Note : Question No. 1 continues on page 2.

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(ii) To achieve a higher green density of 7.6 g/cm3, a proposal is to use a higher

compaction pressure. Explain whether you agree to the proposal. The stainless steel powder has a theoretical density of 8.0 g/cm3

.

(3 marks)

(iii) As for sintering of the green compact, one suggestion is to use methane atmosphere. Share your thoughts on the suggestion and discuss your choice of suitable sintering atmospheres, where appropriate.

(4 marks)

(iv) For die compaction of compacts having multi-levels, density gradient is a concern. Besides using multi-punches with differing displacements so that the compression ratio is the same for each level, briefly discuss three other ways to reduce the density gradient of compacts produce by die compaction.

(3 marks)

(e) Figure I shows four compacts that are to be produced by die compaction. For each of the compacts, explain a possible problem with its present design and the proposed suitable modification/action.

(6 marks)

(i) (ii) (iii) (iv)

Figure 1

2


2 (a) (i)

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A plastic part as shown in Figure 2(a) is to be injection molded. Draw, in the top view as shown in Figure 2(b ), weld lines, if any, that are expected to be formed. Briefly explain your answer.

(5 marks)

0 D Gate

~¢=J 0

(b)

Figure 2

(ii) Four mold cavity layouts for a hexagon shaped thin disc are shown in Figure 3. Classify each mold cavity layout as balanced or unbalanced. Briefly explain your answer. Note that all runners have the same cross-sectional dimensions.

(4 marks)

Note : Question No. 2 continues on page 4. Figure 3 appears on page 4.

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(a) (b)

(c)

(d) Figure 3

(iii) You are injection molding a thin circular disc shown in Figure 4 (diameter 120 mm, thickness 1.2 mm) which is made of polycarbonate (PC) within a total process cycle time of 7 seconds. Assume that your mold has two disc cavities and that the injection pressure in the cavity is 69 MPa. Estimate the minimum tonnage and minimum plasticating rate of the injection molding machine needed for this operation. Show your calculations clearly. The density of PC is 1.2 x 1 03 kg/m3

. Some useful formulae that may be needed are given in the Appendix. (Hint: Estimate suitably the various times available for melting, shaping and cooling).

(5 marks)

15mm 120mm

Figure 4


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(iv) You are now told that there is a problem with injection molding the same disc (Figure 4) in the production shop-floor. The ejector pins are leaving unacceptably deep impressions on the disc. You check the ejection system and it is working fine - so the problem is elsewhere. Determine the nature of the problem and find a solution (show quantitative numbers for your solution). The melt temperature is 300°C and the mold temperature is 93°C. The thermal conductivity of PC is 0.176 W/(mK), Vicat softening temperature is 149°C, the average specific heat of PC is 1.67 kJ/(kgK) and the density is 1.2 x 103

kg/m3. Some useful formulae that may be needed are given in the Appendix.

(b) (i)

(9 marks)

Explain in a few sentences why the speed with which a casting is withdrawn from the furnace is critical in directional solidification process.

(4 marks)

(ii) You are to compare the conventional Bridgman process and that using liquid metal cooling (LMC) for the velocity with which a Nickel-alloy casting is removed from the furnace for directional solidification. Assume that the same mold, Nickel-alloy liquid melt conditions, and furnace conditions are used for the comparison. For the conventional Bridgeman process, the conductive and radiative heat transfer coefficients are 1600 W/(m2K) and 120 W/(m2K), respectively. The mold-shell heat transfer coefficient is 125 W/(m2K) and the resistance due to the gap between the shell and metal can be approximated as a heat transfer coefficient of 250 W/(m2K). For the liquid metal cooling process assume that the heat transfer coefficient in the liquid metal is much larger than any of the other heat transfer coefficients. Determine the ratio of the speeds, for the two cases, with which the casting needs to be removed from the furnace. Some useful formulae that may be needed are given in the Appendix.

(7 marks)

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3 (a) The bolt represented in Figure 5 is to be made by plastic deformation of a cylindrical bar. It is desired to retain the benefits of cold working, and the part is to have equal strain hardening in the head and body sections. Head and body are of simple cylindrical shapes and no other features are present.

200~Di{ 1-· -1-·-·-·-·-·-·-·-·-· t- ·llO.OmmDia.

lc )I I 15.0 mm 75.0 mm

Figure 5

(i) Illustrate with a sketch how the bolt can be formed by using a combination of upsetting and extrusion.

(2 marks)

(ii) What should be the original dimensions (diameter and length) of the cylindrical bar?

(4 marks)

(iii) Assuming that the stress-strain curve of the cylindrical bar follows a power law with K = 500 MPa and n = 0.2, calculate the average flow stress and estimate the maximum force required to produce one bolt in ideal conditions (no friction and redundant work).

(6 marks)

(iv) How does the force calculated in part (iii) above varies if the redundant work of the extrusion process is taken into account? What is the main cause of this redundant work?

[Hints: extrusion pressure with redundant work may be approximated by Pe =

Yavg Qe where Qe = 0.8 + 1.2 s] (4 marks)

(b) This section of the question concerns sheet metal forming.

(i) List the major characteristics of compound dies, progressive dies and transfer dies and explain how these dies are used in practice.

(5 marks)

(ii) Explain the differences and improvements between the conventional blanking and fine blanking processes. On what fundamental principles related to material deformation behaviour are these improvements based on? Use sketches to illustrate your answer.

(5 marks)


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(iii) The component represented in Figure 6 is to be produced by a combination of blanking and punching operations performed in one single step. The material used is a high strength steel with an ultimate tensile strength of 760 MPa and a thickness of 1.5 mm. What is the capacity of the press required to perform this combined operation?

(4 marks)

r=7.5 mm · .. ,@d=7.5~·

. •. ·.@d=7}mfil

. ·• ·1<20~~ . .>1 l5mm1:o 30mm

!OOmm

Figure 6

(iv) If the press available to produce the above component is only capable of delivering 90% of the force required, what solution would you propose to produce the component on that press? Support your answer with calculations where appropriate.

(3 marks)

End of Paper

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Appendix

Some useful formulae that may be needed are given below. Symbols have their usual meanmgs.

Wh'!::.p Q pressure ::::::: lZ j.iL

k a=-

pc

1 1 1 1 1 -=-+--+--+-h,ff he hgap hshell hR

where

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z = L + c(T; - TJ L+c(T; -Tm)


MP4F07 NET SHAPE MANUFACTURING

Please read the following instructions carefully:

1. Please do not turn over the question paper until you are told to do so. Disciplinary action may be taken against you if you do so.

2. You are not allowed to leave the examination hall unless accompanied by an invigilator. You may raise your hand if you need to communicate with the invigilator.

3. Please write your Matriculation Number on the front of the answer book.

4. Please indicate clearly in the answer book (at the appropriate place) if you are continuing the answer to a question elsewhere in the book.


Mp4f0711122

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