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In situ absorptivity measurements of metallic powders during laser powder-bed fusion additive manufacturing
We present in situ measurement of laser light absorption in additive manufacturing.
Process parameters can change absorptivity by a factor of two.
Main influences: scattering in the powder layer, melt pool and keyhole formation.
High scanning velocity facilitates keeping the absorptivity constant.
The effective absorptivity of continuous wave 1070nm laser light has been studied for bare and metal powder-coated discs of 316L stainless steel as well as for aluminum alloy 1100 and tungsten by use of direct calorimetric measurements. After carefully validating the applicability of the method, the effective absorptivity is plotted as a function of incident laser power from 30 up to 540W for scanning speeds of 100, 500 and 1500mms−1. The effective absorptivity versus power curves of the bulk materials typically show a slight change in effective absorptivity from 30W until the onset of the formation of a recoil pressure-induced surface depression. As observed using high-speed video, this change in surface morphology leads to an increase in absorption of the laser light. At the higher powers beyond the keyhole transition, a saturation value is reached for both bare discs and powder-coated disks. For 100m thick powder layers, the measured absorptivity was found to be two times that of the bare polished discs for low-laser power. There is a sharp decrease when full melting of the powder tracks is achieved, followed by a keyhole-driven increase at higher powers, similar to the bare disc case. It is shown that, under conditions associated with laser powder-bed fusion additive manufacturing, absorptivity values can vary greatly, and differ from both powder-layer measurements and liquid metal estimates from the literature.
© 2017 The Authors. Published by Elsevier Ltd.
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