Melt pool stability analysis during the selective laser melting of tungsten

1. Melt pool stability analysis during the selective laser melting of tungsten CIM-Laser One Day Conference, 31th May 2018, Cranfield University

2. Contents • Introduction and Background • Materials Development • Experimental Work • Finite Element Analysis • Conclusions • Future Studies

3. Background and Applications • Applications today include medical implants, rocket nozzles, support hardware, military, electro vacuum, crucible and heating elements • High density of tungsten (W) makes it ideal for radiation attenuation • Pinhole collimators • However, these are difficult to machine because of small dimensions • Interest by industries gradually increasing

4. Selective Laser Melting - SLM • SLM of tungsten powder was carried out using a Renishaw AM125 system which uses a high powered ytterbium fibre laser • Wavelength of 1070 nm • Max. laser power of 200W in continuous wave mode • Max laser scanning speed of 2000 mm/s • SLM of W in argon atmosphere with an initial residual oxygen content of less than 1800 ppm (0.18%)

5. Powder Properties • Powder morphology (a) and powder particle distribution (b) for Tungsten and Tantalum • Angle of Repose measurements confirmed “conical” shape of powder

6. 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

7. Laser Gaussian PDD • Laser beam power density distribution • TEM00 (ISO11146) • Diameter ∼50 µm at 0mm offset • Diameter ∼43 µm at 1mm offset • 25 % increase in Peak I0 Intensity after optimisation

8. Materials Properties • Physical properties of tungsten and CP-Ti • SLM of refractory metalssuch as W difficult due to • high melting point, • high thermal conductivity • high viscosity • oxidation sensitivity • CP-Ti selected as a substrate

9. Process Window – W • ℇ2D is the 2D linear input laser energy density • PLaser is the laser power • 𝑣scan is the laser scan speed • ϕSpot is the laser beam diameter. • SLM of W carried out by melting single layer melt tracks • Laser power range of 100, 150 and 200W • Laser scan speeds of 50 to 400 mm/s before and after Gaussian laser beam profile was optimised • The 2 dimensional (2D) linear input laser energy calculated:

10. Single track melting • Single track melting results of tungsten powder using different scan parameters at laser focus offset of 0mm. • Single track melting results of tungsten powder using different scan parameters at laser focus offset of 1mm.

11. Melt Width v 2D Energy Density • Line width vs 2D line energy density for W powder • Laser Beam diameter, 43 and 50 m • Melt pool width dependent on energy density • Maximum 2D linear energy density of 93 J/mm2, the melt pool width was ~470μm

12. Laser Power v Scan Speed • Laser power vs scan speed for tungsten powder • Laser beam diameter = 43μm • Focus offset = 1mm • Melt pool width dependent on scan speed

13. Thermal Modelling Result indicate that the surface tension coefficient as well as the melt viscosity decrease with increasing temperature • Model based on welding process • Substrate with constant material parameters, line source of heat of Q [W/m] perpendicular to the substrate's surface (semi-infinite) • Q – Line thermal source • v – Thermal source velocity • K – Thermal conductivity •  - Thermal diffusivity • K0 - modified Bessel function of order 0

14. Thermal Modelling • Thermal modelling of temperature structure on Matlab[1] • Laser beam diameters, 43μm and 50μm • Heat source velocity, 50 mm/s and 400 mm/s • Laser Power = 200W • Materials properties: • W powder • W/CP-Ti alloy melt pool defined by rule of mixtures (50/50) • Models based on heat conduction hence limited [1] G.R.B.E. Römer, A.J. Huis in ’t Veld, Matlab Laser Toolbox, Physics Procedia 5 (2010) 413-419. https://doi.org/10.1016/j.phpro.2010.08.068.

15. Thermal Source Profiles • Laser source as a moving line thermal source, where heat is induced by absorbed laser energy • Peak temperature profiles estimating the temperature at the surfaces of the CP-TiW melt pool and on W • Beam is moving in the positive x direction

16. Thermal Contour Profiles • Laser source as a moving line thermal source, induced by absorbed laser energy • Contour temperature profiles temperature profiles estimating the temperature at the surfaces of the CP-Ti/W melt pool and on W • Beam is moving in the positive x direction

17. Isotherm Profiles • Isotherms of the temperature field illustrating heat conduction into material • Isotherms show lack of “keyhole” effects in laser heat source • FFT calculation, optical penetration assumed smaller than heat penetration

18. Thermal Contour Profiles • Thermal profiles estimated on the surface • Temperature profile due to a moving heat source at the surface of the W/CP-Ti single layer melt pool, and on W (layer to layer)

19. Process Window – W • Laser energy was able to melt the tungsten and create a strong bond on a commercially pure substrate. • Example of melt pool break up with balling (left) and radially flattened and broad tungsten pool (right)

20. Hatch Spacing – W • When the hatch spacing is taken into account the 2D linear laser energy density is converted into 2D area laser energy density • Shatch is the hatch spacing

21. Conclusion • The increase of 2D linear energy density that was accompanied by an increase in the melt track width indicated that the melt flow dynamics analogous to the Marangoni effect were significant • Maximum 2D linear energy density of 93 J/mm2, the melt pool width was ~470μm • Thermal effect demonstrated by modelling taking into consideration the properties of CP-Ti and W • The increase in energy was also expected to increase the melt temperature

22. Future Studies • Cross sectional analyses of melt pool • Analyses of composition of melt pool • Look at other models more applicable to SLM • Identify discrepancies • Analysis and FEA of the surface tension and Marangoni effect

23. Thank you for your attention