A Review of Dwell Sensitive Fatigue in Titanium Alloys

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Crystal plasticity analysis of temperature-sensitive dwell fatigue in Ti-6Al-4V titanium alloy for an aero-engine fan disc

Abstract

This study investigates the temperature-dependent dwell fatigue behavior of Ti-6Al-4V blend using crystal plasticity finite element method. The dislocation machinery-based crystal plasticity parameters are calibrated with the information of quasi-static tensile and constant load creep tests at ambient and intermediate temperatures. In particular, the charge per unit-dependent parameters related to skid belongings are employed to institute the relationship between strain rate sensitivity of a soft $\alpha$ -titanium single crystal and temperature. The variation of strain rate sensitivity with temperature influences the stress redistribution occurring inside the hard-soft grain combination, which in plough affects the dwell fatigue sensitivity. Further, the structural analysis for a Ti-6Al-4V fan disc provides the stress fields at takeoff and cruise phases to examine the in-service stress redistribution. The highly localized hoop stress and presence of large macrozones at the diameter of the fan disc, rather than the off-white rate sensitivity of soft macrozones at working temperatures, are responsible for triggering basal stress enhancement and dwell facet nucleation in hard macrozones.

Introduction

Near- $\alpha$ and $\alpha /\beta$ titanium alloys, which possess prominent backdrop of high specific forcefulness, excellent fatigue functioning and creep resistance, have been widely used to manufacture safe-critical components in the cold section of the loftier-bypass ceremonious turbofan engines such as discs and blades of the fan and compressor stages. These rotor components are subjected to cyclic fatigue loading with a constant high mean load hold (dwell) during cruising in each flight cycle. This load-dwell leads to substantial fatigue life reduction (known as "dwell life debit") compared with the prediction obtained from the traditional fatigue life assessment method [one]. The overestimation of the lifetime was repeatedly acknowledged from the premature failures of two near- $\alpha$ alloy IMI685 (Ti-6Al-5Zr-0.5Mo-0.2Si) fan discs of the Rolls-Royce RB211 engine in the 1970s and a near- $\alpha$ Ti-6242 (Ti-6Al-2Sn-4Zr-2Mo) alloy high-pressure compressor disc of the Full general Electric CF6 engine in 1997 [ii], [three]. Like accidents accept propelled considerable research on the dwell fatigue sensitivity of dual-phase titanium alloys [4], [5]. Based on previous studies and from an industrial perspective, IMI685 was replaced by $\alpha /\beta$ Ti-6Al-4V alloy in wrought class, which was initially considered to exist dwell insensitive [1]. In add-on, the volume fraction of the primary $\alpha$ phase in the rotor grade Ti-6242 alloy with bimodal microstructure was optimized to 10–18% through heat treatment to achieve a residue amidst fatigue, pitter-patter, and dwell resistance [half dozen]. All the same, the uncontained failure of a Ti-6Al-4V blend fan hub within the Engine Alliance GP7270 engine in 2017 first demonstrated that the Ti-6Al-4V alloy fan disc is too susceptible to dwell sensitive fatigue during operation [vii]. Thus, information technology is nevertheless imperative to thoroughly sympathize the dwell fatigue in titanium alloys, thereby meeting the claiming of structural integrity evaluation for the aero-engine design process.

Fractographic examinations of dwell fatigue loaded well-nigh- $\alpha$ and $\alpha /\beta$ titanium alloys indicate that the subsurface crack initiation sites are characterized by the basal facets nearly perpendicular to the principal stress direction [eight], which are closely associated with the dwell life debit. Evans and Bache [9] adopted Stroh's dislocation pile-up model to illuminate the early on dwell facet nucleation and subsequently developed an alternative stress redistribution model considering the elastoplastic anisotropy of hexagonal close-packed (HCP) $\alpha$ -titanium [1], [10]. The stress redistribution, or load shedding, was first captured by Hasija et al. [eleven] via rate-dependent crystal plasticity modeling. According to their study, the redistributed stress from the soft (c-axis perpendicular to the loading direction) to the adjacent hard (c-centrality parallel to the loading management) grain during the stress-dwell period is presumed to generate the required stress field (a disquisitional combination of high resolved normal stress on the basal aeroplane and shear stress) for faceting in the hard grain.

