THE FATIGUE RESISTANCE OF PEENED 7050-T7451 ALUMINIUM ALLOY—REPAIR AND RE-TREATMENT OF A COMPONENT SURFACE

Abstract— Metal and glass-bead peening treatments, widely used throughout the aircraft industry to enhance the fatigue performance of many steels and titanium alloys, are now being routinely applied to high-strength aluminium-alloy components. This paper describes the effects of peening on the fatigue life of 7050 aluminium alloy material, which is representative of alloys used for many components in modern military aircraft. Using simulated service loading, two proposed peening/re-peening procedures were evaluated and compared with both the original peened surface and a simple hand-polished surface. The results show that optimisation of peening parameters to reduce surface damage can provide a substantial improvement in fatigue life over both the original peening treatment and the polished surface treatment, however, poor control of peening procedures, or unnecessary "overpeening" can lead to a relatively poor fatigue life. For re-peened surfaces, a procedure incorporating a polishing step, designed to repair any damage from the severe peening applied initially, gave the best fatigue performance. Results are discussed in relation to the stability of the residual surface stresses under fatigue loading, the surface roughness, and the number and types of defects introduced by the peening treatments.     

Recovery of fatigue life using laser peening on 2024‐T351 aluminium sheet containing scratch damage: The role of residual stress

The aim of the current work was to study the effect of laser shock peening (LSP) when applied to 2‐mm thick 2024‐T351 aluminium samples containing scratch‐like defects in the form of V‐shaped scribes 50 to 150 μm deep. The scribes decreased fatigue life to 5% of that of the pristine material. The effect of laser peening on fatigue life was dependent on the specifics of the peen treatment, ranging from further reductions in life to restoration of the fatigue life to 61% of pristine material. Fatigue life was markedly sensitive to near‐surface tensile residual stress, even if a compressive residual stress field was present at greater depth. Fatigue life after peening was also dependent on sample distortion generated during the peening process. Sample distortion modified local stresses generated by externally applied loads, producing additional life changes. Models based on residual stress intensity and crack closure concepts were successfully applied to predict fatigue life recovery.     

Influence of anodization on the fatigue strength of 7050-T7451 aluminium alloy

In recent years, with higher demand for improved quality and corrosion resistance, recovered substrates have been extensively used. Consequently residual stresses originated from these coatings reduce the fatigue strength of a component. Due to this negative influence occasioned by corrosion resistance protective coatings, an effective process like shot peening must be considered to improve the fatigue strength. The shot peening treatment pushes the crack sources beneath the surface in most of medium and high cycle cases due to the compressive residual stress field (CRSF) induced. The aim of this study was to evaluate the influence on the fatigue life of anodic films grown on 7050-T7451 aluminium alloy by sulphuric acid anodizing, chromic acid anodizing and hard anodizing. The influence on the rotating and reverse bending fatigue strength of anodic films grown on the aluminium alloy is to degrade the stress life fatigue performance of the base material. A consistent gain in fatigue life in relation to the base material was obtained through the shot peening process in coated specimens, associated to a residual stress field compressive near the surface, useful to avoid fatigue crack nucleation and delay or even stop crack propagation.     

Laser shock peening on fatigue crack growth behaviour of aluminium alloy

The effect of laser shock peening (LPS) in the fatigue crack growth behaviour of a 2024‐T3 aluminium alloy with various notch geometries was investigated. LPS was performed under a ‘confined ablation mode’ using an Nd: glass laser at a laser power density of 5 GW cm^?2. A black paint coating layer and water layer was used as a sacrificial and plasma confinement layer, respectively. The shock wave propagates into the material, causing the surface layer to deform plastically, and thereby, develop a residual compressive stress at the surface. The residual compressive stress as a function of depth was measured by X‐ray diffraction technique. The fatigue crack initiation life and fatigue crack growth rates of an Al alloy with different preexisting notch configurations were characterized and compared with those of the unpeened material. The results clearly show that LSP is an effective surface treatment technique for suppressing the fatigue crack growth of Al alloys with various preexisting notch configurations.     

Residual life prediction for healing fatigue damaged copper film by laser shock peening

A healing method for fatigue damage was studied by laser shock peening (LSP) with excimer laser for polycrystalline copper film. It is found that work hardening due to LSP could be responsible for the improvement of residual fatigue lives for the damaged and undamaged specimens by LSP, and the hardening degree for the damaged specimen by LSP is obviously higher than that for the undamaged specimen by LSP. In this paper, two basic mechanisms were identified. One is the dissipated energy enhancement mechanism, which improves the fatigue life caused by laser shock stress, and the other is the healing mechanism, which leads to a further improvement. Based on the two mechanisms, a residual fatigue life prediction method is proposed by the view of energy consumption before and after LSP. The predicted lives by the proposed method agree well with the experimental results.     

