# Dense Pure Tungsten Fabricated by Selective Laser Melting
## 1 Introduction  
Tungsten is widely used in the electronic and aerospace industries, and in military applications because of its high melting point (3420 \({}^{\circ}\)C), thermal conductivity (174 W m\({}^{-1}\) K\({}^{-1}\) at 25 \({}^{\circ}\)C), hardness, and strength [1, 2]. It is also recognized as the most promising plasma-facing material for use in future nuclear fusion reactors on account of its excellent radiation-shielding properties against heat and plasma flux [3, 4]. Tungsten parts are commonly fabricated by powder metallurgy methods [5, 6]; particularly, an automatic powder injection molding tool was successfully designed and applied to produce tungsten products [7, 8]. Recently, selective laser melting (SLM) shows considerable advantages in producing metal parts with complex outer and inner structures [9] and provides a promising alternative for tungsten processing.  
During the SLM process, metal powders are deposited and selectively melted by a computer-controlled laser in a layer-by-layer manner. This process has been used successfully to fabricate parts made of titanium alloys [10], stainless steel [11], CoCrMo alloys [12, 13], and copper alloys [14]. In SLM, a continuous molten track needs to be formed in order to achieve densification. If the molten track breaks, balling phenomena can occur, which might reduce the final density of the manufactured parts. [PERSON] et al. studied the stability of a single track in selective laser melting [15] and found that a good wetting condition between the molten pool and the substrate is required to stabilize the molten track.  
The formation of a continuous track is especially difficult during SLM of tungsten because of its high melting point, thermal conductivity, and oxidation tendency. If the tungsten melt solidifies too quickly to spread out completely over the cool substrate, balling effects can occur. In addition,adsorption of oxygen can affect the surface tension of the molten pool and reverse the Marangoni flow direction, which enhances the balling effect. Many researchers have studied the SLM of tungsten. [PERSON] et al. studied tungsten formation by selective laser micro-sintering and their results showed a strong tendency for tungsten to exhibit balling phenomena and that the final density increases with increasing laser power [16]. [PERSON] et al. obtained tungsten bulk with 82% of the theoretical density (TD) by SLM and concluded that continuous molten tracks are essential for achieving densification [17]. [PERSON] et al. also obtained tungsten bulk with 82.9% of the TD by SLM from polyhedral powders and concluded that the balling tendency of tungsten is especially high because molten tungsten droplets tend to solidify before they can completely spread out over the surface of the sample [18]. In addition to these studies, a complex tungsten collimator (89.2% TD) was built by [PERSON] et al. [19]; however, they did not discuss the processing in detail. From the above studies, it can be concluded that improved wetting conditions are required for the SLM of tungsten. Commonly, this is achieved by optimizing the processing parameters [20], such as the laser power and scanning speed. In addition to these optimizations, [PERSON] observed that a spheroidization process can increase the laser absorptivity of AlSi10 Mg powders [21], while [PERSON] et al. observed that increased laser absorptivity increases the molten pool temperature and, thus, decreases the viscosity [22], which is beneficial to the wetting process.  
This aim of this work is to study the densification process of tungsten during selective laser melting. In order to optimize powder properties, raw polyhedral powders were processed into spherical powders by induction plasma equipment. Dense tungsten parts were successfully fabricated under suitable optimized experimental parameters. In situ experiments were conducted to test the laser absorptivity of polyhedral and spherical powders, and the effect of powder spheroidization on densification was then discussed.