Materials Express

2158-5849/2020/10/1032/008 Copyright © 2020 by American Scientific Publishers All rights reserved. doi:10.1166/mex.2020.1722 Printed in the United States of America www.aspbs.com/mex

Effect of shot peening on very high cycle fatigue of 2024-T351 aluminium alloy

Renhui Tian1, Jiangfeng Dong1,2,∗, Yongjie Liu1, Qingyuan Wang2,3,∗, and Yunrong Luo4 1School of Architecture and Environment, Sichuan University, Chengdu 610065, China 2Key Laboratory of Deep Earth Science and Engineering, Ministry of Education, Sichuan University, Chengdu 610065, China 3School of Mechanical Engineering, Chengdu University, Chengdu 610106, China 4School of Mechanical Engineering, Sichuan University of Science & Engineering, Zigong 643000, China

ABSTRACT To investigate the influence of shot peening (SP) on very high cycle fatigue (VHCF) performance of 2024- T351, the specimens with threeIP: surface 192.168.39.151 conditions On: were Fri, perf 01ormed Oct 2021 under 02:41:03 ultrasonic fatigue tests: mechanically- polished without peening (NP), ceramicCopyright: shot American peening Scientific (SP1), steelPublishers and glass mixed shot peening (SP2). The roughness, microhardness, residual stress,Delivered fractogra by phyIngenta measurement and scanning electron microscopy (SEM) were applied before fatigue test to characterize the effective layer induced by the peening treatment. For

Article the failed specimens, the fracture surface were analysed using SEM to study the mechanisms of fatigue crack propagation. In addition, the fatigue life curve in ultra-high cycle region continuously decreased in the three series of specimens. However, the experimental results revealed that fatigue strength improvement resulting from shot peening treatment was negligible in very high cycle regime. Furthermore, the stress intensity factor for the surface crack initiation (SCI) and interior crack initiation (ICI) was discussed based on quantitative anal- ãK ysis on the fracture surface. The average values of fish-eye for NP, SP1 and SP2 specimens are about 2.22, 1.48 and 1.61 MPa m1/2, respectively. · Keywords: Shot Peening, Residual Stress, Aluminum Alloy, Very High Cycle Fatigue.

1. INTRODUCTION In the aerospace and automotive industries, materials are Recently, the behaviour on fatigue of structural materi- commonly treated with cold working processes to improve als in the ultra-high cycle (107 N 109) regime has their fatigue properties [5–7]. The proper shot peening ≤ f ≤ obtained extensive attention due to the general requirement (SP) process is a useful method to improve the material to upgrade the efficiency and reliability of the designed fatigue strength [8–12], which is particular for aluminium machines and structures, and the increases in expected ser- alloys with shot-peened surface on the basis of special vice life beyond ultra-high cycles [1–4]. However, accom- requirements [13]. Furthermore, the shot peening process panied by the investigations of material in the very high induces surface modifications together with the follow- cycle fatigue (VHCF) region, other problems had arisen in ing three effects: (i) surface roughening to postpone the how to improve the fatigue performance of metal alloys in nucleation and early propagation of cracks [14], however, structural components and machines using more efficient the increase in surface roughness will result in the ear- and successful application methods. lier fatigue nuclear and reduce the fatigue life; (ii) surface strain hardening to increase the resistance in crack nucle-

∗Authors to whom correspondence should be addressed. ation and cracks propagation [14, 15]; (iii) compressive

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Table I. Mechanical and physical properties of 2024-T351 at room- indicated that no obvious effect of SP on the VHCF temperature. strength, and both subsurface and interior initiation of the Yield Tensile materials. However, Tanaka’s study [26] suggested that the Elastic Poisson’s Density stress strength Elongation SP treatment could reduce the internal fatigue life due to modulus (GPa) ratio (g/cm3) (MPa) (MPa) (%) the appearance of the tensile stress in the interior region 71.8 0.33 2.77 315 437 14.7 of the specimens. Currently, it is still unclear that how the SP treatment influences the VHCF strength and failure mechanisms. residual stress to retard the crack propagation due to an The present study aims to investigate the influence occur of crack closure stress [9, 10, 14, 16, 17]. of SP on the VHCF behaviour of 2024-T351 alu- In the previous studies, it was reported that SP can minium alloy and evaluate the possibility of fatigue improve the high cycle fatigue performance of steels, alu- strength improvements. Therefore, the specimens with minium alloys [13, 18–21] and titanium alloy [12]. There mechanically-polished surface (marked as NP) and shot- are limited investigations on the influence of SP on ultra- peening with two peening parameters subjected to ten- high cycle fatigue of metallic materials, such as low- sion and compression loading are comparatively tested alloy steel [22–24]. The previous work by Shiozawa [25] using ultrasonic fatigue testing machine with a stress ratio Article

