| The key components of large-scale engineering are usually used in high temperature,high pressure and high corrosion environment for a long time,and their surfaces are easily subjected to corrosion and wear,resulting in failure and damage.Using advanced surface manufacturing techniques to improve the overall performance and service life of key components in harsh service environments,or to repair and remanufacture important damaged components,has become one of the key technologies in high value-added mechanical component manufacturing engineering.Extreme high-deposition-rate laser direct energy deposition(EHLDED)is a method of synchronous powder feeding.The high energy density laser beam is used to melt the added material and form a metallurgical bonding layer on the surface of the substrate material moving at a high rate,which has the characteristics of high efficiency,high precision,low matrix dilution rate and low post-processing cost.Since 2017,it has provided a new way for the high efficiency and high quality manufacturing of the key components of large-scale engineering in China.In this dissertation,the development of EHLDED manufacturing system and key equipment units and the manufacturing process and properties of stainless steel functional deposition layer are carried out.The main research contents and conclusions are as follows:(1)Design of EHLDED manufacturing system and development of key equipment units:The EHLDED manufacturing system was constructed and coaxial powder feeding nozzles and high-flux powder feeding machines suitable for different laser power were developed.The structure optimization design of powder feeding module and powder separation module made the power of the powder feeding nozzle break through from 3400 W to 6000 W.The internal and external nozzle module was further improved,and the protective gas module was creatively added to form a new superimposed structure of"three-layer nozzle",so as to avoid the impact of splash and rebound of powder particles and molten pool droplets on the nozzle.A dual-barrel carrier gas multifunctional powder feeder with mixing module was developed,which could realize the high precision powder feeding with the size range of 20-149μm and the speed of the dust discharging plate 05-8 r/min.(2)Microstructure and properties of stainless steel coatings by EHLDED:Based on the development of a powder feeding nozzle suitable for low and medium laser power(≤3400 W),the effects of different scanning speed on the forming quality,microstructure evolution and corrosion and wear resistance of the deposited coatings were studied.The results showed that with the laser power of 2200 W,the average grain size of the deposited coating was gradually refined as the scanning speed increased from 30 m/min to 60 m/min,and the submicron and nanometer grains were found at 60m/min.The corrosion resistance of the deposited coating increased gradually with the grain refinement.The corrosion current density was 1.64μA/cm~2,the repassivation current density was 0.16μA/cm~2,and the polarization resistance reached 463.185kΩ·cm~2 at the scanning speed of 60 m/min.However,the microhardness and friction coefficient of the deposited coatings prepared at the scanning speed of 50 m/min and60 m/min scanning were similar,which indicated that the microhardness and wear resistance of the deposited coatings reached the threshold when the scanning speed changed with other process parameters are unchanged.(3)Proposing the hybrid manufacturing process of EHLDED combined with extreme high-deposition-rate laser remelting:In view of the metallurgical defects such as surface adhesion of unmelted powder particles and internal pores,the influence of EHLR post-treatment with different defocusing amounts on the surface forming quality and microstructure evolution of the original deposited coatings was studied.The results of laser confocal microscopy and EBSD showed that the roughness of the deposited coatings treated by EHLR decreased significantly,and the remelted layers were dominated by uniformly refined ultrafine grains.Besides,the effect of EHLR treatment with negative defocusing amounts was obviously better than that of positive defocusing.A new extreme high-deposition-rate laser hybrid manufacturing method of EHLDED and interlayer EHLR was further proposed.The improvements of porosity,microstructure and mechanical properties of the coatings before and after the EHLR treatment were compared and analyzed,and the mechanism of interlayer EHLR treatment on mechanical properties of the coatings was revealed.The results showed that the porosity of the deposited coating decreased by 67%,and the microstructure was characterized by typical heterogeneous structure dominated by ultra-fine equiaxial grains after interlayer EHLR treatment.In addition,the interlayer EHLR could induce the generation of new interlayer nanocarbide particles and high density dislocation structure.The yield strength of the EHLDED-EHLR coating reached 549 MPa and the elongation was 49%,which was 25%and 29%higher than that of the original coating,respectively,demonstrating an extraordinary combination of strength and ductility deviating from the traditional strength–ductility trade-off relationship of 316L SS.(4)Application of EHLDED remanufacturing of typical engineering key components:The core component of coal mine hydraulic support,the hydraulic column,was remanufactured using ultra-high speed laser deposition technology,and compared with the deposition layer prepared by traditional laser deposition technology,and the difference of density,organization,microhardness and corrosion resistance of the coatings prepared by the two processes was studied.The microstructure evolution and properties of the coatings was analyzed under the condition of significant difference in manufacturing efficiency.The results showed that the density of the EHLDED coating was 99.46%,which was significantly higher than that of the CLMD coating(99.02%).The average grain size of the EHLDED coating was 1.15μm,which was much smaller than that of the CLDED coating(14.65μm).The average polarization resistance of the EHLDED coating was as high as 709.79 kΩ·cm~2,about twice that of the CLDED coating(about 356.69 kΩ·cm~2).Through copper ion accelerated acetic acid spray corrosion test,the hydraulic column repaired by EHLDED reached GB/T 6461-2002 level 9 after 120 h corrosion test.In this paper,the manufacturing equipment,process and properties of EHLDED technology were systematically studied.The research results can break through the bottleneck of both high deposition efficiency and high forming quality faced by EHLDED,and realize high efficiency and high performance enhanced manufacturing and remanufacturing of damaged parts.It has good engineering application value to meet the long life and high reliability demands of coal mine machinery,large engineering,metallurgical roll,marine equipment and automobile manufacturing industries,and to provide equipment and technology base for promoting remanufacturing of critical and important parts components in our country. |
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