Microstructure and residual stresses in AM metallic parts: Do we know what we do not know?

Transcription

1 10 th International Symposium on NDT in Aerospace Microstructure and residual stresses in AM metallic parts: Do we what we do not? Tobias THIEDE, Tatiana MISHUROVA, Naresh NADAMMAL, Arne KROMM, Johannes BODE, Sandra CABEZA, Giovanni BRUNO BAM Bundesanstalt für Materialforschung und -prüfung, Berlin, Germany Contact ( Abstract The freeform and the revolutionary design possibilities offered by additive manufacturing have skyrocketed the amount of optimization studies in the realm of engineering, and metallic additive manufactured parts are becoming a reality in industry. Not surprisingly, this has not been paralleled by a similar enthusiastic wave in the realm of materials science, and still very little is n about AM materials properties. This has the consequence that, typically, classic materials properties are still used in design and even in simulations. In this talk, I will give a few examples of how necessary it is to dig a lot deeper than at present, in order to understand these new materials classes, and in particular their microstructure and their internal stresses, largely different from their cast or wrought companions. License: 1

2 MICROSTRUCTURE AND RESIDUAL STRESSES IN AM METALLIC PARTS: DO WE KNOW WHAT WE DO NOT KNOW? Giovanni Bruno MICROSTRUCTURE AND RESIDUAL STRESSES IN AM METALLIC PARTS: DO WE KNOW WHAT WE DO NOT KNOW? Tobias Thiede, Tatiana Mishurova, Naresh Nadammal, Arne Kromm, Johannes Bode, Sandra Cabeza, Giovanni Bruno B B - Boss 1

3 Knowledge Knowledge Simplifying the Universe We do NOT that we We that we Awareness We do NOT that we do NOT We that we do NOT 3 Avoid living dangerously We do NOT that we We that we Awareness We do NOT that we do NOT We that we do NOT 4 2

4 Example In Additive Manufacturing everybody is talking about - Free Form - Unconventional Design Ramesh R, PMC Tech - Re-thinking Components - Think out of the box TU HH 5 FEM Simulations However, when it comes to FEM Simulations If we ask the question: Which material properties are we using? The answer is: Literature values, for Conventional Materials Examples from KU Leuven (polymer) If we ask the question: What about residual stress? The answer is: We they are there, We heat treat with conventional HT 6 3

5 Knowledge Knowledge Where do you think we are? We do NOT that we We that we Awareness Common Case We do NOT that we do NOT We that we do NOT Best Case 7 What we should do (our mission at BAM) We do NOT that we We that we Awareness We do NOT that we do NOT We that we do NOT 8 4

6 Case Study: SLM IN718 parts Lots of modeling, little data 9 Macro and Microstructure Optical and Electron Microscopy Electron Backscattering Diffraction (EBSD) Coordinate Measuring Machine (CMM) 10 5

7 Advanced Characterization Methods Neutron Diffraction (ND) Bragg s law d Lattice parameter d 0 Reference Strain Hooke s law Stress 11 Stress Analysis by Diffraction Non-destructive Method The lattice is our gauge length 0 d d> d< d d d d

8 Strain Scanning by diffraction d/d ~ 10-4 Neutron beam I (a.u.) k 0 Q = k - k 0 Diffracting planes (hkl) d 0 d d (Å) 2 k Det Sample d hkl 2 3D Hooke s Law: Strain Stress (Phase Specific) Sampling volume x x = C 13 Samples SLM IN

9 Microstructure Influence of hatching Upskin 3 top layers 500 mm Max. ~12.5 Max. ~6 x10 Hatching = ½ texture intensity Nadammal et al. Mater.Des. 134 (2017) Residual Stress EBSD CMM u(σ L,T,N ) 45 MPa 600µm 600m 16 8

10 Influence of Hatch length Thiede et al. MPC 7 (2018) Possible Scenario SB A B Heat input: Heat output: A = SW W = 10SW WΤ10 p minimum for W = SW B SB BΤ10 A 10SB 10SB BΤ

11 Influence of the support structure Why support structures are important? Preventing cracking and compensate distortion Necessary for overhanging features Facilitating heat flux Easier and more precise removal from base plate 19 Microstructure Bulk sample Support structure Columnar shaped grains with preferred texture (<001>) Contact area between support and sample is small Contact point between support and sample Mishurova et al. Met.Mat.Trans. 49A (2018)

12 Synchrotron X-ray diffraction Subsurface residual stress (penetration around 100µm) Detector BESSY II, HZB, Berlin Sample EDDI beamline 21 RS mapping- von Mises stress Bulk As-built Released High tensile stresses (up to yield ) near the surface RS redistribution and relaxation after removal Support Support structure leads to reduction of RS HT cannot be avoided 22 11

13 Distortion Bulk Support The support structure gives more compliance to the sample and results in larger distortion. The stripe-like pattern correlates with period of the support structure. Lower residual stress corresponds to larger distortion. 23 Insights- 1- d 0 reference Requirement: quantitative assessment of stress Calculation of Strains by diffraction d d d Which reference state needs to be used? 1- Initial powder? Stress-free, but it did not undergo the same HT as the AM part 2- Small cubes extracted from the sample? Not completely stress-free (e.g., cutting) 3- Powders extracted from the sample? Possibly plastically deformed (filing)

14 Insights- 1- d 0 reference Lab XRD results: d 0 scan on the surface ND results on cubes (L, T, N), raw powder (RP), Sample powders (SP) 25 Takeaways- 1- d 0 reference 1- Initial powder? Not suitable: it did not undergo the same HT as the AM part, the chemistry is not the same 2- Small cubes extracted from the sample? Not suitable: Not stress-free 3- Powders extracted from the sample? Plastically deformed, but reproducible and macro-stress-free. OK Deeper analysis is required 26 13

15 Insights- 2- Principal stress axes // Texture In AM the deposition strategy strongly influences the microstructure, through columnar growth 1- What happens to the stress axes, if the hatching has a different geometry? Are the principal axes rotated (like in the case of a weld)? 2- Texture can be very strong. Does it influence the assessment of residual stress? 27 The sin 2 y technique From laboratory XRD we borrow a useful technique that allows determining the influence of texture and the principal axes Sample 2 Q= L 3 y // // y 0 y 0 Shear strain sin 2 y We tilt the sample perpendicular to the scattering plane P 3 y 2 1 // Texture sin 2 y 28 14

16 Insights- 2- Influence of Texture Neutron Diffraction (Bulk) Lab X-ray Diffraction (Surface) All show linear plots : No large influence of texture All show no difference for pos. and neg. tilts : geometrical directions are principal Synchrotron X-ray Diffraction (Sub-Surface) In this case, the classic RS analysis is valid 29 Takeaways 2- Principal stress axes // Texture In AM the deposition strategy strongly influences the microstructure, through columnar growth 1- In the case of IN Ni alloys principal axes are not rotated (unlike in the case of a weld) 2- Texture can be very strong. However, it does not influence the assessment of residual stress by diffraction methods. What happens to other materials/ alloys? 30 15

17 Knowledge Conclusions and Takeaways The large texture does not strongly influence Residual Stress Analysis We do NOT that we We that we Awareness Residual Stress and distortions in SLM IN 718 strongly reflect the hatch pattern and depend on the peculiar thermal history We do NOT that we do NOT We that we do NOT Powders extracted from the AM are the best macro-stress-free reference 31 Competence Centre Safety in Technology and Chemistry Safety creates markets. ONLY Safety creates SUSTAINABLE markets

18 Thank you for your attention. Contact: Giovanni Bruno Head of Division 8.5 Phone: