Fundamentals of selective beam melting: melting

Even though the general aspects of powder bed fusion are intuitively accepted – after all it’s only melting and re-solidification, right? – the detailed understanding of specific mechanisms, their sequences, their kinetics and their respective impact on the final product remain unclear.

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.

Powder-bed selective fusion, such as electron beam melting (EBM) and selective laser melting (SLM), involves different sequential or simultaneous physical processes [1] during manufacturing of components:

• Local transient mechanisms:
  
  
  • Melting;
    
    
  • Solidification;
    
    
• Ongoing residual temperature build up and associated microstructures evolution during SLM.
  
  

First the powder bed absorbs laser or electron beam radiations. Once the powder particles reach melting temperature, they start to liquefy, merge and form a growing melt pool.

Beam absorption and phase change

The beam penetrates the powder bed. In the case of an electron beam, the energy is nearly completely absorbed at the point of contact with the powder. The absorption process for laser radiation is more complex due to multi-reflection of the beam by powder particles [2].

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

Key processing parameters, machining atmosphere and material chemical composition dictate the melt pool transient hydrodynamics and the melt lifespan before solidification. For different melting conditions, the melt pool behaviour can vary from being stable (ie quasistationary) all the way up to behaving like a mini whirlpool where various strong thermally driven forces occur: thermo-capillary forces [], chemically driven diffusion currents [], etc

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 [3], is observed.

Factors influencing melt pool shape: wettability and convection currents

The dependence of the surface tension on the temperature and the high temperature gradients in the melt pool induces a hydrodynamic flow perpendicular to the surface:  the Marangoni convection. It promotes extraction of heat away from the melt pool center, increases the effective heat conduction, andthe melt pool lifespan and influences the melt pool shape. In a powder bed, the shape of the re-solidified melt pool is also strongly dependent on the wetting characteristics of the melt with the powder particles [1].

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References

[1] Das S 2003 Physical aspects of process control in selective laser sintering of metals Adv. Eng. Mater. 5 701–11

[2] Zhou J, Zhang Y and Chen J K 2009 Simulation of laser irradiation to a randomly packed bimodal powder bed Int. J. Heat Mass Transfer 52 3137–46

[3] 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