**1. Introduction**

Titanium alloy has been widely used in the aerospace industry due to its superior advantages of low density, high strength, and anti-corrosion performance [1–4] However, the service life of the titanium alloy structural part is restricted by the insu fficient wear resistance and hardness. Therefore, the surface strengthening of titanium alloy has attracted massive attention from researchers worldwide [5,6]. High entropy alloy (HEA), as a promising multi-component alloy, has drawn considerable attention to the repair of seriously worn aircraft flap slide due to its outstanding comprehensive properties, which is attributed to its e ffect of high entropy, lattice distortion, and slow di ffusion [7–9]. HEA rapidly attracts massive attention in material science since it was first reported [10–12]. HEA has created an unexplored area of alloy compositions and exploited the potential to influence solid solution phase stability through the controlling of configurational entropy.

Preparation of HEA coating on titanium alloy is a feasible method to improve surface hardness and wear resistance of titanium alloy. At present, the main technologies used to prepare HEA coating include thermal spraying [13,14], laser/plasma cladding [15–18], physical vapor deposition [19], and powder metallurgy [20,21]. Among the above HEA coating methods, laser cladding has advantages such as high efficiency, reliable metallurgical bonding, high material utilization ratio, and excellent performance [22,23]. Therefore, laser cladding of HEA coating has been widely adopted for repairing and strengthening the titanium alloy structural parts.

The mechanism of surface strengthening by HEA was reported in the literature. Huang et al. [24,25] investigated the structure and properties of high entropy alloys; results indicated that the wear resistance of the cladding layer was significantly improved due to the second phase strengthening. The effect of the cladding process on the microstructure and properties of the HEA coatings was reported in the literature. Joseph et al. [26] conducted a comparative study between arc melting and laser melting; the microstructure and properties of the HEA coatings were found to be significantly different under different processes, but the compression experimental results were not much different. The effect of chemical compositions on the microstructure and properties of the HEA coatings was reported in the literature. Jiang et al. [27] conducted a comparative study on microstructure evolution and wear behavior of the laser cladding CoFeNi2V0.5Nb0.75 and CoFeNi2V0.5Nb HEA coatings. Cai et al. [28] studied the alloying elements and dilution rates on the microstructure and properties of high entropy alloy cladding layers; results indicated that the migration of the Fe element from the matrix to the cladding layer could make the mixing entropy of CoCrNi coating close to the theoretical mixing entropy of FeCoCrNi high entropy alloy. The effect of laser power on the microstructure and properties of the HEA coatings was reported in the literature. Shu et al. [29] studied the effect of laser power on microstructure, mechanical and chemical properties of the CoCrBFeNiSi HEA coating; results indicated that the amorphous content in the coatings had a significant influence on microhardness, wear resistance, and corrosion resistance. Sui et al. [30] investigated the effect of specific energy on the microstructure and properties of laser cladded TiN/Ti3AlN-Ti3Al composite coating; results revealed that the dilution rate of the coating increased with the increase of specific energy, the microhardness of the composite coating was approximately three times higher than that of the substrate, and the wear resistance was improved remarkably under optimum specific energy 58.3 J/mm2. In summary, previous studies mainly focus on the thermal stability, oxidation resistance, microstructure evolution, mechanical properties, and wear behavior of the HEA coating, however, the effect of specific energy on microstructure, hardness, and wear resistance of HEA coating has not been completed understood in the literature.

Specific energy (E) is a key process parameter during laser cladding of HEA on the TC4 substrate. Specific energy is calculated by using the Equation: E = P/(V × D), where P (W) is the laser power, V (mm/s) is the scanning speed, and D (mm) is the laser spot diameter [30]. In this study, the influence of specific energy on the microstructure, phase transformation, hardness, and wear resistance of laser cladded FeCoCrNi HEA on the TC4 substrate will be systematically investigated, and the mechanism of surface strengthening lying in the laser cladding process will also be revealed, feasible process parameters for preparation of HEA coating will be obtained.
