**2. Materials and Methods**

In this research, the experimental setup for the laser cladding process consists of a laser system (made in Shanghai, China), a 6-axis KUKA robot (made in MS, USA), a water-cooling machine to ensure the normal operation of the laser system, and a protection chamber to control the argon environment with less than 60 ppm oxygen content. Overall, the equipment of laser cladding is shown in Figure 1.

During the laser cladding process, the high entropy alloy powder and the surface substrate are subjected to the radiation of a high-energy laser beam, which quickly melts, diffuses, and solidifies to form a cladding layer that combines well with the substrate. The detailed schematic diagram of laser cladding is shown in Figure 2.

**Figure 1.** Equipment of laser cladding experiments: (**a**) laser cladding system with a KUKA robot, protection chamber, and water-cooling machine; (**b**) laser head; (**c**) laser system.

**Figure 2.** Schematic diagram of the experimental setup for the process.

In this paper, TC4 is used as the matrix material, and self-developed FeCoCrNi high entropy alloy is used as the powder material. The spherical pre-melted FeCoCrNi powders of 15~53 μm were produced by vacuum gas atomization under the Ar atmosphere. The element compositions of the substrate and HEA powder are shown in Table 1.

**Table 1.** Chemical compositions of the substrate and FeCoCrNi powder (wt%).


Meanwhile, reasonable control of the process parameters is essential for achieving the desired performance. However, the properties of laser cladded coating are affected by numerous factors, such as the laser scanning speed and laser power. Therefore, this paper introduces the concept of specific energy, which can reflect the changes in laser power and scanning speed at the same time. The selected laser cladding parameters are shown in Table 2.


**Table 2.** Laser cladding experiment parameters.

After laser cladding, specimens were cut along the transverse sections, then they were ground and polished to 0.06 μm using colloidal silica, and subsequently they were electrolytically etched using the standard Kroll's reagen<sup>t</sup> (HF:HNO3:H2O = 1:2:6) with 10 s. The crystal structure and lattice parameters of the laser cladding products and TC4 substrate were discussed from the corresponding XRD patterns. The XRD was tested on a Bruker D8 ADVANCE (made in Karlsruhe, Germany) Bragg-Brentano diffractometer with an X-ray wavelength of 1.54 Å. The microstructural research and chemical analysis of the laser cladded samples are performed using an FEI Inspect-F Scanning Electron Microscope (made in Hillsboro, OR, USA) equipped with an energy-dispersive spectroscopy (EDS) detector. The microhardness tests were performed using an HVS50 hardness tester, with 15 s load application time and under loads of 1000 g. Meanwhile, the wear experiments were carried out at room temperature using CFT-I type fretting friction and wear machine (made in Shanghai, China) with a normal load of 30 N, a motor speed of 300 t/m, and a wear time of 10 min. Besides, Si3N4 ceramic balls with a radius of 3 mm were used as the friction pair.
