Manufacturing of heavy rings and large copper canisters by plastic deformation

HAMZAH SSEMAKULA

Doctoral Thesis

Division of Materials Forming Department of Production Engineering

Royal Institute of Technology

Stockholm January 2003

TRITA-IIP-2004-2 ISSN-1650-1888 Hamzah Ssemakula Department of Production Engineering Royal Institute of Technology S-100 44 Stockholm

KTH, Royal Institute of Technology

Dept. Of Production Engineering Division of materials Forming

Stockholm Sweden 2004

Manufacturing of heavy rings and large copper canisters by plastic deformation

Hamzah Ssemakula

Doctoral Thesis TRITA-IIP 2004:2 ISSN 1650-1888

Akademisk avhandling

Som med tillstånd av Kungliga Tekniska Högskolan framläggs till offentlig granskning för avläggande av teknisk Doktorsexamen måndagen, 19 januari, 2004 klockan 10.00 i sal M311- Brinellvägen 68, KTH, Stockholm. Fakultetsopponet är Docent Hans Keife, Outokumpu Copper, Fabrication Technology AB, Box 594, 72110 Västerås

Abstract Plastic deformation processes transform material from as-received state to products meeting certain requirements in properties, microstructure and shape. To achieve this transformation, the relationship between material response and process conditions should be understood. This is usually complicated by the complex conditions describing the actual process. Numerous techniques including empirical, physical, analytical and numerical can be employed. In this thesis, numerical technique supported by lab- and full-scale experiments has been employed to analyse the forming parameters. The first part of the thesis is focused on the use of such parameters to predict occurrence of material pores during manufacturing of bearing rings. The second part deals with the influence of forming parameters on the grain size during fabrication of large copper canisters for encapsulation of nuclear waste. The primary task has been to study with the help of commercial FE-codes the magnitude and distribution of forming parameters such as accumulated effective strain, temperature, instantaneous hydrostatic pressure and material flow at different stages of the forming process. In the first part, two types of ring manufacturing routes, which result in pore free and pore loaded rings are studied and compared. Material elements located in different areas of the workpiece have been traced throughout the process. Results of the accumulated strain and instant hydrostatic pressure have been analysed and presented in pressure-strain space. It’s assumed that high hydrostatic pressures together with high effective strains are favourable for pore closure. Area of the workpiece with unfavourable parameters have been identified and compared with ultrasonic test results. Good agreement has been obtained. Based on the results of this analysis, a new concept for avoiding pores in manufacturing of yet heavier rings has been presented. The concept proposes a lighter upsetting in the initial stage of the process and a more efficient piercing which results in higher hydrostatic pressure and bigger and better distributed effective strain. In the second part of the thesis, the influence of forming parameters such as effective strain and temperature on the final grain size of the product has been studied in laboratory scale. As-cast billets of cylindrical shape were extruded at different temperatures and reductions. It has been shown that the grain size in the final product should be small in order to enable ultrasonic tests and to guarantee resistance towards creep and corrosion. Simulations for different material elements located at different distances from the axis of symmetry of the initial cylindrical workpiece have been carried out. In this way, the parameters describing the deformation history of the elements have been determined as functions of time. Experimentally obtained pre- and post deformation grain size in the corresponding locations of the material were determined. It’s concluded that low temperature coupled with high effective strain are conducive for obtaining a small grain size. Based on the beneficial conditions for extrusion of copper, a more detailed FE-analysis of a full-scale industrial process is carried out. A coarse-grained cast ingot of pure copper is heated and by upset forging formed into a cylinder, which is then punched into a hollow blank for subsequent extrusion. The blank is extruded over a mandrel through a 45-degree semi-angle die. Accumulated effective strain and temperature as functions of the tubular wall thickness have been studied at five different locations along the tubular axis. Forming load requirement as function of tool displacement for each stage of the process has been determined. Strain and temperature levels obtained have been related to the grain size interval obtained in the earlier work. It has been concluded that the levels reached are within the interval that ensures a small grain size. A similar analysis has been carried out for forging of large copper lids and bottoms. Die design modifications to improve the grain size in the lid and to optimise the forging process with respect to forging load and material yield have been proposed. A method requiring a small forging load for fabrication of the lids has been analysed Keywords: Pores; grain size; low forging load; effective strain; temperature; hydrostatic pressure; extrusion; forging; canister; lid; rings

