Finite element modeling of distortion during liquid phase sintering

Abstract

Liquid phase sintering (LPS) is a common technique to consolidate materials that are difficult to process by fusion techniques, such as tungsten heavy alloys. One of the major processing difficulties associated with liquid phase sintered alloys is component distortion and loss of component shape. In LPS, this distortion is the result of viscous flow driven by curvature effects and gravity. A finite element model is developed for viscous flow of the semisolid sintering structure using Stokes equations. This model considers solid volume fraction and effective viscosity of the solid-liquid mixture. The simulation predictions are compared to distortion results for microgravity and ground-based sintering experiments, and they show good agreement. The model results indicate that the effective semisolid viscosity is significantly greater than the liquid metal viscosity. Hence, future work needs to quantitatively examine the factors controlling viscosity and the benefits from such high viscosities in liquid phase sintered systems.

Review: liquid phase sintering

“…• Ganesan et al [219] used FEM for an assumed viscous flow of a semisolid LPS structure driven by curvature and gravity. They relied on Stokes equations with consideration of solid volume fraction to estimate the effective viscosity of the solid-liquid mixture.…”

Microstructural effects on distortion and solid-liquid segregation during liquid phase sintering under microgravity conditions

“…The data are given for the tops and bottoms of the ground-based samples, but only average values are given for the microgravity samples since they were free from statistically significant microstructural gradients. [18] Sample microstructures from various regions of both sample types are given in Figure 3 and 4 to illustrate the difference in microstructural gradients. Due to the homogeneity of the microgravity samples, rigidity criteria can be directly related to microstructural parameters in comparison to the distortion that accompanies solid-liquid segregation in ground-based experiments.…”

“…[18,33] However, the effect of grain size on bond rigidity and the critical solid volume fraction is less clear. Capillary forces from the wetting liquid at the necks of particles help hold the particles together if there are pores.…”

“…[33] Distortion rates have been theoretically predicted for W-Ni-Fe using a squeeze-flow model [33] and finite element analysis. [18] In both of these approaches, a critical solid volume fraction was assumed and an apparent viscosity was back calculated to take into account particle bonding and other microstructural effects. Thus, a better understanding of the links between microstructural parameters and viscous flow is needed to refine predictions of distortion rates.…”

Finite Element Simulation of Liquid Phase Sintering with Tungsten Heavy Alloys

“…Raman and German 11) considered the influence of gravity on the distortion of tungsten heavy alloys and proposed a rheological model to predict the shape. Ganesan et al 12) and Binet et al 13) considered a densified LPS component to be a viscous mass and modeled distortion behavior using Navier-Stokes equations. Olevsky et al 3) considered the effect of gravity and friction on distortion assuming that the material obeys power law creep behavior under uniaxial loading.…”