Analytical model for balling defects in laser melting using rivulet theory and solidification

TL;DR

This paper develops an analytical model for balling defects in laser melting, focusing on fluid instabilities and solidification competition, providing insights into defect formation and the influence of various parameters.

Contribution

It introduces a novel analytical formalism that accounts for fluid instabilities and solidification, including substrate effects, to predict balling behavior in laser melting.

Findings

Model predicts instability growth at higher wavelengths

Strong sensitivity to solidification front curvature

Deviations highlight importance of fluid flows and heat transport

Abstract

In the laser welding and additive manufacturing (AM) communities, the balling defect is primarily attributed to the action of fluid instabilities with a few authors suggesting other mechanisms. Without commenting on the validity of the fluid instability driven \textit{mechanism} of balling in AM, this work intends to present the most realistic analytical discussion of the balling defect driven purely by fluid instabilities. Synchrotron-based X-ray radiography of thin samples indicate that fluid instability growth rates and solidification can be comparable in magnitude and thus compete. Neglecting the action of fluid flows and heat transport, this work presents an analytical formalism which accounts for fluid instabilities and solidification competition, giving a continuous transition from balling to non-balling which is lacking in current literature. We adapt a Rivulet instability model from the fluid physics community to account for the stabilizing effects of the substrate which the Plateau-Rayleigh instability model does not account for, and estimate the instability growth rate. Our model predicts instability growth at higher wavelengths and shallower melt pool depths relative to width, as well as strong sensitivity to the solidification front curvature. Deviations between model predictions and our experimental results demonstrate the importance of fluid flows and heat transport in the balling process. Our experiments further demonstrate at least one mechanism by which the melt pool length and balling wavelength are not equivalent, as commonly claimed.