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Laser cladding microstructure and properties of die steel
(1) Cladding depth. As the laser power increases, the depth of the single pass cladding layer increases rapidly, but after the power reaches 1.3 kW, the depth increases less and basically reaches the limit depth. The data regression process yields a curve fitting equation of D=-0.0929P2+0.9091P+0.776, PÃŽ(700,1300), D is the cladding depth, mm; P is the laser power, W. When the lap joint rate is 10% and multi-pass cladding is performed with different laser parameters, the cladding depth is 1.65~2.62mm, the depth is the most uneven without laser preheating, and the WC is added to the cladding material. The unevenness of the coating is more serious, that is, the unevenness of the depth of the cladding layer is aggravated.
(2) Hardness of the cladding layer. Regardless of the alloy powder and laser process, the hardness of the surface layer after cladding treatment is high, the hardness of the subsurface layer is the highest, which can reach 945HV0.2; after adding 25% to the cladding alloy powder, the hardness is not significantly improved. After laser cladding, the microstructure of the cladding layer is not uniform, the surface layer is in the form of cast structure, and the subsurface layer and the bottom of the molten pool near the substrate are quenched structures, and the matrix still maintains the original tempered structure. Therefore, the hardness peak appears on the subsurface, not on the surface. The cladding layer is mainly hardened by solid solution strengthening, fine grain strengthening, and dispersion strengthening of the second phase.
(3) Wear resistance. Under the same experimental conditions, the wear of the matrix sample is the largest, reaching 39.4g, and the wear resistance of the laser cladding surface is greatly improved. The absolute wear is at least 9.3g, and the relative wear resistance is the highest. The previous 4.24 times indicates that laser cladding can significantly improve the wear resistance of the surface. The wear resistance of the surface before and after the addition of the powder in the cladding alloy did not change significantly. There are many facets on the wear surface of the clad specimen, and there are slender scratches in the same direction as the sliding direction, indicating that the laser cladding surface is not only subjected to adhesive wear during the friction test, but also subjected to abrasive wear. The measured amount of wear is the result of a combination of these two types of wear.
(4) Organizational structure. Regardless of whether or not the alloy powder is added, the microstructure of the cladding layer is very similar, and it is divided into two types: a mixed structure of a granular, short rod-like distribution on a nickel-chromium-silicon solid solution and a low-melting-point nickel-based eutectic matrix near the bottom of the molten pool. It is a typical planar epitaxial growth structure; the other is the dendritic structure in the middle of the molten pool and the surface growing along the direction of the heat flow, and the entire cladding layer is a mixed structure of planar crystals and dendrites. Under the scanning electron microscope, the eutectic structure of the cladding layer is more obvious, and the fine dendrites are arranged neatly. The addition of tungsten carbide did not change the structure and no desired tungsten carbide superhard spots were observed. During the cladding cooling process, a part of tungsten forms a composite phase with chromium, boron, etc., and a small portion is solid-dissolved in the eutectic matrix. Spectral analysis between the branches and dendrites shows that the branch zone is a nickel-based solid solution and contains a certain amount of chromium, while the tungsten content is low, but the tungsten content between the dendrites is high, indicating that the tungsten carbide is at high temperature. After being melted and cooled, the tungsten carbide disappears and is distributed between the dendrites in the form of other second phases such as W3.2Cr1.8B3.
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