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«Microstructural and Mechanical Property Characterization of Laser Additive Manufactured (LAM) Rhenium by Robbie Adams A Dissertation Presented in ...»

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Microstructural and Mechanical Property

Characterization of

Laser Additive Manufactured (LAM) Rhenium

by

Robbie Adams

A Dissertation Presented in Partial Fulfillment

of the Requirements for the Degree

Doctor of Philosophy

Approved April 2012 by the

Graduate Supervisory Committee:

Nikhilesh Chawla, Chair

James Adams

Stephen Krause

ARIZONA STATE UNIVERSITY

May 2012

ABSTRACT

This report will review the mechanical and microstructural properties of the refractory element rhenium (Re) deposited using Laser Additive Manufacturing (LAM). With useable structural strength over 2200 °C, existing applications up to 2760 °C, very high strength, ductility and chemical resistance, interest in Re is understandable. This study includes data about tensile properties, fatigue and microstructure including deformation systems and potential applications of that information. The bulk mechanical test data will be correlated with nanoindentation and crystallographic examination. LAM properties are compared to the existing properties found in the literature for other manufacturing processes. The literature indicates that Re has three significant slip systems but also twins as part of its deformation mechanisms. While it follows the hcp metal characteristics for deformation, it has interesting and valuable extremes such as high work hardening, potentially high strength, excellent wear resistance and superior elevated temperature strength. These characteristics are discussed in detail.

i

ACKNOWLEDGMENTS

I would like to thank Prof. Nikhilesh Chawla for his guidance and assistance with this research. The following past and present group members provided assistance: Jason Williams and Ling Jiang of the ASU Mechanical Laboratory, Grant Crawford, Martha Dudek, Danny Singh, Xin Deng, Kyle Yazzi, Eric Padilla and Nick Chapman. I would also like to thank the Honeywell management for funding a portion of this research and the following Honeywell Materials Laboratory staff: Mr. Jim Hartman, Mr. Russ Bartos, Mr. Don Benjamin and Mrs. Diana Chapa. Dr. ZhenQuan Liu of the ASU Center for Solid State Materials provided significant assistance with microscopy. I would like to acknowledge the assistance of AeroMet Corporation for early funding and Mr. Frank Arcella of AeroMet for guidance and support in this project. Mr. Todd Leonhardt of Rhenium Alloys Inc. (RAI) also provided guidance. Also, RAI provided raw material to AeroMet. Initial ultra-high tensile testing was performed at Southern Research Institute (SORI) in Birmingham AL and early scanning electron micrcroscopy was performed at the University of AL Birmingham under the direction of SORI. Finally I would like to thank my family for their support in this endeavor.

–  –  –

LIST OF FIGURES

1. INTRODUCTION

2. LITERATURE REVIEW

Processing

Rhenium Microstructure

Dislocation Systems in Rhenium

Dislocation Systems in Rhenium Single Crystals

Stacking Fault Energy

Twinning

Work Hardening (Stress vs. Deformation)

Rolling Effects

Metallographic Characteristics

Rhenium Physical and Mechanical Properties

–  –  –

Indentation Testing

3. RESEARCH OBJECTIVES AND APPROACH

Characterize Microstructure

Grain Size and Morphology

Crystal Orientation

Determine Nanoindentation Properties

–  –  –

Testing at Ultra-High Temperatures and Fractography

Fatigue Testing and Fractography

4. CHARACTERIZATION

Introduction

Chemical Analysis

Materials and Experimental Procedure

Results and Discussions

Microstructural Characterization

Materials and Experimental

Results and Discussion

Orientation Image Mapping

Materials and Experimental Procedure

Results and Discussion

Nanoindentation

Tensile Testing

Materials and Experimental Procedure

Results and Discussions

Fatigue Testing

Materials and Experimental Procedure

Results and Discussion

Fractography of Tensile and Fatigue Specimens Using Electron Microscopy....... 64 Materials and Experimental Procedure

