Fabrication And Numerical Simulation Of Cobalt-based Composite Coating On Titanium Alloy Surface By Laser Cladding

With the development of China’s aerospace industry, titanium and titanium alloys have been widely used because of its own merits, such as low density and high specific strength. However, titanium alloy is always limited in its application by low hardness and poor wear resistance. In recent years, laser cladding can provide a very promising approach to improving the surface properties of titanium alloys, which is one of the research hotspots in the field of surface modification. Laser cladding can form a composite coating of high hardness and good wear resistance onto the surface of titanium and its alloys, to improve its surface defects effectively as well as maintain the matrix’s good performance. In laser cladding process, there are heat conduction and chemical changes in the small molten pool, which are extremely complex and responsive. This phenomenon directly influences the quality and mechanical properties of cladding layer. Therefore, it is particularly important to control the temperature field of molten pool. It is of great significance in practical application to grasp the temperature distribution of the molten pool by using computer numerical simulation. It can guide the optimization of cladding process parameters and the prediction of coating quality defects, in order to improve the quality of cladding layer.In this paper, a cobalt-based composite coating was prepared on TC4 surface by laser cladding. First of all, the temperature distribution of molten pool was explored by using the finite element software ANSYS, and laser power parameter was optimized. Then laser cladding was carried out according to the optimized parameters. There were two kinds of cladding materials:pure KF-Co50 self-fluxing powder and cobalt-based mixed powder which doped with 10% ZrO2. Finally, microstructure and mechanical properties of two samples’ cladding were analyzed. Specific studies are as follows:The model of preset laser cladding was established by FEM software ANSYS. In addition, the mobile laser heat source was loaded by using parametric design language APDL, The numerical simulation of temperature field shows that:the molten pool of laser cladding is ellipsoidal and the position of molten pool’s maximum temperature is slightly lagging behind laser spot center. The isotherms of molten pool is spoon-shaped, and there is large temperature gradient in front of the spot (the isotherms are intensive) while the temperature gradient is small behind the spot (the isotherms are sparse). The results of the study on the temperature distribution of molten pool at different laser powers show that the maximum temperature of molten pool and cladding layer depth increase with the increase of laser power. In order to obtain a small dilution rate of cladding, it is better that laser power is between 1000Wto1100W.The experiment of preset laser cladding was carried out on TC4 alloy by semiconductor laser, which model was Laserline LDF 4000-100. The experimental parameters are as follow: laser power is 1000W, scanning velocity is 5mm/s, and the diameter of laser spot is 3mm. After that, XRD、 EPMA and SEM were used to analyze the microstructures and phase structures of cladding layer. The results show that the phase species of A1 and A2 are similar. TiB2 and WB with the shape of strip or block, in-situ synthesized TiC particles and hard reinforced phases such as CoTi、Cr23C6、CrB, were distributed in the y-Co with a little β-Ti solid solutionA DHV-1000 type Digital Vickers hardness tester was used to measure the micro-hardness values of the cladding layer. The micro-hardness distribution curves of cladding cross section show that the micro-hardness of cobalt-based composite coating is improved significantly compared with the titanium alloy substrate, which is about three times higher than that of the substrate. The value of the micro-hardness show a gently downward tendency from cladding layer surface to the substrate. Horizontal comparison of two specimens’micro-hardness distribution curve displays that the specimen of adding zirconia ceramic particles do not significantly improve the hardness of the cladding layer. The addition of ZrO2 aims at reducing cracks in the cladding layer and arresting its propagation, as well as improving the strength and toughness of the cladding layer.