Effect of surface mechanical attrition treatment on high cycle and very high cycle fatigue of a 7075-T6 aluminium alloy
Introduction
The fatigue durability of mechanical components is highly important due to increasing demand for security and economic issues. In general, fatigue damage of mechanical components occurs at surface defects that play the role of stress concentrators. To improve their in-service performance and life, one of the usually used approaches is to alter the surface properties by means of mechanical surface treatment, such as shot peening [1], [2], [3], severe shot peening [4], [5], deep rolling [6], [7], [8], laser shock peening [9], [10] and surface mechanical (or ultrasonic) rolling treatment [11], [12], [13]. Shot peening is one of the effective surface engineering techniques for improving fatigue resistance of metallic components, and has been widely used in aerospace and automotive industries. This technique can strengthen the near surface region of mechanical parts and increase the resistance to crack initiation and propagation [1], [14]. Surface mechanical attrition treatment (SMAT) is one of the most promising derivatives of shot peening, and has been receiving considerable attention in recent decades [15], [16], [17], [18]. During SMAT, spherical shot is confined in an enclosed chamber and propelled by a vibrating generator working at a frequency range of 0.05–20 kHz [19]. When the vibration generator operates at a frequency above 16 kHz, the process can also be called ultrasonic shot peening (USP) by some researchers [20], [21], [22]. The surface of a sample is repeatedly impacted by the shot with high kinetic energy in multiple direction. These impact loadings generate local plastic deformation, which progressively leads to the formation of a strengthened near surface region. Several studies indicate that SMAT can increase the fatigue life of various metallic materials [21], [23], [24], [25]. For example, Roland et al. [23] investigated the effect of SMAT on the fatigue performance of a 316L steel and showed that its fatigue limit is improved by ~21%. Li et al. [25] reported that the fatigue strength at 5 × 106 cycles of a carbon steel treated by SMAT is enhanced by ~13% with respect to its untreated counterpart. Nevertheless, some authors mentioned harmful effect of SMAT on fatigue strength for several materials under certain fatigue loading conditions [16], [26]. The controversial effects of SMAT on fatigue performance could be related to different factors involved in the process. In order to optimize the process parameters, to reduce detrimental effects and improve fatigue performance, it is vital to have insight into the influencing mechanisms of SMAT on fatigue behaviour.
In general, similar to shot peening, the positive effect of SMAT on fatigue performance is associated with superficial compressive residual stresses and work hardening induced by the inhomogeneous plastic deformation caused by multiple impacts during the process. In addition, a nanostructured layer can be formed when the impact intensity is high enough [19], [27] and it could reinforce the positive effect of SMAT. The compressive residual stresses and work hardening allow to delay or even avoid the growth of surface cracks under fatigue loadings [28]. In some cases, the crack initiation site is shifted from the sample’s surface to the interior, typically beneath the compressive residual stresses and the work hardened layer [28], [29]. SMAT can also cause detrimental effect on fatigue performance, and this negative effect is mostly related to surface defects. Severe SMAT conditions may lead to surface discontinuities in the form of deep dimples, micro-cracks and folds, even though increased compressive residual stresses and deepened work-hardened layer can be obtained. These surface defects could give rise to stress concentration and consequently degrade the fatigue performance [30].
Another important aspect regarding the involvement of the effect of surface treatment is related to the applied stress amplitude. The results presented in the literature are sometimes controversial. For example, Zhou et al. [16] reported that the enhancement of the fatigue life due to shot peening is progressively reduced with a decrease in fatigue load. However, Kumar et al. [21] indicated that the improvement of fatigue life induced by ultrasonic shot peening became more effective with a decreased applied stress. The correlation between the effect of surface treatment on fatigue life and applied stress seems to be more obvious in high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regimes. Shiozawa and Lu [31] reported that the fatigue strength of a steel is significantly improved by shot peening in HCF regime, while the strengthening effect is extinguished and even decreased in VHCF regime. However, Trsko et al. [32] observed an opposite effect of severe shot peening on fatigue life, and they reported an improvement of about 23% on fatigue strength in VHCF regime but a much less evident effect in HCF regime. The different effects of surface treatment on fatigue performance might be attributed to the difference in damage mechanism between HCF and VHCF regimes. This difference has not yet been well identified to date, since most of the previous investigations concerning the effect of surface treatment on fatigue properties were performed in LCF and HCF regimes. While several studies focused on the effect of conventional shot peening on fatigue strength in VHCF regime [32], [33], [34], very limited work has been conducted to assess the effect of SMAT on VHCF properties.
In this paper, the fatigue properties of a 7075-T6 aluminium alloy processed by SMAT are studied through experiments in the range of HCF and VHCF. First, the material characteristics and the experimental procedure are briefly described in Section 2. Surface morphology, residual stress and hardness variation caused by SMAT, as well as fatigue test results are then presented in Section 3. In this section, the fatigue resistance of the samples treated with different SMAT conditions is compared to that of the untreated samples in order to highlight the effect of SMAT. Furthermore, fatigue cracking mechanisms are analysed by observing the fracture surfaces at both macroscopic and microscopic scales. In Section 4, the effects of SMAT are analysed and discussed by taking into account different factors possibly involved in the cracking process such as surface state, residual stresses, work-hardened layer, and stress amplitudes. Finally, some conclusions are drawn based on the results, the analyses and the discussion.
Section snippets
Material
A commercial 7075-T6 aluminium alloy was investigated in this work. It is a high-strength aluminium alloy typically used in aeronautical or light-weight structures. This alloy has a good mechanical resistance/weight ratio and thus a low environmental impact for transportation sector. In this work, the material was received in the form of cylindrical bar with a diameter of 15 mm, and its nominal chemical composition (in wt.%) is shown in Table 1. The microstructure observed using Optical
Surface morphology and roughness
In order to investigate the role of surface state in fatigue behaviour, surface roughness profiles of samples treated by electropolishing and SMAT were measured over 4 mm evaluation length in the middle of samples using contact profilometer (Mahr Marsurf M300). Typical surface profiles of samples are illustrated in Fig. 3. It can be seen that for the samples treated by SMAT (Fig. 3b and c), the maximum height of the profiles is much greater compared to the electropolished sample (Fig. 3a). This
Discussion and analysis
It is widely accepted that fatigue life and the corresponding damage process of samples treated by mechanical surface treatment techniques depend on surface condition, residual stresses and work hardening. Surface roughness is considered as a negative factor, as it generally causes local stress concentration that promotes crack nucleation from surface and thus degrades the fatigue performance of materials [46]. The effect of rough surface on fatigue properties can be treated as notch effect and
Conclusions
In this work, the fatigue properties of a 7075-T6 aluminium alloy treated by SMAT using two different conditions (Steel-2 and Steel-3 conditions) under HCF and VHCF loadings were investigated. The fracture surfaces were observed to study the change in damage mechanisms induced by SMAT, especially the cracking process of this alloy in crack initiation and early propagation stages. Based on the analyses of the results presented in this paper, the following conclusions can be drawn:
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SMAT
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.
Acknowledgements
Financial support from the University of Technology of Troyes (UTT), Grand Troyes and Conseil Départemental de l'Aube through Z. Sun’s Tenure Track position is greatly appreciated. H. Xue gratefully acknowledges the financial supports provided by Northwestern Polytechnical University (111 Project, Grant No. B13044). T. Gao would like to express his cordial gratitude to Analytical & Testing Centre of Northwestern Polytechnical University for SEM observations.
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