激光冲击近β型钛合金的室温拉伸和高周疲劳性能及其断裂机理

Tensile and High Cycle Fatigue Properties and Fracture Mechanism of NearβTitanium Alloy Strengthened by Laser Shock Peening

目的提高近β钛合金的强度和高周疲劳性能。方法对两相区固溶时效热处理后的TB6和Ti55531钛合金锻态材料表面进行了激光冲击强化(LSP),并对强化前后的试样进行室温拉伸和高周疲劳试验。采用扫描电镜(SEM)对拉伸和高周疲劳断口进行了观察和分析。结果与未LSP的样品相比,LSP后TB6和Ti55531钛合金的抗拉强度(Rm)分别提高了25 MPa和25 MPa,提高比例分别为2.26%和2.02%;屈服强度(Rp0.2)分别降低了48 MPa和30 MPa,降低比例分别为4.58%和2.54%;断面收缩率(A)、延伸率(Z)和弹性模量略有提升。在低应力水平下,LSP后Ti55531合金的疲劳寿命高于TB6合金,而在高应力水平下,TB6合金具有略高于Ti55531合金的疲劳寿命。结论经过和未经LSP的TB6和Ti55531钛合金的拉伸断裂模式均为微孔聚集型韧性断裂与沿晶脆性断裂混合的断裂模式,表面激光冲击不改变其拉伸断裂模式,近β钛合金在不同应力状态的疲劳寿命差异与材料显微组织差异导致的疲劳裂纹萌生和扩展速率不同有关。

Advanced aircraft has put forward the requirements of lightweight,high strength and high reliability for the materials used in its key structural parts.Near-βtitanium alloy has the characteristics of high strength,good fracture toughness,excellent fatigue resistance,large quenching depth and so on,is widely used in the manufacture of key bearing parts such as aircraft landing gear,helicopter rotor system central parts and connectors.TB6(nominal composition Ti-10V-2Fe-3Al)and Ti55531(Ti-5Al-5Mo-5V-3CR-1Zr)are two typical near-βtitanium alloys.Laser shock strengthening(LSP)is a surface treatment that can improve the fatigue life by inducing residual compressive stress(CRS),which cause change of microstructure and increase of dislocation density on the surface of the material,thus reduce the probability of fatigue crack initiation and propagation at surface.In this study,laser shock processing(LSP)was carried out on the surface of as-forged TB6 and Ti5553 titanium alloy after solution aging treatment in two-phase region.X-ray diffractometer and Vickers hardness tester were used to measure the residual stress and Vickers micro-hardness on the surface and along the depth of the sample.The results display that surface residual stress of TB6 sample is‒639.06 MPa,and that of Ti55531 sample is‒588.24 MPa.The surface hardness of Ti55531 and TB6 after LSP reaches 452HV0.1/10 and 400HV0.1/10,respectively,the value increases by 11.3%and 12.1%respectively compared with that before LSP.The hardness of the TB6-LSP sample and Ti55531-LSP sample at the depth of 900μm and 700μm respectively is consistent with the hardness of the matrix material.Tensile tests at room temperature and high cycle fatigue tests were carried out on the specimens before and after laser shock processing.The tensile and high cycle fatigue fractures were observed and analyzed by scanning electron microscopy(SEM).Compared with the sample without LSP,the tensile strength(Rm)of LSP’ed TB6 and Ti55531 titanium alloys increased by 25 MPa(2.26%)and 25 MPa(2.02%),respectively.The yield strength(Rp0.2)decreased by 48 MPa(4.58%)and 30 MPa(2.54%),respectively.The percentage reduction of area(A),elongation(Z)and elastic modulus(E)were slightly improved.Fatigue life of Ti55531 alloy after LSP is higher than that of TB6 alloy at low stress level,while the fatigue life of TB6 alloy is slightly higher than that of Ti55531 alloy at high stress level.The increase of micro-hardness can be attributed to the severe plastic deformation of the material caused by the highintensity shock wave in the process of laser surface shot peening,and there are a lot of high-density dislocations in the plastic deformation region,which leads to the increase of the hardness of the material.The tensile fracture modes of TB6 and Ti55531 titanium alloys with and without LSP are microporous aggregated ductile fracture and inter-granular brittle fracture.Surface laser shock processing does not change the tensile fracture mode of the alloys in this study.The difference of fatigue life of nearβtitanium alloy under different stress states is related to the difference of fatigue crack initiation and propagation rate caused by the difference of material microstructure.