On the influence of mechanical surface treatments—deep rolling and laser shock peening—on the fatigue behavior of Ti–6Al–4V at ambient and elevated temperatures

https://doi.org/10.1016/S0921-5093(03)00069-8Get rights and content

Abstract

It is well known that mechanical surface treatments, such as deep rolling, shot peening and laser shock peening, can significantly improve the fatigue behavior of highly-stressed metallic components. Deep rolling (DR) is particularly attractive since it is possible to generate, near the surface, deep compressive residual stresses and work hardened layers while retaining a relatively smooth surface finish. In the present investigation, the effect of DR on the low-cycle fatigue (LCF) and high-cycle fatigue (HCF) behavior of a Ti–6Al–4V alloy is examined, with particular emphasis on the thermal and mechanical stability of the residual stress states and the near-surface microstructures. Preliminary results on laser shock peened Ti–6Al–4V are also presented for comparison. Particular emphasis is devoted to the question of whether such surface treatments are effective for improving the fatigue properties at elevated temperatures up to ∼450 °C, i.e. at a homologous temperature of ∼0.4T/Tm (where Tm is the melting temperature). Based on cyclic deformation and stress/life (S/N) fatigue behavior, together with the X-ray diffraction and in situ transmission electron microscopy (TEM) observations of the microstructure, it was found that deep rolling can be quite effective in retarding the initiation and initial propagation of fatigue cracks in Ti–6Al–4V at such higher temperatures, despite the almost complete relaxation of the near-surface residual stresses. In the absence of such stresses, it is shown that the near-surface microstructures, which in Ti–6Al–4V consist of a layer of work hardened nanoscale grains, play a critical role in the enhancement of fatigue life by mechanical surface treatment.

Introduction

The control of failures due to high-cycle fatigue (HCF) in turbine engine components is one of the most critical challenges currently facing the US Air Force [1], [2], [3]. In general, components such as blades and disks are subjected to HCF loading associated with the high frequency (1–2 kHz) vibrations within the engine, superimposed on to a low-cycle fatigue (LCF) component associated with the start and stop cycles. In order to increase the resistance of such components to the initiation and early growth of fatigue cracks under such conditions, especially in the presence of foreign-object damage, mechanical surface treatments are widely used. Such treatments, such as deep rolling, shot peening and laser shock peening, are known to significantly improve resistance to wear and stress corrosion, and in particular to enhance the fatigue strength of highly-stressed metallic components used in a variety of engineering applications [4], [5], [6].

Though shot peening has traditionally been used for most aircraft components, deep rolling (DR) offers several attractive advantages due to the generation of a deeper ‘case’ of compressive residual stresses1 and a work hardened microstructure as well as a relatively smoother surface finish [7]. The process involves plastic deformation of the near-surface layers of the metallic work-piece by a spherical or cylindrical rolling element under a controlled force or pressure. In contrast to roller burnishing, which is performed with lower pressures and solely serves to achieve a smooth surface topography, the aim of this treatment is to induce deep compressive residual stresses and work hardened surface layers.

Although deep rolling is restricted to certain component geometries, the process is relatively inexpensive and has the advantage that the lower surface roughness levels achieved act to lower the chance of fatigue crack initiation whereas the compressive residual stresses act to retard subsequent crack propagation [8], [9], [10]. For this reason, deep rolling is increasingly utilized to enhance the fatigue strength and service lives of steel components, such as crankshafts [11], especially in the presence of detrimental notches.

In the present work, we examine the effectiveness of deep rolling for improving the fatigue behavior of a commonly-used turbine-engine alloy, Ti–6Al–4V, with special emphasis on the near-surface residual stress states and microstructures, before and after mechanical and/or thermal exposure. Results are compared with those obtained using laser shock peening (LSP), where a short, high-power laser pulse is used (under a water blanket) to vaporize a sacrificial coating on the component, thereby producing a shock wave that propagates inwards from the surface (see [12] for details). Akin to DR, LSP offers many advantages compared to shot peening in that the process is capable of introducing deep cases of compressive residual stresses and cold work in the near-surface layers without compromising the surface roughness (which remains relatively unaffected by LSP). Moreover, this process is not restricted by component geometry, as is the case with deep rolling.

The observed improvement in the fatigue lifetime due to mechanical surface treatments is known to depend predominantly on the amount, depth distribution and stability of the induced residual stresses and work hardening in the near-surface regions [13]. Consequently, but for a few exceptions [14], [15], elevated service temperatures are generally believed to be detrimental to the fatigue response of surface treated components, owing to the partial (or, in some cases, total) relaxation of the induced residual stress state. In fact, there is a paucity of results in archival literature dealing with the effectiveness of such mechanical surface treatments at temperatures above ambient [16], [17], [18]. For this reason, an objective of this work is to investigate the effectiveness of mechanical surface treatments at increasing temperature. Specifically, the cyclic fatigue performance of Ti–6Al–4V is evaluated, before and after DR and LSP, at both ambient and elevated temperatures up to 450 °C, i.e. between ∼0.15 and 0.4T/Tm, where Tm is the melting temperature. Results are discussed in the context of the stability of the residual stresses and near-surface microstructure with increasing temperature.

Section snippets

Materials

The Ti–6Al–4V alloy used in the present study originated as double vacuum-arc remelted forging stock, produced by Teledyne Titanium (Pittsburgh, PA) specifically for the US Air Force-sponsored joint government-industry-academia High-Cycle Fatigue Program. The chemical composition of this alloy (in wt.%) is given in Table 1 [19].

The original, 63.5 mm diameter bar-stock was sectioned into 400 mm long sections, preheated to 940 °C for 30 min and forged as glass-lubricant coated bars at this

S/N behavior

A comparison of the stress/life (S/N) fatigue behavior (at R=−1) of the virgin and the deep rolled Ti–6Al–4V at temperatures of 25 and 450 °C is shown in Fig. 3. It can be seen that deep rolling leads to a significant enhancement in the observed fatigue lifetime at room temperature, particularly in the high-cycle fatigue regime; indeed at σa=500 MPa, lives are increased by roughly two orders of magnitude. However, this beneficial influence of deep rolling is still apparent, although reduced, at

Discussion

The results of this work clearly indicate the beneficial effect of mechanical surface treatment, specifically by deep rolling and laser shock peening, in enhancing both the ambient-temperature HCF- and LCF resistance of Ti–6Al–4V. The results further show that the effect, which is associated with an increased lifetime and lower initial crack-propagation rates, can be attributed to the creation of a favorable compressive residual stress state within 500–1000 μm of the surface, coupled

Conclusions

Based on an experimental investigation of the effect of mechanical surface treatments, specifically deep rolling and laser shock peening, on the HCF- and LCF behavior of Ti–6Al–4V at temperatures from ambient up to 450 °C (T/Tm∼0.4), the following conclusions can be made.

Deep rolling can enhance the HCF and LCF strength of Ti–6Al–4V. While laser shock peening also resulted in an improvement in fatigue strength, deep rolling was found to be more effective for the process parameters investigated.

Acknowledgements

This work was supported in part by the US Air Force Office of Scientific Research under Grants No. F49620-96-1-0478 (under the auspices of the Multidisciplinary University Research Initiative on High Cycle Fatigue) and No. F49620-02-1-0010 to the University of California at Berkeley. Thanks are also due to Metal Improvement Company, Livermore, CA, for performing the laser shock peening and to DFG (Deutsche Forschungsgemeinschaft) for financial support to Dr. I. Altenberger under Grant No. AL

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