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Acknowledgements This work was carried out at the Department of Production Engineering, Division of Materials Forming at the Royal Institute of Technology, KTH Sweden. The research program was financed partly by Ovako Steel AB, Hofors and by The Swedish Nuclear Fuel and Waste Management Co (SKB). I wish to express my gratitude to my supervisor, Professor Ulf Ståhlberg for his expert advice, support and guidance. This work would never have been possible if not Ovako Steel AB had given me financial support, carried out full scale experiments and supplied me with useful data for simulations. Special gratitude goes to Thore Lund, Maria Nars and Astrid von Platen for stimulating discussions and optimistic attitudes. I would like to extend my sincere thanks also to Sven Gunnar Persson and Töre Svall for availing me with geometrical data and carrying out ultrasonic tests respectively. I am indebted to SKB in general for financial support and extremely grateful to Claes-Göran Andersson, Marika Westman, and Martin Burström and Peter Eriksson who have given me so freely of their time and have been candid in their opinions. I would like to thank Kent Öberg and his entire team at Scana Steel Björneborg AB. I am particularly gratified that even in an area so close to industrial application as this is, these colleagues in industry have nevertheless felt able to be supportive and informative. I thank also Christer Eggertsson at the Swedish Institute for Metals Research (SIMR) for manufacturing the device used in preheating the extrusion container used in laboratory experiments at KTH. I am also grateful to the entire staff at the department and in particular to all my colleagues at the division of Materials Forming for all the fun and memorable moments we have experienced together. It has been a great honour sharing this time with you all. Finally, I wish to extent my sincere gratitude and heartfelt appreciation and love to my Dear wife Zilpah Mukidi and our children Moses and Mariam for the unrelenting support and encouragement throughout the years. Darling, thank you for being YOU!

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Dissertation This dissertation contains a summary and the following papers

Paper A H. Ssemakula and U. Ståhlberg, A study of pore closure in the

manufacturing of heavy rings, Journal of Materials Processing Technology 84 (1998) 25-37

Paper B H. Ssemakula and U. Ståhlberg, A new pore closure concept

in the manufacturing of heavy rings, Journal of Materials Processing Technology 110 (2001) 324-333

Paper C H. Ssemakula and U. Ståhlberg, Grain size as influenced by

process parameters in copper extrusion, Scandinavian Journal of Metallurgy 30 (2001) 232-237

Paper D H. Ssemakula, Manufacturing of large copper canisters by

extrusion, International Journal of Engineering and Simulation 2 (2001) 11-17

Paper E H. Ssemakula, M. Jacobsson, M. Magnusson and U.

Ståhlberg, Improving the grain size in the forging of large copper canisters. Submitted for publication to the Journal of Materials Processing Technology

Paper F H. Ssemakula, U. Ståhlberg and Kent Öberg, Close-die

forging of large Cu-lids by a method of low force requirement (TRITA-IIP-04-02) ISSN 1650-1888

iii

iv

CONTENTS

1. Introduction…………………………………………………… 1 2. Manufacturing of large bearing rings…………………………….. 2

3. Manufacturing of large copper canisters and lids……………… 3

4. Relationships between the papers of the thesis…………………….5

5. Summary of appended papers

5.1 . Paper A: A study of pore closure in the manufacturing of heavy rings………………………………. 6

5.2. Paper B: A new pore closure concept for the manufacturing of heavy rings ……………….. 10

5.3 Paper C: Grain size as influenced by process parameters in copper extrusion…………………………. 14

5.4 Paper D: Manufacturing of copper canisters by extrusion… 16

5.5 Paper E: Improving the grain size in the forging of large

copper lids……………………………………. 17

5.6 Paper F: Closed-die forging of large Cu-lids by a method of low force requirement …………… 21

6. Concluding remarks…………………………………………… 23

7. References…………………………………………………… 25

Appendix: Papers (A - F)

1. Introduction Numerous components in metal industry today are manufactured by metal forming processes that involve plastic deformation of the workpiece to obtain the desired shape. Components such as bearing rings and large copper canisters are manufactured by a variety of forming processes including upset forging and/or closed-die piercing and extrusion. These processes can be configured in different ways to fabricate parts that have the requisite shape, size and mechanical properties to properly function in critical applications. In the past advances in forming technology took place through innovative design of forming equipment and an experience-based approach to process development on the shop floor. With the advances in numerical techniques like FEM, simulations of such processes have now become a vital tool in process design, control of the forming processes and qualitative analysis, the emphasis being on lower cost and high quality parts.

