Solidification Microstructure And Stress Control Of Ni-based Alloy Coatings By Mechanical Vibration Assisted Laser Cladding

Laser cladding can make metal materials rapid melting-coagulation, but the influence of coating cracking in the process of preparing high crack sensitivity nickel-based alloy coatings that has hindered the development of the technology in surface engineering field. To overcome this difficulty, a new technology of mechanical vibration assisted laser cladding is proposed in this dissertation, which extends the microstructure forming mechanism of nickel-based alloy coating and the stress control theory. The research contents will cover the organization solidification characteristics, macro-appearance, cracking mechanism and behavior, stress control, mechanical properties. The method of finite element numerical simulation combining with the theoretical analysis and actual test results was used. The stress distribution of the coating surface, interface, substrate heat-affected zone and influence of mechanical vibration were discussed. The research has theoretical innovation and practical significance to the stress control, improve and eliminate of Ni-based alloy coating crack.Build the ANSYS finite element model in the basis of the solidification alloy coating final size without and with mechanical vibration Ni60coating. The stress field of coating along the laser scanning direction (Z-axis) with the material thermal physical parameters those changes over time was simulated. The distributing characteristic of stress and strain field with the mechanical vibration was discussed. The optimum mechanical vibration process parameters are predicted. Results show that the strain peak value dropped from4.36×10-4to3.17×10-4in coating surface,2.88×10-4to2.1×10-4in interface and from4.24×10-5to3.09×10-5in the heat affected zone. The stress peak value of Z axis direction (parallel to the laser scanning direction) dropped from114MPa to81.3MPa (residual tensile stress) in the surface of coating, from197MPa to165MPa (residual compressive stress) in the interface and from74.8MPa to50.5MPa (residual tensile stress) in heat affected zone. The improvement of the stress distribution showed that the mechanical vibration in cladding process can play a role of stress control and improve the stress and strain distribution in and around the Ni60alloy laser cladding coating effectively. The Z axis residual stress distribution in coating surface, interface and substrate heat affected zone was measured by X-ray diffraction method. Results show that residual tensile stress value in coating surface and residual compressive stress in the interface and substrate heat affected zone showed a decreasing trend. By comparison finite element numerical simulation and actual measurements show that the simulation and measured results have almost the same property of residual stresses in different regions. The stress change rules of simulation numerical conform better to measured values. The feasibility of numerical simulation was proved. Residual stress is251.1+60MPa (tensile stress) in the centre of coating surface,121.6MPa (compressive stress) in the interface,13.5MPa (compressive stress) in the substrate heat affect zone. It will contribute to prevent the new crack formation and improve the stability of coating in the use process. The reasonable difference of analysis results between numerical simulation and the ideal simulation conditions was more accordant with the actual situation. And this has instructional significance.The solidification characteristics of nickel-based alloy coating with mechanical vibration were analyzed by OM, SEM, EDS, XRD technology synthetically. The results reveal that mechanical vibration can improve the distributed uniformity of solidification structure and refine the grain size, the dendrite-to-equiaxed grains transition in matrix phase; and the quantity and size of carbides has increased significantly; the continuity of matrix phase and the integrity of crystal structure are improved under the separation of intercrystalline network phase in Ni60/TiC composite coating. The gross dendrites are converted into dendrite and equiaxed. hard phase uniform distribution, the pores is reduced in Ni60CuMoW alloy coating.The solidification process of laser cladding is affected by vibration energy from the mechanical vibration. The solidification mechanism of the coating liquid was affected. The ionized core is increased in the front of the solid-liquid interface; the advancing velocity slowed down and the cellular and equiaxed grains solidification mode. The flow of the molten pool impels the formation of "stratosphere". The primary dendrite was swept away that increases free core number in the front of the solid-liquid interface; the composition undercooling was alleviated on the front edge of crystallization; the transition of cellular to dendrites was inhibited; the floatation speed of nucleation particles with the solid-liquid interface was slow down; the concentration fluctuation of element in molten pool was alleviatied. The effects on the article size of hard phase and distribution was improved; the continuity of matrix phase was enhanced; the crack sensitivity of the Ni-based alloy coating was reduced.The "stratosphere" alleviates convective overturn in molten pool, the surface corrugation are replaced by smooth distribution on coating. The macroscopic crack distribution of Ni60CuMoW composite coating with mechanical vibration (limited amplitude) is investigated. The observations show that crack cross is reduced on surface coating, and the crack number of the unit length decreased from0.17crack/mm to0.08crack/mm. There is no crack appears in the alloy coating when the vibration of frequency150Hz and amplitude0.17mm. The cracking behavior of nickel-based alloy coating from interface cracks initiation and expands to the surface of coating by the transgranular and intergranular cracking ways, and crack cross exists in coating. However, the crack cross is instead of a single through cracks in coating with mechanical vibration, the disruptive impact of cracks was weakened.The corrosion potential of mechanical vibration Ni60CuMoW alloy coatings compared with no mechanical vibration coating moves about1134.9mVSCE, and corrosion current density dropped from0.076to0.008μm·cm-2, almost an order of magnitude. The corrosion resistant performance was improved significantly. The average microhardness value rose from720to835HV0.5on cladding zone, increased by nearly16%, and the microhardness value curve was slowed down. The microhardness value drops rapidly from468on heat affected zone to182HV0.5on substrate with no fluctuation. The average wear mass loss of alloy coating dropped from9.2to7.6mg in wear test, declined by17%; the average friction coefficient dropped from0.081to0.068, decreased by16%; the fluctuation of curve was weakened. The wear surface show a uniform distribution of shallow furrow shaped without deep ditch, but a small amount of adhesive wear; the bonding strength between matrix phase and particle was reinforced; the wearing resistance and stability of Ni60CuMoW alloy coating has been optimized with mechanical vibration.