Fatigue life recovery in aluminium alloy aircraft structure

Abstract This paper presents the results of several DSTO research programmes which investigated the effectiveness of the fatigue life enhancement method used on RAAF F/A‐18 aircraft – glass bead peening. The research identified ways in which process improvement could enhance this effectiveness, and developed a procedure for mid‐life reworking of critical airframe parts to effectively restore the original fatigue life. The procedure included removing a very thin layer of material (and with it, any undesirable manufacturing features and accumulated fatigue cracking) from the surface. Further life recovery can be achieved, if required, by applying an optimized peening procedure. This process has been developed to allow restoration of fatigue life to critical airframe components which are thought to be accumulating fatigue crack damage faster than desired.     

A Critical Review of Laser Shock Peening of Aircraft Engine Components

Many aviation accidents are caused by the failure of aircraft engine components, and engine blades are especially vulnerable to high-cycle fatigue fracture in severe working environments as well as to impact damage caused by foreign objects. To address this problem, the United States took the lead and has been successful in implementing laser shock peening (LSP) as a surface treatment for aircraft engine components to enhance their fatigue performance. This review provides an overview of the development of LSP for use in treating aircraft engine components over the past three decades, with a brief introduction to the development of high-energy pulsed lasers for LSP. A particular focus of this review is on the limitations and challenges associated with the application of LSP for treating critical aircraft engine components. It is hoped that this review serves as a reference for future research and development that can lead to better performance of these components.     

喷丸成形压力对7B50−T7751铝合金力学性能的影响

目的 研究7B50−T7751铝合金在不同喷丸成形压力下力学性能的变化规律,探究喷丸成形压力对材料表面形貌、疲劳寿命及静力性能的影响。方法 在不同的喷丸成形压力(0.42、0.50 MPa)下对7B50−T7751铝合金进行处理,分析材料的表面形貌。在此基础上,通过细节额定疲劳基准值和截止值进行计算,并进行压缩试验,结合铝合金材料在喷丸前后应变层的位错密度和形态,分析喷丸成形压力对合金材料疲劳寿命和静力性能的影响。结果 与未喷丸试件相比,在0.42 MPa的成形压力下,合金材料的疲劳寿命和静力性能均有所提高。喷丸成形之后,材料表层引入了一定深度的残余压应力层,形成位错密度较大的加工硬化组织,阻碍裂纹扩展,宏观上提高了材料的强度。在0.50 MPa的成形压力下,材料表面更加粗糙,裂纹易在晶粒连接薄弱处萌生,导致合金材料的疲劳寿命有所降低。结论 随着喷丸成形压力的增大,合金材料的疲劳寿命先增大后减小,抗压强度有所增大。在0.50 MPa的成形压力下,部分裂纹易于在弹坑边缘萌生,在一定程度上会降低合金材料的疲劳强度。     

Investigation of the fatigue life of pre- and post-drilling hole in dog-bone specimen subjected to laser shot peening

The influence of processing sequence of laser shot peening (LSP) on the fatigue properties of fastener hole was investigated with finite element method and experiments. The results show that different processing sequences lead to different residual stress distributions and different fatigue lives. The compressive residual stresses (CRS) are squeezed into two-sided surface layers of fastener hole by two sided laser shot peening, and the ellipse CRS fields are found on both sided surfaces of sample. However, when the pre-drilling hole in dog-bone specimen is subjected to LSP, the tensile stresses appear at its mid-thickness region, while the CRS distribute in the entire thickness region of the post-drilling hole after LSP. The fatigue crack initiation of specimens treated by LSP stems from the subsurface layer of hole edge. The fatigue striation spacing of specimen with post-drilling hole after LSP is narrower in comparison with that of case with pre-drilling hole before LSP. The fatigue life of post-drilling hole is longer than that of the pre-drilling hole.     

Laser peening and shot peening effects on fatigue life and surface roughness of friction stir welded 7075-T7351 aluminum

The effects of laser peening, shot peening, and a combination of both on the fatigue life of Friction Stir Welds (FSW) was investigated. The fatigue samples consisted of dog bone specimens and the loading was applied in a direction perpendicular to the weld direction. Several laser peening (LP) conditions with different intensities, durations, and peening orders were tested in order to obtain the optimum peening parameters. The surface roughness resulting from various peening techniques was assessed and characterized. The results indicate a significant increase in fatigue life using LP compared to shot peening when tested on their native welded specimens.     

