On the primary spacing and microsegregation of cellular dendrites in laser deposited Ni–Nb alloys

Abstract

In this study, an alloy phase-field model is used to simulate solidification microstructures at different locations within a solidified molten pool. The temperature gradient G and the solidification velocity V are obtained from a macroscopic heat transfer finite element simulation and provided as input to the phase-field model. The effects of laser beam speed and the location within the melt pool on the primary arm spacing and on the extent of Nb partitioning at the cell tips are investigated. Simulated steady-state primary spacings are compared with power law and geometrical models. Cell tip compositions are compared to a dendrite growth model. The extent of non-equilibrium interface partitioning of the phase-field model is investigated. Although the phase-field model has an anti-trapping solute flux term meant to maintain local interface equilibrium, we have found that during simulations it was insufficient at maintaining equilibrium. This is due to the fact that the additive manufacturing solidification conditions fall well outside the allowed limits of this flux term. ‡ Corresponding author.

Uncertainty analysis of microsegregation during laser powder bed fusion

“…The melt pool profile obtained from this simulation was used to extract the solidification parameters G and V (following their mathematical expressions given in Refs. [3,39]) at the liquidus temperature isotherm where the solid-liquid phase transformation begins. The ranges of G and V (Fig.…”

Predictive modeling of solidification during laser additive manufacturing of nickel superalloys: recent developments, future directions

“…The solidification boundary represents different temperature gradients and solidification rates. Typically in simulations, the temperature gradient varies between ≈ 10 5 K m −1 and 10 7 K m −1 and the solidification rate varies between ≈ 0.01 m s −1 and 0.5 m s −1 for a laser scan speed on the order of 1 m s −1 [26,27]. Temperature gradient times the solidification velocity is the cooling rate.…”

Formation of Nb-rich droplets in laser deposited Ni-matrix microstructures

“…3b). The microsegregation or the composition gradient between the cell core and the periphery of individual cells is extracted by a compositiondistance profile across the cells and reported in [24]. The rejection of Nb by the growing cells increases the Nb content in the liquid.…”

“…The rejection of Nb by the growing cells increases the Nb content in the liquid. During terminal solidification, close to the bottom of the simulation box, as the roots of the solid cells grow toward each other and coalesce in the mushy zone at a low temperature, Nbrich liquid in the intercellular channels is separated into isolated droplets, as in [24,[29][30][31]. Since the diffusion path is absent at lower temperatures, the Nb content in these droplets increases rapidly with a reduced residual liquid fraction with increasing distance below the cellular growth front.…”

“…Phase-field simulations begin with a thin layer of solid at the bottom of the simulation box with initial Nb compositions in the solid and the liquid, as in[24]. Small, random amplitude perturbations are applied at the initial solid-liquid interface, from which stable perturbations grow with time and break into steady state γ cells.…”