This thesis is divided into two parts. The first part deals with the study of pore closure during manufacturing of heavy bearing rings [1-4]. The second part analyses manufacturing of large copper canisters and lids for encapsulation of nuclear waste. In both cases commercial FE-codes have been utilized to simulate the forming processes and to analyse forming parameters [5-10]. A systematic approach has been adopted consisting of simulation of the processes and validation of the results by laboratory and full-scale trials. This has enabled prediction of material flow during plastic deformation and estimations of areas where material defects and the undesirable large grain sizes can be found. Ring manufacturing is some times associated with residual pores, which can be considered as volumetric discontinuities in the material. They are generally due to gas entrapment and metal shrinkage during solidification. If not detected and taken care of, pores can degrade not only material’s mechanical properties but also adversely affect its performance in service. They can, for instance decrease the fatigue properties by initiating cracks and consequently lead to failure of the component. Bulk forming is one way of minimizing detrimental effects of pores. It is assumed that a combination of high effective strain together with high hydrostatic pressure is favourable for pore closure. The second part of the thesis deals with manufacturing of large copper canisters and lids, [11-13]. The canister consists of a pressure bearing insert

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of nodular cast iron and an outer corrosion resistant barrier of almost pure copper to provide protection against corrosion in the geo-chemical environment foreseen in the deep repository in Sweden. The purpose of the disposal container is to isolate and contain the radioactive waste from people and the environment. To achieve this, a multi-barrier system is envisaged of which the copper canister is just a part. The canister has a diameter of approximately 1000 mm and a height of about 5000 mm. It has been shown that the grain size in the final product, canisters and lids, should be small in order to enable ultrasonic tests and to guarantee high resistance towards creep and corrosion. Further more the canister should be able to withstand the mechanical loads anticipated in the deep repository. In the present work, the influence of process parameters during forming of the canisters and the lids such as temperature and strain on the final grain size has been studied. Forming tool geometries are analyzed and the processes optimized with respect to strain distribution, forming load and material yield. Alternative tool geometries are proposed to promote higher strains of better distribution and to achieve die filling with a minimum-forming load. 2. Manufacturing of large bearing rings Many researchers have studied pore closure in different forming processes. In this work the likelihood for existence or pores has been predicted by analysing the magnitude and distribution of the accumulated effective strain and the instant hydrostatic pressure in the material at different stages of the forming process. Paper A Two types of manufacturing routes, which according to full-scale experiments result in pore free and pore-loaded rings are compared. Material elements located in different areas of the workpiece have been traced throughout the process. Results of the accumulated effective strain and the instant hydrostatic pressure have been analysed and presented in a pressure–strain space. It’s assumed that a combination of high effective strain together with high hydrostatic pressure is favourable for pore closure. Results are compared with ultrasonic tests of the workpiece at corresponding stages of forming. In this way pore occurrence and forming parameters are correlated. In order to determine boundary conditions between workpiece and tools, theoretically determined shapes of the workpiece after different forming steps are compared with results of full-scale trials. Paper B

2

Manufacturing of a yet heavier ring, with serious problems regarding remaining pores, is analysed. Accumulated effective strains- and instant hydrostatic pressure distributions are studied by FE-simulations [14-21]. The results show small strains close to the outer surface of the ring. This is consistent with ultrasonic tests and micrographic examinations. Based on the results obtained in earlier analysis, Paper A, a new concept to avoid pores is presented. The concept proposes a lighter upsetting in the initial stage of the process and a more efficient piercing which results in higher hydrostatic pressure and bigger and better distributed effective strains. 3. Manufacturing of large copper canisters and lids Microstructural changes, which occur during manufacturing processes, have a significant impact on material properties, which in turn affect the performance and life cycle of the product. In the canisters for encapsulation of nuclear waste, the permissible average grain size should not exceed 360 µm according to “The Swedish Nuclear Fuel and Waste Management Co”, (SKB). This is necessary to satisfy the requirement of mechanical and corrosion resistance in the environment prevailing in the deep repository and to enable ultrasonic tests. A considerable amount of theoretical and experimental investigations have been done on tube extrusion of different copper alloys including composite materials [16-20]. There is however little published work dealing specifically with how forming parameters influence the microstructure evolution of pure copper during hot extrusion and forging. The main objective of this work is to study this influence. The investigations were done theoretically by FE-simulations and experimentally by laboratory and full-scale trials. Paper C Simulations for different material elements determined temperatures and accumulated effective strain during extrusion of a cylindrical copper billet. Material elements located at various distances from the central axis of the initial workpiece are traced from the inlet to the outlet of the die. In this way parameters describing the deformation history of the element were determined as functions of time. Experiments corresponding to the simulations were carried out and the resulting grain sizes were evaluated. Different reductions in area and initial temperatures were treated. Paper D A full-scale industrial process for manufacturing of large copper canisters is analysed. The process involves production of a seamless tube by hot

