In the current work we employ multi-phase-field simulations to understand the effect of remelting on microstructure evolution, especially on nucleation of new grains during selective electron beam melting (SEBM) of Ni-based super alloy. The phase-field model is coupled to both mass and heat transport phenomena including release of latent heat of solidification. We run remelting simulations in both as cast and homogenized conditions. Experimental observations show that remelting triggers the nucleation of new grains at the melt pool border. The simulation results shed more light on the local conditions at the melt pool border thereby enhancing our understanding of the mechanisms responsible for the nucleation. The simulation results are validated with experimental results obtained for the Ni–20.5 mol pct Al model binary alloy.

In this work, we present phase-field (PF) simulations directly coupled to thermodynamic and kinetic databases in three dimensions. The direct coupling allows consideration of the full alloy complexity of the CMSX-4 superalloy over a large range of temperatures. The simulation conditions are chosen for additive manufacturing utilizing Electron Beam Melting (EBM). Transformation of interdendritic liquid into eutectic $$\gamma '$$γ ′ is considered. The simulation results confirm the unique segregation behavior of all the alloying elements. It is demonstrated that the treatment of the full complexity of alloy composition is superior to all approximations with quasi-binary or -ternary approximation and justifies the significantly increased computational effort. Our results demonstrate that multi-component simulations must become a standard for phase-field applications to real material systems.

Computational Fluid Dynamics (CFD) simulations are used in this work to study the dynamic behavior of the melt pool and heat transfer during the single-track laser melting process of a nickel-based superalloy (CMSX-4). To include the effects of powder inhomogeneities and obtain a realistic distribution of the powder layer on the bed chamber, the CFD model is coupled with a Discrete Element Method (DEM) solver. The coupled model is implemented in the open-source software package OpenFOAM. In the CFD model’s governing equations, some key physical mechanisms, such as the Marangoni effect and recoil pressure, are considered. With the help of the coupled CFD-DEM model, we have investigated the effect of key process parameters, such as laser power, scanning speed of the laser, powder size, and powder shape, on the size and homogeneity of the melt pool. From the simulation results, it was discovered that high laser power and slow scanning speed create a deep and narrow keyhole that leads to porosity. In contrast, balling defects are found to be caused by a small melt pool obtained from fast scanning speeds and inadequate laser power.

Recent breakthroughs resolving open questions in materials science by phase-field simulations are reported. They relate to solidification structure formation in additive manufacturing, carbon redistribution during bainitic transformation, and the onset of damage during high-temperature creep of superalloys. The first example deals with the balance between epitaxial growth and nucleation in solidification. The second relates to the controversy regarding diffusion control and dominance of massive transformation in bainite transformation. The third relates to directional coarsening (rafting) in superalloys as a diffusion-controlled phase transformation: loss of coherency of precipitates marks the onset of damage associated with rotation of the crystal lattice and topological inversion. Technical details of the phase-field method are reviewed as necessary, and limitations of the approach are discussed. Graphical

A novel model is proposed to describe the columnar to equiaxed transition (CET) in continuous casting. The model bases on Rappaz and Thevoz’s solute diffusion model for equiaxed dendritic growth, combined with a 1-Dimensional solidification model normal to the slab surface. The unique feature of the proposed model is the combination with a mixing term between interdendritic and extradendritic melt, representing long-range solutal mixing by convection. The model can also be applied to predict equiaxed to columnar transition (ECT), i.e. the chill zone thickness. The model consists of modules such as nucleation, growth kinetics, solute and heat balance, and a solute mixing module. Nucleation is considered with a fixed nucleation undercooling. The growth kinetics of the dendrites are treated according to the LGK model. A finite difference scheme is employed for solving 1-Dimensional heat transfer equations and finally, volume averaged solute balance equations are solved in a staggered scheme. Mixing of inter- and extradendritic liquid is, as a first step, treated ideally fast. When applied to Fe-C binary system with the thermo-physical properties obtained from literature and CALPHAD simulations, the model successfully predicts recalescence, phase fraction evolution, and concentration profiles in different phases. Realistic boundary conditions of the continuous casting process are obtained from macroscopic FEM simulation.