Objective With the release of “Energy Planning for the Core Area of the Silk Road Economic Belt” in 2018, Xinjiang's coal industry is about to enter an unprecedented significant leap-forward development. In the manufacturing industry, it is paramount to develop new coating technologies for wear and corrosion protection of large and high-quality components. The most common process for wear and corrosion protection is hard chromium plating. Its biggest drawback concerns environmental protection. Moreover, the electrochemical processes consume much energy and become less economical as electricity cost increases. Therefore, new alternatives to hard chromium plating are under investigation, and high-speed laser cladding (HSLC) is such an alternative. The benefits of laser cladding (LC) are low heat input, low dilution with the substrate, less material consumption, and overall good performance. HSLC overcomes the efficiency obstacle of conventional LC technology, as well as provides an environmentally friendly and cost-effective production mode for the fabrication of thin coatings on large parts. To meet the tough health and environmental demands along with industries need for lower costs, high-quality iron-based alloy coating material suitable for HSLC was prepared. Through the analysis of its macro-morphology, microstructure, hardness, and corrosion resistance, the basis for improving the corrosion resistance and service life of hydraulic props is provided.
Methods In this study, 45 steel was selected as the substrate. Iron-based alloy powder was used as coating materials. A ZKZM-2000 fiber laser system was employed. A defocus of 15 mm was adopted. The diameter of the beam spot was 1.2 mm. The powders were fed by a powder feeder, and argon was employed as the carrier gas. The main distinctions between HSLC and conventional LC are the melting mode of powder and the formation mode of the molten pool. For the former, the focal planes of the powder stream and laser beam are above the molten pool. Under such conditions, most of the powders are heated and melted before being injected into the molten pool. For the latter, the powders are mainly melted in the molten pool. At 50% overlapping rate, 260 μm thickness and ~7.1% dilution rate of the coating can be obtained. Macroscopic features and microhardness were investigated using an ultra-depth three-dimensional microscope and a microhardness tester, respectively. Test specimens were etched using aqua regia to analyze the microstructure and phase components of the coatings using scanning electron microscopy, energy dispersive spectrometry, and X-ray diffractometry (XRD). The corrosion behavior of the coating was evaluated using CHI660E system at room temperature (20 ℃). The medium was 3.5% NaCl solution.
Results and Discussions The cladding efficiency of the HSLC process could reach up to 0.243 m 2/h. Under the same process parameters, the growth laws and trends of single-layer, double-layer, and four-layer coatings are basically the same, indicating that the increase in the number of cladding layers has little effect on the microstructure of the coating. The HSLC samples were formed uniformly at the macro-level, and the surface roughness was controlled at 21.38 μm, which was less than 10% of the conventional LC samples (Fig. 2). The geometric dilution ratio is ~7.1%, which is uniform and dense, in addition to good metallurgical bonding to the substrate. The low dilution ratio obtained from the HSLC process is due to low heat input and specific metallurgical forms. The result of the XRD pattern for HSLC coating indicates that the coating mainly consists of α and γ phases (Fig. 7). The ultrafine dendrites with an average grain diameter of less than 2.8 μm are formed (Fig. 6). The difference between grain sizes is mainly determined by the cooling rate. High cladding speed contributes to increasing cooling rate, which causes low dilution ratio and dendrites refinement. The microhardness of the coating is about three times as high as the substrate. In a 3.5% NaCl solution, the corrosion current of the coating dropped by two magnitudes compared to substrate, which indicates that a more uniform microstructure of HSLC coatings leads to a higher corrosion resistance.
Conclusions Cladding layers of iron-based alloy powders were prepared on 45 steel surface using HSLC. The macroscopic features, microstructure, and corrosion resistance were comparatively investigated. The crack-free layers obtained with HSLC present good metallurgical bonding with the substrate and a high degree of uniformity and compactness. At a scanning speed of 3600 mm/min, the coating thickness is up to 260 μm, with a dilution rate of ~7.1%. Compared with the dendrite characteristics by messy dendrites of the conventional LC, the microstructure of the coating prepared by HSLC is mostly dendrites. Moreover, its microstructure of dendrite is finer, the difference in composition between grains is smaller, and the distribution of grains is more uniform. The microhardness of the HSLC coating is three times as high as the substrate under the joint action of grain refinement and solid solution strengthening. The corrosion behavior in 3.5% NaCl solution indicates that the HSLC coating has good corrosion resistance, and its corrosion current density Icorr is two and three orders of magnitude lower than that of conventional LC coating and hard chromium plating, respectively. Therefore, the coating obtained from HSLC can satisfy the tough health and environmental demands. In addition to improving the work efficiency, the waste of follow-up processing materials and resources is reduced.
