**1. Introduction**

Aluminum alloys are widely used in automobiles, ships, aerospace, and other fields because of their low density, high specific strength, and good corrosion resistance. However, low hardness and poor wear resistance restrict their application in various fields [1,2]. Preparing particle-reinforced metal matrix composite coatings on aluminum alloys via surface engineering techniques is an effective way to improve their surface properties [3]. Laser technology has been a research hotspot in recent years [4,5]. Ayers et al., first proposed laser melt injection (LMI), which injects the additive particles into the laser melt pool directly, and then particle-reinforced metal matrix composite coatings can be formed on various metal substrate surfaces [6]. Compared with laser cladding, LMI has the advantages of low particle solubility, high surface performance, and low cracking tendency [7]. Ayers et al., prepared TiC- and WC-reinforced metal matrix composite layers on aluminum alloy substrates, and the wear resistance of the aluminum alloys was improved [8–12]. Vreeling et al., prepared SiC/Al composite layers on Al substrates via LMI, and found that preheating the Al substrate is an effective means of injection of SiC into the Al melt [13]. Wang et al., modified Al substrate surfaces via LMI using CeO<sup>2</sup> particles, and the microstructure of the surface was suitably modified in terms of corrosion resistance [14].

In the existing literature, the most commonly used injection particles are ceramics, such as WC, SiC, TiC, etc. Ceramics are well known for their high hardness and good wear resistance, but low room-temperature toughness. Compared with ceramics such as WC, SiC, TiC, etc., W has better room-temperature toughness, and W/Al have better interface compatibility and smaller thermal and physical differences, making W an ideal reinforcing particle for aluminum alloys. Over the past few years, high-performance W/Al composite layers have been prepared via stirring friction, laser metal deposition, and laser alloying [15–17]. However, no studies on the laser melt injection of W-particle-reinforced metal matrix composite layers have been reported to date.

In this study, a W-particle-reinforced aluminum matrix composite layer was prepared via LMI, and the microstructure and wear behavior of the composite layer were studied.

#### **2. Materials and Methods**

Tungsten (W) particles with diameter of 5–25 µm were selected as the injection particles (Figure 1), and a 7075 aluminum alloy block with dimensions of 200 mm × 150 mm × 50 mm was used as the substrate, the chemical composition of which is shown in Table 1.

**Figure 1.** SEM of the W particles.

**Table 1.** Chemical composition of the 7075 Al alloy (wt.%).


As shown in Figure 2, the LMI apparatus included a 6 kW continuous-wave fiber laser (IPG, YLR-6000, IPG Photonics, Oxford, MA, USA) with a laser wavelength of 1.06 µm, a homemade laser head, a 6-axis robot, and a powder feeder (HUST-III, Huazhong University of Science and Technology, Wuhan, China).

**Figure 2.** Equipment for the laser melt injection.

During the LMI process, a laser beam with a diameter of 3 mm irradiated the Al substrate's surface, the W particles were injected into the tail of the laser molten pool, and argon with a flow rate of 4 L/min was used as the delivering and shielding gas. As the laser head scanned, the W particles were captured by the molten pool, and a W/Al composite layer was finally formed, with an overlapping ratio of 50%. In order to investigate the effect of powder feeding rate on laser melt injection, the powder feeding rate was set to 7 g/min, 10 g/min, 13 g/min, or 16 g/min, and the laser power was 3000 W, while the laser scan speed was 700 mm/min. The specimens were machined using an electric spark CNC machine (DK7750, Taizhou Zhongxing CNC Machine Tool Plant, Taizhou, China), and transverse sections of the samples were ground, polished, and then etched with Keller's reagent for 2–3 s at room temperature. The microstructure of the W/Al composite layer was characterized by scanning electron microscopy (SEM, JSM7600F, Shanghai Baihe instrument Technology Co., Ltd., Shanghai, China) and energy-dispersive spectroscopy (EDS, IncaXMax50, Oxford instruments co., Ltd., Oxford, UK). The chemical composition and elemental distribution of the W/Al composite layer were analyzed with an electronic probe microanalyzer (EPMA, EPMA-8050G, Shimadzu Corporation of Japan, Shimadzu, Japan) equipped with a wavelength-dispersion spectrum (WDS). The phases in the W/Al composite layer were identified with an X-ray diffraction meter (XRD). The hardness of W/Al composite layer was tested using a microhardness tester (HXD-1000TM, Shanghai changfang optical instrument co., ltd., Shanghai, China) with a t load of 2.94 N and holding time of 20 s. The hardness distribution was measured along its depth direction. Roomtemperature sliding wear was tested using an abrasion tester (UMT TriboLab, Brooke Technology Co., Ltd., Billerica, MA, USA), as shown in Figure 3, where a Si3N<sup>4</sup> ball with a diameter of 6.3 mm was slid on the specimen with a test load of 15 N and speed of 10 mm/s, the wear length was 5 mm, and the test duration was 20 min. Each group of tests was repeated three times, and the micromorphology of the wear samples was characterized using a confocal microscope (KEYENCE, VK-X2500, Keens Japan Ltd., Osaka, Japan).

**Figure 3.** Equipment for the wear test.
