Selective Laser Melting of an Oxide Dispersion Strengthened Steel: Microstructure and Tensile Testing

1. Thomas Boegelein1,2, Sébastien N. Dryepondt1, Karl Dawson2, Amit Pandey1, Gordon J. Tatlock2 2nd ODDISEUS workshop March 2015 Selective Laser Melting of an Oxide Dispersion Strengthened Steel: Microstructure and Tensile Testing 1Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 2 Centre for Materials and Structures, School of Engineering, University of Liverpool, L69 3G, UK

2. Limitations of ODS materials Introduction • Relatively high costs of ODS materials • Inflexible sample configuration • Anisotropic properties • ODS precipitates cannot be retained when conventionally welded [1] [1] ODS processingcost!!! [1] I.E. Anderson et al., Workshop on Oxide-Dispersion-Strengthened Alloys for Nuclear Applications, University of Oxford, 24-26 September 2012. 2

3. Selective laser melting Introduction PM2000 (wt.%) All work presented was done on as-MA PM2000 • Additive layer by layer rapid prototyping technique • Repeated deposition of thin layers (50 μm) by successively laser melting • Fully dense solid freeform components from powder beds 3

4. Selective laser melting Introduction (real time) ~10mm • Additive layer by layer rapid prototyping technique • Repeated deposition of thin layers (50 μm) by successively laser melting • Fully dense solid freeform components from powder beds 4

5. Motivation of this work Introduction 5

6. Complex thin walled structure Previous work ~10mm • Parameters were not optimised • Build defects such as cracks, inclusions, segregation of Y-rich slag (probably a fractured scale on the wall surface (~30% of total Y in the alloy) The build was produced by Dr E. Louvis, Manufacturing Science and Engineering Research Centre , University of Liverpool 6

7. Complex thin walled str. Previous work 1200°C /1h (Cu grid) Annealing results to the transformation into single phase Y-Al-O (mainly YAM) Multiphase particle containing O, Al, Ti, Cr, Fe, Y.. (as-grown condition) (annealed condition) (extracted particles) 7

8. Complex thin walled structure Previous work • ODS particles retained with a size distribution similar to conventional PM2000 (as-grown) (annealed) New small dispersoids • Annealing results in smaller dispersoids, probably due to nucleating and grain growth with remaining Y in atomic solution • Coarsening is present in parallel • Ultrafine precipitates perhaps surving MA and melting might act as nuclei Particle coarsening • Particle formation in both, molten and liquid material 8

9. Parameter development Presentation of the builds to be tested • Energy input ↓ • Surface quality ↑ • Build defects ↓ • Producing walls with different thicknesses, coatings[1], solid builds [1] T. Boegelein et al., Proc. HTCPM (Les Embiez) 2012. 9

10. Overview of test builds Presentation of the builds to be tested SLM builds to be tested: Walls fabricated with 1, 2, 3 parallel laser scans per deposit layer (>99.5% dense) Solid blocks (>98.5% dense) Strategy 10

11. Overview of test builds Presentation of the builds to be tested SLM builds to be tested: Walls fabricated with 1, 2, 3 parallel laser scans per deposit layer (>99.5% dense) Solid blocks (>98.5% dense) Strategy Vertical ribs for build stabilisation 11

12. Overview of test builds Presentation of the builds to be tested Columnar grains Scan line Double-crescent grain shape 12

13. EBSD study Presentation of the builds to be tested 13

14. Dispersoids Microstructural characterisation • Fine homogeneous distribution of ODS particles retained for all builds • Agglomerations of particulates were never observed • Idea: Ultrafine precipitates perhaps surviving MA and melting might act as nuclei (SLM wall 1) 14

15. Dispersoid size distribution Microstructural characterisation • Thin walls show a size distribution similar to conventional PM2000 and the SLM hexagon structure • A certain amount of Y is probably still in solution in the matrix or present in the form of ultrafine precipitates 15

16. Dispersoid size distribution Microstructural characterisation • Thicker walls (3x) show significantly finer dispersoids • This is perhaps due to nucleation and particle growth due to an increased number of repeated heating cycles compared to SLM wall 1 16

17. Dispersoid size distribution Microstructural characterisation • Blocks have the highest energy input of the fabricated builds • Dispersoids have significantly coarsened (Ostwald ripening) • Reservoir of Y leading to particle nucleation and growth is probably exhausted 17

18. Overview of test builds Presentation of the builds to be tested SLM builds to be tested: Walls Solid blocks Conventional PM2000: As-extruded sheet taken from barRecrystallized sheet taken from bar As grown / annealed 1200°C/1h Microtensile specimen 4 mm 18

