Multi-Scale FEM Simulation of Selective Laser Melting Process

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Multi-Scale FEM Simulation of

Selective Laser Melting Process

Nils Keller, Vasily Ploshikhin

Airbus Endowed Chair for Integrative Simulation and

Engineering of Materials and Processes

Prof. Dr.-Ing. Vasily Ploshikhin

European Altair Technology Conference 2013

Torino / Italy, 23.04.2013

ISEMP BCCMS

Contents

Contents

- Challenge

- FEM preprocessing

- Simulation models

- Simulation example

- Conclusion

2

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Challenge

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Challenge

Computing time estimation

4

Example ALM process: 10cm x 10cm x 10cm cube

Calculation time estimation:

• Elements: 2.500.000.000 + base plate

• Time increments: 250.000.000 (10.000 increments per layer)

Realistic process simulation is impossible!

Aim: Prediction of temperature field, distortion and residual stress

Base plate

Part

• Layer size: ~40µm

• Melt pool diameter: ~100µm

• Process time: ~7h (10s per layer)

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Challenge

Simplifications

5

Simplifications must be made in order to enable thermo-mechanical FEM

simulations:

Powder Material

Laser / Heat flux

Base Plate

Layer quantity

Size reduction

- effective heat capacity

Boundary Conditions:

- fixed displacement

- negative heat flux

Summarization

Hatch sections Complete layers

Effective heat conductivity Isolation / Convection

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Challenge

Requirements for thermo-mechanical FEM

simulations of ALM processes

6

Layer-based mesh • Each layer has the same height

• Layers are solid and match the slice contour

• Connection of layers at nodes

Heating

Cooldown

Next Layer

Adaptive time steps • Small time steps for heating phase

• Increasing time steps while powder feed

Process input • Use of real process parameters

• Reproduction of hatching strategies

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FEM Preprocessing

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FEM preprocessing

From STL to FEA

8

Preprocessing Software FE Meshing & Job

FEM Job

CAD / STL

FEM-ALM Interface Tool: CAD Import, Slicing, Meshing, Job generation:

8

CAD Source: BEGO Medical GmbH

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ALM FE Meshing

9

FEM preprocessing

Meshing of segments with hex8 elements with connection between layers:

Segmentation FE Mesh Slice Stack

Active Layer

Nodes

Elements Grid

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FEM preprocessing

ALM FE Meshing Generation of meshes of entire components (close to the CAD)

Example:

Layer thickness: 400 µm

Layers: 160

Elements: ~26.000

10

Thermo-mechanical FEA for

realistic layer sizes possible

Mapping of the contour with

less elements (no „voxels“)

FEA

- Residual Stress

- Distortion

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Simulation models

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Simulation models

FEM Simulation of ALM processes

• Deactivation of all elements of the part at

process start

• Sequential activation of single

elements/layers or material change (powder

consolidation)

• Heat flux on activated elements

• Calculation of temperature field (and strains)

Q

Layer activation

Sequential activation

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Simulation models

Layer model thermo-mechanical FEA

- Mesh of the geometry

- Activation of layers

- Simultaneous heat flux per

layer

Hatching model Thermal / thermo-mechanical FEA

- Mesh of the geometry

- Sequencially activation of

elements

- Usage of real hatchings

Powder model Thermal FEA

- Mesh of the complete

building space (powder)

- Consolidation at melting

temperature

- Usage of real hatchings

µm mm cm

Simulation models

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Simulation models

Layer model thermo-mechanical FEA

- Distortion

- Residual stresses

- Process stability

Hatching model Thermal / thermo-mechanical FEA

- Local temperature field

- Residual stress

tendencies of different

hatchings

Powder model Thermal FEA

- Micro defects (unmolten

powder)

- Powder attachments at

surfaces

- Melt pool propagation

µm mm cm

Simulation models

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Simulation example

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Simulation example

CAD Build-Job FEA-Job

Preprocessing: supports and meshing

„Proof of concept“-simulation of a free choosen part

16

CAD Source: GrabCAD.com

Preprocessing

- Magics

- AutoFAB

- …

ALM-FEM Integration

- ISEMP Tool

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Simulation example

17 17

Process: thermo-mechanical FEA

Fixed displacement

Convection: 5e-2

Simultaneous heat flux - Material: AA6056

- Elements: 43.392

- Layer thickness: 200µm

Calculation Time: ~26 h (1 core)

T > Tmelt

T ≈ T0

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Simulation example

18 18

350 MPa

0 MPa

Aft

er

AL

M

Aft

er

po

st-

mil

lin

g

Results: residual stress

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Simulation example

19 19

500 µm

0 µm

Results: distortion

Deformation factor: 3

Aft

er

AL

M

Aft

er

po

st-

mil

lin

g

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Conclusions

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Conclusion

Potential of FEM simulation of ALM processes

21

Thermo-mechanical FEA

• Tendencies for residual stress

and deformation during ALM and

process after post-processing

FEM simulations for prediction of residual stresses and for derivation of

strategies for optimal thermal management

Computer-based process optimization

Thermal FEA

• Knowledge basics for thermal

process management

• Investigations on critical local

environments

Support structures, orientation, hatching strategies, …

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Conclusion

Convergence problems by instable process

22

Thermo-mechanical FEM simulations break by instable ALM processes:

Reasons:

1. Bad supported overhangs 2. Local overheatings

• Too high laser power

• Low heat conductivity of support structure

• Bad geometry or orientation

Break because of missing mechanical boundary conditions

Break because of too large strains

z

part

activated

deactivated

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Conclusion

Need for integration of process constrains

by topology optimization

23

Support structures • Thermal and mechanical aspects restrict freedom of

designs

• Stresses after post-processing (milling)

Consideration of the manufacturing process for topology optimization of

light weight parts

Source: EADS IW

Integration of process related issues to the topology

optimization process due to advanced guidelines

Residual stresses • Stresses while process hot cracks

• Stresses after process lower fracture resistance

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Nils Keller Group Leader Additive Layer Manufacturing and Joining

Airbus endowed Chair for Integrative Simulation and

Engineering of Materials and Processes (ISEMP)

Faculty 1 / Physics

University of Bremen

Am Fallturm 1, Entrance A, Room 3.28

28359 Bremen

Tel.: +49-(0)421-218-62325

E-Mail: [email protected]

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Contact information