Microstructure, microhardness and corrosion resistance of Stellite-6 coatings reinforced with WC particles using laser cladding
Introduction
Modern methods and techniques of surface engineering enable the production of different types of composite and complex coatings. The most frequently reported in the literature are thermal spraying, plasma and laser remelting of galvanic coatings, diffusion layers or precoat in the form of pastes [1], [2], [3], [4], [5], [6]. Using laser technology it is possible to modify properties of the surface of various materials such as light metals and various species of steels and cast iron [7], [8]. Laser cladding method [9], [10], [11], [12] is utmost importance for various industrial branches wherein a suitably designed nozzle simultaneously emits the laser beam and adds powder mixtures to it. It is thus possible to easily produce a coating with new unique properties. In industry, 5-axis CNC laser processing centers which allow production at coating only in zones where they are required are being used. An important problem often discussed in studies is wear resistance. This problem was described for example in the paper [13]. Less frequent meets the information focus on wear in soil environment. The production of wear-resistant coatings on agricultural tools may be a good example for using laser cladding technology. During exploitation such tools wear away only in some zones, so modifying their entire surface is economically unjustified. Increased durability only in the zones exposed to wear seems to be a good solution. Laser cladding may be a successful alternative to diffusion methods and other methods which are more energy- and time-consuming.
There are many publications where the authors describe metal matrix composite coatings (MMC). Due to the presence of hard particles of carbides such as WC, VC, etc. located in metallic matrix, MMC coatings have a much better wear resistance. Microstructure of this type of coatings is similar to microstructure of sintered materials. So far, majority of studies were focused on production of MMC coatings on Fe-based alloys or Ni-based alloys [14], [15]. However, there is far fewer papers concerning production of MMC coatings with Stellite-6 as a matrix [16], [17], compared to the amount of papers describing Stellite-6 coatings without hard particles [18], [19], [20], [21], [22], [23]. This alloy is often used for production of coatings on machine components which operate in conditions requiring high strength as well as good wear and corrosion resistance. Increased temperature does not change these properties. High durability of Stellite-6 is associated with its chemical composition (Table 1) and its microstructure. The chromium content contributes both to increasing corrosion resistance and wear resistance, which are a result of formation of two types of carbides M7C3 and M23C6. Alloying additives like tungsten and molybdenum cause formation of MC and M6C carbides and intermetallic phases. Carbides are embedded in the solid solution matrix, which is characterized by dendritic microstructure with high chromium content. Co-based alloys may be a good alternative to Ni-based alloys primarily due to a relatively lower price. Available publications mainly concentrated on production of MMC coatings using a CO2 [22] and Nd:YAG laser [23]. Laser production of coatings with matrix of Stellite-6 reinforced mainly by tungsten carbides may find a number of applications in mechanical and automotive industry and oil and gas mining.
The paper presents results of studies on Stellite-6/WC composite coatings on low carbon steel using Yb:YAG laser disk. Microstructure, chemical and phase composition were investigated. Additionally, microhardness as well as resistance to wear and electrochemical corrosion was measured. Bonding between tungsten carbide particles and metal matrix was also examined.
Section snippets
Materials
Low-carbon steel specimens with dimensions of 20×20×7 mm3 were investigated. In this study, mixtures consisting of commercially available Co-based alloy (Stellite-6) powder and tungsten carbides powder (WC) in two different proportions were used. Chemical composition of the steel used is shown in Table 1, while chemical composition of Stellite-6 powder is shown in Table 2. The Stellite-6 powder particles were spherical with size in the range of 25–53 µm, whereas WC powder particles were irregular
Macroscopic tests and description of laser parameters
Surface topography was characterized by surface waviness and surface inequality typical for the coatings produced with high-energy methods. However, the laser cladding method decreased surface inequality with higher powers of laser beam. This is due to melting of powder particles. When laser beam power was low, powder particles on surface were not completely melted, but only were glued. Such a surface is shown in Fig. 4. The SEM image shows the Stellite-6 and tungsten carbide particles which
Results of microhardness tests
The results of microhardness measurements are shown in Fig. 12, Fig. 15. These measurements were made along a straight line only in the Stellite matrix. The influence of carbide particles on the matrix hardness was determined. For MMC Stellite-6/30% WC coatings the irregular nature of the microhardness profiles was observed (Fig. 12). A significant difference in microhardness was found between the zone close to primary carbide and the zone far away from the carbide. Fig. 13 shows the change in
SEM/EDS X-ray microanalysis
The study of chemical composition of the produced Stellite-6/30% WC coatings demonstrated an influence of the addition of tungsten carbide on the increased amount of tungsten in the matrix. The results of EDS microanalysis are presented in Fig. 16. In the analysis of the chemical composition it was concluded that the spherical participates (Fig. 8) were probably complex carbides (Co, W, Cr, Fe)7C3. With increasing distance of the measurement points from carbide particles, a gradual decrease in
Phase analysis
Results from XRD patterns of specimens obtained sequentially with 30% WC and 60% WC are shown in Fig. 18, Fig. 19. XRD studies were carried out for the coatings produced using the following parameters: the laser beam power of 700 W and the scanning Speer of 340 mm/min. In both these cases the phases were the same, whereas their intensity had changed, and consequently their occurrence frequency changed too. In the coatings produced using the mixture containing 30% WC, WC and W2C phases were found.
Electrochemical corrosion
The important tests in the case of MMC coatings are testing of corrosion and erosion resistance. This paper applies to research on electrochemical corrosion resistance. For the MMC coatings containing 30% and 60% WC, the dependence curves of current density for a given potential were determined. These are shown in Fig. 20. Corrosion resistance of the MMC coatings was compared to the corrosion resistance of the Stellite-6 coating made in the same method using the same parameters. The
Conclusions
- 1.
Metal matrix composite coatings of Stellite-6 with the addition of 30% and 60% tungsten carbide WC produced using the laser cladding method were characterized by enhanced lower electrochemical corrosion resistance as compared to layers produced using pure-Stellite 6 powder. With increasing content of carbide particles in the coating, the number of possible corrosion cells increased. This causes destruction by electrochemical corrosion.
- 2.
The addition of tungsten carbide causes changes in
Acknowledgments
The authors wish to thank Ph.D. M. Tuliński for his help in XRD analysis.
Dariusz Bartkowski is a scholarship holder within the project “Scholarship support for Ph.D. students specializing in majors strategic for Wielkopolska׳s development”, Sub-measure 8.2.2 Human Capital Operational Programme, co-financed by European Union under the European Social Fund.
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