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Fig. 1. Microstructure of AA 2024-T351: (a) Longitudinal section of NP specimen; (b) transverse section of NP specimen.

Fig. 2. Microstructure and section topographies near surface and surface micrography of AA 2024-T351 at different shot peenings: NP specimen (a) and (d); SP1 specimen (b) and (e); SP2 specimen (c) and (f).

Mater. Express, Vol. 10, 2020 1033 Materials Effect of shot peening on very high cycle fatigue of 2024-T351 aluminium alloy Express Tian et al.

Table II. The surface treatment parameters of fatigue specimen and surface roughness.

Shot Diameter of Almen Air Label type bead (Œm) intensity (mmA) pressure (MPa) Coverage (%) Ra (Œm)

NP – – – – – 0.443 SP1 Z150 300 0.1 0.2 200% 2.123 0.32 ± SP2 S110 GB250 300 250 0.1 0.15 0.25 200% 200% 2.734 0.43 + + + + ±

of R 1. In addition, the effect of roughness, residual was firstly cast using at an Almen intensity of 0.1 mmA, stress=− and the location of crack initiation on the VHCF then the glass bead GB250 was carried out at an Almen behaviour of 2024-T351 are analysed. Furthermore, the intensity of 0.15 mmA with a coverage of 200%. fatigue fracture mechanisms based on the analysis of scan- ning electron microscopy (SEM) fractographys and evalu- 2.3. Microhardness and Residual Stress ation of stress intensity factor are discussed. The depth distributions of micro-Vickers hardness on the cross transverse plane for all the specimen are shown 2. MATERIALS AND METHODS in Figure 3, under a load of 25 g and a dwell time of 15 s. The residual stress field for all specimens mea- 2.1. Materials sured by X-ray diffraction (XRD) analysis is shown in The material used in this work is 2024-T351 aluminium Figure 4. It can be seen that the residual compressive stress alloy, with a chemical composition (in % weight) of Si decreased with increase the depth from the sample sur- 0.11%, Cu 4.43%, Mg 1.36%, Al 93.11%, Mn 0.6%, and face, and approached approximately zero when the depth Fe (balance). The mechanical properties of 2024-T351 are presented in Table I. The optical micrographs of the typically-etched microstructure using “Keller” reagent are shown in Figure 1. The microstructure micrograph on SP specimens and surface micrographs for all the specimens are shown in Figure 2. IP: 192.168.39.151 On: Fri, 01 Oct 2021 02:41:03 Copyright: American Scientific Publishers Delivered by Ingenta 2.2. Surface Treatment Three series of AA 2024-T351 specimens were treated Article by shot peening processes with different parameters, as shown in Table II. One series of specimens with NP was set as control. One series of SP1 was treated by shot peen-

ing with a ZrO2 ceramic shot (0.3 mm in diameter) and an Almen intensity of 0.1 mmA. One series of SP2 was treated with mix shot peening, in which steel shot S110

Fig. 4. Residual stress distributions of the specimen treated by two types of shot peening.

Fig. 5. The basic components of the ultrasonic fatigue system and test Fig. 3. In-depth microhardness profiles of samples. setup.

1034 Mater. Express, Vol. 10, 2020 Effect of shot peening on very high cycle fatigue of 2024-T351 aluminium alloy Materials Tian et al. Express

Table III. The coefficients for regression curve of alloy 2024.

