microstructure and residual stresses in am metallic parts: do we … · 2019-02-03 · 3 example in...

10th International Symposium on NDT in Aerospace

1 License: https://creativecommons.org/licenses/by/4.0/

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

BAM Bundesanstalt für Materialforschung und -prüfung, Berlin, Germany

Contact e-mail: [email protected]

(

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 known 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.

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1

www.bam.de

MICROSTRUCTURE AND RESIDUAL

STRESSES IN AM METALLIC PARTS: DO

WE KNOW WHAT WE DO NOT KNOW?

Giovanni Bruno

25.10.2018

www.bam.de

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 BrunoB

25.10.2018

B - Boss

2

NDT in Aerospace- Dresden 3

We do NOT know that we know

Simplifying the Universe

We do NOT know that we do NOT know

Know

ledge

We know that we know

Awareness

We know that we do NOT know

25.10.2018

4


Avoid living dangerously


Know

ledge


Awareness


NDT in Aerospace- Dresden25.10.2018

3

Example

In Additive Manufacturing everybody is

talking about

- Free Form

- Unconventional Design

- Re-thinking Components

- Think out of the box

5NDT in Aerospace- Dresden25.10.2018

TU HH

Ramesh R, PMC Tech

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…

6

If we ask the question:

What about residual stress?

The answer is:

We know they are there,

We heat treat…with conventional HT

Examples from KU Leuven (polymer)


4

7


Where do you think we are?


Know

ledge


Awareness


Common

CaseBest Case


8


What we should do (our mission at BAM)


Know

ledge


Awareness



5

Case Study: SLM IN718 parts

Lots of modeling, little data


Macro and Microstructure


Electron

Backscattering

Diffraction (EBSD)

Coordinate

Measuring

Machine (CMM)

Optical and Electron

Microscopy

6

Advanced Characterization Methods

Neutron Diffraction (ND)

d

Bragg’s law

Lattice

parameter

Strain Stress

Hooke’s lawd0 Reference


0d0

02

•Non-destructive Method•The lattice is our gauge length

Stress Analysis by Diffraction

0

0

d

dd

12

d< d0 0

22

d> d0 0


7

Strain Scanning by diffraction

d0

I (a.u.)

2d (Å)

3D Hooke´s Law: Strain

Stress (Phase Specific)

x

Q = k - k0

Neutron beam

Det

Sampling volume

Sample

Diffracting planes (hkl)

dhkl

k0

k

2

d/d ~ 10-4

d

= C

13

x


Samples – SLM IN718


8

Microstructure

Influence of hatching

Upskin – 3 top layers

x10 Hatching = ½ texture intensity

500 mm

15

Max. ~12.5 Max. ~6


Nadammal et al. Mater.Des. 134 (2017) 139-150

Residual Stress

𝑢(𝜎𝐿,𝑇,𝑁) ≤ 45 𝑀𝑃𝑎

16

600m600µm

EBSD

CMM


9

Influence of Hatch length

17NDT in Aerospace- Dresden25.10.2018Thiede et al. MPC 7 (2018) 20170119

Possible Scenario

𝑆𝐵 10𝑆𝐵

𝐵

Τ𝐵 10

B

10𝑆𝐵

SB

Τ𝐵 10

𝐴

𝐴


𝐴 = 𝑆𝑊 ∙ 𝑊 = 10𝑆𝑊 ∙ Τ𝑊 10

𝑝 minimum for 𝑊 = 𝑆𝑊Heat input:

Heat output:

10

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


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) 3038-3046

11

EDDI beamline

Detector

Sample

Synchrotron X-ray diffraction

BESSY II, HZB, Berlin

Subsurface residual stress (penetration around 100µm)


RS mapping- von Mises stress

•High tensile

stresses (up to

“yield”) near the

surface

•RS redistribution

and relaxation after

removal

• Support structure

leads to reduction

of RS HT cannot

be avoided

Bulk

Support

As-built Released


12

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.


Insights- 1- d0 reference

24

0

0

d

dd Calculation of Strains by diffraction

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)

Requirement: quantitative assessment of stress


13

Insights- 1- d0 reference

25

Lab XRD results:

d0 scan on the surface

ND results on cubes (L, T, N), raw powder

(RP), Sample powders (SP)


Takeaways- 1- d0 reference

26

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


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Insights- 2- Principal stress axes // Texture

27

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?


The sin2y technique

From laboratory XRD we borrow a useful technique that allows

determining the influence of texture and the principal axes

Q= L3

Sample

y

2

We tilt the sample perpendicular to the scattering plane

//

1

2

y

0 0.1 0.2 0.3 0.4 0.5 0.6 0.70.8 10.9

sin2y

//

TextureP3

0 0.1 0.2 0.3 0.4 0.5 0.6 0.70.8 10.9

sin2y

//

y 0

y 0Shear strain


15

Insights- 2- Influence of Texture

29

Neutron Diffraction (Bulk) Lab X-ray Diffraction (Surface)Synchrotron X-ray Diffraction

(Sub-Surface)

All show linear plots :

No large influence of texture

All show no difference for pos. and neg. tilts :

geometrical directions are principal

In this case, the classic

RS analysis is valid


Takeaways 2- Principal stress axes // Texture

30

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?


16

31


Conclusions and Takeaways


Know

ledge


Awareness

We know that we do NOT know Powders extracted from the AM are

the best macro-stress-free reference

The large texture does not strongly

influence Residual Stress Analysis

Residual Stress and distortions in

SLM IN 718 strongly reflect the

hatch pattern and depend on the

peculiar thermal history


Safety creates markets.

Competence Centre

Safety in Technology and Chemistry

32

ONLY Safety creates

SUSTAINABLE markets.


17

www.bam.de

Thank you for your attention.

Contact:

Giovanni Bruno

Head of Division 8.5

Phone: +49 30 8104-1850

Email: [email protected]


mailto:[email protected]

microstructure and residual stresses in am metallic parts: do we … · 2019-02-03 · 3 example in...

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