The stern shaft is an important device for the power transmission of ships. However, corrosion is the main failure mode for stern shafts, which are subjected to the attack of Cl- and microorganisms in seawater for a long time in a marine environment with high salt and humidity. The vibration and shock of the ship stern shaft aggravate the wear of the stern shaft. The traditional anticorrosion strengthening of the ship stern shaft surface involves coating its surface with anticorrosion coatings, such as ethylene resin, epoxy resin, and chlorinated rubber. Although these anticorrosion coatings protect ship stern shafts to a certain extent, most are toxic and harm the natural environment, which seriously violates the current trend of green development. Therefore, developing a green, clean, and pollution-free surface modification method for ship stern shafts has not only economic value, but also broad environmental value. An attempt is being made to develop a eutectic high-entropy alloy based on laser cladding technology to provide an effective method for green anticorrosion and wear-resistant modification of ship stern shaft surfaces.
The base material was 42CrMo steel. In an argon atmosphere, FeCoNiCrNb0.5Mo0.25 high-entropy alloy cladding layers were prepared using a laser with a spot diameter of 4 mm and a scanning speed of 3 mm/s at five different laser powers (1200, 1300, 1400, 1500, and 1600 W). The phase compositions of the cladding layers were analyzed using X-ray diffraction. The microstructures of the cladding layers were observed by scanning electron microscopy. The hardness values of the cladding layers were measured using a Vickers hardness tester. Friction and wear experiments were conducted using a multifunctional friction and wear tester. The corrosion resistances of the cladding layers were tested using an electrochemical workstation.
With the increase in laser power, the molten pool depth of the high-entropy alloy cladding layers increases (Fig. 3). FeCoNiCrNb0.5Mo0.25 entropy alloy cladding layers prepared using different laser powers are composed of an incomplete eutectic structure of FCC and Laves phases. With an increase in laser power, the content of lamellar nano-eutectic structure first increases and then decreases. The eutectic microstructure can be promoted by increasing the laser power appropriately, but too high laser power results in a stronger dilution effect of Fe in the substrate on the high-entropy alloy cladding layer, which weakens the promoting effect of Mo and Nb on the eutectic microstructure. The microstructure of the high-entropy alloy cladding layer prepared using a laser power of 1400 W is better than that of high-entropy alloy cladding layers prepared using other laser powers, and the microstructure is nano-eutectic with a lamellar spacing of approximately 86 nm. The increase in the laser power reduces the average hardness of the cladding layer, and the high-entropy alloy cladding layer sample prepared at laser power of 1200 W has the highest microhardness of 665.8 HV1.0, which is approximately 2.34 times that of the substrate (Fig. 5). With an increase in the laser power, the wear resistance of the FeCoNiCrNb0.5Mo0.25 high-entropy alloy cladding layer first increases and then decreases (Fig. 11). The high-entropy alloy cladding layer (1400 W cladding layer) sample owns excellent eutectic structure and the best wear resistance, with the lowest wear rate of 0.079 mm3·N-1·m-1. Compared with the substrate, the FeCoNiCrNb0.5Mo0.25 high-entropy alloy cladding layers have better corrosion resistance. However, there is no obvious linear relationship between the laser power and corrosion resistance of the high-entropy alloy cladding layer (Table 7). The lowest self-corrosion current density of the FeCoNiCrNb0.5Mo0.25 high-entropy alloy cladding layer is 1.716×10-6 A·cm-2. The existence of a eutectic structure reduces the corrosion resistance of the cladding layer to some extent. The corrosion resistance of the 1400 W cladding layer with a better eutectic structure is poor, and the self-corrosion current density is 4.332×10-6 A·cm-2.
Laser power affects the microstructure by changing the content of the cladding layer elements and solidification conditions. Properly increasing the laser power can promote the eutectic microstructure, but too high laser power strengthens the dilution effect of Fe in the matrix on the high-entropy alloy cladding layer and weakens the promotion effect of Mo and Nb on the eutectic microstructure. With an increase in the laser power, the microhardness of the cladding layers decreases owing to the increase in substrate dilution. The wear mechanisms of high-entropy-alloy cladding layers include oxidation wear, adhesion wear, and abrasive wear. The oxide film on the worn surface plays a significant role in protecting the lower metal. The eutectic structure with alternating soft and hard distributions reduces the material loss, and the cladding layer prepared at 1400 W has the lowest wear rate. In 3.5% NaCl solution, corrosion occurs around the oxide on the cladding layer surface, and the existence of a eutectic structure intensifies galvanic corrosion and reduces corrosion resistance.
