Investigation On Microstructure Characteristics And High Temperature Oxidation Resistance Of Al₂O₃/CoNiCrAlY Coatings By Laser Cladding

With the development of aviation technology,the flow ratio,thrust to weigh t ratio and turbine inlet temperature of aviation Engine is further increased,result ing in current superalloy difficult to meet the new requirements.Thus,in add-iti on to researching new superalloys,using thermal barrier coating which consists of YSZ top and MCrAlY bonding layer can greatly increase gas turbine e-ngine operating temperature.Typically,MCrAlY?M stand for Co,Ni or Co+Ni?coating can reduce the thermal expansion coefficient mismatch between the top YSZ cera mic and substrate,and high-temperature oxidation properties of the su-bstrate can be further improved.Thus,high temperature oxidation resistance of MCrAlY coat ings are considered as life controlling factors of thermal barrier co-atings.High t emperature failure mechanism and further improving its high temperature oxidati on resistance,has attracted wide attention of researchers.In this paper,CoNiCrAlY coating and Al₂O₃/CoNiCrAlY coatings were respectively deposited on K438 superalloy by laser cladding.Research laser processp arameters and Al₂O₃ content on the microstructure of two coatings,and based it,study their oxidation behavior at 1100?.The main work accomplished in this paper and conclusion is as follows:?1?Using appropriate laser process parameters on K438 superalloy,no crack and cavity,good quality of coaings can be obtained.When the laser power is too small,the cladding powder due to lack of energy has not been fully melted and solidified convection,and internal cladding layer will crack.The laser power is too large,the dilution of the coating is too large,resulting in element diffusion between the coating and the substrate,destroy the good performance of the original coating.It was found that the optimum process parameters: P=2kW,D=3.5mm,Vg=0.8g/min,V=600mm/ min.?2?After Al₂O₃ particles were incorporated into CoNiCrAlY coating,the coating of macroscopic size change.compared with CoNiCrAlY coating,Al₂O₃/CoNiCrAlY coating width significantly increased,significantly reduced thickness.In addition,the coating dilution rate increased exponentially,adding 25% Al₂O₃ particles of coating is 4.33 times of CoNiCrAlY coatingwithout adding Al₂O₃ particles.?3?CoNiCrAlY cladding prepared by different process parameters consists of three areas: the cladding zone,binding region and the heat affected zone.It is a metallurgical bond between the coating and the substrate.CoNiCrAlY coating of microstructure sequentially from the bottom ot up is planar crystal,columnar dendrites or cellular dendrites,equiaxed.?4?Preparation of the different laser parameters CoNiCrAlY coating phase contained ?-Co,?-?Ni,Cr?,?'-Ni3 Al,Cr2Ni3 composition,besides,the relative content of each phases is deferent.When the laser power is increased,?'-Ni3 Al decreased,?-Co and ?-?Ni,Cr?content increased;when the increased scan speed,increasing the ?'-Ni3 Al content;when the powder feed rate increase,?'-Ni3 Al and ?-Co content is reduced,Cr2Ni3 and ?-?Ni,Cr?content increased.If other process parameters keep the same,increasing scanning speed,the more Columnar crystals will be obtaind.?5?Compared with no Al₂O₃ coating,incorporated Al₂O₃ coating the columnar crystals are thinner,phase composition is the same,the cladding layer of Al₂O₃ particle size of about 3um,which is 0.06 to 0.04 times of original powder.?6?K438 alloy oxidized at 1100 ? accord with linear law,without Al₂O₃ incoporated coating and Al₂O₃/CoNiCrAlY coating are both in line with the parabolic law.Superalloy K438 oxidized phases by the ?-Co,?-?Ni,Cr?,?'-Ni3 Al,?-Al₂O₃,Cr2O3,NiO,?Co,Ni??Al,Cr?2O4 and TiO2.WithoutAl₂O₃ incoporated coating and Al₂O₃/CoNiCrAlY coating has similar phase composition,consist of ?-Co,?-?Ni,Cr?,?'-Ni3 Al,?-Al₂O₃,Cr2O3,NiO,?Co,Ni??Al,Cr?2O4 composition.With Al₂O₃ incoporated coating spinel phase diffraction peaks is weaker,indicating that Al₂O₃ coating added to contribute to suppress the growth of harmful spinel oxide.?7?within a certain range,increasing the laser power helps to improve the oxidation resistance of the coating.Oxidation resistance of Coating prepared by 2.5kW power is more excellent than the coating prepared by 2kW power.The smaller the scanning speed,the better the coating oxidation resistance.?8?Not added Al₂O₃ particles of coating at 1100 ? isothermal oxidation for 200 h does not occur the internal oxide,while the inner oxide appeared in Al₂O₃/CoNiCrAlYcoating,due to the high dilution.Besides,AlN is foud in the substrate.