Three-dimensional granular model of semi-solid metallic alloys undergoing solidification: Fluid flow and localization of feeding

Abstract

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues.Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. AbstractA three-dimensional (3-D) granular model which simulates fluid flow within solidifying alloys with a globular microstructure, such as that found in grain refined Al alloys, is presented. The model geometry within a representative volume element (RVE) consists of a set of prismatic triangular elements representing the intergranular liquid channels. The pressure field within the liquid channels is calculated using a finite elements (FEs) method assuming a Poiseuille flow within each channel and flow conservation at triple lines. The fluid flow is induced by solidification shrinkage and openings at grain boundaries due to deformation of the coherent solid. The granular model predictions are validated against bulk data calculated with averaging techniques. The results show that a fluid flow simulation of globular semi-solid materials is able to reproduce both a map of the 3-D intergranular pressure and the localization of feeding within the mushy zone. A new hot cracking sensitivity coefficient is then proposed. Based on a mass balance performed over a solidifying isothermal volume element, this coefficient accounts for tensile deformation of the semi-solid domain and for the induced intergranular liquid feeding. The fluid flow model is then used to calculate the pressure drop in the mushy zone during the direct chill casting of aluminum alloy billets. The predicted pressure demonstrates that deep in the mushy zone where the permeability is low the local pressure can be significantly lower than the pressure predicted by averaging techniques.

Prediction of Hot Tear Formation in Vertical DC Casting of Aluminum Billets Using a Granular Approach

“…The hydromechanical coupled model for solidification and semisolid deformation, [3][4][5] which was recently developed by Sistaninia et al uses a granular approach to fulfill the above requirements and simulate hot tearing. The use of a granular approach enables each grain to exist as a discrete element while allowing for efficient simulation of an ensemble of grains making up the microstructure.…”

Measurement of Mechanical Coherency Temperature and Solid Volume Fraction in Al-Zn Alloys Using In Situ X-ray Diffraction During Casting

“…These bumps are associated with the formation and coalescence of grain clusters as reported by Verne`de et al [26] and Sistaninia et al [27] As soon as those grain clusters tend to weld together, macroscopic tensile strain develops owing to the constrained configuration of the dog-bone-shaped casting and leads to the formation of hot tears as shown in Figure 3. Stresses and strains are then relaxed and the lattice parameter continues to decrease.…”

“…With the in situ XRD measurements, the lattice parameter is expected to exhibit such a behavior as long as individual grains or grain clusters grow without transmitting any macroscopic tensile strains. [26,27] At mechanical coherency, solid bridges are well established between grains and grain clusters and macroscopic strains and stresses start to develop possibly leading to the formation of micropores and hot tears, as reported in Figure 3.…”

Hot tearing in polycrystalline Ni-based IN738LC superalloy: Influence of Zr content

“…Results showed that carbide bridging improve cast ability for the same reason. Sistaninia et al (2012) studied grain boundaries as they are the location for solidification cracking. Rappaz et al (2003) showed that intragranular coalescence occurs at a low solid fraction whereas grain coalescence (intergranular) takes place much deeper in the mushy zone at a higher solid fraction around 99%.…”