3

extrusion of a hollow blank over an internal mandrel. The entire forming process is simulated and at every stage of the forming process, the distribution and magnitude of the accumulated effective strain and temperature is analysed. Material flow and tool geometries in critical forming stages is studied [21-22]. Forming loads during piercing and extrusion are predicted and plotted as a function of ram displacement. Paper E Forging of the lid for the large canisters is studied. According to full-scale experiments, a current forging method results in a product of coarse-grained microstructure close to the final mid contact surface. The analysis shows that this area is associated with the formation of “dead metal zones”. An alternative-forging tool is proposed to increase the effective strain in the area for decreasing the grain size [23]. The flat dies used during upset forging are furnished with a central spherical protrusion. In the second stage of the current forming, the cogging operation needed to fill the die cavity is replaced by ordinary axisymmetric closed-die forging. Thus an indentation is created, which is then flattened out. Both these operations increase the strain levels in the product, especially in the central critical region. The material yield is improved since the product after closed-die forging is near the desired product shape. The forming load however becomes moderately higher. Paper F The analysis is now focused on a method for fabrication of the large lids by a comparatively small forging load. Scana Steel Björneborg AB, a forging company in Sweden specializing in heavy forging, proposed the method. It is built up by a flashless forging sequence of the lids, which roughly can be divided into two phases. The first phase is axisymmetric and includes axial upsetting of a cylindrical billet, followed by open-die piercing with a spherical punch and finally a conventional axisymmetric closed-die forging up to the maximum available press load of 4600 ton. The second phase is non-axisymmetric where a tool of rectangular cross-section and parallel to the bottom die is used to bite off small areas of the workpiece peripheral surface and forge them into the deep cavity of the bottom die. The bottom die is rotated about 450 after each such pressing. Diagonal pressings finish this kind of forging. FE-simulations are carried out to analyze the load requirement at different stages of the process, the accumulated effective strain and the final product temperature distribution [24-28]. The results are co-related to the grain size evaluation from the full-scale trials. It’s concluded that 4600 ton is adequate to form the lids and that high-

4

accumulated effective strains and a low finishing temperature are coincidental with a fine grain size. From optical metallographic examinations it is clear that the method results in acceptable grain size throughout the product. 4. Relationships between the papers of the thesis

Paper F

Low forging load

and small grain size

Improving the grain size in

forging of lids

Paper E

Pore closure. Proposed concept to avoid pores

Paper C

Beneficial conditions

COPPER

STEEL

Manufacturing of large copper canisters and lids

Grain size as influenced by process parameters

Simulation and experiments

Pore closure. Comparison between manufacturing routes

Beneficial conditions

Paper A Paper B

Paper D

5

Figure 1. Principle scheme showing the relationship between the papers of the thesis 5. Summary of appended papers 5.1 A study of pore closure in the manufacturing of heavy rings (Paper A) The analysis is focused on explaining the occurrence of pores in heavy rings. Two forming routes, which principally differ significantly have been analyzed and compared. One route “bad ring” results in rings that are associated with pores while the other “good ring” does not. Figure 2 is a schematic picture showing the workpiece geometries after different forming sequences. Theoretically determined shapes of the workpiece of the workpiece after different manufacturing steps are compared with full-scale experiment to ascertain geometrical accuracy and ensure correct interfacial boundary conditions, figure 3. By using the FE-code Qform, accumulated strain distributions and instant hydrostatic pressures in the workpiece have been studied. It is assumed that high effective strains and high hydrostatic pressures are favorable for achieving pore closure. In both forming routes, nine material elements at different locations within the workpiece have been selected and traced backwards throughout the entire forming process, figure 4. Their accumulated strain and instant hydrostatic pressure are progressively analyzed. For each forming step, the results are presented as a pressure–strain graph. Figure 5 shows a bar graph of strains obtained for different material elements after the piercing operation for both the “good” and “bad” ring. Ultrasonic tests to reveal pore locations in the ring were carried out. For the bad ring, pore are concentrated in the portions located in the upper region from the cutting edge, figure 6. Low strains and low hydrostatic pressure characterize this area. The good ring does not show any pores.

