Effects of fabrication conditions on the microstructure, pore characteristics and gas retention of pure tungsten prepared by laser powder bed fusion

Highlights

• The laser irradiation conditions were optimized to achieve W densification.

• The maximum relative density of W bulk reached 98.58 ± 0.25% (19.03 ± 0.05 g/cm³).

• Trapping of Ar gas used as shielding gas in nanopores was observed.

• Remarkably reduced Ar retention and nanoprore density by heat treatment at 1900 °C.

Abstract

In this study, the effects of fabrication conditions on the microstructure, pore characteristics and gas retention of pure tungsten specimens prepared by laser powder bed fusion (LPBF) were investigated. The LPBF specimens contained micro- and nano- sized pores as internal defects. By optimizing the laser irradiation conditions, the formation of micropores was suppressed. The densest LPBF specimen was obtained when the input energy density was adjusted to be 411 J/mm³, and the relative density of the specimen measured by utilizing Archimedes' principle reached 98.58 ± 0.25% (19.03 ± 0.05 g/cm³). Heat treatment at a high temperature (1900 °C) was effective to reduce the number density of nanopores. The compositions and amounts of internal gas in the specimens were examined using a quadrupole mass spectrometer. The results indicated that the specimens contained the argon (Ar), which was used as the shielding gas. The Ar retention was correlated with the number density of nanopores in the specimens and not with the density of micropores. The Ar retention decreased to almost half that in the as-fabricated specimen after the heat treatment at 1900 °C. These observations suggest that Ar gas was trapped in nanopores in the LPBF material. It was demonstrated that a part of the trapped Ar dissolved and diffused in grains during the heat treatment and was released to the outside.

Introduction

Tungsten (W) has unique physical properties, such as a high melting point (3693 K), high density (19.3 g/cm³ at 298 K) and high thermal conductivity (174 W/mK at 300 K). Furthermore, at high temperatures, W has a low coefficient of thermal expansion and excellent strength [1]. Because of these characteristics, W has been widely utilized in various applications, such as radiation beam collimators for medical instruments [2], filaments for electron beam generation, and crucibles for single crystallization. In particular, in the engineering design of fusion reactors such as ITER and the demonstration power plant (DEMO), W is considered the most promising candidate for plasma-facing materials (PFMs) exposed to high heat loads caused by plasma and neutron irradiation [3]. This is due to the specific advantages of low tritium retention, high sputtering resistance and high recrystallization temperature [2], in addition to the generally favourable thermophysical properties discussed above. W is a brittle, body-centred cubic (bcc) metal with a distinct high brittle-ductile transition temperature [4]. Therefore, powder metallurgy (PM), hot rolling, swaging, and hot isostatic pressing (HIP) are applied in W manufacturing processes. However, even with machining or deformation, it is difficult to create complex structures, such as integrated internal fins for a high-performance heat exchanger, using these conventional methods. For wider application of W materials, it is necessary to establish a new processing method that provides high structural flexibility.

Additive manufacturing (AM) is a layer-by-layer processing technology, otherwise known as 3D printing, based on slice data prepared from 3D computer-aided design (CAD) information. The AM technology can be used to produce complex structures that are difficult to fabricate using conventional processes such as casting, machining, and deformation. Recently, the laser powder bed fusion (LPBF) AM method has attracted attention as a new approach to fabrication of high-density metallic components. The LPBF process involves repeated formation of a melted metal layer by selective laser irradiation and material feeding to a powder bed. This technique allows highly accurate manufacturing of complex parts by laser processing. LPBF has potential as a fabrication method that could improve the design and efficiency of thermal devices requiring high heat exhaust performance. Development of an LPBF processing method for W, which will be subject to severe heat loads in fusion devices, may facilitate the creation of a novel plasma-facing component in the future.

In LPBF, optimization of the laser irradiation conditions and material powder is important to obtain high-density metal parts. Appropriate control of laser irradiation conditions, to suppress the formation of defects inside the material, could enhance the mechanical properties of LPBF parts. Sufficient densification (>99% relative density) has been achieved for several materials including titanium alloys [5], aluminium alloys [6,7], and stainless steels [8,9] using LPBF. There have been several studies on pure W materials fabricated using LPBF [[10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]]. Tan et al. prepared pure W with a density of 19.01 g/cm³ (relative density: 98.50%) by LPBF using spherical particles [12]. In addition, Chen et al. applied HIP to pure W prepared by LPBF as a post-fabrication treatment to increase the density (as-fabricated: 96.4 ± 0.3%, after-HIP: 96.7 ± 0.3%) [14]. However, it is still difficult to increase the relative density of W to a level comparable to those of other major materials fabricated using LPBF; greater understanding of the effect of fabrication conditions on density is needed.

Cracking and nanopore formation are the main phenomena inhibiting the densification of pure W prepared by LPBF (LPBF-W). Wang et al. described how nanopores caused by the evaporation of tungsten oxide may promote cracking [19]. These defects can degrade mechanical and thermal properties. To achieve densification of LPBF-W, the formation mechanism of nanopores should be further investigated. One phenomenon related to the generation of pores in LPBF parts is entrapment of the shielding gas [21]. Argon (Ar) gas is commonly used as a shielding gas to protect the powder bed from oxidation during the LPBF process. Since it has very low solubility in metal [22], the Ar becomes trapped in cavities, i.e. in microscopic spherical pores with sizes of the order of micro- to nanometers [21,23]. The presence of Ar inside LPBF parts causes serious problems. For example, during HIP processing of titanium LPBF parts, the pressure inside the pore increases as the pore radius decreases and reaches the order of GPa [23]. Although the high temperature environment under HIP slightly increases the solubility of Ar in the material, the trapped Ar inhibits complete densification. For W in particular, the high melting point and extremely high cooling rate (approximately 5 × 10⁶ K/s) [17] associated with its high thermal conductivity may promote gas trapping. Therefore, the amount and composition of internal gas in LPBF-W should be measured to clarify the mechanism of pore generation. Development of effective methods to reduce gas retention should improve the thermomechanical properties of LPBF materials.

In this study, we investigated the effect of fabrication conditions on the density of LPBF-W prepared using pure W powder. Gas retention in LPBF-W was investigated by both microstructure observation and gas analysis. An effective method for increasing the density of LPBF-W is discussed.