One of the main challenges in the laser powder bed fusion (LPBF) process is making dense and defect-free components. These porosity defects are dependent upon the melt pool geometry and the processing conditions. Power-velocity (PV) processing maps can aid in visualizing the effects of LPBF processing variables and mapping different defect regimes such as lack-of-fusion, under-melting, balling, and keyholing. This work presents an assessment of existing analytical equations and models that provide an estimate of the melt pool geometry as a function of material properties. The melt pool equations are then combined with defect criteria to provide a quick approximation of the PV processing maps for a variety of materials. Finally, the predictions of these processing maps are compared with experimental data from the literature. The predictive processing maps can be computed quickly and can be coupled with dimensionless numbers and high-throughput (HT) experiments for validation. The present work provides a boundary framework for designing the optimal processing parameters for new metals and alloys based on existing analytical solutions. 

Predictive process mapping for laser powder bed fusion: A review of existing analytical solutions

The laser direct deposition, one kind of additive manufacturing technologies, was used to produce W‒Cu composite material. To explain the formation of the deposited microstructure, the process of laser-powder interaction and particle transport phenomena during laser direct deposition were investigated through experiment and simulation works. The parameters of the coaxial powder flow (the distribution and the velocity of flying particles) and the particle motion on the melt pool were demonstrated with high-speed imaging. By using these parameters, a laser-powder flow interaction model was established to calculate the laser intensity attenuated by the powder flow and the temperatures of W and Cu particles. Based on the calculated results, a three-phase fluid dynamics model was constructed to simulate the transport phenomena of W and Cu particles on the substrate surface during laser deposition. The calculated results indicate that the temperatures of W and Cu particles were mainly in the solid state but with huge difference in temperature while arriving at the deposition area. The captured images show that these solid particles can be deposited until the substrate surface was slightly melted. The simulation results suggest that the heat of W particles is conducted to Cu particles rapidly after being deposited on the substrate surface. Coupled with the laser irradiation, Cu particles are melted and form a melt pool. Subsequently, a fallen W particle can float on the melt pool since the inertial force of the falling W particle and the surface tension between Cu and W are roughly balanced. In comparison, if a Cu particle falls on a floating W particle, the Cu particle will be melted and then flow into the melt pool in 300 μs. The combination of high-speed imaging and our simulation explained the deposited microstructure, i.e., the distribution of W particles in the deposition layer.

Characterization of laser-powder interaction and particle transport phenomena during laser direct deposition of W‒Cu composite

Tungsten and tungsten-based alloys are the leading material choices for the divertor plasma facing components (PFCs) in future fusion reactors Recrystallization may occur when they undergo high heat loads, drastically modifying the predesigned grain structures and the associated desired mechanical properties However, the influence of recrystallization on the thermal fatigue behavior of tungsten PFCs still remains unclear In this study, ITER-grade tungsten was simultaneously exposed to a high-flux hydrogen plasma (~5×1024 m-2s-1) and high-cycle (104-105) transient heat loads in the linear plasma device Magnum-PSI By correlating the surface temperature distribution, obtained by analyzing temperature-, wavelength-, and surface-dependent emissivity, and the surface modifications of the plasma exposed specimens, the crack initiation heat flux factor threshold was found to be ~2 MWm-2s05 (equivalently, ~007 MJm-2 for a 1 ms pulse), which slightly decreases with increasing surface temperature (~1550 K) and increasing pulse number Based on electron backscatter diffraction (EBSD) analyses of cross-sections near the crack initiation sites, faster recrystallization kinetics near the surface compared to literature was observed and the surface cracks preferentially initiated at high angle grains boundaries (HAGBs) Upon recrystallization, the yield strength decreases which entails increasing cyclic plastic strains The HAGBs fraction is increased, which constrains the transfer of plastic strains at grain boundaries The recrystallization decreases the dislocation density, which promotes heterogeneous deformation All these mechanisms explain the reduced crack initiation threshold of recrystallized tungsten compared to its as-received counterpart The results provide new insights into the structural failure mechanisms in tungsten PFCs exposed to extreme fusion plasmas

Recrystallization-mediated crack initiation in tungsten under simultaneous high-flux hydrogen plasma loads and high-cycle transient heating

Study on melting and solidification behaviors of tungsten loaded by high heat flux for divertor in tokamak fusion reactor

In-situ study of the processes of damage to the tungsten surface under transient heat loads possible in ITER

Laser energy absorption coefficient and in-situ temperature measurement of laser-melted tungsten

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Laser energy absorption coefficient and in-situ temperature measurement of laser-melted tungsten