6

“ GOOD” RING (B-22348-C) “BAD” RING (B-23168-C)

BEFORE AND AFTERUPSETTING

BEFORE AND AFTERUPSETTING

AFTER CLOSED DIEPIERCING

AFTER OPEN DIE PIERCING

(INCLUDING CUTTINGAWAY A THICK CENTRALDISC)

AFTER CLOSED DIEUPSETTING

(INCLUDING CUTTINGAWAY THE THIN BOTTOMDISC)

AFTER OPEN DIEUPSETTING

(FLATTENING OF THEUPPER PART OF THE RING)

AFTER RINGRO LLING

AFTER RINGROLLING

Φ 170

286

185

Φ 220 Φ 275

196

Φ222

Φ85

20

Φ 275 Φ110

167

Φ 223

Φ 85

Φ 285

Φ110

Φ 335.4

Φ 232.4

Φ 437.6

Φ 327.7

Figure 2. Schematic picture showing the workpiece geometries after different steps

7

THEORY EXPERIMENT

DURING UPSETTING

DURING OPEN DIE PREPIERCING

"BAD” RING

AFTER OPEN DIE PIERCING

AFTER FLATTENING

"GOOD” RING

THEORY EXPERIMENT

DURING UPSETTING

DURING PIERCING IN CLOSED DIES

AFTER PIERCING IN CLOSED DIES

AFTER FINISHING FORMING Figure 3. Comparison between theory and experiment regarding shapes after different forming steps for the “good” and “ bad” ring

8

Traced points

”Good” ring” ”Bad ring

9

3 2 1

65 4

9 8 7

9 8 7

6 5 4

3 2 1

9 8 7

6 5 4

3 2

1

9 8 7

6 5 4

3 2 1

3 2 1

6 5 4

9 8 7 7

6 5 4

3 2 1

9 87

A

B

C

.

Figure 4. Backward tracing of points from final position (C) to initial position (A) for the

“good” and “bad” ring

After piercing

"Good" ring "Bad" ring

3,4

1,8

3,2

2

4,1

2,42

1,31,8

0,94

3

1,5

Point number1 2 4 5 7 8

0

1

2

3

4

5

Area of pores

Figure 5. Comparison between the” good” and the” bad” ring after piercing

Figure 6. Results from ultrasonic testing of a ring. Large grey areas including white

dots indicate pores 5.2 A new pore closure concept in the manufacturing of heavy rings (Paper B) The proposed concept is based on the analysis of a manufacturing route of a heavy ring with serious pore closure problem. By using a commercial FE-code Qform, the entire hot forming process has been analysed with respect to accumulated effective strain and instant hydrostatic pressure. It is shown that the current route results in small strains close to the outer surface of the ring. This area is therefore prone to remaining pores. Figure 7 is a schematic diagram showing the steps in the forming process. The existence of pores has been confirmed both by ultrasonic tests and metallographic examinations,

10

figure 8. The manufacturing route begins with a heavy upsetting of a cylindrical billet by 54%. The workpiece is then pierced in a closed die by a punch, which penetrates it to within 28.6 mm of its base. With the mandrel still in place the workpiece is upset by an upper flat part forcing it to fill the die cavities. The remaining bottom disc is discarded by shearing in a separate operation.

200

∅ 128.3

∅ 225

∅ 369 ∅ 362

438

184.1165.1

28.6 Figure 7. Current manufacturing route: (a) Initial billet (b) billet after upsetting (c) End of piercing (d) Discarding the bottom disc

“GOOD” RING

“BAD” RING

AA7

8

Figure 8. Ultrasonic test results. White areas reveal pores. Approximately 6/7 of the “ bad” ring contains pores. The “good” ring is pore free

11

Through the analysis, piercing has been identified as a major step in creating

quipped with this knowledge, an alternative-forming route for the ring has

he FE-analysis of the new concept showed a great improvement in the

large strains in the workpiece. Material flow during this process has been analysed and is shown by velocity vectors in figure 9. In step 4 figures 9, it’s clear that the bulk of the material that is in contact with vertical part of the die remains stationary, indicating that the strains obtained in this region are low.

