| Remanufacturing is an advanced production technology that repairs used products to maintain the performance of old products or exceed the quality of new products based on performance failure,life assessment and other analysis.Among them,laser cladding as a green manufacturing technology has been widely used in the remanufacturing industry.However,the application of laser cladding in the field of hot work mold repair is partly restricted because the external field of laser cladding process is a single heat source and the different thermal properties between the clad material and the substrate material can lead to problems such as high susceptibility to internal cracking during the solidification of the melt pool.Therefore,to address the problems of H13 hot work mold steel surface repair/reinforcement and laser clad layer defects,research on the clad layer forming quality,microstructure and mechanical properties based on ultrasonic-assisted laser clad remanufacturing process is carried out to provide theoretical basis and technical support for ultrasonic-assisted laser repair of large non-planar dies.The main research contents are as follows:First,three-factor,three-level single pass orthogonal experiments were designed to investigate the effects of laser power,powder feeding speed and scanning speed on the macroscopic morphology of the melted layer;the effects of different process parameters on the microstructure and mechanical properties of the coating were investigated by multi pass lap experiments;four-factor,three-level ultrasonic-assisted laser melting orthogonal experiments were designed to investigate the effects of ultrasonic power on the macroscopic morphology and microstructure of the coating.Secondly,the effect of ultrasonic power on elemental segregation is studied by the content of precipitated phase,distribution mode and precipitation morphology;the effect of ultrasonic power on elemental distribution is studied by tracer method;the effect of ultrasonic power on mechanical properties of coating is studied by porosity,microhardness and high temperature wear resistance;the mechanism of ultrasonic vibration on coating is analyzed based on the basic theory of ultrasonic action on melt pool.Finally,a multi-objective optimization model was established to optimize the ultrasonic-assisted laser cladding process parameters.The unevenness of the coating,microhardness and wear volume are taken as the optimization objects,and the comprehensive objective function of the optimization object is obtained based on the linear weighting method,and the principal component analysis is performed by SPSS to determine the weight coefficients of the sub-objectives;the standardization of the experimental data is realized by gray correlation analysis and the comprehensive gray correlation degree of the objective function is obtained;the optimization is performed by BP-PSO neural network in Matlab software.The genetic algorithm was used to find the optimal results and to perform experimental validation.The verification results showed that the unevenness of the coating was 0.0241,which was 16.61%lower than the unevenness of the coating before optimization;the wear volume was 1.3834×10~8μm~3,which was 5.01%lower than before optimization;the average microhardness was 371.28 HV,which was 20.74%higher than the average microhardness of 307.52 HV of he coating before optimization.The experimental results show that the BP-PSO neural network genetic optimization algorithm has unique advantages in multi-objective optimization of melting process parameters,and the algorithm achieves integrated regulation of morphology and properties of ultrasonic-assisted laser melting of superalloy,which lays the foundation for improving the comprehensive performance of laser cladding remanufactured parts.This thesis has 69 pictures,25 tables and 104 references. |
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