Laser-matter interactions in additive manufacturing of SS316L and 13-93 bioactive glass revealed by in situ X-ray imaging

Chu Lun Alex Leung, Sebastian Marussi, Michael Towrie, Jesus del Val Garcia, Robert C. Atwood, Andrew J. Bodey, Julian R. Jones, Philip J. Withers, Peter D.Lee - Department of Mechanical Engineering, University College London; Research Complex at Harwell, Science & Technology Facilities Council, Rutherford Appleton Laboratory; School of Materials, The University of Manchester; Central Laser Facility, Research Complex at Harwell, UK Research & Innovation; Science & Technology Facilities Council, Rutherford Appleton Laboratory; Applied Physics Department, University of Vigo; Diamond Light Source Ltd, Harwell Science & Innovation Campus; Department of Materials, Imperial College London

Laser-matter interactions in laser additive manufacturing (LAM) occur on short time scales (10-6 – 10-3 s) and have traditionally proven difficult to characterise. We investigate these interactions during LAM of stainless steel (SS316 L) and 13-93 bioactive glass powders using a custom built LAM process replicator (LAMPR) with in situ and operando synchrotron X-ray radiography. This reveals a range of melt track solidification phenomena as well as spatter and porosity formation. We hypothesise that the SS316 L powder absorbs the laser energy at its surface while the trace elements in the 13-93 bioactive glass powder absorb the laser energy by radiation conduction. Our results show that a low viscosity melt, e.g. 8 mPa s for SS316 L, tends to generate spatter with a diameter up to 250 µm and an average spatter velocity of 0.26 m s-1 and form a melt track by molten pool wetting. In contrast, a high viscosity melt, e.g. 2 Pa s for 13-93 bioactive glass, inhibits spatter formation by damping the Marangoni convection, forming a melt track via viscous flow. The viscous flow in 13-93 bioactive glass resists pore transport; combined with the reboil effect, this promotes pore growth during LAM, resulting in a pore size up to 500 times larger than that exhibited in the SS316 L sample.

How Amira-Avizo Software is used

For the XCT dataset, we quantified the pore size distribution of both melt tracks in 3D using Avizo 9.1 (Thermo Fisher Scientific, US) and the method described in the literature [13,56]. We discarded any segmented objects with a volume fewer than five voxels (equivalent to a diameter of 6.75 µm) to minimize quantification errors induced by image noise.