Cracking Mechanism Of Laser Cladding Ni-based Alloy Coatings

Currently the major obstacle of the industrialization of Ni-based laser cladding layer is the high cracking susceptibility. To solve this problem, the thesis combined structure analysis with stress analysis, and systematically analyzed the crack behavior, crack properties and the cracking mechanism of Ni-based laser cladding layers; By micro structure, elemental composition and phase analysis, as well as fracture observation, the structure factor of cracking mechanism was investigated; Stress analysis was accomplished by newly-designed crack susceptibility testing method for laser cladding technology, to standardize cracking susceptibility of laser cladding material by quantitative indicators; Finally, the thesis investigated the influence of rare earth elements added into Ni-based alloy, and prepared crack-less laser cladding layers by adding proper dosage.The macro cracks of Ni-based laser cladding layer consist of transverse perforative crack, longitudinal perforative crack, herringbone crack, herringbone bifurcation crack, herringbone branches- like crack, inclined perforative crack, inclined non-perforative crack, reticular crack, and arc-shaped crack, etc. With the increase of scanning speed, the crack rate increased, and cracks gradually shifted from perforative crack to the herringbone and reticular pattern; With the increase of laser power, crack rate was slightly reduced, without obvious change on micro morphology. Most of the cracks of Ni-based laser cladding layer originates from or near the surface, and expand downwardly throughout the cladding layer, ultimately end within the matrix; Minority of cracks sprouted in recent fusion line; Very small amount of micro-cracks along crystal or crystalline form of wear, providing a source of macro crack; Herringbone crack initiates from the edge of the cladding layer, expanding through the cladding layer and ends at another crack or inside the cladding layer; Ni-based cladding layer shows cleavage cracking characteristics on Cr3C2 particles and coarse dendrite, while quasi-cleavage characteristics is found in intergranular structure; Cracks of Ni-based cladding layers are low-plasticity brittle cracks resulting from the mutual effect of high stress and low plasticity of the micro structure.The structure factor of Ni-based cladding cracks is analyzed. The results indicate that Ni-based cladding layer contains carbon, boron, iron, silicon, tungsten, chromium, nickel and other elements, which in the cladding layer formed ductile matrix phase such as γ-(Fe, Ni), Ni3 Fe, and other hard phase like Cr B, Cr7C3, Cr23C6 and Fe3 B, ensuring high hardness and good wear and corrosion resistance of the cladding layer. However, too much hard phases largely enhances the brittleness of the cladding layer, which will increase the probability of cracks under high stress.According to the idea of welding crack test RRC, together with the characteristics of laser cladding process, a brand- new crack test Variable Thickness Crack Test was designed. By evaluating the critical thickness Hcr, crack sensitivity is assessed by quantitative indicators; Test plate deflection is also measured as a stress evaluation basis; Theoretical analysis shows that under the same process parameters, the longitudinal contraction of the cladding layer is a fixed value, the vertical deflection of the test plate was calculated. the thickness and inversely proportional to the square of the amount of deformation; Experimental results show that critical thickness Hcr of Ni-based alloy was approximately 3mm; As the test plate thickness increases, the number of cracks increases; As the test plate thickness increases, the deflection of the test plate decreases, and the variation of the deflection was in accordance with the theoretical analysis; it is concluded that stress within Ni-based laser cladding is large, and is mainly longitudinal stress, when plate thick is 3mm, the inner residual stress reaches a critical stress, leading to cracking of the cladding layer.REE was added into Ni-based alloy to prepare crack less cladding layer. The results show that with the increase of the rare earth element content, the cracking rate of Ni-based laser cladding firstly increased and then decreased, with 0.6% La2O3 as the best dosage to reduce cracking rate; The micro structure of Ni-based layer with 0.6% La2O3 addition was denser, with finer grain size; No new phase is found with the addition of REE. Chromium carbide particles still exist, but the quantity is much smaller; Effect of rare earth on the grain refinement increased toughness of the coating; while purification effect of rare earth on the micro structure reduces the hard phase quantity, thus reduces the brittleness and crack source, thereby reducing the crack sensitivity; REE improves macro-hardness of Ni-based cladding layer; Rare earth elements has little effect on the friction coefficient of Ni-based cladding layer, but it can moderately reduce wear loss, thus improve anti-wear ability; Adding 0.6% La2O3 into Ni-based cladding also increases the the corrosion resistance of N-based layer by 44.9%.