Surface treatment type ‘f′ (MPa) b NP 571 0.105 − SP1 675 0.118 − SP2 406 0.095 −

Basquin function as [27]:

b Fig. 6. Geometry of the specimen used for ultra-fatigue tests (in mm). ‘ ‘ ′ 4N 5 (1) a = f × f

where ‘a is the stress amplitude, Nf is failure fatigue at around 140 Œm below the surface. For the samples cycles, ‘a′ is the toughness coefficient of fatigue, and b is peened by the ceramic beam and mix shot peening, such the exponent of the fatigue life curve. Table III lists the of the residual compressive stress at surface is 150 MPa regression coefficients of all series of specimens. and 165 MPa, respectively. ∼ ∼ It can be seen from Figure 7 that a continuously descending pattern of S–N curves for all the specimens, 2.4. Fatigue Tests except for that of with a stress amplitude of 70 MPa at The fatigue tests in the study were performed in air by the fatigue cycles from 106 to 109. In addition, the results using an ultrasonic fatigue machine of USF-2000 under the also indicate that the fatigue strength for specimens with frequency of 20 kHz and the stress ratio of R 1 at room SP treatment lower than that of NP ones. The decreasing =− temperature. Figure 5 shows the basic components for such amplitudes of fatigue strength as compared with that of ultrasonic fatigue used and test setup in the research. The NP specimens are summarized in Table IV. maximum stress under fatigue test will exists at the middle of the specimen, and the maximum displacement will be 3.2. Fracture Mechanism Analysis Article measured at the free end of the specimens. In addition, For the specimens subjected to fatigue test, the fracture compressed air was used for cooling.IP: 192.168.39.151 The geometry of On: the Fri,surfaces 01 Oct of2021 all failed02:41:03 samples with NP and SP treatments Copyright: American Scientific Publishers fatigue specimens satisfied with resonance conditionDelivered was bywere Ingenta observed by scanning electron microscopy. In gen- shown in Figure 6. eral, two types of crack initiation mechanisms, i.e., surface crack initiation (SCI) and interior crack initiation (ICI), 3. RESULTS AND DISCUSSION were observed during the test. The typical surface initiations of the 2024-T351 alu- 3.1. S–N Curves minium alloy with NP treatment are shown in Figure 8. S N Figure 7 shows the – results for the specimens of The SCI morphology in an NP sample is shown in NP, SP1 and SP2. By means of the least square method Figure 8(a), with a stress amplitude of 91 MPa after 8.83 (Eq. (1)), the results could be approximated by using the 106 cycles. The high magnification of the crack origin site× and subtle fatigue striations in a stable crack growth area are shown in Figures 8(b) and (c). Figure 9 shows the fracture surfaces of the SP1 ceramic shot peened specimen at a stress amplitude of 90 and 80 MPa. It can be seen that the cracks are initiated from the surface at a stress amplitude of 90 MPa (Fig. 9(a)). Contrary to stress amplitude of 80 MPa, crack initiation site was located at subsurface with a depth of 100 Œm,

Table IV. Fatigue strength for N 107, 108 and 109 based on the = regression curves.

Surface Fatigue Fatigue Fatigue treatment strength for strength for strength for type N 107 (MPa) N 108 (MPa) N 109 (MPa) = = = NP 105 82 65 SP1 100 76 58

Decrease ã‘a 5 6 7 Fig. 7. S–N diagrams of 2024-T351 aluminium alloy under NP, SP1 SP2 88 70 56 Decrease 㑠17 12 9 and SP2 treatments (R 1, f 20 kHz, T 20 5 žC). a =− ≈ = ±

Mater. Express, Vol. 10, 2020 1035 Materials Effect of shot peening on very high cycle fatigue of 2024-T351 aluminium alloy Express Tian et al.

Fig. 8. SEM images of NP specimen failed by surface crack initiation, N 8.83 106 cycles, ‘ 91 MPa: (a) Low magnification, (b) enlargement f = × a = of crack origin in (a), (c) the minor striations of crack source region.

