Spheroidal Particle Stability in Semi-Solid Processing

TL;DR

This paper introduces a model for the growth and stability of spherical particles in semi-solid processing, predicting conditions under which particles retain their shape during cooling, supported by experimental validation.

Contribution

A new analytical and numerical model for spherical particle growth and stability in semi-solid alloys, enabling prediction of maximum cooling rates for shape retention.

Findings

Growth velocity peaks and then decreases with overlapping solute fields.

High particle density and solid fraction improve stability.

Model predictions align well with experimental data for Al-Cu alloy.

Abstract

A model for diffusion-controlled spherical particle growth is presented and solved numerically, showing how, on cooling at sufficient rate from a given fraction solid, growth velocity first increases, and then decreases rapidly when solute fields of adjacent particles overlap. An approximate analytical solution for the spherical particle growth velocity is then developed and shown to be valid until the solute fields begin to overlap. A particle stability model is next presented, building on the above analytic solution. This model permits prediction of the maximum cooling rate at which a semi-solid slurry or reheated semi-solid billet can be cooled while still retaining the spherical growth morphology. The model shows that particle stability is favored by high particle density, high fraction solid and low cooling rate. The predictions of the stability model are found to be in good quantitative agreement with experimental data collected for Al-4.5wt%Cu alloy. Engineering applications of the results obtained are discussed.