1 3

2 4

Figure 9. Material flow during piercing operation

Ebeen proposed. The aim is to increase the accumulated strain in the product especially in critical areas close to its outer surface. The new concept therefore proposes initially a lighter upsetting of the cylindrical billet figure 10. This results in a higher but slender “pan cake”. The subsequent piercing is carried out in a die with a smaller diameter so that the walls of the die, figure 10c, support the envelope surfaces of the workpiece. Using the same punch as before, the workpiece is then pierced. The heavy shear created by the punch should get a better chance to spread out throughout the whole thickness of the ring. Tcritical areas. The strains obtained are approximately 160% larger. The concept however implies a risk for the formation of a fold on the inner surface of the ring during closed die upsetting, figure 11.

12

165.1

369

b)

∅ 310.8

274

∅ 310.8

274 c)

a)

∅ 225

438

e)∅ 310.8

280.8

d)28.6

∅ 310.8

280.8

∅ 128.3

184.1

Figure 10 New manufacturing concept route: (a) initial billet (b) billet after upsetting( c) billet before piercing (d) end of piercing (e) discarding the bottom disc ( f) heavy closed die upsetting

a) b) c) d) Figure 11. New concept for upsetting in a closed die: (a) formation of gap close to the punch (b) closure of the gap (c) rapid filling of the gap and formation of flash (d) sealing of the gap.

13

5.3. Grain size as influence by process parameters in copper extrusion (Paper C) The objective of this work is presented in figure 12. Results from this analysis are meant as guidelines for paper D. In this work the influence of process parameters like temperature, strain and strain rate on the final grain size during hot extrusion of coarse-grained copper billet was analysed. Using FE-code Qform, simulations for different material elements located at different distances from the axis of symmetry of the initial billet figure 13 were carried out. Parameters describing the deformation history of the elements were traced out throughout the process for different workpiece temperatures and reduction ratios. Results are presented as functions of time. An example is presented in figure 14.

( ) ( ) ( )tTtt ;; εε &

Benificial working condition

FE- simulation

FE- simula-tion

Experi-ment

Materi-al flow

Grain size

Micro structure

Figure 12. Picture showing the principle aim of the work

ABC

Figure 13. Initial positions of material elements A, B and C

14

0

1

2

3

4

5

0 1 2 3 4

Time [s]Ef

fect

ive

stra

in

A

BC

Figure 14. Effective strain as a function of time for material elements A, B, C.

T= 650 0C, A0/A1= 10 Experiments corresponding to the simulations were carried out. Microstructural changes during high temperature deformation for the selected material elements were characterized by optical microscopy and the grain size was evaluated by the linear intercept method. The results are presented with corresponding effective strain and temperature. The smallest grains were found close to the extrudate surface. The most fine-grained microstructure was found for the highest extrusion ratio and lowest workpiece temperature. For these conditions the as-cast average grain size was reduced from d= 1.9 mm to within the interval of 23 µm ≤ d ≤ 28 µm. It has been concluded that low workpiece temperature together with high strain is favourable for fine microstructure. The results of the work are summarized in figure 15.

010

203040

50

6070

80

90100

0 1 2 3 4 5 6Strain

Tem

pera

ture

incr

ease

[0 C

]

A

B

C

25 µm < d < 31µm T0 = 650 oC 2.53 < ε < 4.57

26 µm < d < 37 µm T0 = 750 oC 2.48 < ε < 4.8

23 µm < d < 29µm T0 = 550 oC 2.65 < ε < 4.06

A0/A1= 15

A0/A1 =10

Figure 15. Influence of temperature increase and strain on the final grain size for elements along A, B, C after various extrusion conditions

15

5.4. Manufacturing of copper canisters by extrusion (Paper D) The objective of this work is to analyze the current manufacturing process of large copper canisters by hot extrusion with respect to temperature and strain distribution in the product. The analysis was carried out by sectioning the workpiece at every stage of forming into five sections. Strain and temperature were recorded and the results are presented as functions of workpiece radius figures 16 and 17. Levels obtained in the analysis were compared to those obtained earlier in a similar work [1], which ensured a fine microstructure in the product. Forming loads during piercing and extrusion have been determined. Modifications on the geometry of the punch and on the semi die angle were carried out. Results due to modifications in terms of strain distribution and load are compared with the current process.

