喷丸与CuNiIn涂层复合处理对高温合金榫试样微动疲劳性能的影响

Effect of Shot Peening and CuNiIn Coating Composite Treatment on Fretting Fatigue Properties of Superalloy Tenon Specimens

汽轮机叶片榫结构接触部位易发生微动疲劳失效,但行业内仍缺乏有效解决措施,因此开展了不同载荷下不同表面改性技术处理后10705BX铁基高温合金榫结构微动疲劳试验研究。分别对原始未处理(AS)、喷丸处理(SP)及喷丸与CuNiIn涂层复合处理(SC)的10705BX铁基高温合金榫结构试样的微动疲劳性能进行测试分析,在微动疲劳试验前后,对原始、喷丸处理及复合处理后的10705BX铁基高温合金的表截面形貌、断口形貌及力学性能进行表征分析。结果表明:原始、喷丸处理和复合处理试样表面粗糙度Sa分别为0.08、3.38和13.65μm。喷丸处理后表面硬度提高了16%,加工硬化层深度约为80μm,微动疲劳寿命相较原始试样提高了7.8倍。复合处理的涂层平均厚度约为50μm,微动疲劳寿命相较原始试样提高了4.2倍,相比较喷丸处理来说,复合处理后材料的微动疲劳寿命提升较弱。原始、喷丸处理和复合处理试样的裂纹均为多疲劳源萌生,但是喷丸和复合处理后的裂纹源数量明显减少。喷丸处理和复合处理后裂纹的扩展速率均显著提高。喷丸后试样表层获得加工硬化层并且引入残余压应力,主要提升了裂纹萌生寿命。喷丸处理及喷丸与CuNiIn涂层复合处理方法有望成为提升汽轮机叶片服役寿命的重要备选途径。

The 10705BX iron-based superalloy has the advantages of high yield strength,corrosion resistance,and oxidation resistance.It is widely used for manufacturing high-temperature components,such as turbine discs,blades,and fasteners.However,under actual working conditions,the tenon structure of a turbine is subjected to alternating cyclic loads,resulting in fretting fatigue cracks until failure,which eventually leads to accidents.Surface modification technology forms a work-hardening layer and introduces residual stress;thus,the fretting fatigue properties of materials can be improved using this method.To date,the fretting fatigue properties of 10705BX iron-based superalloys and related surface-strengthening techniques have not been reported.Fretting fatigue tests were performed on 10705BX iron-based superalloy tenon structures treated using different surface modification techniques under different loads to extend the service life of the turbine.The fretting fatigue tests were conducted using a high-frequency fatigue-testing machine.The specific test parameters were as follows:a peak load of 22 kN,stress ratio of 0.1,and frequency values of 128–138 Hz.The fretting fatigue properties of the 10705BX iron-based superalloy dovetail structure were compared after as-received(AS),shot peening(SP),and shot peening+coating composite treatments(SC),respectively.Subsequently,the surface and cross-sectional morphologies,fracture characteristics,and mechanical properties were analyzed.The results showed that the surface roughness Sa values of the AS,SP,and SC samples were 0.08,3.38,and 13.65μm,respectively.The surface hardness increased by 16.28%after SP.The hardened layer depth was approximately 80μm.Based on the KAM chart of EBSD analysis,the stress in the 0–80μm region from the surface depth of the sample after SP treatment was relatively high owing to the high residual compressive stress introduced by shot peening.The fretting fatigue lifetime after SP was 7.8 times longer than that of AS.The fretting fatigue lifetime after SC was 4.2 times longer than that after AS.The average coating thickness was approximately 50μm.Compared to the SP treatment,the fretting fatigue life of the SC treatment was low.This was because,after the SC treatment,the hardness of the hardened layer below the coating decreased slightly,reducing the anti-fretting fatigue performance.During plasma-spraying CuNiIn coating,the thermal effect of thermal spraying can somewhat reduce the residual stress introduced by shot peening,reducing the anti-fretting fatigue property of the sample.However,because the CuNiIn coating was soft,it exhibited good fretting wear resistance.When subjected to cyclic loading,ductile deformation occurs easily,and it has a lubricating effect,reducing stress concentration,and improving fatigue life.The cracks in the AS,SP,and SC samples were initiated by multiple fatigue sources.However,the number of crack sources after SP and SC decreased significantly.The fretting fatigue fracture mechanism of 10705BX did not change after SP and SC treatments but expanded in the cleavage mode.However,the crack propagation rate increased.The fatigue band spacings in the rapid expansion zone of the AS,SP,and SC treatments were 0.39,0.83,and 0.84μm,respectively.An increase in the fatigue-band spacing indicates an increase in the crack growth rate.The crack initiation lifetime increased because of the formation of a hardened layer and the compressive residual stress after SP.SP and SC treatments are expected to be important alternative methods for extending the service life of turbine blades.