Improving the fretting performance of aero-engine tenon joint materials using surface strengthening

This paper summarises the regulation mechanism of the traditional and emerging surface strengthening treatments – mechanical shot peening (SP) and laser shock processing (LSP) treatment, respectively – on the fretting fatigue behaviour of the tenon joint materials of aero-engine cold-end parts, as surface strengthening treatment contributes to the improvement of service performance. From the point of view of the actual service environment of turbine components, this paper also expounds on the key problems faced by surface strengthening treatments in improving the mechanical properties of joint materials for aero-engine hot-end components. The application of LSP in the fretting fatigue life extension of turbine joint materials has strong feasibility and development prospects.     

高强度合金抗疲劳应用技术研究与发展

评述了超高强度钢、高强度Al合金和Ti合金表面完整性抗疲劳应用技术的研究和发展。高强度合金疲劳性能对应力集中敏感，不适当的加工工艺和切削热等造成的表面损伤和高拉应力使其疲劳和应力腐蚀性能损失殆尽。先进的表面完整性加工尤其是表面改性可显著提高疲劳性能，如激光冲击使7475-T761拉-拉疲劳寿命提高约89％，7075-T6裂纹扩展速率降低到原来的1／1500；超声喷丸使超高强度钢低周疲劳强度提高约50％，Ti7Al4Mo合金高周疲劳强度提高约15％；表面超硬化可使Vasco X-2M齿轮钢接触疲劳寿命提高30～35倍等。     

Einfluß der Oberflächenbeschaffenheit auf die Schwingfestigkeit von Aluminium-Druckguß

Effect of Surface Condition on Fatigue Strength of Die-Cast Aluminium Surface processing like milling or shot peening involves modifications of the surface and subsurface condition of a material that can be described by changes of the residual stresses, the hardness, and the surface roughness. Moreover, there is the possibility of introducing additional surface defects. In this paper the influence of these modifications on the fatigue behaviour of the aluminium die-casting alloy GD-AlSi8Cu3 is presented. S-N curves are determined for four surface conditions produced by milling, shot peening, and a combined treatment. The improvement of fatigue limit by shot peening is confirmed for this material. The additional effect of mean stresses is determined. Investigations of the fatigue fracture surfaces show that pores and pipes as well as surface defects induced by shot peening act as fatigue crack initiation sites.     

A comprehensive investigation on the effects of laser and shot peening on fatigue crack growth in friction stir welded AA 2195 joints

The effects of various surface treatment techniques on the fatigue crack growth performance of friction stir welded 2195 aluminum alloy were investigated. The objective was to reduce fatigue crack growth rates and enhance the fatigue life of welded joints. The crack growth rates were assessed and characterized for different peening conditions at a stress ratio (R) of 0.1, and 0.7. The surface and through-thickness residual stress distribution were also investigated and presented for the various regions in the weld. Tensile residual stresses introduced during the welding process were found to become significantly compressive, particularly after laser peening. The effect of the compressive stresses was deemed responsible for increasing the resistance to fatigue crack growth of the welds. The results indicate a significant reduction in fatigue crack growth rates using laser peening compared to shot peening and native welded specimens. This reduced fatigue crack growth rate was comparable to the base unwelded material.     

The effect of excimer laser surface treatment on the pitting corrosion fatigue behaviour of aluminium alloy 7075

An excimer laser (KrF) operating at a wavelength of 248 nm was used to modify the surface microstructure of 7075-T651 aluminium alloy. The aim was to improve both the corrosion resistance and the pitting corrosion fatigue resistance of the alloy by means of laser surface melting (LSM). The microstructure and the phases of the modified surface structure were analysed, and the corrosion behaviour of the untreated and the laser-treated specimens were evaluated by immersion test. The fatigue resistance of the 7075 alloy has been presented in the form of S/N curves.A microscopical examination and the transmission electron microscopy (TEM) study revealed that LSM caused a reduction both in number and size of constituent particles and a refinement of the grain structure within the laser melted zone. As a result, the corrosion resistance of the aluminium alloy was improved. There was a significant reduction in the number of corrosion pits and shallow attack occurred. The fatigue test results showed that under dry fatigue conditions, the total fatigue life of the laser treated specimens, in which the crack initiation period is of considerable significance, was lower than that of the untreated specimens. However, after shot peening, the fatigue life of the laser treated specimens was recovered. This was primarily attributed to the elimination of surface defects, but also be in part, due to the introduction of compressive residual stresses in the surface layer of the specimen. The fatigue resistance of the shot peened laser-treated specimens, tested in 3.5 wt% NaCl solution with 48 hrs prior immersion, was greater than the untreated specimens with an increase of two orders of magnitude in fatigue life. This was primarily due to the elimination of surface defects and the reduction of corrosion pits.     

