Study Of Crack Control And Heat Treatment Of Laser Cladded IN939 Nickel-Based Superalloy

Gas turbine is the heart of aircraft,power station and large ship,also known as the"pearl on the crown"of equipment manufacturing industry.Its turbine blades are usually made of nickel-based superalloy materials,which can withstand extremely high temperature and various complex loads.However,when the blades have served for a long time in extreme environment,they will suffer various damage attacks such as corrosion,cracking,large deformation,etc.,which results in the blades scrapped.Total replacement of these blades is costly,because the nickel-based superalloy blade material is expensive and the well designing of the blades also cause high cost.Therefore,repairing of the blades with remanufacturing technology has become more attractive with great reduction of the life cycle cost of gas turbines and the waste of relevant resources.Laser cladding technology is one of the typical repair technologies for damaged parts,which displays the characteristics of high energy density,good combination,high machining accuracy,wide range of material selection,etc.However,in the process of laser cladding,rapid heating and cooling can induce large stress in the cladding layer,thus resulting in the initiation and propagation of cracks.Additionally,most of the nickel-based superalloys are precipitation strengthening alloys,while laser cladding process cannot completely precipitate the strengthening phase and achieve the best strengthening effect.As a result,the present research is focused on the crack control and heat treatment of laser cladded IN939 nickel-based superalloy,which can provide theoretical basis and experimental support for the application of laser cladded nickel-based superalloy,in particular,for the crack control and heat treatment processes in remanufacturing of nickel-based superalloy parts.In this research,the effects of laser power,scanning velocity and powder feeding rate on the morphology of laser cladded IN939 molten pool are investigated using orthogonal test method,and then the optimal process parameters are selected for overlap testing to analyze the advantages and disadvantages of laser cladding with different overlap rates.The microstructure and phase compositions of laser cladding IN939 superalloy are examined,and the mechanisms of microstructure transformation are analyzed.The formation mechanism and influencing factors of the cracks in multi-layer cladding are studied.It has been found that the formation and propagation of cracks can be suppressed by heat input controlling and cross cladding.The laser cladded IN939 superalloy can be strengthened by two-stage heat treatment?1160?/4h+850?/16h?.The precipitation mechanism of??phase and the transformation rule of carbides are studied.The effects of??phase and carbides precipitation on the microhardness and corrosion resistance of IN939 superalloy are analyzed.The main conclusions of this research are summarized as follows:?1?The laser cladded IN939 superalloy from the optimal process has good morphology and without obvious metallurgical defects.From the bottom to the top of the cladded layer,the microstructure changes from fine dendrites to columnar dendrites until equiaxed dendrites.The transformation from columnar dendrite to equiaxed dendrite occurs at the top of the cladded layer.In addition,the phases in the laser cladded IN939 superalloy include?phase matrix,MC carbides and a small amount of??phase.The liquidus and solidus temperatures of the alloy are 1259?and 1335?,respectively.The dissolution temperature of MC carbide is about 1314?.?2?The cracks in the multi-pass and multi-layer cladded IN939 superalloy have typical feature of intergranular cracking.It is observed under scanning electron microscope that the fracture front of the cracks is relatively smooth,which indicates that the fracture front is formed by liquid phase at the grain boundary.It is inferred that these cracks are liquation cracks.The cracks in the IN939 alloy are mainly caused by the heat effect between passes.After five passes and three-layer cladding,the cracks do not appear in the first layer,but occur in the middle of the second layer and third layer.?3?The evolution of liquation crack in the IN939 superalloy can be described in several steps.When the laser energy is re-acting on the cladding,eutectic reaction occurs between second phase particles???precipitations,MC carbides?and?phase matrix in the heat affected zone,resulting in low melting point eutectic products.After the liquefaction of the low melting point eutectic products,unbalanced solute liquid membranes are formed at grain boundaries,and the liquid grain boundaries with poor tensile resistance are pulled apart by stress,causing liquation cracks.By reducing the scanning velocity and heat input,the crack sensitivity of the laser cladded IN939 alloy can be reduced to a lower level.Interlayer cross cladding can offset part of the internal stress,reduce the grain size,and thus suppress crack propagation.With the same process parameters,the number of cracks in the interlayer cross cladded IN939 alloy is 50%of that in the interlayer parallel cladding one,and the average crack length is reduced by105.7?m.?4?Solution treatment can dissolve most of the primary carbides,and the reprecipitated carbides after cooling are evident,similar to the primary carbides and evenly distributed.Aging treatment can promote the transformation of MC carbides to??and M₂₃C₆ carbides near the grain boundaries,and the M₂₃C₆ carbides are distributed at the grain boundaries like chains.In addition,due to high temperature of solution treatment and sufficient diffusion of elements,the constituents of??phase are far away from each other in air cooling,and their respective nucleation and precipitation lead to the formation of??phase with small size.The temperature of aging treatment is suitable for the nucleation and growth of??phase,and the particle size of??phase obtained after cooling is larger.The aging treatment after solution treatment can cause the adjacent??phase particles to fuse with each other and grow up continuously,thus to obtain large-size??phase.?5?Due to the precipitation strengthening of??phase,the microhardness of the laser cladded IN939 superalloy after heat treatment is improved,but the increase of microhardness by the solution treatment is only 4.9%.Aging treatment and complete heat treatment improve the microhardness better,with 10.2%and 10.9%,respectively,higher than the cladding sample before heat treatment.Due to the positive effect of M₂₃C₆ carbides at the grain boundaries on the corrosion resistance of the laser cladded IN939 superalloy,the polarization resistance of the sample is increased after single aging treatment and complete heat treatment,the self-corrosion current density and corrosion rate are decreased.The corrosion resistance of the laser cladded IN939superalloy is significantly improved by the heat treatments.