Resolving thermal gradients and solidification velocities during laser melting of a refractory alloy

TL;DR

This paper presents a high-speed IR imaging method to measure thermal gradients, cooling rates, and solidification velocities during laser melting of refractory alloys, aiding microstructure control in additive manufacturing.

Contribution

It introduces a novel in situ IR imaging technique with high temporal resolution to accurately characterize thermal fields during laser melting of refractory alloys.

Findings

Thermal gradients decrease from edge to center of melt pool.

Solidification velocity increases from edge to center.

Transition from epitaxial to equiaxed grains correlates with thermal gradients.

Abstract

Metal additive manufacturing (AM) processes, such as laser powder bed fusion (L-PBF), can yield high-value parts with unique geometries and features, substantially reducing costs and enhancing performance. However, the material properties from L-PBF processes are highly sensitive to the laser processing conditions and the resulting dynamic temperature fields around the melt pool. In this study, we develop a methodology to measure thermal gradients, cooling rates, and solidification velocities during solidification of refractory alloy C103 using in situ high-speed infrared (IR) imaging with a high frame rate of approximately 15,000 frames per second (fps). Radiation intensity maps are converted to temperature maps by integrating thermal radiation over the wavelength range of the camera detector while also considering signal attenuation caused by optical parts. Using a simple method that assigns the liquidus temperature to the melt pool boundary identified ex situ, a scaling relationship between temperature and the IR signal was obtained. The spatial temperature gradients (dT/dx), heating/cooling rates (dT/dt), and solidification velocities (R) are resolved with sufficient temporal resolution under various laser processing conditions, and the resulting microstructures are analyzed, revealing epitaxial growth and nucleated grain growth. Thermal data shows that a decreasing temperature gradient and increasing solidification velocity from the edge to the center of the melt pool can induce a transition from epitaxial to equiaxed grain morphology, consistent with the previously reported columnar to equiaxed transition (CET) trend. The methodology presented can reduce the uncertainty and variability in AM and guide microstructure control during AM of metallic alloys.