Difficulty arises from the overall process spanning across different scales – micro-meso-macro. This is combined with the ultra-rapid transient local melting/resolidification behaviour.
As a consequence, process assessment and repeatability studies are typically based on experimental knowledge and a posteriori characterisations of mechanical properties rather than on complete understanding and control of physical phenomena.
Local transient mechanisms:
- Ongoing residual temperature build up and associated microstructures evolution during SLM.
Beam absorption and phase change
As the beam energy is absorbed in the powder bed, the powder temperature increases and the thermal energy spreads by heat diffusion. When the temperature exceeds the solidus temperature of the metal, the solid–fluid phase transformation starts consuming latent heat L. When the local liquid phase fraction exceeds a given threshold value, the solid starts to behave as a liquid. The liquid material is governed by the Navier–Stokes equations.
As the powder changes state, so does the value of its laser beam absorptivity: this increases dramatically and varies non-linearly with temperature.
Heat transport in the liquid is either by diffusion or by convection. Radiation and convection of heat from the liquid surface are neglected so that the excess heat of the liquid must be dissipated by heat conduction into the powder bed in order to re-solidify the melt pool. The neglect of convection is justified since the EBM process is under a vacuum. Radiation, vaporization and marangoni convection, variation of absorption characteritics with phase change can have an essential effect.
Melt flows and melt pool shapes
The shape of the melt pool – its width, its length and the fractal dimension (~roughness) of its top surface - changes accordingly to the strength and direction of these hydrodynamic movements occurring during the melting phase and as long as the material remains liquid.
This can range from little deviations in the case of a stable, quasistationary melt pool to significant changes in geometry. Sometimes the disintegration of the melt pool into spherical droplets, called balling and commonly denoted as Rayleigh instability , is observed.
Factors influencing melt pool shape: wettability and convection currents
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 Gusarov A V, Yadroitsev I, Bertrand Ph and Smurov I 2007 Heat transfer modelling and stability analysis of selective laser melting Appl. Surf. Sci. 254 975–9