| Shield hobs are usually in service under extreme harsh conditions such as heavy load and strong wear,resulting in a high rate of wear and scrap,leading to major economic losses and waste of resources.Because of the advantages of high energy density,small heat affected zone and no environmental pollution,laser cladding technology has gradually become the preferred method for surface strengthening and remanufacturing repair of damage-critical parts.It is consistent with the high-end remanufacturing concept proposed by "Made in China 2025" and meets the requirements of green manufacturing in the "14th Five-Year Plan".Powder and laser are important components in the cladding process,and the interaction between them will have an important influence on the melting effect of powder and the forming quality of cladding layer.In order to meet the quality requirements of important parts for precise forming,it is of great significance to conduct in-depth research on the influence of laser-powder interaction.This thesis focuses on the remanufacturing repair process of shield hobs of typical vulnerable parts,using a combination of simulation,theory and experiment.The interaction between powder,laser and matrix is studied layer by layer from the three aspects of nozzle powder convergence characteristics,laser-powder interaction effect and its influence on single-pass cladding forming.The main research work of this thesis is as follows:(1)Fluent was used to simulate the influence of process parameters on the powder convergence characteristics of coaxial annular nozzle.Results show that the powder flow field at the nozzle outlet can be divided into: circulation zone,convergence zone and divergence zone.The powder feeding rate mainly affects the convergence concentration,the laser beam shielding gas velocity mainly affects the focus position,and the powder particle size and carrier gas velocity both have an effect on the concentration and position.If the velocity of carrier gas is too large or the velocity of protective gas is too small,vortices will appear in the circulation zone.(2)Based on the simplified model,the expression of attenuation rate was deduced.High-speed camera system was used to collect and analyze the image of laser-powder interaction.Results show that the attenuation rate is directly proportional to the powder feeding rate and inversely proportional to the carrier gas flow rate under the coaxial impinged jet.The physical state change of powder particles can be divided into four stages according to brightness and volume,and the molten powder will produce "tailing" phenomenon at the front end of the flight direction due to the effect of vaporization and airflow.Its length increases first and then decreases with the increase of the distance between the nozzle.With the increase of laser power or powder feeding rate,the number of powder melting increases and the bright area increases,but the influence of carrier gas flow rate on it is opposite.(3)A single-pass cladding experiment was performed using Fe-Cr-B alloy powder,and the influence of process parameters on the macro morphology,cross-sectional morphology,powder utilization rate and microstructure properties of the cladding layer was studied.Results show that the surface of the cladding channel is prone to the phenomenon of sticky powder.The utilization rate of powder gradually increases with the increase of laser power or powder feeding rate.When the laser energy density increases,the microstructure at the fusion interface becomes coarse and the elements diffuse sufficiently.Due to the effect of fine grain strengthening and solid solution strengthening,the microhardness of the cladding layer can reach700?800HV.This thesis includes 47 pictures,5 tables,and 83 references. |
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