Selective Laser Melting of Refractory Metals CIM-Laser One Day Conference 9 th May 2017 Post Graduate Centre, Heriot-Watt University Edinburgh
Contents • Introduction and Background • Materials Development – Experimental Work – Results • Case Studies • Future Studies
Refractory Metals - Properties • Physical properties of Properties of Refractory Metals Tungsten Tantalum tungsten and tantalum Density at 25 °C (g/cm^3) 19.2 16.69 • SLM of refractory metals Liquid Density (g/cm^3) 17.6 15 difficult due to Melting Point (°C) 3422 2996 – high melting point, Thermal Conductivity (W.m^-1.K^-1) 174 57.5 – high thermal Specific Heat (J.kg.K^-1) 134 140 conductivity Thermal Diffusivity (m^2/s) 0.068 0.025 – high viscosity Atomic mass 183.88 180.94 – oxidation sensitivity. Tension Force (N/m) 2.361 2.07
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 250 4000 13.5% Beam radius (µm) 3500 Irradiance (kW/cm^2) 200 13.5% Beam radius (µm) 3000 Irradiance (kW/cm^2) 2500 150 2000 100 1500 1000 50 500 0 0 -5 -4 -3 -2 -1 0 1 2 3 4 5 Focus Offset (mm) • Laser beam profiling on the Renishaw AM125 machine • Schematic overview of the selective • Sufficient intensity for melting Refractory metals can be laser melting (SLM) process reached only for the centre part of the geometry • Renishaw AM125, ytterbium fibre, (diameter ∼ 43 µm) 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 500 500 450 450 Line Width - Mean (µm) 400 Line Width - Mean (µm) 400 350 350 300 300 250 250 200 200 Focus Offset=0 (100% Power) 150 150 Ta - Focus Offset=1mm 100 100 Focus Offset=1 (100% Power) W-Focus Offset=1 (100% Power) 50 50 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 5000 1D line energy density (J/m) 1D line energy density (J/m) • • Line width vs 1D line energy density Line width vs 1D line energy density for tungsten (W45) powder for tantalum (Ta45) powder • – 1D Energy Density = Laser Power/ Scanning speed) Laser focus offset study
Process Window – W45 and Ta45 250 250 200 200 Laser Power (W) 150 150 Laser Power (W) 100 100 Very wide lines Very wide lines 50 Wide Lines 50 Wide Lines Smaller line width Small line width or breaks Thin lines or breaks 0 0 0 100 200 300 400 0 100 200 300 400 Scan Speed (mm/s) Scan Speed (mm/s) • • Laser power vs scan speed for Laser power vs scan speed for tungsten (W45) powder tantalum (Ta45) powder • • CP-Ti base plate CP-Ti base plate
Process Window – W and Ta • Single layer hatch patterns • Single layer hatch patterns for for tungsten (W45) using 4 tantalum (Ta45) using 4 different scanning different scanning strategies strategies
Process Window – W45 Laser Power = 200W, Point Hatch Apparent 3D volume Exposure Time = 200µs Distance Space Speed (mm/s) energy Layer Thickness= 30µm (µm) (mm) density (J/mm 3 ) 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 • Less evidence of cracks in Tantalum – XY Horizontal top surfaces – ZY Vertical side surfaces – 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) A B W45 - Powder W(110 ) W(200) W(211) W(220) Intensity (cps) 30 40 50 60 70 80 90 2 theta (deg.) • X-ray diffraction plot showing W powder and SLM processed traces and peaks
Density of SLM – W45 • • Build-direction (z-y) view Cross-section view (x-y) view 19.5 100 19.5 100 99 99 19 19 98 Density -xy (g/cm^3) 98 Density -zy (g/cm^3) 18.5 18.5 97 97 Density - zy (%) Density - xy (%) 96 96 18 18 95 95 17.5 17.5 94 94 93 17 93 17 92 92 16.5 16.5 91 91 16 90 16 90 A B C D A B C D Density -zy (g/cm^3) Density - zy (%) Density -xy (g/cm^3) Density - xy (%) • 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
EBSD Pole 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
Applications - W • 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
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
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