*3.1. Microstructure*

Figure 4 shows the macroscopic surface and cross-section of the W/Al composite layer with a powder feeding rate of 16 g/min. The XRD results in Figure 5 show that W, Al, and Al4W are the constituent phases in the W/Al composite layer. The peaks from the

aluminum alloy phases are very weak, and the main peak from aluminum is strong in the W/Al composite layer. Figure 6 shows the scanning electron microscopy (SEM) images of the W/Al composite layer. As shown in Figure 6, all of the W/Al composite layers were composed of white particles, dark blocks, and a black matrix. The EPMA results of the W/Al composite layers with powder feeding rates of 7 g/min, 10 g/min, 13 g/min, and 16 g/min are given in Figure 7. The EPMA results show that the composition of the white particles was 100W (at. %), that of the dark block was 79.3Al-20.7W (at. %), and that of the black matrix was 96.5Al-1.3Mg-1.7Zn-0.5Cu (at. %). Based on the EPMA and XRD results, it can be concluded that the white particles are W, the dark blocks are Al4W, and the black matrix represents aluminum alloys. The fraction of the reinforcing phase is an important factor that affects the performance of the composites layer. Here, the area fraction of W and Al4W in the W/Al composite layer was measured using Imaging-plus 6.0 software (Pro Plus 6.0, American Media Cybernetics image technology company, Rockville, MD, USA), as shown in Figure 8, and the area fraction of W and Al4W increased with the increase in the powder feeding rate. When the powder feeding rate was 7 g/min, the area fraction of W particles and Al4W in the W/Al composite layer was only 6.3% and 14.6%, respectively. As the powder feeding rate increased to 16 g/min, the area fraction of W particles and Al4W increased to 46.2% and 35.9%, respectively.

**Figure 4.** Macroscopic surface (**a**) and cross-section (**b**) of the W/Al composite layer with a powder feeding rate of 16 g/min.

**Figure 5.** X-ray diffraction spectrum of the W/Al composite layer.

**Figure 6.** Microstructure of W/Al composite layers with powder feeding rates of (**a**) 7 g/min, (**b**) 10 g/min, (**c**) 13 g/min, and (**d**) 16 g/min.


**Figure 7.** Microstructure and elemental distribution of the W/Al composite layers with powder feeding rates of 7 g/min, 10 g/min, 13 g/min, and 16 g/min.

**Figure 8.** Area fraction of W, Al4W, and Al alloy in the W/Al composite layer.

#### *3.2. Hardness*

As shown in Figure 9, the hardness of the W/Al layer increased with the increase in the powder feeding rate. When the powder feeding rate was 7 g/min, the hardness of the W/Al composite layer was almost the same as that of the 7075 Al substrate. As the powder feeding rate increased to 16 g/min, the hardness of the W/Al composite layer could reach up to 350 HV, which is 2.5 times higher than that of the 7075 Al substrate (142.3 HV).

**Figure 9.** Hardness distribution of the W/Al composite layer.