A

B

C

D

E

0 20406080100Figure 16. Sections A-E of the workpiece after piercing

1

1,5

2

2,5

3

3,5

4

0 20 40 60 80 100

Percentage wall thickness

Stra

in

EA

DCB

500

520

540

560

580

600620

640

660

680

700

0 20 40 60 80 100

Percentage wall thickness

Tem

pera

ture

ED

C

B

A

Figure 17. (a) Strain distribution after piercing (b) Temperature distribution after piercing

16

Closed-die piercing was identified as a vital stage in the forming process. Large strains are introduced in the workpiece by the action of the punch. Due to complex material flow dictated by the geometry of the punch on addition to temperature and interface boundary conditions, strain distribution at this stage is highly inhomogeneous. The inner surface of the workpiece close to the punch is more deformed than the peripheral regions. The temperature profile is also uneven. Areas close to the tools are relatively colder because of heat dissipation to the tools. During direct extrusion, the peripheral areas with low strains are subjected to heavy shearing. This raises the strain levels in those areas with the result that the final product after extrusion has strains that fall within the interval, which ensures a fine post-deformation microstructure. It is concluded that the strain levels in the tubular after extrusion are within the interval that ensures a fine microstructure according to [1]. A 450-semi die angle results in a more homogeneous than a 900-semi die angle. It also requires a smaller load at the beginning and at the end of the process 5.5 Improving the grain size in the forging of large copper lids (Paper E)

In this paper an alternative way of forging the lids is presented. Based on the recent full- scale experiments, the locks had been fabricated by a two-step forging method; upsetting of a cylindrical ingot between two parallel flat tools followed by cogging to fill die cavity. The initial ingot measured ∅350 mm and 1400 mm in height. This geometry resulted in buckling during upsetting and made repeated straightening necessary. This forging procedure showed a poor material yield and resulted in coarse-grained microstructure in the lids close to the final midpoint contact surfaces. The alternative way is focused on improving the microstructure by promoting higher strains in the critical area. In this project, the following is carried out. To avoid buckling, and make the process faster, an initial ingot of large diameter of ∅500 mm instead of ∅350 mm is proposed and analyzed for upsetting and ordinary closed die forging. The coarse grained microstructure is thought to be due to the dead metal zone. To increase the strain levels in this region and thus eliminate this dead zone, the flat tools are furnished with a central protruding part of spherical shape. Figure 18 shows layers on which strain is analysed while figure 19 presents the effective strain distribution on the layers before modification of the tools. Figure 20a presents the modified tool and 20b the effective strain distribution after upsetting with it.

17

Figure 18. Material layers L1, L2 and L3

r

L1L2

L33

1

1,5

2

2,5

Effe

ctiv

e st

rain

0,5

0

L1L2L3

0 500 100 200 300 400 r [mm]

Figure 19. Effective strain distribution after upsetting using flat tools from a height h0= 538 mm to h1= 140 mm

18

Effe

ctiv

e st

rain

L1

L3

L2

r [mm] 400 500 300100 2000

3

2,5

2

1,5

1

0,5

0

b)

Rd a)

Figure 20. Picture showing (a) the definition of the radius R and penetration depth d (b) the strain distribution after upsetting with R= 50 mm and d= 30 mm from h0= 538 mm to h1= 140 mm Dies for closed-die forging have been designed starting from the geometry and dimensions of the finished machined product figure 21a. Almost pure copper is readily forged and therefore a single cavity die is considered. The design incorporates generous fillet and corner radii where the lines defining the forging geometry intersect in order to avoid die wear and premature die failure and to eliminate abrupt directional changes. To ensure easy separation of the dies and the workpiece without any griping or locking, all the vertical lines defining the forging profile are replaced with sloped lines representing the draft angles. Machining allowance is also included in the design figure 21b. Figure 21c illustrates the profile of the dies generated by FEM with the inserted workpiece ready for forging.

19

h1

125 mm

5o

10o 15o

R

c)

t0

b)

69

∅ 1104

∅ 814

140 83.5

∅ 976a)

Figure 21. Schematic picture for die design (a) Machined product (b) As-forged semi-finished product including machining allowance and draft angles (c) Die cavities with a workpiece inserted, including notations It is concluded that it is possible to get rid of the coarse-grained microstructure close to the surface centres of the lids by furnishing the flat tools with a central protruding part of spherical shape. The proposed method encompasses a change in the geometry of the initial billet from a diameter of 350 mm and a height of 1400 mm to the one of 500 mm and height of 540 mm. The proposed die profiles in closed-die forging result in a product with flash, that is close to the dimensions of the final product. The load requirement for the operation is approximately 25 000 ton i.e. less that the specified limit of 30 000 ton.