Results and Discussion

–  –  –

6. FUTURE WORK

REFERENCES

APPENDIX A. Re TENSILE TOOLING CALCULATIONS

–  –  –

2. Summary of Re Twinning Systems.1, 2, 3

3. Hexagonal Metal Average Critical Resolved Shear Stress Metals.2

4. Summary of Typical Properties. 1, 5, 6, 7

5. Comparison of Re Properties to Other hcp Materials.1, 5, 6

6. Room Temperature Properties of Re.31

7. Elevated Temperature,1371 °C, Properties of Re.31

8. Biaglow RT Fatigue Tests.

9. Chazen RT Fatigue Tests.

10. Direction Cosines of Selected Grains.

11.Tensile Data Provided by Southern Research Inc. using Carbon Tooling................... 56

12. Pure Re Transverse Tensile Tests at 1925 °C performed at Honeywell..





13. Fatigue Test Results.

–  –  –

2. Major components of a Laser Additive manufactured system.17, 18.

3 Photograph that shows AeroMet Laser Additive Manufacturing system.18................... 33

4. XPS Spectrograph Graph showing the presence of tungsten.

5. WDS and EDS clearly reveals the presence of W in the WDS graph but not the EDS graph. The large white peak is the Re EDS spectrum and the gray peaks are the WDS output..

6. As-deposited LAM Re plate

7. Typical wrought equiaxed grains found in PM or PM HIPed shapes. Used as the starter base for the plate deposited in Figure 4

8. LAM Re microstructure in the Y-X plane that shows approximately equiaxed grains.

9. LAM Re microstructure in the X-Z plane, shows a characteristic wavy pattern in the build direction.

10. Cross-section of LAM Re microstructure in the X-Z plane.

11. Three dimensional view of pure LAM Re microstructure

12. Schematic of the electron back scattered diffraction (EBSD) arrangement.62............ 42

13. OIM image showing local areas of misoreintation with associated planes and modulus with additional plane data.

14. OIM image showing local areas of misoreintation with associated planes and modulus

–  –  –

17. Comparisons of metallographic, OIM matching crystal orientation and a SEM photograph of same segment.

18. Comparison of nanoindentation measured Young’s modulus and values calculated to date for Re.

19. Sample locations for mechanical property testing of pure Re

20. High temperature furnace and load train

21. Schematic of SORI tensile specimen and view of location within the LAM plate prior to extraction

22. Honeywell tensile specimen drawing used for ultra-high temperature testing........... 55

23. LAM pure Re strength and strain to failure tested at SORI properties of pure Re.

24. Graphical representation of stress vs. apparent strain seen in pure Re when tested at 1925 °C

25. Welded fatigue specimens

26.Plot of fatigue results from Table 12 for pure Re.

27. Stress strain curve from first cycle of RT fatigue of pure LAM Re specimen 1042-2-5

28. Graphical representation of stress vs. apparent strain is seen in pure Re sample 1042-2-6 when tested at RT at various cycles. Note the change in width of the hysteresis loop. The smaller width in the later loop may indicate strain hardening.

29. Evidence of strain hardening due to fatigue cycling is shown by the valley to peak distances. Note in the early cycles the total strain is increasing.

30. Micrograph of pure LAM Re fracture features tested at 1925 °C and at 1371 °C respectively.

–  –  –

32. Tensile fracture features found at 1925 °C.

33. Photomicrograh shows grain morphology of fatigue specimen 1042-2-6 after completion of test. Deformation from test shows grain relief.

Note grains are roughly parallel to fatigue specimen axis................... 68

34. Overview of specimen 1042-2-6 with the specimen axis transverse to the deposition (travel) direction but parallel to the build direction labeled axial. Note a very different fracture morphology that the previous specimen.

35. Fracture features showing tearing features resulting from fatigue stress transverse to the specimen axis and the elongated columnar grains in fatigue specimen 1042-2-6.

36. Optical picture at fracture showing orientation of grain shape as transverse to 1042-1-5 specimen axis but parallel to build direction

37. Optical photo graph shows fracture of specimen 1033-2 with a more obvious fracture at the grain boundary.

38. Note undulating fracture surface of specimen 1042-1-5. See Figure 39. Note that this specimen axis was transverse to the grain axis

39. Fatigue fracture surface of specimen 1042-1-5. Note the fine ridges in the lower left picture. They appear to be striations.

