Laser Powder Bed Fusion of Molybdenum and Mo-0.1SiC Studied by Positron Annihilation Lifetime Spectroscopy and Electron Backscatter Diffraction Methods

Abstract

Positron annihilation lifetime spectroscopy (PALS) has been used for the first time to investigate the microstructure of additively manufactured molybdenum. Despite the wide applicability of positron annihilation spectroscopy techniques to the defect analysis of metals, they have only been used sparingly to monitor the microstructural evolution of additively manufactured metals. Molybdenum and molybdenum with a dilute addition (0.1 wt%) of nano-sized silicon carbide, prepared via laser powder bed fusion (LPBF) at four different scan speeds: 100, 200, 400, and 800 mm/s, were studied by PALS and compared with electron backscatter diffraction analysis. The aim of this study was to clarify the extent to which PALS can be used to identify microstructural changes resulting from varying LPBF process parameters. Grain sizes and misorientation results do not correlate with positron lifetimes indicating the positrons are sampling regions within the grains. Positron annihilation spectroscopy identified the presence of dislocations and nano-voids not revealed through electron microscopy techniques and correlated with the findings of SiO₂ nanoparticles in the samples prepared with silicon carbide. The comparison of results indicates the usefulness of positron techniques to characterize nano-structure in additively manufactured metals due to the significant increase in atomic-level information.

Keywords: additive manufacturing; laser powder bed fusion; microstructure; molybdenum; nanoparticles; positron annihilation lifetime spectroscopy; selective laser melting; silicon carbide.

Figure 1

Probability functions for the implantation depths of positrons from a ²²Na source are plotted for the three linear absorption coefficients referenced from previous literature [25,26,28].

Figure 2

EBSD inverse pole figure (IPF) map for traditionally manufactured molybdenum with determined texture intensity.

Figure 3

KAM map for traditionally manufactured molybdenum.

Figure 4

EBSD IPF maps and corresponding IPFs for the pure molybdenum specimens in the observation direction for all four scan speeds: (a) 100 mm/s; (b) 200 mm/s; (c) 400 mm/s; (d) 800 mm/s.

Figure 5

EBSD IPF maps and corresponding IPFs for the Mo-0.1SiC specimens in the observation direction for all four scan speeds: (a) 100 mm/s; (b) 200 mm/s; (c) 400 mm/s; (d) 800 mm/s.

Figure 6

KAM maps for the pure molybdenum specimens in the observation direction at all four scan speeds: (a) 100 mm/s; (b) 200 mm/s; (c) 400 mm/s; (d) 800 mm/s. Grain orientation spread graphs for the pure molybdenum specimens in the observation direction at all four scan speeds: (e) 100 mm/s; (f) 200 mm/s; (g) 400 mm/s; (h) 800 mm/s.

Figure 7

KAM maps for the Mo-0.1SiC specimens in the observation direction at all four scan speeds: (a) 100 mm/s; (b) 200 mm/s; (c) 400 mm/s; (d) 800 mm/s. Grain orientation spread graphs for the Mo-0.1SiC specimens in the observation direction at all four scan speeds: (e) 100 mm/s; (f) 200 mm/s; (g) 400 mm/s; (h) 800 mm/s.

Figure 8

Constrained fitting for LPBF molybdenum. The I₁, I₂, and I₃ correspond to the three fixed lifetime components: 115, 135, and 430 ps, respectively.

Figure 9

Constrained fitting for LPBF Mo-0.1SiC. The intensities correspond to the three fixed lifetime components: 135, 261, and 430 ps.