20

5.6. Closed-die forging of large Cu-lids by a method of low force requirement (Paper F) The analysis focuses now on studying of a method of fabrication of the lids requiring a small forging load. Proposed by Scana Steel Björneborg AB, a forging company in Sweden specializing in heavy forging, a flashless forging sequence of the lids is divided into two phases. The first phase is axisymmetric and includes axial upsetting of a cylindrical billet, followed by open die piercing with a spherical punch and finally axisymmetric closed die forging up to the maximum available press load of 4600 ton, figure 22 a-d. The second phase is non-axisymmetric where a tool of rectangular cross-section and parallel to the bottom die is used to bite off small areas of the workpiece peripheral surface and forge them into the deeper cavity of the bottom die. The bottom die is rotated about 450 every after such pressing, figure 22f. FEM-simulations for the process are carried out to analyse accumulated effective strain and temperature distribution in the final product and to predict load requirement at different stages of the process. Full-scale trials have been carried out. Post forming grain size from different specific locations on the vertical cut coinciding with the line of symmetry, has been evaluated by the standard comparison procedure. These results are roughly co-related to the effective strain and temperature distributions in the product obtained through simulations. It’s concluded that 4600 ton is adequate to form the lids and that high-accumulated effective strain and low temperature are coincidental with fine grain size. The method results in an average grain size throughout the product, which is less than the specified limit of d< 360 µm, figure 12.

21

d a

Peripheral cogging e

b Diagonal

cogging

c f

Figure 22. Schematic picture showing changes in workpiece geometry and how the forming sequences are performed. (a) Initial billet (b) Billet after upsetting (c) after piercing (d) before closed die forging (e) after closed die forging (f) Cogging of the peripheral part of the workpiece followed by “diagonal forging” including the central protrusion

22

in

. Concluding remarks

g processes through computer simulation has ecome essential to ensure that those parts that have the requisite shape, size

l codes Qform-2D and Deform-3D have been sed to analyse metal forming in different industrial processes. The objective

has been to get better insight of the processes and the parameters that govern

gure 12. Evaluated grain size and photomicrographs for specified locations of the ished product

Location 5 Grain size: 64µm

Location 12 Grain size: 127µm

Location 13 Grain size: 90µm

100µm

100µm

100µm

Ffi

6 Optimisation of metalworkinband mechanical properties are produced. Considerable effort and attention has been devoted to the development of constitutive models, simulation software and other systems. In this work, two commerciau

23

them. The majority of the processes analysed take place at elevated temperatures and high strain. Effective simulation of these forming operations requires input of accurate data relevant to the processing conditions. The results so obtained have to be validated by experiments if they are to be reliable. Two industrial processes have been analysed. The first one is the manufacturing of heavy bearing rings with pore closure problem. In this

rocess, Qform-2D has been utilized to locate areas of low strain and low

waste. rom other research works it has been shown that a fine microstructure is

phydrostatic pressures in the product. Such areas in the deforming workpiece have been shown to be prone to remaining pores. Ultrasonic tests and metallographic examinations have confirmed predictions made using FEM. By taking less pre-forming of the workpiece and altering the geometry of the intermediate tool, it has been shown that pore prone areas gain in effective strain and thereby minimizing the likelihood of having residual pores. The second process analysed is the manufacturing of large copper canisters of φ≈ 1000 mm and ≈ 5000 mm in height, for encapsulation of nuclearFvital for corrosion and creep resistance. The specified limit for such canister is d < 360µm. The current manufacturing involves hot extrusion of copper ingots to a seamless tube. During piercing heavy strains are obtained close to the inner surface of the workpiece and after the subsequent extrusion the whole cross-section of the tube has been up to high values of the accumulated strains. The obtained microstructure was found to be fine-grained. The forces necessary for this manufacturing sequence is however big. Qform has been used to study the influence of process parameters such as temperature and strain on the final grain size. On a laboratory scale, the influence of the process parameters on the final grain size has been studied. Simulations of different material elements located at different distances from the axis of symmetry have been traced out throughout the process. Experiments corresponding to the simulations have been carried out by metallographic examinations. In that way specific grain size has been mapped on specific process parameters. This knowledge has been extended to analysing the full-scale industrial process mentioned above. It is concluded that hot extrusion results in a tubular with a strain distribution that ensures a fine microstructure. - It is also shown that the large lids for the canister can be manufactured with a low forging load. The process includes upsetting from a cylindrical billet followed by open-die piercing. The as-pierced billet is then closed-die forged axisymmetrically till the maximum forging load of 4600 ton is reached. For filling the deeper part of the cavity with the low force, peripheral and the diagonal cogging is used. No flash is

24

formed. Because of the fact that the cogging means that material elements are deformed many times, the grain size is found to be acceptably small. 7. References

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