| High temperature wear is a common failure mode of critical mechanical components, such as hot-work die,hot roll,valve seat, and the engine cylinder.Correspondingly, Co based wear-resistant coating by laser cladding can prolong the service time of such components. With the largescale of industrial production and the rapidlyupgrading of technology, the service condition of mechanical components is more demanding, which means the hardness, wear resistance properties, especially high temperature wear resistance performance of the materials need to strive for improvement to ensure a long-term stable and effective operation. Conventional unitary Co based coating is unable to meet the above requirements, it is therefore urgent to research and develop a coating with ideal high temperature wear and oxidation resistance. Current research and production practice shows that, hard phase such as WC within the laser cladding Nibased coating can improve the wear resistance, but similar work is not sufficient in Co based coating, especially in the optimization of cladding process and properties of WC/Co-Cr coating at high temperature. As a consequence, it is of scientific significance and application prospect to explore the design and preparation technology of high quality coating with good wear-resistant at high temperature. The innovation of this paper is:To ensure the optimal processing parameters for laser cladding, WC/Co-Cr composite coating was prepared on the 45 steel substrate by YAG laser cladding. The current, frequency, pulse width, scanning speed was set as thevariables.Orthogonal test with four factors and three levels was put forward. The results are based on the evaluation of the qualities of the as-prepared coatings, and show that the effect of current on coating quality is the most significant, followed by frequency and laser scanning speed. Pulse width is insignificant. With the increase of WC content, the laser pulse width should be increased, and the laser scanning speed should be reduced. With the mass fraction of WC be 10%, the optimal cladding process is: current of 380 A,frequency 40 Hz, pulse width of 1 ms, scanning speed of 8 mm/s. With the mass fraction of WC increase to 20%, the optimal cladding process is: current of 380 A, frequency 40 Hz, pulse width of 1.5 ms, scanning speed of 6 mm/s.WC/Co-Cr composite coatings were characterized by X-ray diffraction (XRD),TEM, X-ray fluorescence (XRF), and Energy Dispersive Spectrometer (EDS). The main valence states of the samples were analyzed by XPS, and the morphology of the wear debris was analyzed by using an iron spectrum analyzer. The results show that after laser cladding, the phase composition of the composite coating is changed compared with the original powder mixture, some strengthening phases such as carbides and intermetallic compounds are created during cladding. The elements show a gradient distribution at the interface, which is beneficial to the comprehensive performance of coating. The as-prepared coating is composed of Co-Cr solid solution and (?) distributed strengthening phase such as Cr7C3, Cr3C2 and WC. The hardness gradually decreases from the surface to the base, and the maximum surface hardness was HV 1160.Wear characteristics of laser cladding WC/Co-Cr coating at room temperature (RT)were studied by sliding wear test. The wear mechanism of the composite coating was studied by comparing the wear weight loss, friction coefficient and wear surface of the coating under different wear conditions,was turned out to be a typical abrasive wear.With the increase of WC content, the average friction coefficient of the coating decreased, and the amount of wear and wear scars decreased dramatically. When the WC increased to 20 wt.%, the brittleness of coating increased, followed by the increasing of the amounts of surface micro crack, which is not conducive to improve the properties of such material.High temperature wear behavior of the coatings was investigated by MMU-5G high temperature wear testing machine. Scanning electron microscope (SEM) was used to observe and analysis of the worn surfaces. The corresponding wear mechanisms were proposed combined with the analysis of the composition on the worn surface. The results show that the wear resistance of WC/Co-Cr composite coating is better than H13 steel.The wear mechanism of coating for the wear test from RT to 200? is the combination of abrasive wear and adhesive, and turn into minor abrasions and oxidation wear at 200?500?, then turn into plastic smear at 600?.High temperature oxidation characteristic of coating was studied using a static oxidation method carried at 600—. The rules of high temperature oxidation were revealed by thermogravimetric and morphology analysis. The results show that the oxidation resistance of H13 steel after cladding with coating is significantly improved.The increasing in weight and oxidation speed of H13 steel after cladding are reduced.Oxidation rate constants (KP) decreases from 0.0109 to 0.0014. The microhardness of WC/Co-Cr coating before high temperature oxidation is HV1100, significantly higher than that of H13 steel, and this value still keeps higher than H13 steel after high temperature oxidation test, which is 650 and 200 HV200 for the coating and H13 steel respectively. The Fe2O3 phase appears in H13 steel. Correspondingly, WC/Co-Cr coating is mainly composed of CW3,Co6W6C, ?-Co, Cr23C6,and Cr7C3 phase. s-Co phase converts into a-Co phase after high temperature oxidation test. Carbides phase remains unchanged due to their excellent antioxidant properties. |
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