| Laser cladding technology is one of pivotal technologies for remanufacturing engineering dueto the superior technical characteristics such as high energy density, high processing accuracy,wide materials selection and excellent metallurgical bonding between coating and substrate.Remanufacturing engineering takes the prolonging service life of component with added value asinstruction, and takes the improvement surface properties of the waster components as the goal,which meet the demand of national strategic development for constructing cyclic economy. Thelaser cladding technology has wide application in remanufacturing field of turbine blade for powergeneration because the laser cladded coating has the advantage of favorable grain growthorientation and superior mechanical properties than wrought and as-cast alloys.In the paper, the remanufacturing engineering is taken as the research background, and thelaser cladded Ni-based superalloy coating is taken as the research object. The laser claddedNi-based superalloy was fabricated using high power diode laser cladding system. The researchcontent mainly includes the optimize process parameters for laser cladding and the microstructureof the coating, effect of cooling rate on the microstructure and segregation, the oriented growthbehavior and the crack morphology and crack sensitivity factors, effect of heat treatment on themicrostructure, segregation and mechanical properties. The studies provide scientific andtheoretical bases for the remanufacturing engineering of laser cladding on Ni-based superalloy.Some of main experimental results and conclusions are listed as follows.The microstructure of the epitaxial deposited coating was studied. The cladded coating withgood shape, low dilution rate, without defect and good metallurgical bonding with the substratewas fabricated with optimized process parameters. The direction of maximum temperaturegradient in the cross section of the coating varied from perpendicular to substrate surface in thedown region of the coating to paralleling to the direction of laser beam. The planar crystal, cellularcrystal, columnar dendrite and equiaxied crystal were formed from down region and up region ofthe coating. The precipitation phases in the cladded coating were dendritic (Nb, Mo)-rich Lavesphase and a small amount (Nb, Ti)-rich carbide and nitride. The effect of cooling rate on the microstructure and segregation in the cladded coating wasinvestigated. The constitutional supercooling Csand the temperature gradient Gmwere improvedby increasing cooling speed, and then the Laves was refined, and the concentration of dendriticLaves a was decreased, indicating that Nb segregation in the coating was restrained, and more Nbatom played the solution strengthening on the matrix. The Laves concentration in the liquid nitridecooled coating was reduced to3.5vol.%, and Nb concentration in Laves was reduced to8.5~14mass.%, which were both lower than the air-cooled coating. The Nb concentration in the austenitewas increased to3.5~7.5mass.%that was higher than the air cooled coating. The Nb segregationwas alleviated by the rapid cooling rate provided by liquid nitride.The granule complex phase Nb(Al, Ti) with0.2~0.9μm was precipitated at about999℃during the laser remelted coating. Al and Ti atoms were redistributed between Laves and thegranule complex phase via the uphill diffusion, and the intermetallic was precipitated from thesupersaturated Laves phase when the temperature was decreased to the spontaneous decompositiontemperature. Carbide (Nb0.12Ti0.88)C1.5and nitride (Nb0.88Ti0.12)N1.5were formed due to alloyingelements interdiffuse and redistribute between MC/MN and supersaturated Laves under the largerthe degree of supercooling and temperature gradient. The carbide and nitride were segregated withNb and Ti and were precipitated along the Laves. The average hardness and the average elasticitymodulus of (Nb0.12Ti0.88)C1.5and (Nb0.88Ti0.12)N1.5were much higher than the austenite, and thestrengthening effects of second-phase particles carbide and nitride are134.34MPa.The grain growth behavior of the epitaxial deposited coating was studied. EBSD analysisresults showed that strong texture with various growth directions was formed in the down regionof the cross section of the coating. Most grains was formed as texture with grain misorientationangle about2°and growth direction towards to <100> in the horizontal cross-section, while thegrain was coarsened due to the thermal cycle. In the middle region of the deposition coating, thedirectional dendrite was formed as the strong texture along <001> with misorientation angle about2°under the controlling of direction of maximum temperature gradient. In the top region of thecoating, the texture in the cross section was formed with large grain size, and strong texture in thehorizontal cross section with small misorientation angle about2°.The tips of small solidification crack with stress concentration was activated by the transversetensile stress, and the crack was propagated with the coarsening Laves and carbide along thedirection of deposition and laser beam. The liquation crack was form before the liquid alloy fillingthe dendrite clearance, and was healed at the topmost the deposition coating and the ending of thelaser beam. The propagation path of the crack was provided by the coarsening Laves and carbide eutectic with the effect of transverse residual stress that was increased with increasing the numberof deposition layers.The effect of heat treatment on the microstructure and composition segregation in the coatingwas studied, and the mechanical properties of the coating were test. Only a small portion of Nb inIN718alloy coating was precipitated as the constituent of the strengthening phase. The rest of Nbplayed the role of solution strengthening and segregated in the form of Laves in the laser-claddingprocess due to the slow cooling rate. Laves phase in the coating was dissolved during the elevatedtemperature solid solution, and the Nb was redissoluted in the austenite and precipitateddispersedly as strengthening phase γ″-Ni3Nb with dimension of15~25nm during the double agingtreatment. The mechanical properties of the coating were improved by γ″-Ni3Nb due to the largemismatch between γ″-Ni3Nb and austenite γ. Laves concentration in the coating and Nbconcentration in Laves increase along the deposited direction. Laves concentration and Nbconcentration in Laves were increased along the deposition direction. Part of Laves was dissolvedby the thermal cycle during the successive laser deposition, concentration gradient between Lavesand austenite, lattice imperfection of the coating and residual stress.The tensile strength of the standard heat treated coating at ambient temperature was1334MPa that was higher than the as-deposited coating918MPa as well as the wrought alloy and castalloy. The coating was destroyed as transgranular fracture. The tensile strength of the standard heattreated coating at650℃was higher than that of980STA coating and DA coating. Theanisotropic tensile property of the coating was generated by the texture formed during the epitaxialdeposition. The tensile strength of the standard heat treated coating along the deposition directionat650℃was938MPa that was about the same to the wrought alloy and was higher than903MPa along the direction of laser scanning and780MPa along the direction of overlapping. Thefracture mode of the coating along the direction of laser scanning and the direction of overlappingwere mixed-mode of ductile fracture and brittle fracture, and was changed to ductile fracture alongthe deposition direction. |
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