A critical look at the prediction of the temperature field around a laser-induced melt pool on metallic substrates

Yi Shu,Nancy Yang,Daniel Galles,Wei Cai,Brandon A Mcwilliams,Ottman A Tertuliano,Adrian J Lew

doi:10.1038/s41598-021-91039-z

Abstract

The study of microstructure evolution in additive manufacturing of metals would be aided by knowing the thermal history. Since temperature measurements beneath the surface are difficult, estimates are obtained from computational thermo-mechanical models calibrated against traces left in the sample revealed after etching, such as the trace of the melt pool boundary. Here we examine the question of how reliable thermal histories computed from a model that reproduces the melt pool trace are. To this end, we perform experiments in which one of two different laser beams moves with constant velocity and power over a substrate of 17-4PH SS or Ti-6Al-4V, with low enough power to avoid generating a keyhole. We find that thermal histories appear to be reliably computed provided that (a) the power density distribution of the laser beam over the substrate is well characterized, and (b) convective heat transport effects are accounted for. Poor control of the laser beam leads to potentially multiple three-dimensional melt pool shapes compatible with the melt pool trace, and therefore to multiple potential thermal histories. Ignoring convective effects leads to results that are inconsistent with experiments, even for the mild melt pools here.

Highlights

The study of microstructure evolution in additive manufacturing of metals would be aided by knowing the thermal history
A key observation about the problem of computing the thermal histories in the solid region of the substrate gives rise to the question we examine in this paper
In the frame of the laser beam, the steady state temperature of the solid region of the substrate can be described by a nonlinear convection–diffusion equation, with the convection given by the velocity of the laser beam, c.f. (1), and the nonlinearities arising from the dependence of the thermal diffusivity on the temperature

Summary

Introduction

The study of microstructure evolution in additive manufacturing of metals would be aided by knowing the thermal history. The fraction of the energy of the laser beam absorbed by the substrate is a result of both the temperature-dependent absorptivity of the metal and the geometry of the liquid surface, typically ranging from 20–30% for mild melt pool surfaces to above 80% in the presence of a cavity generated by a keyhole[3]. In the latter case, the motion of the liquid surface is often fast and non-smooth, so mixing and heat transport are enhanced.

Methods

Results

Conclusion