1. Solid Freeform Fabrication Symposium 2012
Benchmarking capabilities of SLM and EBM to build overhangs without supports
Pratik Vora
Advanced Additive Manufacturing Group, The University of Sheffield 19

2. Polymer powder bed processes
AM at Sheffield
Dr C. Majewski
Polymer powder bed processes
Prof N. Hopkinson
(Head of Group)
Prof I.Todd
Dr K. Mumtaz
Metal powder bed processes
Dr P. Smith
Dr F. Claeyssens
Dr M. Gilbert
Prof. R. Van Noort
Design optimisation
Dental applications
Photo curing
processes
Other Activities
Advanced Additive Manufacturing

3. AM at Sheffield Mechanical Engineering Department Materials Department
3D Systems SLS Formiga
High Speed Sintering
Object Eden 260V
MicroFab Jetlab
Materials Department
Arcam EBM S12
Arcam EBM A2
Renishaw SLM 125
Optomec Aerosol Jet
Dental School
Object Printer
V Flash

4. Overview 1) Requirement for supports in metal AM (powder Bed)
- Powder Bed Metal Additive Manufacturing
- Why Parts Warp, what size of overhang can be built?
2) EBM/SLM Overhang Experiments
- EBM/SLM Technologies
- Part Design/Warp Measurement
3) Reducing the Development of Stress within a Build
- Heated Platforms
- Novel methods (Anchorless Selective Laser Melting)

5. Metal AM (Powder Bed)
Most widely and extensively used AM process for the production of metal parts
Uses thermal energy (laser or electron beam) to selectively fuse/melt regions of a powder bed
Produces high density fully functional parts in one step
Good material variety and part properties

6. Requirement for supports
Some geometries require support/anchors due to thermal warpage
Rapid heating/melting and cooling/solidification
Large thermal variations
Stresses/warpage
Limits geometric freedom,
Incurs post processing and cost for anchor removal

7. Requirement for supports
Warpage
10mm
10mm
Metal Anchors/Supports 7

8. What size overhang?
- Warping is a well known phenomenon in powder bed processes
- What size unsupported overhang can be built using EBM and SLM processes?
- Is one process better than the other at producing overhangs?
- How can stresses be reduced?

9. EBM/SLM Overhang Expt. EBM S12 (Arcam) SLM 125 (Renishaw)
Electron Beam
Operates in vacuum
Powder bed pre-heating (~750◦C)
Laser Beam
Inert atmosphere
Powder bed pre-heating (<140◦C)

10. Test Part Design
Overhang length
1mm-15mm
Thickness
0.5mm-3mm z x

11. Warp Measurement L
Data points
Warp Height
Warp Length

12. EBM Overhang Ti6Al4V Parameters Beam Current: 10-15mA
0.5mm thickness
1mm thickness
Parameters
Beam Current: 10-15mA
Preheating Temp- 750ºC
Layer thickness: 70µm
Beam Speed: 500m/s
2mm thickness
3mm thickness

13. EBM Effect of Thickness

14. EBM Warp Height
15mm overhang
10mm overhang
5mm overhang

15. SLM Overhang AlSi12 z x Parameters Laser Power: 200W
140ºC Bed Temperature
0.5mm thickness
1mm thickness
2mm thickness
3mm thickness
25ºC Bed Temperature
Parameters
Laser Power: 200W
Preheating Temp: 25/140ºC
Beam Speed: 300m/s
Layer Thickness: 50µm

16. EBM/SLM capabilities
EBM and SLM were only capable of building 2mm overhangs unsupported without warpage.
Thicker structures develop more stress leading to increased warpage/geometric distortion
SLM: Preheating of 140ºC did not show improvement.

17. Stress Reduction, how?
Warpage is clearly an issue for large overhanging geometries in both EBM and SLM processes.
How to reduce stress and remove requirement for supports
Reducing thermal gradient in powder bed.
Better understanding of processing parameters to reduce residual stress.

18. Custom Designed Heated Platform, Developed with Renishaw
Stress Reduction, how?
Heated Powder Bed
Mechanical Testing
Hot Bed Bench Testing
Custom Designed Heated Platform, Developed with Renishaw
Renishaw SLM 125

19. Eutectic Material Composition
Stress Reduction, how?
Novel Methods: Anchorless Selective Laser Melting (ASLM)
Conventional SLM
+Heated powder bed
ASLM
(patented method)
Eutectic Material Composition

20. ASLM Atomic Percent Silicon Liquid Temperature, ºC Forms eutectic at
Al melts at 660°C
Si melts at 1414°C
Liquid
Temperature, ºC
Forms eutectic at
Al 88%Wt. & Si 12%Wt.
Eutectic solidification temperature 577°C
660.45ºC
577 ºC
12% Si Al
Weight % Silicon Si
Eutectic
Hyper
Hypo
* Source: ASM Handbook

21. ASLM Z X Stage 1 Stage 2 Stage 3 Bed temp T (mix of metal A & metal B)
Alloy of metal A & B formed
Laser melts metal A & B forming a new eutectic (or hypo/hyper eutectic) alloy
Stage 2
Bed temp T
Formed alloy held at elevated temperature in a stress reduced state
(mix of metal A & metal B)
Bed temperature held
at temperature T. Processed material cools uniformly
Powder A & B
remain solid
Stage 3
Heated powder bed, mix of un-alloyed powders
A & B
Bed temp T Z
(mix of metal A & metal B) X

22. ASLM Low Melt Materials
ASLM Component, BiZn Eutectic Alloy

23. ASLM Mid Temp Materials
Aluminium casting alloys
Binary Phase Diagram
Alloy Composition – Al x
Solidus
% when mixed A B C D E 80 10 2 6
500
Split alloy into 2 powders
Powder 1 X
600 60
Powder 2 x
700 40
Thermodynamic Modelling
Processing Window:
= 600 – 500 = 100oC

24. Summary
Already known that certain overhanging geometries require supports when built using EBM/SLM
Initial findings indicate overhangs greater than 2mm cannot be built (warp free) using EBM/SLM
Stress reduction can be achieved by a combination of powder bed pre-heating and eutectic material selection

25. Thank you for listening
Acknowledgments
Renishaw PLC
LPW Ltd