Study On The Microstructure And Properties Of Laser Cladding Gradient WC-Ni Composite Coating

The Ni based and Ni based WC single and multi-layer composite coating were fabricated by CO2 laser cladding on the surface of Q345 R steel. The content of WC is 10%, 30% and 50% in composite coating. The weight fractions of the adding WC rang is 10%~30%~50% of each layer in the multi-layer cladding is changing in gradient. SEM, EDAX, X-ray diffraction and optical microscopy were used to observe and analysis the microstructure, composition distribution, phase composition, surface structure and wear surface morphology of the clad layer. Microhardness test of cladding layer was conducted by HVS-1000 A microhardness. The test of the wear coefficient and wear resistance of the laser cladding was conducted by using MM-U10 G high temperature friction and wear tester.The test results of the single laser cladding layer show that layer has good surface quality.With the amount of WC addition increased, the rate of dilution of cladding layer becomes larger;γ-(Ni, Fe)、Ni2B and Cr23C6 are the main phases in the Ni60 cladding layer, The mainly phases of10% WC cladding layer are γ-(Ni, Fe), Ni2 B, Cr3C2, W2 C, FeNi3 and C, 30%WC of the cladding layer is mainly composed of γ-(Ni, Fe), Cr3C2, W2 C, FeNi3, W2 B and C, 50%WC of the cladding layer is mainly composed of Cr3C2, W2 C, Fe Ni3, WC, Fe4 C and C. Varied carbide morphology occurs and the uniformity of the coating area becomes larger due to the addition of WC particles.The microhardness of the clad layer is nearly 2 times of the substrate. with the increase of WC content in the cladding layer, the hardness of cladding layer increased.The surface morphology of the multipass composite coating is good. Uniformity of the laser cladding layer lap is more obvious. The microstructure of the overlapping zone is smaller. WC particles have various forms of burning loss, which solution diffusion type is the main burning form of WC. Hardness of overlapping zone compared with is slightly lower than that of non overlapping zone. Wear resistance of the cladding layer is significantly higher than that of the substrate, reduce the abrasion loss, and friction coefficient becomes larger. The wear mechanism of substrate is mainly adhesive wear, while the abrasive wear is the main wear mechanism for the Ni-WC coating.The results of the multi-layer laser cladding experiments indicate that the surface of composite cladding layer is flat and a good metallurgical bonding with the substrate. The microstructure of different layers is similar to that of the single cladding layer. No obvious demarcatioat is found at the junction of the cladding layer. According to the composition analysis of 4 layer gradient composite coating, there is a significant dilution effect in the first cladding layer and the substrate, and WC particles in the first cladding layer has decomposed, but the decomposition rate is not great. Moveover, structure composition in the junction of the first and the second coating is not much difference, and has a good combination. However, strong inhomogeneous composition is found at the third and fourth coating due to WC accumulation.Microhardness of 3 and 4 layer from the surface of the gradient coating layer to the substrate showed continuous decrease and no obvious step.