Selective Laser Melting of Refractory Metals · 2017-05-16 · Selective Laser Melting of Refractory Metals CIM-Laser One Day Conference 9th May 2017 Post Graduate Centre, Heriot-Watt

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

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/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

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ean (

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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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Lin

e W

idth

-M

ean (

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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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er P

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Wide Lines

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er P

ow

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Very wide lines

Wide Lines

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• 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

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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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Density -

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Density -xy (g/cm^3) Density - xy (%)

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A B C D

Density -

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Density -

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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