Dissimilar Ti-6242 blend, the disappearances of subsurface facets and dwell life debit in Ti-6246 (Ti-6Al-2Sn-4Zr-6Mo) alloy from 293 Thou to 423 Thousand have confirmed the temperature sensitivity of dwell fatigue in different titanium alloys [12], [13]. It has been reported that the dwell life debit in Ti-6Al and Ti-6242 alloys is largest at around 393 Yard and almost fades out at temperatures higher than 503 M, merely Ti-6246 alloy suffers the severest life reduction betwixt 723 Grand and 823 K because more interstitial Mo in the $\alpha$ stage of Ti-6246 alloy causes an increase in activation energy for dislocation nucleation [14], [15]. These experimental findings are consequent with the discrete dislocation plasticity (DDP) predictions that stress redistribution in Ti-6Al and Ti-6242 is most notable at 393 K and drops to a depression level at 503 Yard, whereas the redistributed stress in Ti-6246 alloy is the greatest at a much higher temperature of 573 Chiliad [sixteen]. Therefore, the discrepancy of dwell fatigue sensitivity betwixt Ti-6242 and Ti-6246 is presumably attributed to the temperature-dependent stress redistribution phenomenon [17]. As the cold creep (T < 0.4T_m) in titanium alloys is an important factor affecting the stress redistribution and concentration, Yazar et al. [18] paid attention to the influence of temperature on creep deformation and carried out dwell fatigue tests on virtually- $\alpha$ IMI834 (Ti-five.8Al-4Sn-iii.5Zr-0.7Nb-0.5Mo-0.3Si-0.06C) alloy at temperatures ranging from room temperature (RT) to 633 K. They found that the strain aggregating was highest at 393 G, followed by RT, 513 K, and 633 Thousand under the aforementioned normalized stress level. Similarly, Peng et al. [19] conducted constant load pitter-patter tests for commercially pure titanium (CP-Ti) at low and intermediate temperatures. The pitter-patter strain was pointed out to increase with temperature from 273 G to 388 K, and then decreased gradually and diminished at 498 K. Given that the macroscopic creep is dominated by microscopic dislocation skid, Harr et al. [xx] further used high-resolution digital image correlation (Hr-DIC) technique to quantitatively analyze the onset of slip traces in Ti-6242 alloy under dwell fatigue. Their observations suggested that the slip trace aggregating at 393 Yard was greater than those at RT and 473 K. To fill the gap between the crystal slip and macroscale creep, the stress-related strain charge per unit sensitivity (SRS) of individual slip was linked with the temperature-dependent pitter-patter response in Ti-6Al alloy [21]. In addition to common cold creep, the plastic anisotropy of the HCP lattice plays a vital role in the temperature sensitivity of the stress redistribution [22]. At depression temperatures, plastic deformation along the c-axis is difficult for hard grains because the critical resolved shear stress (CRSS) required to actuate pyramidal <c +a > slips is $2-\mathrm{half-dozen}$ times higher than that for prism <a > sideslip [23]. Hence, the soft grain sheds stress onto the abutting hard grain during the dwell menses to ensure the deformation compatibility near the hard-soft grain purlieus. With elevating temperature, the noticeable decrease in CRSS for pyramidal <c +a > slips results in a competitive strength between prism <a > and pyramidal <c +a > slips. The continuous slip manual at difficult-soft grain boundaries tin coordinate the local deformation, resulting in the diminishment of stress redistribution [24]. The above studies regarding the issue of temperature on dwell fatigue take focused on most- $\alpha$ titanium alloys, while very limited work has been committed to $\alpha /\beta$ titanium alloys. Further investigations into the temperature-sensitive dwell fatigue in Ti-6Al-4V alloy are crucial because the partitioning outcome of a different $\beta$ -stabilizer V on $\alpha$ phase may alter the temperature-dependent SRS [25], and Ti-6Al-4V aero-engine components also undergo complex temperature history during takeoff, climbing, cruise, descent, and landing (due east.thousand., the Ti-6Al-4V fan disc in the GP7270 engine is reported to be exposed to temperatures from 294 One thousand to 343 K [seven]).

Apart from the influence of temperature, rotor components in aero-engines are subjected to multiaxial loading (e.g., centrifugal load, aerodynamic strength, pressure, and sometimes maneuver load) [26], [27], which is distinct from the typical uniaxial dwell fatigue tests under laboratory environment. To consider the stress multiaxiality, Bache et al. [9] performed biaxial dwell fatigue tests for IMI685 blend under tension and torsion loadings. Information technology was found that dwell life debit under biaxial stress was less than that under the pure tension-tension loading. Doquet et al. [28] replaced the torsion load with internal pressure to reverberate the in-service loading on compressor discs and concluded that biaxial loading could delay the facet nucleation and resist fatigue crevice propagation in Ti-6Al-4V alloy. Additionally, the permissible stress for titanium alloy rotors is well below the yield strength to ensure a profitable service hour, whereas the specimens are mainly tested at maximum dwell stress of over $\mathrm{0.nine}{\sigma }_{0.\mathrm{ii}}$ considering the efficiency and cost of tests [29]. Attributable to the aspects mentioned to a higher place, the effect of loading continues to be a concern when the laboratory results obtained from specimens are applied to actual parts.