–  –  –

Rhenium (Re) is a refractory metal with many attractive properties and valuable commercial uses but with manufacturing and availability challenges.1-7 It is classified as a refractory because it has a melting point of 3180 °C.1 About 70 percent of Re is used as an alloying element in super alloys to maintain strength at jet engine operating temperatures.8 Oil refineries employ another 20 percent as a catalyst in combination with platinum.1, 4, 7, 8 The combined uses account for approximately 90 percent of Re production.8 Other applications include use in ultra-high temperature rockets and heavy wear and/or arc erosion electrical switch components, thermocouples. Alloying rhenium with tungsten and molybdenum significantly increases the ductility of both materials at room temperature.7 The optimum combination of strength and ductility occurs with 20 to 30 percent of Re alloyed with tungsten and 40 to 50 percent alloyed with molybdenum.6 Knowledge of Re begins with Walter and Ida Noddack (Tacke) and O. Berg when they discovered it in 1925 in Germany.4 It was the last natural element found. Re is a byproduct of copper and molybdenum mining and its ore is produced in Arizona, Germany, Siberia, South America and other locations.4, 8 Re metal is produced from several processes and one commonly used process is extraction from molybdenum ore.

Molybdenum flue dust containing rhenium is oxidized, trapped in a filter then liquefied in an aqueous solution, mixed with potassium chloride, and neutralized with ammonium hydroxide. Ammonium perrhenate (NH4ReO4) is precipitated from the resultant solution.1, 4 The precipitate is then converted to pure rhenium metal in a hydrogen atmosphere.4, 7 Re has mechanical properties that are attractive compared to other elevated temperature materials.1, 4, 7, 9, 10 Unlike some refractory metals such as tungsten and molybdenum, Re is ductile at room and cryogenic temperatures having no brittle-toductile transition temperature.1, 4 Yet between room temperature and 1200°C, Re has twice the strength of tungsten.1 In the annealed state at room temperature (RT), Re has strength and ductility analogous to 300 series stainless steels (SS). Many SS have an ultimate tensile strength (UTS) to yield strength (YS) ratio of approximately two and an elongation ranging from 30 to 50 percent. Annealed Re UTS to YS ratio is approximately 3 and elongation of conventionally produced Re at room temperature has been reported as higher than 30 percent.1, 4, 7 Such a spread can be indicative of high ductility but also high work hardenability. In fact rhenium Re work hardens at a very high rate with a strain-hardening exponent of 0.353.1, 4, 7 The ductility combined with high hardenability allows it to be worked to a UTS of over 2 GPa.1, 4, 7. It also has a high elastic modulus typically reported as 460 GPa.1, 4, 7 That is much higher than steel with a modulus of 206 GPa. Another of its outstanding mechanical properties is useable strength in neutral or reducing atmospheres approaching its melting point i.e. a strength of 48 MPa at 2204 ○C.10 Some other commercial alloys would be liquid at these temperatures.

Surprisingly, since it is very ductile, Re resists wear effectively.1, 2, 4, 7, 11, 12 Normally ductile materials are not wear resistant. Re is also very resistant to chemical attack in many environments including sulfuric and hydrochloric acid1, 4, 7 and it is highly resistant to seawater attack.1, 4 In neutral or reducing environments Re has been shown to be highly erosion resistant to arcs.4, 7 However, it is readily dissolved by oxidizing acids such as nitric.4, 7 Unfortunately, Re is not oxidation resistant. The pure metal oxidizes at low temperature and produces a volatile oxide, rhenium heptoxide, Re2O7, at approximately 360 °C.1, 4, 7 At higher temperatures in oxidizing environments, it may oxidize catastrophically. Re alloys that are resistant to oxidation at the low portion of the useful elevated temperature ranges have been created but are not the subject of this study.

These excellent combinations of properties result from its electronic structure. Re is a member of the Group VII B in the sixth period of the periodic table and is part of the group considered transition metals.1, 4 It has an atomic number of 75 with a valence state between +7 and –1. Its atomic weight is approximately 186.3 and a theoretical density of



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