Research On Microstructure And Properties Of Laser Cladding High Chromium Iron/Ti Coatings

Holding the idea of“using the best steel on the cutting edge”,this thesis is intended to address the following issues.The high-performance alloy powder is melt-coated at the cutting edge by utilizing laser coaxial powder feeding technology,with the ordinary3Cr13 martensitic stainless steel as its matrix,thereby,improving the production status of domestic large-scale and low-quality knives,enhancing the international competitiveness of knives,and breaking the current situation of foreign high-end knives monopoly.High-chromium-based alloy powder with high hardness and high wear resistance is selected,which is also widely used on the surface of some wear-resistant workpieces such as petroleum,mining,cutting tools,etc..However,two main problems exist in the cladding process on account of the high content of Cr and C in the composition:?1?A strict demand of the cladding process is required for that the coatings are easy to crack.?2?The corrosion resistance of the coatings do not meet the requirements of high-end knives under the optimal process parameters.Thus,it is necessary to optimize the cladding process by designing orthogonal test,adjusting the laser power P,the powder feeding speed V_f,and the laser scanning speed V_g;The effect of the addition of Ti on the microstructure and properties of the coatings are investigated by adding different amounts of Ti powder to the powder of high chromium iron base.The results indicate that the melting width and melting height of the coatings are closely related to the laser cladding parameters?laser power,powder feeding s peed,scanning speed?.Under the optimal cladding process,the microstructure is divided into three regions due to different solidification conditions in the coatings:dendritic region,fine-grain eutectic region,and coarse-grained region.Thus,the hardness of the cladding layer is stepped.The crack propagation path of the crystal region is also different.The crystal is fractured near the fusion line and in the middle of the coating,since the emergence of the stress concentration caused by the needle-like morphology of the carbide in this area;whereas there is a transgranular fracture in the top region of the cladding layer,since a relatively large carbide is formed in that area.With the addition of different contents of Ti to the high chromium iron-based coatings,the Thermo-Calc software Fe-alloy TCFE7 database is used to accurately calculate the phase,precipitation order and precipitation mechanism of the high-chromium iron-based/Ti alloy in equilibrium.There is no M₂₃C₆ phase in the high chromium iron-based coatings.It is considered that the formation of TiC and M₇C₃ phases in the molten pool consumes a large amount of C atoms,resulting in insufficient C atoms in the liquid phase to form M₂₃C₆.At the same time,according to Bramfitt's two-dimensional lattice mismatch theory,M₇C₃ is moderately effective with TiC as the core in nucleation.Thus,the M ₇C₃carbide refines the M₇C₃ type carbide during the crystallization process by using TiC as the core nucleation.The calculation results are in good agreement with experimental data.When 3.0wt%Ti is added to the high chromium iron-based alloy,the corrosion resistance of the coatings is increased,and the corrosion crack is suppressed;simultaneously,the average hardness of the cladding layer is the highest,which is about twice as big as the matrix.