**1. Introduction**

Tungsten heavy alloys (WHA) have been used in many fields, including aerospace, defense, military, nuclear, electronics, and marine industries, due to their high melting point and density, excellent thermal conductivity, low thermal expansion, good corrosion resistance, and superior comprehensive properties at high temperatures [1,2]. They have also been proven to be ideal facing-plasma materials in the nuclear industry [3,4]. The 90W-7Ni-3Fe allow, a typical tungsten heavy alloy, which consists of tungsten (90 wt.%), Nickel (7 wt.%) and Iron (3 wt.%), possesses good mechanical properties due to the addition of Ni and Fe, while retaining high-density. In addition, Ni and Fe play a key role in the suppression of cracks in the fabrication of tungsten because of the nature of the brittleness of tungsten at room temperature [5]. Traditionally, 90W-7Ni-3Fe is processed by solid and liquid sintering. A mixture of tungsten, nickel, and iron powder was prepared and then sintered at a certain temperature, such as 1400 ◦C, for a fixed time in a vacuum or inert gas atmosphere [6,7]. Usually, sintered samples need to be treated by isostatic pressing or other heat treatments to reduce their porosity and avoid hydrogen embrittlement [8–10]. However, it is di fficult, or even impossible, to obtain complex components of a tungsten heavy alloy by conventional processes. Thus, additive manufacturing (AM) may be an alternative manufacturing process for high melting point refractory metals, such as tungsten and its alloys.

Selective laser melting (SLM), as a metal additive manufacturing technology, has been proven to be a promising technology in the high-accuracy and integrated fabrication of metallic components. SLM employs a high-energy laser beam to melt metallic powder layer by layer, according to 2D slice data. SLM is a multidisciplinary cross-technology involving mechanical engineering, material science, optics, software, and has attracted much attention from many fields. Deprez et al. [11] produced a complex high-density tungsten collimator. The produced collimator was geometrically accurate, and the tested values of sensitivity and resolution were close to the expected results of the CAD design. The relative density of the SLM pure tungsten prepared by Zhang et al. [12] reached 82%, who found the formation of nanocrystalline in the tungsten samples fabricated by SLM. Zhou et al. [13] analyzed the balling phenomena in the process of SLM-tungsten and proposed a competitive mechanism of spreading and solidification to explain the balling phenomena. Enneti et al. [14] investigated the e ffects of scan speed and hatch spacing on the relative density of SLM pure tungsten, but the highest relative density was only 75%. The relative density of the SLM tungsten prepared by Wen et al. [15] reached 98.7%, and they studied the e ffects of process parameters on the surface morphology, microstructure, and properties of SLM pure tungsten. Similarly, Tan et al. [16] also obtained SLM tungsten with a relative density of 98.5% and analyzed the e ffects of di fferent laser powers and scan speeds on the SLM's surface morphology, microstructure, and properties. All the previous studies on SLM tungsten indicated that the crack phenomenon was inevitable due to the inherent brittle nature of tungsten [17–19]. Therefore, several measures of crack-suppression for SLM pure tungsten were adopted and reported by Li et al. [20] and Ivekovi´c et al. [21]. Their results showed that the addition of secondary-phase nanoparticles or Tantalum could reduce cracks in the process of SLM pure tungsten, but a crack-free sample was still not available. In addition, the addition of low-melting-point metals might be beneficial to the fabrication of crack-free tungsten alloy components. Li et al. [22] investigated the fabrication of W-10Cu by SLM and obtained an optimized combination of laser power and scan speed. Their results showed that the forming mechanism of SLM W-10Cu was liquid phase sintering. Ivekovi´c et al. [23] obtained a 90W-7Ni-3Fe sample with a high relative density (>95%). Their results indicated that the high densification of 90W-7Ni-3Fe samples required a high energy density. In addition, they observed three major binding mechanisms: liquid phase sintering, partial melting, and complete melting. Preheating contributes to complete melting. After heat treatment, the tensile strength decreased slightly, but the elongation was significantly improved. Li et al. [24] studied the e ffect of process parameters on the densification and microstructure of 90W-7Ni-3Fe in SLM. They found that a lower scan speed, narrower scan interval, and thinner layer thickness can improve the densification process. Similar findings were also reported by Zhang et al. [25], who established an e ffective 3D model based on finite element analysis theory and temperature distributions under di fferent process parameters. As mentioned above, tungsten and its alloys are of interest to researchers, but SLM 90W-7Ni-3Fe has been rarely discussed.

In this work, 90W-7Ni-3Fe samples were formed by SLM. The influences of the process parameters on the densification, phase constitution, and microhardness of the 90W-7Ni-3Fe samples were discussed. In addition, the microstructure and tensile properties of the 90W-7Ni-3Fe samples fabricated by SLM were investigated and discussed.

## **2. Materials and Methods**

#### *2.1. Experimental Equipment and Preparation*

All experiments were carried out with an SLM 280 HL (SLM solutions, Germany), equipped with two 400 W Nd: YAG lasers. Figure 1a depicts the schematic diagram of SLM 280 HL. SLM 280 HL has two kinds of substrates: 250 mm × 250 mm and 100 mm × 100 mm. A small substrate was utilized in this work. During the process of SLM, tungsten heavy alloy powder particles fell from the powder container under gravity, and then the fallen powder particles were evenly spread on the substrate under the action of a powder scraper. The bi-directional movement of the powder scraper was adopted for the sake of enhancing the building e fficiency. All samples with a size of 10 mm × 10 mm × 5 mm were prepared on a 304 stainless steel substrate, which was preheated at 150 ◦C in order to prevent cracking and warping. In the whole SLM process, the chamber was filled with argon in order to avoid oxidation, and the content of oxygen was kept below 400 ppm. Figure 1b illustrates the building process of the SLM tungsten heavy alloy.

**Figure 1.** (**a**) Schematic diagram of selective laser melting (SLM) equipment; (**b**) the building process of the SLM.
