Abstract

Two high-purity tungsten powders, produced via different manufacturing techniques, were characterized to determine size distribution, morphology, thermal properties, and flow characteristics and, thus, the likely suitability for Laser Powder Bed Fusion (LPBF) production. Specimens from duplicate builds were produced with the two powders and characterized for density, defect mechanisms, and thermal penetration into the substrate plate to compare apparent power densities. The first powder was a chemically reduced powder with irregular morphology and the second, a plasma spheroidized powder with highly spherical morphology. The latter was found to have tighter morphological control and size distribution, having a third of particles at the modal particle size in comparison to a fifth of the chemically reduced powder. This led to better flow characteristics, and an increase of 1.5 g cm−3 (1500 kg m−3) in the packing densities seen in the powder bed which corresponds to 57 pct theoretical density vs 50 pct theoretical density in the chemically reduced powder. As a result, the specimens produced from the plasma spheroidized powder had higher densities (97.3 vs 88.5 pct) and the dominant defect mechanism moved from lack of fusion dominated in the chemically reduced powder to cracking dominated in the plasma spheroidized. The plasma spheroidized powder also showed higher apparent power densities (effective absorptivities) as evidenced by an 80 pct deeper penetration of the laser into the substrate plate.

Highlights

  • TUNGSTEN is a candidate material for the plasma facing components (PFCs) within a nuclear fusion reactor as a result of its high melting point (3420 °C, 3695 K), high thermal conductivity (164 WmÀ1 KÀ1), and high density (19250 kg mÀ3).[1]

  • Archimedes’ density testing was conducted on the samples produced via Laser Powder Bed Fusion (LPBF); Figure 7 shows the variation of sample density as a function of area energy density (AED)

  • There appears to be a large decrease in the density for samples prepared from chemically reduced powder from AED values of 3.6 9 10À3 to 3.76 9 10À3 J mmÀ2, but that appears to be a secondary effect compared with the differences in density between the two powder types

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Summary

Introduction

TUNGSTEN is a candidate material for the plasma facing components (PFCs) within a nuclear fusion reactor as a result of its high melting point (3420 °C, 3695 K), high thermal conductivity (164 WmÀ1 KÀ1), and high density (19250 kg mÀ3).[1] These allow the components to survive the operating temperatures as well as providing effective radiation shielding and conduction of heat through the components. There are difficulties associated with the processing of tungsten as a result of its high melting point and intrinsic brittleness (Ductile–Brittle Transition Temperature (DBTT) ~ 400 °C, 673 K).[3] Conventionally, powder metallurgy methods including sintering have been used, but as final machining is challenging, the complexity of component geometries has been limited.[4] The current divertor monoblock design can be seen in Figure 1; its simple shape is largely dictated by manufacturing issues

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