as shown of a rectangle in Figure 9(b). This was caused quasi-circular region, which is called a “fish-eye” area. by the surface hardened layer thickness about 100 Œm in Figure 11(b) shows a high magnification of the “fish-eye” depth, which lead to the crack tend to originate from the area, in which some voids exist. interior of the sample. Figure 10 shows the fracture surface with some typical 3.3. Stress Intensity Factor surface crack initiation sites, crack origin sites are marked According to the defect type and size (Ainc), the initiation as dotted black line. It is shown that much more number of sites of fatigue crack could be confirmed and classified into surface crack initiation sites in the series of SP1 than that surface defect, inclusion defect, void defect, and discon- of NP specimen. This may be related to the lower stress tinuous microstructures defect, etc. In general, the fracture intensity factor at the crack tips of SP1 than that of NP surface for the specimens could be set as surface and inte- samples. rior failure based on the quantitative analysis of fractogra- Typical interior crack initiation of the specimens phy. Figure 12(a) shows the typical fracture surface of the is shown in Figure 11. FigureIP: 11(a) 192.168.39.151 shows a black On: Fri, 01 Oct 2021 02:41:03 Copyright: American Scientificfatigue surface Publishers failure, together with the depth of the crack Delivered by Ingenta Article

Fig. 9. Details of fatigue crack initiation of SP1 specimen, (a) ‘ 90 MPa, N 6.78 107 cycles; (b) ‘ 80 MPa, N 2.73 107 cycles. a = f = × a = f = ×

Fig. 10. SEM fractographys showing fracture surface of the specimens: (a) NP, ‘ 82 MPa, N 9.52 107 cycles; (b) SP1, ‘ 100 MPa, a = f = × a = N 8.17 106 cycles. f = × 1036 Mater. Express, Vol. 10, 2020 Effect of shot peening on very high cycle fatigue of 2024-T351 aluminium alloy Materials Tian et al. Express

Fig. 11. SEM micrographs showing of 2024-T351 NP specimen with interior crack initiation, N 4.48 108 cycles, ‘ 73 MPa: (a) Overall f = × a = view; (b) enlargement of the crack origin marked by dash lines in (a).

(a) (b) Article

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Fig. 12. Schematic diagrams of fracture morphology. (a) Surface failure; (b) interior failure. nucleation area (CNA) and the depth of the stable growth or NP specimens and 2.9 to 3.4 MPa m1/2 for SP1 · area (SGA). Also, the depth of GBF (DGBF), GBF radius specimens, respectively. In the ICI mode, the ãKfish eye − (RGBF) and fish-eye radius (Rfish eye) were demonstrated values for NP, SP1 and SP2 specimens are in aver- and illustrated in Figure 12(b). − ages of 2.22, 1.48 and 1.61 MPa m1/2, respectively. As known from the above results in fracture mecha- · nisms analysis, SCI and ICI crack initiation schematics are observed from the fractography test. In conformity to Murakami [28–30], the stress intensity factors could be calculated by using the following formula:

ãK C1ã‘q√area (2) = where the parameter of C1 is set as 0.65 and 0.50 for the surface and internal defects, respectively. The parameter of 㑠is the fatigue stress amplitude, and the parameter of √area is the square root of the projected area of the orig- inating inclusion failure plane perpendicular to the main principal stress. Figure 13 shows the correlation between the calcu- lated results of ãK values and the fatigue life Nf . The stress intensity factor ãK significantly changes with the increase in fatigue life. In the SCI mode, the variation Fig. 13. The relationship of ãK value and the fatigue life Nf in all of ãK is in the range of 2.41 to 3.38 MPa m1/2 series specimens. SCI · Mater. Express, Vol. 10, 2020 1037 Materials Effect of shot peening on very high cycle fatigue of 2024-T351 aluminium alloy Express Tian et al.

The value of ãKfish eye in NP specimens is slightly an aluminum alloy: Role of sheet thickness. Engineering Fracture higher than that of SP− specimen, which indicates that Mechanics, 180, pp.105–114. the NP specimens have higher fatigue strength than 8. Tekeli, S., 2002. Enhancement of fatigue strength of SAE, 9245 steel by shot peening. Materials Letters, 57(3), pp.604–608. that of SP specimen at the ultra-high cycle fatigue 9. Wang, S.P., Li, Y.J., Yao, M. and Wang, R.Z., 1998b. Fatigue limits zone. of shot-peened metals. Journal of Materials Processing TecRehnol- ogy, 73(1–3), pp.57–63. 10. Wang, S.P., Li, Y.J., Yao, M. and Wang, R.Z., 1998a. Compressive 4. CONCLUSIONS residual stress introduced by shot peening. Journal of Materials Pro- (1) Surface hardness and residual compressive stress in cessing Technology, 73(1–3), pp.64–73. 11. 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Received: 16 October 2019. Accepted: 19 February 2020. Article

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