Selective Laser Melting of Refractory Metals
CIM-Laser One Day Conference9th May 2017
Post Graduate Centre, Heriot-Watt UniversityEdinburgh
Contents
• Introduction and Background
• Materials Development
– Experimental Work
– Results
• Case Studies
• Future Studies
Refractory Metals - Properties
Properties of Refractory Metals Tungsten Tantalum
Density at 25 °C (g/cm^3) 19.2 16.69
Liquid Density (g/cm^3) 17.6 15
Melting Point (°C) 3422 2996
Thermal Conductivity (W.m^-1.K^-1) 174 57.5
Specific Heat (J.kg.K^-1) 134 140
Thermal Diffusivity (m^2/s) 0.068 0.025
Atomic mass 183.88 180.94
Tension Force (N/m) 2.361 2.07
• Physical properties of tungsten and tantalum
• SLM of refractory metals difficult due to
– high melting point,
– high thermal conductivity
– high viscosity
– oxidation sensitivity.
Background and Applications
• Applications today include medical implants, rocket nozzles, support hardware, military, electro vacuum, crucible and heating elements
• High density of tungsten makes it ideal for radiation attenuation– Pinhole collimators
• However, these are difficult to machine because of small dimensions
• Refractory SLM process being driven slowly by industries
Laser Beam Profiling
• Laser beam profiling on the Renishaw AM125 machine
• Sufficient intensity for melting Refractory metals can be reached only for the centre part of the geometry (diameter ∼43 µm)
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-5 -4 -3 -2 -1 0 1 2 3 4 5
Irra
dia
nce
(kW
/cm
^2)
13
.5%
Bea
m r
adiu
s (µ
m)
Focus Offset (mm)
13.5% Beam radius (µm)
Irradiance (kW/cm^2)
• Schematic overview of the selective laser melting (SLM) process
• Renishaw AM125, ytterbium fibre,1070nm
Process Window – W and Ta
• Single track melting results of tungsten and tantalum powder using different scan parameters at 200W Laser Power
• 100 to 200mm/s speed
Line Width v 1D Energy Density
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Lin
e W
idth
-M
ean (
µm
)
1D line energy density (J/m)
Focus Offset=0 (100% Power)
Focus Offset=1 (100% Power)
• Line width vs 1D line energy density for tungsten (W45) powder
• Laser focus offset study
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0 1000 2000 3000 4000 5000
Lin
e W
idth
-M
ean (
µm
)
1D line energy density (J/m)
Ta - Focus Offset=1mm
W-Focus Offset=1 (100% Power)
• Line width vs 1D line energy density for tantalum (Ta45) powder– 1D Energy Density = Laser Power/ Scanning speed)
Process Window – W45 and Ta45
• Laser power vs scan speed for tungsten (W45) powder
• CP-Ti base plate
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Las
er P
ow
er (
W)
Scan Speed (mm/s)
Very wide lines
Wide Lines
Smaller line width
Thin lines or breaks0
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0 100 200 300 400
Las
er P
ow
er (
W)
Scan Speed (mm/s)
Very wide lines
Wide Lines
Small line width or breaks
• Laser power vs scan speed for tantalum (Ta45) powder
• CP-Ti base plate
Process Window – W and Ta
• Single layer hatch patterns for tungsten (W45) using 4 different scanning strategies
• Single layer hatch patterns for tantalum (Ta45) using 4 different scanning strategies
Process Window – W45Laser Power = 200W,
Exposure Time = 200µsLayer Thickness= 30µm
Point Distance
(µm)
Hatch Space (mm)
Apparent Speed (mm/s)
3D volume energy density (J/mm3)
A C2 (sub 0) 20 0.115 100 578
B C2 (sub 6) 20 0.155 100 434
C C2 (0) 29 0.115 145 399
D C2 (6) 29 0.155 145 299
SLM of Refractory Blocks
• Evidence of cracks in Tungsten– XY Horizontal top surfaces
– ZY Vertical side surfaces
• Less evidence of cracks in Tantalum – XY Horizontal top surfaces
– ZY Vertical side surfaces
SLM of Tungsten – SEM and EDS
• SEM and EDS analysis of a tungsten (W45) SLM sample
• Sample B – XY Build Direction, etched
• SEM and EDS analysis of a tantalum (Ta45) SLM sample– ZY Build
Direction, block
XRD of Tungsten (W45)
• X-ray diffraction plot showing W powder and SLM processed traces and peaks
30 40 50 60 70 80 90
Inte
nsi
ty (
cps)
2 theta (deg.)
A
B
W45 - Powder
W(1
10
)
W(2
20)
W(2
11)
W(2
00)
Density of SLM – W45• Cross-section view (x-y) view • Build-direction (z-y) view
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A B C DD
ensity -
xy (
%)
Density -
xy (
g/c
m^3
)
Density -xy (g/cm^3) Density - xy (%)
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A B C D
Density -
zy (
%)
Density -
zy (
g/c
m^3
)
Density -zy (g/cm^3) Density - zy (%)
• Optically determined density of the cross-section (z-y) view of four tungsten (W45) samples fabricated using different parameters
• Highest density – Sample A (Pd=20µm, hatch=115µm), x-y view
SLM of Tungsten – Grain structure
SLM Tungsten SEM’s showing grain structures– cross sectional lateral x-y view
– build direction cross-sectional z-y view
EBSDPole figure of the 115 µm hatching space sample, suggesting a strong <111> preferential growth along the build direction
• Maximum intensity of 10 times random
Pole figure of the 155 µm hatching space sample, suggesting a relatively weaker <111> preferential growth along the build direction
• Maximum intensity 7.1 times random
• The Nuclear physics instrumentation group previously had a choice of 1mm or 2mm collimation
• SLM was used to fabricate a finer collimator which resulted in a narrower beam spot (0.6 mm nominal)
• More accurate scan results but at the expense of number of gamma rays per second
• The SLM Tungsten 0.6mm collimator allowed higher resolution scans giving better detector characterisation results
Applications - W
SLM of Refractory Metals
Outlook and future work• Transmission Electron Microscopy (TEM) • 3D Xray Tomography
– Collaboration with Manchester University
• Elimination of cracks– Heat treatment, heated bed or alloying
• SLM of Tungsten sub 25 µm powder– Effect of powder particle size
• SLM of Tantalum• System modification
Thank you for your attention
Acknowledgements - University Of Manchester