| Nickel-based single crystal superalloys are the material of choice for aero-engine turbine blades based on their high temperature strength,creep resistance and oxidation resistance.Designing a large number of cooling holes on the turbine blades can effectively improve the blade cooling efficiency and thus reduce the blade wall temperature.However,the existence of the cooling holes will have a certain impact on the fatigue strength and service life of the turbine blades.Therefore,it is of great significance to study the effects of cooling hole on the creep properties and film cooling characteristics of nickel-based single crystal superalloys.In this paper,nickel-based single crystal superalloy DD6 is studied.Firstly,the micro-evolution process of cubic γ’ phase under tension/pressure creep conditions was studied by finite element analysis technique on the nanometer scale.Then,the plate sample with and without cooling hole was tested by high temperature tensile creep test.The microstructure and fracture morphology of the plate specimens after fracture were compared and analyzed.The influence of the cooling hole on the creep properties of the plate specimens was discussed.Finally,the radiusing-type hole generated during the laser processing of the cooling hole was taken as the object.The film cooling characteristics of radiusing-type hole under different blowing ratios were studied and analyzed.The finite element analysis shows that during the high temperature tensile/compression creep of the [001] oriented single crystal unit,the crystal lattice shrinkage or expansion occurs in the different crystal faces of the cubic γ’ phase under the action of the principal stress component of the crystal plane.The change of the crystal lattice will repel or adsorb the atoms such as Al and Ti,and promote the γ’ phase to grow in the normal direction of the lattice expansion,and finally form N-type or Ptype directional coarsening.The high temperature tensile creep test showed that cooling hole had a significant effect on the creep properties of the nickel-based single crystal superalloy DD6 plate specimens.The addition of the cooling hole not only changes the fracture style of the specimen,causing it changes from microporous aggregate fracture to mixed fracture with cleavage-like and dimple fracture characteristics,and it also affects the creep lasting performance of plate specimen.The presence of the cooling hole results in a stress concentration state around the hole,a large increase in dislocations and cut the γ’ phase,the creep lifetime of DD6 was reduced from 94 h to 57 h.The addition of the cooling hole causes a change in the type of γ’ phase rafting around the hole,so that the γ’ phase rafting direction is 65° from the tensile stress axis,and N-type rafting no longer occurs.The study of film cooling efficiency shows that under the same blowing ratio,the film cooling efficiency of the radiusing-type hole is greatly improved compared with cylinder hole.By studying the effect of the blow ratio on the cooling efficiency of radiusing-type hole,it is found that the cooling efficiency of the radiusing-type hole is different in different blowing ratios: During the process of changing the blowing ratio from 0.3 to 0.48,the lateral and exhibition direction cooling efficiency of the radiusingtype hole are significantly improved.When the blowing ratio is changed from 0.48 to 0.8,in the range of 0<X/D<5,the lateral cooling efficiency is higher than that of the blow ratio of 0.48;when 5<X/D<30,the cooling efficiency of the lateral direction with a blow ratio of 0.8 is lower than when the blow ratio is 0.48. |
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