Laser Cladding Cobalt-based Alloy And Its Composite Coatings

Because the titanium alloys have less hardness and are susceptible to result in the adhesive wear, their excellent performance (such as high-intensity, better corrosion-resistance) are not fully realized. In order to improve the surface hardness and wear-resistance of titanium alloy. We will use laser surface modification technology to laser cladding cobalt-based alloys, c cobalt-based alloys +TiC, cobalt-based alloys +TiN and B₄Con the Ti₆Al₄V alloy surface.The experiment results showed that the laser cladding cobalt-based alloy coating include the cladding, combined and heat-affected area in the organizational structure. The primary phases present the dendrite, massive and granular several shapes because of the composition and temperature differences in the different parts of coating; The coating achieve the better metallurgical combination with the substrate. And, in order to optimize the parameters of laser cladding, we also analyze the influence of the thermodynamic analysis and convection factors on the surface quality of cladding coating. In the metallurgical combination between the coating and substrate, we should try to reduce the rate of the coating material to substrate. The rate is generally believed it should be between 5% and 10% in order to ensure excellent performance of surface coatings.The laser cladding cobalt-based alloy composite coating is composited of TiC particles and Ti₃SiC₂ solid solution, in addition small amount of NiC_x, V₆Si₅, Cr₇C₃ etc. We also analyze the solidification behavior of the cobalt-based alloy +TiC coating, and the influence of solidification velocity R and temperature gradient G on the crystal structure and morphology after the solidification. The temperature gradient G and solidification rate R (G/R) is the control parameters of the morphology of solidification structure. With distance from the bottom of the pool, G/R drop rapidly, the changes between cellular and dendrite occur, and form the coating characteristics which is constituted of cellular or dendrite primary phase and eutectic composition. We analyze the influence of cladding process parameters on the micro-structure of coating. The experiments show that the power is higher; the size of the dendrite is greater. When the scanning speed is relatively small the laser cladding process, the coating can fully absorb heat, the growth of micro-structure will be more fully formed and can form the typical morphology of dendrite. The laser cladding cobalt-based alloys +TiN mainly include carbide (such as TiC, Ni₃C etc.), nitride (such as TiN, Ni₄N etc.), solid solution (such as Ti₃SiC₂, Co₁₆Ti₆Si₇. CrSi₂, TiFeSi₂, etc.), and inter-metallic compounds (such as Cr₃Ni₂ etc.). Through the backscattered electron microprobe composition analysis of transition area in the cladding coating, we can discover that the distribution of elements in the coating. In addition, the crack and porosity defects of cladding are analyzed. The cracks can be attributed to thick pre-placed coating, uneven heat and caused stress in the reaction process. The crack in the TiN coating may is because that the plastic prosperity of TiN lower than TiC.The analysis of coating surface performance show: The lasers cladding micro-hardness has been remarkable increase over the matrix, the micro-hardness up to 1500HV, and presents a smooth transition gradient from the surface to substrate. The Strengthen mechanism mainly includes the fine-grain, hard phase dispersion, and supersaturated solid solution strengthening etc. In laser cladding TiC and TiN+ cobalt-based alloys have Ti, Cr solid solution, also have carbide, nitride phase etc.. The precipitation phase is relatively small and present homogeneous distribution. Hence, the resistance of TiC and TiN cladding coating is better than resistance of cobalt-based alloy coating.