Spatially resolved temperature measurements and understanding the local heat-affected zone (HAZ) are of urgent importance in metal additive manufacturing (AM). Many experiments have used infrared (IR) temperature measurements at the build surface, but there is little experimental work measuring temperatures below the surface. Measurements below the top surface are vital to understand how the heating of additional build layers affects the temperature history of previously printed layers, especially in thin geometries. A thin metal plate with incident laser power can reasonably represent such thin geometries, for example, an overhang feature of an actual part undergoing laser powder bed fusion (LPBF). As a part of this work, we describe the design, construction, and calibration of an experimental setup that allows direct measurement of temperature distributions on the bottom side of a metal substrate. This is accomplished using a high-speed IR camera (2000 fps) positioned below a thin Inconel 625 substrate of known thickness (500 μm) and emissivity. The top side is irradiated with a high-power ytterbium fiber laser, while temperature distributions are measured on the bottom surface. The local temperature on the bottom side is resolved with a spatial resolution of 200 μm in the x and y directions. Local temperature distributions and HAZ dimensions are reported for single-track scans as a function of the laser power. The insights derived from this study will ultimately establish a fundamental link between laser processing parameters, local thermal history, and printed part properties.

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