Laser Cladding Gradient Coating And Its Microstructure And Properties

With the continuous development of technology and industry,higher performance requirements such as better operational stability,higher processing efficiency and longer service life for die-cutting equipment have been proposed.Deformation,cracking,wear and other problems in the working area contact surface are the primary cause of part failure.Laser cladding technology has many advantages,such as greatly reducing the cost of raw materials,processing costs and production cycle of parts,and strengthening and repairing cutting tools.Therefore,it has become the preferred choice for die cutting tool processing and manufacturing.By adjusting the microstructure of single cladding layer,a cladding layer with good structure and composition distribution,excellent hardness,wear resistance and stress distribution can be obtained at one time,which has important theoretical and practical significance for the one-time forming of small-size die cutting edge.At the same time,it provides theoretical basis and guiding significance for the multi-pass lap laser cladding and even for the regulation of the forming quality and structure and properties of metal 3D printing.In this paper,the experimental results are explored from three aspects: optimization of process parameters,effect of hard phase size and ternary composite of WC-BN-Ni60.The main conclusions are as follows:Through orthogonal test,three key parameters of laser power,scanning speed and defocusing amount are selected to optimize,and a set of optimal process parameters are obtained.Then,the single-factor verification test was carried out on scanning speed which is the most significant factor,and the best process parameters were finally obtained as follows: laser power 1000 W,scanning speed 1mm/s,defocusing amount 12 mm.Under the optimal process parameters,the overall morphology of the cladding layer is good,the defect rate is the lowest,the hard phase content is gradually reduced from the top of the cladding layer to the matrix,and the microstructure and properties are improved.Based on this parameter,follow-up experiments were carried out.The cladding layers with different microstructures and properties were obtained by choosing different WC particle sizes,changing the density and chemical activity of hard phase.Experiments show that when the WC particle size is reduced to nanometer level,the settlement phenomenon which is not conducive to the mechanical performance is restrained.After decomposition of WC particles with high chemical activity,the needle-like structure was recrystallized in the actual working area above the cladding layer,and the overall performance was greatly improved.The top microhardness reaches 70 HRC.The hardness transition of the interface between the coating and the substrate is excessively smooth,the friction coefficient is reduced to about 0.2,the wear volume is reduced by 75% compared with the micron level,and the peak stress of the interface is reduced by 150 MPa.In order to further enhance the mechanical properties of the gradient coating and improve the morphology,a certain proportion of WC hard phase was replaced by h-BN(x=10%~30%)on the basis of the previous experiments.Experiments have shown that h-BN increases the nucleation probability of WC,transforms the needle-like structure into fine granular structure,which improves the friction performance,and eliminates the stress concentration at the needle tip.At the same time,the B element is beneficial to the deoxidation and exhaust of the molten pool,completely eliminating the pores.The optimum ratio is(20% BN+20%WC+60%Ni60A),which forms a good gradient structure and obtains excellent performance.The friction coefficient is reduced to about 0.1,The residual tensile stress at the interface between the cladding layer and the substrate is reduced to 350 MPa,and the residual stress distribution trend of the section is the most gradual.