In the present enquiry, the effect of temperature on the dwell fatigue behavior of Ti-6Al-4V blend is numerically investigated to address the dwell fatigue sensitivity of an aero-engine fan disc in operation. Firstly, temperature-dependent crystal plasticity parameters of the $\alpha$ stage are calibrated from a serial of tensile and creep tests at ambience and intermediate temperatures ranging from 293 K to 664 K. And so, the relationship between the strain rate sensitivity of a single $\alpha$ crystal in Ti-6Al-4V alloy and temperature is analytically established. Based on this relationship, a polycrystalline aggregate model is generated to predict the stress redistribution inside the hard-soft grain combination and assess the event of temperature on dwell fatigue sensitivity. Furthermore, structural analysis of a Ti-6Al-4V 3-web fan disc is performed nether two operating weather condition, i.east., takeoff and cruise phases. The results are applied to make up one's mind the roles of temperature, loading land, and macrozone morphology in local stress redistribution and dwell facet nucleation under the service environment.

Section snippets

Crystal plasticity constitutive model

The rate-dependent crystal plasticity constitutive law used in this study was originally formulated by Huang [30] based on the finite strain theory. The current deformation gradient $\mathit{F}$ is given by $\begin{array}{c}\mathit{F}={\mathit{F}}^{\mathrm{east}}\bullet {\mathit{F}}^p\end{array}$ where ${\mathit{F}}^e$ refers to lattice stretching and rigid body rotation, and ${\mathit{F}}^p$ denotes the deformation caused by plastic shear of the material.

As the deformation twinning action is suppressed by high aluminum content (greater than 5%) [31], information technology is assumed that the plastic deformation results from all

Temperature-dependent strain rate sensitivity

Dwell fatigue tests from previous work propose that a combination of high SRS and low strain hardening leads to large macroscopic strain accumulation in IMI834 blend in the temperature range of 293 M to 513 Grand [18]. To farther understand the creep behavior and temperature sensitivity of dwell fatigue at the grain calibration, the human relationship betwixt the rate-dependent characteristic of individual skid systems and the SRS should exist established. To this end, the conventional SRS exponent $m$ ( $m=\frac{d\mathrm{50}n\sigma }{\mathit{dln}\dot{\epsilon }}{|}_{}$

Effect of temperature on stress redistribution

The variation of SRS of the prism sideslip-orientated soft grain with temperature alters the time-dependent dislocation pile-ups at hard-soft grain boundaries during dwell fatigue. As a powerful tool, crystal plasticity finite element method is applied to quantitatively evaluate the magnitude of the pile-up stress and determine the dwell fatigue sensitivity.

A 2D microstructure containing $71$ primary $\alpha$ grains is extruded forth the z-axis using an ABAQUS plugin Neper2CAE [55] to develop a quasi-3D

Dwell fatigue sensitivity of a fan disc

An in-depth comprehension of the temperature-sensitive SRS and stress redistribution is beneficial to recognize the dwell fatigue failure of different titanium alloy rotor components in service. Besides, the stress level, introduction of stress multiaxiality, and macrozone morphology after manufacturing (due east.m., dice forging and machining for compressor discs [lx], cross-rolling and machining for blades [seven]) affect the dwell facet nucleation. To comprehensively elucidate the upshot of these factors,

Discussions

In general, the fan disc is operated below 373 K, whereas the dwell fatigue sensitivity of Ti-6Al-4V alloy is supposed to be the highest at 416 K, every bit described in Section three. Thus, the working temperatures of this component cannot cover the dwell sensitive temperature range, unlike the near- $\alpha$ titanium alloy discs or bliscs at the first several stages of the high-pressure compressor, which are operated over 573 Grand (out of the dwell sensitive temperature range) during the cruise phase [57]. Since

Conclusions

In this report, the effect of temperature on grain-scale strain rate sensitivity and pitter-patter behavior in Ti-6Al-4V alloy is investigated to empathise the dwell fatigue sensitivity of a fan disc at operating states, with attending focused on the stress redistribution phenomenon occurring at hard-soft grain boundaries. The following conclusions can be summarized:

(one)

The activation energy ( $\mathrm{\Delta }F$ ) and activation volume ( $\mathrm{\Delta }5$ ) of the $\alpha$ phase in Ti-6Al-4V alloy increase with rising temperature. The higher

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors gratefully acknowledge the support of ISIJ Research Promotion Grant "Establishment of the principle to design microstructure of ductile ii-stage Ti alloys having strong resistance for fracture" within the Iron and Steel Institute of Japan. Liangwei Yin acknowledges the contribution of Mr. Yan Xu, Mr. Baiyang Li from AECC Commercial Aircraft Engine Co. Ltd., Mr. Xu Liu from AECC Shenyang Engine Design and Research Found, Mr. Mingxuan Cai from AECC China Gas Turbine

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Source: https://www.sciencedirect.com/science/article/pii/S0142112321005302

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