Simultaneous high-speed x-ray transmission imaging and absolute dynamic absorptance measurements during high-power laser-metal processing

Absorptivity measurements during laser powder bed fusion of pure copper with a 1 kW cw green laser

“…7) clearly indicate an increase in absorptance when a keyhole is created. Recent studies [12,13,28] show a similar increase of absorptance for other materials and using an IR laser. Other works [24,29] have considered already a model of multi-reflection of rays on a black body proposed by Gouffé [30], and considering the influence of the black body aspect ratio χ i.e.…”

“…A 160 mm diameter Ulbricht integrating sphere was used for reflectivity R measurements, as already used by Simonds [12] and Allen [13]. Such a setup allows considering all specular and diffusive laser reflections on a prepositioned sample shifted at an 8 • angle.…”

“…Such time-resolved absorptance variations should be different from those occurring in a real L-PBF machine where a lateral gas flow cleans the atmosphere above the laser-melt-pool interaction zone. Another possible contribution for the time variations of the absorptance can also be due to the inherent keyhole instability near the keyhole threshold (approximately when the keyhole depth = laser diameter), as indicated by Simonds [12], Fabbro [24] and Mayi [25] -Absorptance increases with an increase of hatch distance (25-50 % with H = 100 µm, 35-55 % with H = 200 µm, and 50-75 % with H = 500 µm). That indicated absorptance decreases when the laser interacts on the denudated area and/or on the last fusion bead.…”

“…Even if there is still a lack of data on laser-material interaction for visible wavelengths, and in L-PBF regime, the Ulbricht sphere technique has been reported several times as an efficient way to investigate lasermatter energy coupling on various kinds of metals or alloys in welding regime [10]. Such a time-resolved technique, combined with postmortem analysis or real-time synchrotron X-ray radiography of fusion tracks [12,13] provides useful information on coupling efficiency near transition regimes or process instabilities for which laser absorptivity sharply increases or decreases. Very recently, Lane et al [14] and Simonds et al [15] have also considered this technique for analysing the IR laser absorptance during L-PBF on a In625 and a 316L powder bed together with simultaneous melt-pool monitoring, and correlation with the tracks' stability.…”

Absorbance measurement for in situ process regime identification in laser processing

“…Even for powder material, this keyhole formation includes an increase in absorption, as Trapp et al demonstrated with direct calorimetric measurements [25]. Simultaneous measurements of laser power absorption and high-speed x-ray imaging were made to correlate time-resolved keyhole geometry with laser energy absorption on laser welds of Ti6Al4V [26]. They investigated the periodic nature of absorptance, which was traced back to protrusions of the keyhole sidewalls that either narrowed the keyhole opening or reduced the overall keyhole area.…”