Fig. 1

Time-series radiographs acquired during LAM of an Invar 36 single layer melt track (MT1) under P = 209 W, v = 13 mm s⁻¹ and LED = 16.1 J mm⁻¹ (see Supplementary Movie 1). a The melt track morphology at three key stages of LAM. b The formation of a molten pool and a denuded zone (yellow dotted line). The laser beam causes metal vaporisation, generating a recoil pressure at the interaction zone (blue dotted arrows) whilst indirectly heating up the surrounding argon gas (red arrows). The molten pool/track grows while enlarging the denuded zone by c molten pool wetting and d vapour-driven powder entrainment (orange dotted semi ellipse) which can lead to the formation of e powder spatter (purple dotted circle) and droplet spatter (its trajectory path is indicated by the green arrows). After the laser switches off at t = 334 ms, f pores nucleate, coalesce and collapse, resulting in an open pore (pink dotted line). All scale bars = 250 μm