Basic Research Of Repairing Of Gas Turbine Blade By Laser Additive Manufacturing Technology

The nickel-base supperalloy K435 representing excellent high temperature performance could be widely used as turbine blades and turning vanes for Industrial Gas Turbine blades.The manufacturing technique of the blade is complicated and the service environment of the blade is adverse,this easily cause the blade to be discard.Therefore,the challenge is to find a proper method to accomplish high quality repairing relating to blades which were used in gas turbine and aerospace engines.Laser cladding technology,with some characteristics such as low heat input and high automation,can make the structural dimension and mechanical properties of the complex damaged components be recovered with less impacted on the base metal.However,the research on the common fundamental problems for laser cladding technology is not deep.The interaction between laser,powder and molten pool are much complicated.The research approach about them is mainly through simulation and theoretical calculation.However,only through this approach,it is difficult to restore the real process.Therefore,to further explore the potential of the technology and improve the forming quality of the parts has important research significance and economic significance.The high-speed camera system combined with image processing was used to investigated the laser cladding process from the following perspectives: powder concentration distribution characteristics;the interaction between laser and powder particles;the interaction between powder particles and the molten pool.Through the research and analysis of the above question,the thesis aim to find out the factors which affect the surface adhering powder.Combined with optimized processing parameters and mechanical properties of repaired parts,the high-accuracy repair of turbine blade root in Ni based superalloy K435 was achieved.A systematic method that described the powder concentration distribution using gray level was presented.,and the feasibility of the method was also evaluated in detail.The powder distribution is an almost symmetrical Gaussian distribution near the focal region.The position of this point is significantly affected by the coaxial and carrier gas,but the influence they exhibited is different.The critical value of the carrier gas flow is 5 L/min,and the larger the flow rate is,the shorter the focal length is.Above the critical value,the focal length is almost unchanged.while the larger the coaxial gas flow is,the longer the focal length is.This phenomenon can be mainly attributed to the two force direction at particles is different.However,the position of the peak value is approximately not affected by mass flow rate,but the larger the mass flow rate is,the greater the concentration gradient of the focal region is.The intensity and duration time of powder irradiated laser have great influence on its existence.Plasma is easily produced by laser,which will have a very strong recoil force on the particle,and this will has a great effect on the velocity and trajectory of the powder.Carrier gas moves the plasma right in front of the particle,which forming a bright tail is the essential reason for the "tail" phenomenon.The direction and the length of the "tail" were highly consistent with the direction and acceleration of the powder particles.The number and area of powder particles irradiated by laser are strongly affected by process parameters,such as,gas flow rate,defocusing distance,powder feeding rate and particle size distribution.The fundamental cause of adhering particles mainly because the powder particles fed into the partially solidification zone around the molten pool,and its remaining heat is not enough to melt the particle immediately.The amount of adhering particles could be effectively reduced by changing the interaction point of the powder flow and the melt pool,which can be achieved by adjusting the process parameters and adding the assistant airflow.The rising height ?Z along the deposition direction is equal to the height of the single cladding layer H is a sufficient and necessary condition to ensure that the interaction position between laser,powder particles and the molten pool is not changed.If rising height ?Z is greater than the single layer height H,the cladding process can not be carried out stability.If rising height ?Z is lower than the single layer height H,the process was stable,but the surface accuracy will be decrease with the gap between them is increases.Considering the surface smoothness of cladding layer and the interface bonding strength between cladding layer and substrate,the overlap rate shoule be moderate.The interface bonding strength of K435 alloy is higher than that of base metal after repaired by laser cladding.The tensile strength of the deposition layer along the deposition direction and cladding direction were 1530 MPa and 1350 MPa,which increased by 69.4% and 49.5% in comparison with the base metal.The tensile ductility of them could reach 23% and 18% respectively,which increased by 187.5% and 125% in comparison with the base metal.The mechanical properties of the deposited have a slight difference in the deposition direction and cladding direction.The structure dimension and deformation of the repaired turbine blade basically meet the needs of the manufacturers.