19. Overview of test builds Presentation of the builds to be tested SLM builds to be tested: Walls fabricated with 1, 2, 3 parallel laser scans per deposit layer (>99.5% dense) Solid blocks (>98.5% dense) Conventional PM2000: As-extruded sheet taken from barRecrystallized sheet taken from bar Microtensile specimen 4 mm 19

20. Overview of test builds Presentation of the builds to be tested SLM builds to be tested: Walls fabricated with 1, 2, 3 parallel laser scans per deposit layer (>99.5% dense) Solid blocks (>98.5% dense) Conventional PM2000: As-extruded sheet taken from barRecrystallized sheet taken from bar (cross sectioned/etched) Microtensile specimen 4 mm 20

21. The typical tensile behaviour Microtensile testing at room temperature Approx 3x accelerated (load-unload-reload tests) • The typical fracture behaviour for SLM builds and reference material: elastical deformation → plastical deformation (incl. necking) → fracture 21

22. The typical tensile behaviour Microtensile testing at room temperature Conv. PM2000 as-extruded Conv. PM2000 recrystallized Typical SLM wall as-grown Typical SLM wall annealed • The resulting type of the stress-strain plot was typically for both, SLM builds and recrystallized reference material. 22

23. Key results Microtensile testing (RT) SLM walls SLM blocks Reference material Summary of 0.2% offset yield strength data Standard deviation is smaller than the data points 23

24. Key results Microtensile testing (RT) SLM walls SLM blocks Reference material • SLM blocks are subject to non-randomly oriented porosity, which affected YS 24

25. Key results Microtensile testing (RT) 57 Mpa 53 Mpa SLM walls SLM blocks Reference material • Testing perpendicular to the grain orientation increased YS • Perhaps due to changes in the orientation of slip planes due to a texture 25

26. Key results Microtensile testing (RT) Recrystallised reference material SLM walls SLM blocks Reference material • Annealing significantly increases the YS of SLM walls in both directions • Post-build dispersoid formation and perhaps changes in matrix-particle coherency (strongest effect for walls produced with the lowest heat input) 26

27. Microtensile testing at room temperature Fracture surface Build direction 25 27

28. Microtensile testing at room temperature Fracture surface Build direction Brittle fracture surface (transgranular) no effect of annealing Very similar to conv. recr. extruded material 25 28

29. Microtensile testing at room temperature Fracture surface Build direction 25 29

30. Microtensile testing at room temperature Fracture surface Ductile fracture with dimples (transgranular) Anisotropy; Dispersoids act as stress-concentration features Anisotropy / Dispersoids act as stress-concentration features Build direction effect of annealing Varying ductile fracture types (transgranular) Explanation: New fine strengthening precipitates / change in particle-matrix coherency relationships 25 30

31. Microtensile testing at room temperature Fracture surface Build direction Both ductile and brittle regions (transgranular) when tested in both X and Z More variety in the direction of grain orientation High level of porosity in the fracture surface Pores act as stress concentration features and reduce the cross sectional area 31

32. Conclusions • SLM-ODS is feasible • Annealing of walls increased YS up to conventional recrystallised PM2000 • Formation of new small strengthening dispersoids during annealing (post-build or during deposition) • Source of Y might be Y still in solid solution in the as-grown state or present in the form of ultrafine particles • YS and fracture mechanism were anisotropic due to a strong [001] fibre texture in build direction • Further work: • Studies on particle morphology and their formation•SLM of other ODS alloys •Developing new strategies/alloys to eliminate build defects and control the precipitate size and grain structure 32

33. Acknowledgements • The staff at the Manufacturing Science and Engineering Research Centre at the University of Liverpool in particular Dr J. Singh and Dr J. Robinson • U.S. Department of Energy, Office of Fossil Energy managed by U.T.-Battelle, LLC • EPSRC grant EP/H018921/1 (Materials for Fusion and Fission Power) 33

34. More information T. Boegelein, S.N. Dryepondt, A. Pandey, K. Dawson, G.J. Tatlock (2015) Mechanical Response and Deformation Mechanisms of Ferritic Oxide Dispersion Strengthened Steel Structures Produced by Selective Laser Melting. Acta Materialia, 87, 201-215. T. Boegelein, et al., Characterisation of a Complex Thin Walled Structure fabricated by Selective Laser Melting using a Ferritic Oxide Dispersion Strengthened Steel, manuscript in preparation. 34

35. Overview Presentation of the builds to be tested • Double-crescents around the scan line • Very low level of porosity 35

36. Overview Presentation of the builds to be tested Top view Side view • Higher level of (equiaxed) porosity and inclusions • Stirred grains due to the scan strategy (altered direction of the heat flow) 36