| Featured with excellent corrosion resistance,oxidation resistance,creep resistance and fatigue resistance,Inconel 718 superalloy is applied as an indispensable structural material in aerospace,energy power,petrochemical and nuclear power industries.Due to the poor service environment of Inconel 718 alloy,it is inevitable to cause damage due to various reasons,resulting in huge energy and economic losses.Laser additive remanufacturing technology has the characteristics of controllable energy input,small deformation and high process reliability.It has a good application prospect in the remanufacturing of high value parts.Therefore,based on the research background of high performance and rapid response repair demand of Inconel 718 high temperature parts after service damage,this paper focuses on the repair characteriaztion,heat treatment mechanism,control of heat affected zone and the subsequent hot corrosion mechanism of laser repaired Inconel 718 alloy,to provide a theoretical basis for high performance repair of Inconel 718 alloy parts.The research work mainly focuses on the following aspects:First,the model prediction and parameter optimization of the geometry characteristics of the laser additive remanufactured repair layer,and the interaction effect of the process parameters on the geometry characteristics;Second,the microstructure characteristics and heat treatment mechanism of laser additive remanufactured Inconel 718 alloy,with emphasis on the Laves phase dissolution mechanism and theδphase transformation mechanism;The third is the analysis of the interface characteristics and mechanical properties of laser repaired Inconel 718 alloy,which reveals the influence of energy inputs and heat treatment regime on the microstructure and properties of the interface;The fourth is the study of hot corrosion mechanism and associated mechanical evolution behavior of laser remanufactured Inconel 718 superalloy in simulated salt molten salt.The main conclusions are as follows:（1）The regression models between the remanufacturing processing parameters and the geometric characteristics of the repaired layer were createtd by response surface methodology（RSM）,which can be used for the rapid prediction of the geometric dimensions of repaired Inconel 718 layer.The maximum error of the mathematical model is less than 8.46%.Futhermore,based on the goal of the minimum dilution ratio and the W/H greater than 5,the optimized processing parameters were obtained from the RSM.（2）The microstructure changes and precipitation/dissolution mechanism of the precipitates during the heat treatment of laser repaired Inconel 718 alloy were obtained by different heat treatment methods.After solution treatment,recrystallization occurs and the grains are refined but uneven.At the same time,the Laves phase between dendrites gradually eliminates.The higher the solution temperature is,the faster the dissolution rate is,and the more conductive to the uniform precipitation of strengthening phase.In the process ofδaging treatment of 800℃,the morphology and law ofδphase precipitation in the substrate and the repair area are different.Theδphase in the repaired layer nucleates and grows on the basis of the stacking fault of theγ"phase on the clase-packed plane through the shearing mechanism,presenting fine needle like precipitation.However,theδphase in the substrate preferentially nucleates and grows at the grain boundaries and eventually grows in parallel within the grain.Although the aging treatment can effectively improve the microhardness and tensile strength of the repaired zone and the substrate,the microhardness and mechanical properties decrease as rising of the aging time.Futhermore,with different aging treatments,all the tensile fractures of the repair parts are located in the repair zone.The fracture surface is flat,showing typical brittle fracture characteristic.（3）According to the actual needs of the repair parts,980STA（980℃/1 h+720℃/8 h+620℃/8 h）and DA（980℃/1 h+720℃/8 h+620℃/8 h）can be selected as the post heat treatments.After the DA treatment,the microstructure is similar with that of the as-deposited sample.A small amount of Laves phase dissolves and a large number ofγ"andγ′strengthening phases precipitate around the Laves phase.However,after 980STA treatment,the microstructure is refined obviously and a small amount ofδphase also precipitates at the grain boundary besidesγ"andγ′strengthening phases.Under the applicationn of vertical scanning stategy,the room temperature tensile strength of the as-deposited sample is 762.03 MPa.After DA and 980STA heat treatment,the room temperature tensile strength of the repaired samples reach 1076.85MPa and 1174.10 MPa respectively,which increases 41.3%and 54.1%respectively.（4）The study on the microstructure and properties of the transition zone between the repair zone and the substate shows that the element distribution between the repair zone and the substate is uniform,and there is no macrosegregation,but the microhardness is obviously excessive.In the HAZ,the coarsening of grains and the dissolution of some precipitates occur at the same time,and the larger the heat input is,the larger of the HAZ is.Compared with the as-deposited and DA samples,recrystallization occurs at the repaired interface after 980STA heat treatment,and the grains in the HAZ grow up.DA and 980STA heat treatment can effectively improve the microhardness distribution at the interface of the repaired area,so that the microhardness of the HAZ is equivalent to that of the substate.The average interfacial shear strength of the as-deposited,DA and 980STA samples are 608.87 MPa,893.27MPa and 948.82 MPa,respectively.（5）A cyclic hot corrosion test of Inconel 718 alloy is peformed at 650℃.The element mapping results of the corroded surfaces reveal that the corrosion products of as-deposited Inconel 718 include two distinct layers,of which the outermost layer is mainly composed of Ni2Cr O4,Ni V2O6,Cr2O3 and Fe3O4,while the inner layer contains Ni3S2.The laser deposited Inconel 718 alloys with different heat treatments behave differently in the hot corrosion.The hot corrosion performance of 1100STA sample is better than that of DA samples,while the latter has better hot corrosion resistance than the as-deposited samples.According to the hot corrosion test results and theoretical analyses,the hot corrosion behavior of as-deposited Inconel 718 alloy with different heat treatments in the salt mixture solution is closely related to basic fluxing mechanism,sulphidation as well as oxidation. |
