*3.3. Wear*

As shown in Figure 10, all of the W/Al composite layers exhibited lower friction coefficients than that of the 7075 Al substrate. The average friction coefficient of the 7075 Al alloy substrate was about 0.442; with the increase in the W powder feeding rate from 7 g/min to 16 g/min, the average friction coefficient decreased from 0.404 to 0.367.

**Figure 10.** (**a**) Friction coefficient curves and (**b**) average friction coefficients of the W/Al composite layers and 7075 Al substrate.

Figure 11 shows the wear surface of all of the samples; as shown in Figure 11a, the abrasion width and depth of the 7075 Al substrate were 1368.4 µm and 114.7 µm, respectively, while all of the W/Al composite layers exhibited lower abrasion width and depth than those of the 7075 Al substrate. With the increase in the W powder feeding rate from 7 g/min to 16 g/min, the average abrasion width decreased from 1102.2 µm to 617.3 µm, and the average abrasion depth decreased from 50.7 µm to 20.7 µm. According to wear rate, ε = V/(G·L). The wear volume is V, the test load is G, and the wear scar length is L. As shown in Figure 12, all of the W/Al composite layers exhibited better wear resistance than that of the 7075 Al substrate. The wear rate of the 7075 Al alloy substrate was 4.74 mm3/N m; with the increase in the W powder feeding rate from 7 g/min to 16 g/min, the wear rate decreased from 1.64 mm3/N m to 0.40 mm3/N m.

**Figure 11.** Wear surface morphology of (**a**) 7075 Al alloy and W/Al composite layers with W powder feeding rates of (**b**) 7 g/min, (**c**) 10 g/min, (**d**) 13 g/min, and (**e**) 16 g/min.

**Figure 12.** (**a**) Wear scar distribution curves and (**b**) wear rates of 7075 Al alloy and W/Al composite layers.

#### **4. Discussion**

During the laser melt injection, the W particles enter the high-temperature melt pool, and W atoms diffuse into the laser melt pool. According to the Al–W binary phase diagram, during the cooling process of the melt pool, W particles react with Al solution to form intermetallic compounds. Khoshhal, Niu, and Wang pointed out that Al4W was first formed due to its low generation enthalpy, and further calculations show that the low generation enthalpy can be attributed to the fact that Al4W has a smaller n(Ef) (the Fermi level) than Al12W [18–20]. According to the solid–liquid reaction mechanism, Al4W is formed by a peritectic reaction between W and aluminum at 1327 ◦C. At the beginning of the laser melting, the liquid will quickly adhere to the surface of the W, forming an adherent layer with a certain concentration gradient, and the concentration of aluminum gradually decreases from the outside to the inside [21,22]. As the Al concentration in the inside layer of the diffusion layer increases, the solute atom W reacts with the solvent atom Al to form an Al4W intermetallic compound enveloping the W particles [23]. There are also partially escaped W and Al reactions to form free Al4W intermetallic compounds in the matrix between the W particles. During the process of LMI, the cooling rate is very fast (about 2.8 <sup>×</sup> <sup>10</sup><sup>3</sup> ◦C/s), and there is not enough time for the Al4W to react with the molten Al and form Al5W and Al12W [24]. Therefore, the W/Al composite layer consists of W, Al, and Al4W.

During the process of wear testing, the temperature of the wear sample is raised due to the friction heat. The 7075 Al alloy has low hardness, and strengthens at elevated temperature; when temperature of the Al alloy reaches its flashpoint, the 7075 Al alloy is welded with the Si3N<sup>4</sup> ceramic ball, and tears under the action of shear force (Figure 13a–c). Thus, the friction coefficient and wear rate of 7075 Al are larger. Compared with the 7075 Al alloy, the W and Al4W in the W/Al composite layer have higher hardness at elevated temperature, which can enable them to effectively resist the extrusion of the Si3N<sup>4</sup> ceramic ball, and reduces the wear (Figure 13d–f). Compared with the 7075 Al substrate, the friction coefficient and wear rate of the W/Al composite layer are smaller. This shows that the W/Al composite layer has excellent wear resistance [25].

**Figure 13.** Wear surface (**a**,**b**,**d**,**e**) and cross-sectional view (**c**,**f**) of 7075 Al alloys (**a**–**c**) and W/Al layer with a powder feeding rate of 16 g/min (**d**–**f**).

#### **5. Conclusions**


**Author Contributions:** Conceptualization, D.W.; methodology, Z.X. and D.W.; validation, Z.X. and W.S.; formal analysis, Z.X. and C.T.; investigation, Z.X., P.S. and J.Y.; writing—original draft preparation, Z.X.; writing—review and editing, D.W., Q.H. and X.Z.; supervision, D.W. and X.Z.; funding acquisition, X.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Key Projects of the National Natural Science Foundation of China (No. 92066201).

**Acknowledgments:** The authors would like to thank the State Key Laboratory of Material Processing, Die & Mould Technology in HUST, and the Analytical and Testing Centre of HUST for XRD, SEM, and wear tests.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

