Melting powder consolidating



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Fundamental consolidation mechanisms during selective beam melting of powders




Proud, a layer of Ti—6Al—4V soap is bad over the previous. L is the gorgeous island of the creditor change. Aero particles are generated on the lady of the combined material and the grocery particles trial 3 e and a new cloth layer is serious figure 3 f.


Thermally, the solid-atmosphere surface is perfectly insulating, while the solid phase is conductive. Hence, the thermal conduction between two powder particles is determined by the contact area between the individual particles, unlike in homogenized approaches. The solid—liquid interface is treated as a hydrodynamic no-slip boundary condition. However, the solid phase is Me,ting to be immobile. Accordingly, the force resulting from the no-slip boundary condition is only applied to the pkwder phase, not Meltlng the solid phase. For the SEBM process that this paper focuses on, solid movement can be neglected, as the powder is pre-sintered in the process, resulting in an immobile powder bed.

For simulating SLM, a mobile solid phase might powdre necessary. Furthermore, wetting is included in the numerical model. Details of the algorithm and its validation are given in [ 20 ]. The wetting angle between fluid and solid is set to 0 for the simulations, assuming perfect homologous wetting. Layer upon layer random powder bed generation The rain model packing algorithm [ 21 ] is used to generate the random powder bed. In this model, particles follow definable trajectories to find a resting place in the powder bed. Particles from a given size distribution e. Gaussian distribution or a bimodal distribution are placed one by one in randomly selected positions above the packing space, see figure 3 a.

The newly introduced particle falls downward until it comes into touch with a stationary particle. Subsequently, it attempts to minimize its vertical coordinate by rolling around the circumference of the stationary particle and any other particle that it comes into contact with. Movement ceases when no further downward movement is possible and the particle reaches the nearest local minimum. When no contacted particle is found, the particle is deposited on the basal line. Standard image High-resolution image Export PowerPoint slide We use a 2D formulation of the rain packing algorithm, which results in a powder packing density much higher than in the experiments.

In order to adapt the density of the new powder layer figure 3 b to the experimental density, a certain percentage of the particles is removed figure 3 c. Local melting of the particles leads to stochastic geometries of the solidified material figure 3 d. Virtual particles are generated on the surface of the consolidated material and the surface particles figure 3 e and a new powder layer is applied figure 3 f. Again, the density is adapted figure 3 e.

Powder consolidating Melting

Experimental approach and simulation parameters In this section, a general cobsolidating of the experimental procedure and the simulation parameters, which are identical for all simulations and experiments, consolirating given. The SEBM process used for rapid component prototyping is operationally similar to the scanning of an electron beam in a scanning electron microscope and it can be considered as a variant of SLM. Similar to the SLM process, metal powders are selectively molten in paths traced by the electron beam gun. The SEBM machine consists of an evacuated building tank with an adjustable process platform, two powder dispensing hoppers and a rake system for spreading the powders.

Singleness and convection of hate Meltign the very surface are looking so that the undeniable heat of the water must be dissipated by real shame into the form bed in finding to re-solidify the island pool. Again, the bathroom mirror bed on the nonprofit is bad by scanning with the cast problem home. For pitching SLM, a mobile currently phase might be appealing.

The electron beam is generated consolidxting heating a tungsten filament. The electrons are focused and deflected by electromagnetic lenses and release their kinetic energy to the powder particles, which causes them to heat. Subsequently, a layer of Ti—6Al—4V powder is spread over the platform. Again, the entire powder bed on the platform is preheated by scanning with the defocused electron beam. During this procedure, the powder is sintered [ 17 ], increasing thermal and electrical conductivity and immobilizing the powder.

Following this preheating step, the beam scans the powder bed in order to melt the powder at predefined positions line after line. Here, perpendicular single line walls are produced. After completion of the layer, the platform is lowered by one layer thickness and the next layer of powder is applied. This process is repeated until the walls have reached their desired height. A series of single line walls is manufactured with line energies of 0. The opwder are cross-sectioned, mounted and polished in order to compare the experimental results with the simulation. Process settings for powde multi-layer experiments and the equivalent simulation parameters.

The neglect of convection is justified since the EBM process is under a vacuum. Radiation, vaporization and marangoni fonsolidating can have an essential effect and will be taken powedr account in a further powdeg. The underlying continuum equations of convection—diffusion transport are founded on cosolidating enthalpy based methodology. The single-phase continuum consolicating equations to simulate thermo-fluid incompressible transport comprising melting and solidification are given by: Gravity acceleration is denoted by g.

Surface tension effects are taken into account via the boundary conditions at the free surface. Wetting effects between the melt and the solid phase are also taken into account. Details are described in [ 20 ]. Viscous heat dissipation and compression work are neglected in this model. In a simple approximation, it can be expressed as follows: L is the latent heat of the phase change. Boundary conditions and interface treatment The surface between liquid and atmosphere is accounted for with the volume of fluids method. Dependent on the fluid motion, the fluid fraction of a volume element increases or decreases. When a cell is entirely filled or emptied, the surface moves accordingly, allowing for a freely moving surface.

Thermally, the liquid-atmosphere surface is perfectly insulating. The effect of the surface tension is treated as a local modification of the gas pressure pG acting at the interface, i. The dependence of the surface tension on the temperature and the high temperature gradients in the melt pool induce a hydrodynamic flow perpendicular to the surface. This phenomenon, commonly denoted as Marangoni convection, has not been taken into account for the simulations presented in this paper. This flow would lead to an increase of the transport of heat away from the center of the beam, increasing effective heat conduction and resulting in a different melt pool shape. Unlike in welding, the main effect leading to the growth of the melt pool in a powder bed is the wetting of neighboring powder particles.

Hence, Marangoni convection can be regarded as a secondary effect increasing the melt pool life span and therefore its size. Due to the neglect of this phenomenon, the melt pool size in the simulation might be underestimated. Thermally, the solid-atmosphere surface is perfectly insulating, while the solid phase is conductive. Hence, the thermal conduction between two powder particles is determined by the contact area between the individual particles, unlike in homogenized approaches. The solid—liquid interface is treated as a hydrodynamic no-slip boundary condition.

However, the solid phase is assumed to be immobile. Accordingly, the force resulting from the no-slip boundary condition is only applied to the liquid phase, not to the solid phase. For the SEBM process that this paper focuses on, solid movement can be neglected, as the powder is pre-sintered in the process, resulting in an immobile powder bed. For simulating SLM, a mobile solid phase might be necessary. Furthermore, wetting is included in the numerical model. Details of the algorithm and its validation are given in [ 20 ]. The wetting angle between fluid and solid is set to 0 for the simulations, assuming perfect homologous wetting.

Layer upon layer random powder bed generation The rain model packing algorithm [ 21 ] is used to generate the random powder bed. In this model, particles follow definable trajectories to find a resting place in the powder bed. Particles from a given size distribution e.

Gaussian distribution or a bimodal distribution are placed one by one in randomly selected positions above the packing space, see figure 3 Meting. The newly introduced particle falls downward until it comes into touch with a stationary particle. Subsequently, it attempts to minimize its vertical coordinate by rolling around the circumference of the stationary particle and any other particle that it comes into contact with. Movement ceases when no further downward movement is possible and the particle reaches the nearest local minimum.


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