Research On Creep Properties Of Nickel-based Single-crystal DD6 With Misorientation And Cooling Hole

Nickel-based single-crystal superalloy is the first choice of materials for turbine blade in advanced aircraft engines.Designing a lot of cooling holes in turbine blade improves cooling efficiency of turbine blade and thrust-weight ratio of advanced aircraft engines.However,cooling holes affect the properties of nickel-based singlecrystal superalloy.There is always an angle of deviation between axial directions of turbine blade and crystallization direction of nickel-based single-crystal superalloy.Because of the anisotropy of nickel-based single-crystal superalloy,the misorientation has a great effect on the properties of nickel-based single-crystal superalloy.Therefore,the purpose of this paper is to study the misorientation and cooling hole on nickel-based single-crystal superalloy.This paper takes the domestic second-generation nickel-base single-crystal superalloy DD6 as the research object to make experimental research.Firstly,the laser drilling of cooling holes are conducted on nickel-base single-crystal superalloy DD6 using picosecond and femtosecond pulse laser.Afterwards,uniaxial tensile creep tests at high temperature are carried on different nickel-base single-crystal superalloy DD6.Finally,the fracture morphology and microstructure evolution of different DD6 specimens are observed using SEM.The results of the picosecond laser drilling experiments show that,the laser power is the main influence factor of drilling cooling holes,but the repeat frequency is not.The qualities of cooling holes are poor using picosecond laser drilling,and the recast layer,which is made up of oxides generated by alloying elements and oxygen elements in the air,is formed around the hole.The results of the femtosecond laser drilling experiments show that,the single pulse energy,pulse count and defocusing amount all are the main influence factors of drilling cooling holes.The qualities of cooling holes are relatively good;no recast layer is formed,and oxides are little.The results of tensile creep tests at high temperature show that,the higher the stress is,the worse the creep properties of nickel-base single-crystal superalloy DD6 are;misorientation has a great influence on creep time of DD6,and the creep properties of [001] orientation DD6 is better than that of 5° misorientation DD6;the cooling holes cause worse creep properties of nickel-base single-crystal superalloy DD6.At high temperature,the fracture morphologies of all specimens display dimples.The smaller the stress is,the bigger the size of the dimple is.The misorientation influences the fracture morphologies,and the dimples of [001] orientation DD6 are more obvious than that of 5° misorientation DD6.Due to the stress concentration,the dimples of the specimens with cooling hole are concentrated around the hole.On the(001)crystallographic plane,γ′ precipitates of nickel-base single-crystal superalloy DD6 display square,but on crystallographic plane deviated 5° with [001] orientation,γ′ precipitates display rectangle.The higher the stress is,the bigger the size of rafting γ′ precipitates near fracture is,and when the time is enough,γ′ precipitates behave serious distortion.The rafting type of the γ′ precipitates of DD6 will change because of the existence cooling hole,and the γ′ precipitates won’t coarsen directionally perpendicular to applied stress direction.The rafting type of the γ′ precipitates of DD6 won’t change because of misorientation,but with cooling hole,the angle,which the direction of γ′ precipitates rafting deviates from perpendicular direction of applied stress,will change.