复合喷丸提高等轴状TC4钛合金的疲劳性能

首先对等轴状TC4钛合金疲劳试样表面进行了不同时间的高能喷丸,制备出一定深度的纳米表层,然后采用小尺寸弹丸进行表面损伤修复喷丸,提高纳米表层质量,最后对不同喷丸状态的试样进行了疲劳试验。结果表明:复合喷丸使等轴状TC4钛合金的疲劳强度相比未喷丸状态提高了34%,在单纯高能喷丸纳米化方法的基础上进一步提高了12%;高能喷丸在试样表面形成的损伤阻碍了表层纳米化提高疲劳强度的效果,通过修复喷丸可以修复部分高能喷丸损伤。     

Effect of laser shock processing on the fatigue crack initiation and propagation of 7050-T7451 aluminum alloy

The aim of this paper was to identify the effect of laser shock peening (LSP) on the fatigue crack initiation and propagation of 7050-T7451 aluminum alloy. The laser shocked specimen in which residual compressive stress is mechanically produced into the surface showed a very high dislocation density within the grains. This was evident throughout the LSP region. The spacing among the fatigue striations in the LSP region was narrow, which indicated that LSP had an obvious inhibitory action to fatigue crack initiation and growth. In contrast, the region without LSP exhibited an extremely low dislocation density. And LSP improved 7050-T7451 alloy specimens’ fatigue intensity.     

Experimental results in fretting fatigue with shot and laser peened Al 7075-T651 specimens

This work focuses on determining the effect of shot and laser peening on fretting fatigue in the Al 7075-T651 alloy. These surface treatments generate a residual compressive stress field near the treated surface where contact under fretting fatigue produces high stress levels. The fretting fatigue resistance of shot and laser peened specimens was assessed in a series of tests involving measurements of the residual stress field, residual stress relaxation under the action of cyclic loads, the friction coefficient, surface roughness and material hardness. The obtained results are compared with those for untreated specimens. The tests show the beneficial effect of the compressive residual stresses and the improvement that surface roughness causes in fretting fatigue life, especially in shot peened specimens. Another important effect observed, is the partial residual stress relaxation produced during the fretting fatigue tests.     

On the application of laser shock peening for retardation of surface fatigue cracks in laser beam‐welded AA6056

The present study aims to investigate the extent to which the fatigue behaviour of laser beam‐welded AA6056‐T6 butt joints with an already existing crack can be improved through the application of laser shock peening. Ultrasonic testing was utilized for in situ (nondestructive) measurement of fatigue crack growth during the fatigue test. This procedure allowed the preparation of welded specimens with surface fatigue cracks with a depth of approximately 1.2 mm. The precracked specimens showed a 20% reduction in the fatigue limit compared with specimens without cracks in the as‐welded condition. Through the application of laser shock peening on the surfaces of the precracked specimens, it was possible to recover the fatigue life to the level of the specimens tested in the as‐welded condition. The results of this study show that laser shock peening is a very promising technique to recover the fatigue life of welded joints with surface cracks, which can be detected by nondestructive testing.     

Studies on fatigue life enhancement of pre-fatigued spring steel specimens using laser shock peening

SAE 9260 spring steel specimens after enduring 50% of their mean fatigue life were subjected to laser shock peening using an in-house developed 2.5 J/7 ns pulsed Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG) laser for studying their fatigue life enhancement. In the investigated range of process parameters, laser shock peening resulted in the extension of fatigue life of these partly fatigue damaged specimens by more than 15 times. Contributing factors for the enhanced fatigue life of laser peened specimens are: about 400 μm thick compressed surface layer with magnitude of surface stress in the range of −600 to −700 MPa, about 20% increase in surface hardness and unaltered surface finish. For laser peening of ground steel surface, an adhesive-backed black polyvinyl chloride (PVC) tape has been found to be a superior sacrificial coating than conventionally used black paint. The effect of repeated laser peening treatment was studied to repair locally surface melted regions and the treatment has been found to be effective in re-establishing desired compressive stress pattern on the erstwhile tensile-stressed surface.