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Effect of Cold Expansion on High Cycle Fatigue of 7A85 Aluminum Alloy Straight Lugs
Rare Metal Materials and Engineering Volume 44, Issue 10, October 2015 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2015, 44(10): 2358-2362.
ARTICLE
Effect of Cold Expansion on High Cycle Fatigue of 7A85 Aluminum Alloy Straight Lugs Wen Shizhen,
Liu Cuiyun,
Wu Ruolin,
Ma Chaoli
Key Laboratory of Aerospace Advanced Materials and Performance of Ministry of Education, Beihang University, Beijing 100191, China
Abstract: The cold expansion method was proposed to improve the high cycle fatigue (HCF) property of 7A85 aluminum alloy straight lugs. The microstructure, the microhardness and the residual stress near the hole wall of cold expanded lugs were investigated. The results indicate that a plastic deformation layer with about 150 μm in thickness is formed on the surface of the hole wall. Within the plastic deformation layer, the microhardness and the compressive residual stress are high, and the grains are stretched along the moving direction of the mandrel and compressed perpendicular to it. The fatigue life and fatigue strength were found to be improved significantly by cold expansion (CE). The fatigue crack initiation and propagation of cold expansion specimens were analyzed compared with those of the specimens without cold expansion (WCE). The mechanism of the improved fatigue property for cold expanded 7A85 aluminum alloy straight lugs was discussed. Key words: cold expansion; 7A85 Al alloy; lugs; high cycle fatigue (HCF)
Weight saving and reliability are two primary goals in aircraft. The high specific strength and fatigue property of materials are favorable to structural design. Al-Zn-Mg-Cu alloys (7xxx aluminum alloy) are a good choice for structural materials in aerospace due to their low density, excellent stress corrosion resistance and fracture toughness. Recently, a new high strength thick plate 7xxx aluminum alloy named 7A85, with a similar composition with 7085, has been developed in China. 7A85 alloy shows excellent hardenability besides the advantages above for 7xxx aluminum alloy. If its fatigue property can also meet design requirement, it will be a perfect candidate for spar and joint structure manufacture of aircraft. There are some constraints which may reduce the fatigue property of 7A85 structure. To improve wear and corrosion resistance for aluminum alloy components in the aircraft industry, anodization is an essential technology [1,2]. But Chaussumier et al [3] pointed out that multiple cracks may initiate from the surface flaws and defects caused by anodization, and the fatigue property will be reduced
substantially. On the other hand, for the joint structure, for example lugs, the holes are often drilled into plates in order to accommodate the fasteners. Fatigue cracks tend to initiate from the hole because of stress concentrations, and the fatigue property is quite different from the unnotched specimens. Therefore, not only the high performance of material is required, but also special treatment is used to improve the reliability of the joint component [4-6]. It is well accepted that a compressive residual stress near the surface of fatigue weak area is beneficial to improve the fatigue property, and various surface treatment techniques have been used to introduce the compressive residual stress. Cold expansion (CE), having been used in aeronautical industry for more than 40 years [7,8], can evidently enhance the fatigue life of fastener joints with the benefit of no addition of extra material and cost saving. But it is difficult to improve the fatigue property by deformation strengthening if the strength of material is high. For high strength aluminum alloy, it is not obvious whether cold expansion can significantly decrease the influence of crack
Received date: October, 14, 2014 Foundation item: National Key Basic Research Development Program of China (“973” Program) (2012CB619503) Corresponding author: Wen Shizhen, Master, Key Laboratory of Aerospace Advanced Materials and Performance of Ministry of Education, School of Materials Science and Engineering, Beihang University, Beijing 100191, P. R. China, Tel: 0086-10-82339772, E-mail: [email protected] Copyright © 2015, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
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Wen Shizhen et al. / Rare Metal Materials and Engineering, 2015, 44(10): 2358-2362
initiation induced by anodization and resist stress concentrations of joint structures [2]. In this paper, we studied the effect of cold expansion on the high cycle fatigue (HCF) property of 7A85 lugs.
1
Experiment
7A85 forged Al alloy was provided by the Southwest Aluminum (Group) Co., Ltd, China. The chemical composition of this alloy (wt%) is Zn 7.64, Mg 1.41, Cu 1.57, Zr 1.00, Fe 0.56, Si 0.013, Ti 0.54 and Al Bal. The straight lugs were firstly machined from the forged alloy. A part of straight lugs were then processed by cold expansion (CE), in which an oversized mandrel was forced through the holes of the lugs, and the holes were expanded. The degree of cold expansion (DCE) can be calculated as follows: D - d0 (1) DCE= 100% d0 where, D represents the diameter of the oversized mandrel and d0 is the diameter of the hole before CE. In this study, the diameter of the oversized mandrel is 20 mm, and the DCE is 0.77%. All the specimens were anodic oxidized after being machined into the dimension shown in Fig.1a. S amp l es f o r mi cro s t ru ct u re an d mi cro h a rd n es s measurements were cut from the sections near the hole, shown in Fig.1b. Polishing was performed with SiC sand paper and electrochemical etched in a 5vol% HBF4 solution. a
155
10
45
Φ20
b Cross-section selected to be tested and observed
Y ZX
Fig.1
Shape of the lug specimen: (a) planar graph (all dimensions in mm) and (b) 3D sketch of 1/4 lug
The hardness profile along the Y axis was measured by FM-800 microhardness tester. The applied load and holding time were 0.098 N and 10s, respectively. The residual stress along the Y axis induced by CE was measured layer by layer with Stresstech X3000 X-ray Stress Analyzer. Fatigue tests of the straight lugs were carried out using a conventional high frequency fatigue testing machine under a sinusoidal wave load. The stress ratio (R=σ min/σ max) was 0.06, and the frequency was from 150 Hz to 170 Hz. Six stress levels were selected, and three specimens at least were tested for each stress level. The tests were not finalized until the specimens fractured or the cycle number reached 1 ×106. After fatigue test, the fractures were cut by an electrical discharging machine and cleaned with acetone for fractographic analysis. The fracture surfaces were examined with optical microscopy and scanning electron microscope (SEM).
2
Results and Discussion
2.1 Microstructure Optical microstructures of the samples after etching are shown in Fig.2. They show that the microstructure near the hole of the sample without cold expansion (WCE, Fig.2b) is almost the same as the original microstructure (Fig.2a). After the CE process (Fig.2c), the microstructure near the surface region of the hole is much different from that of the original region. Because of the compression stress and shear stress caused by the oversized mandrel during the CE, the microstructure on the surface of the hole is deformed due to the mandrel movement. It can be found that a plastic deformation layer with about 150 μm in thickness is formed in the CE sample (Fig.2c). Within the plastic deformation layer, the grains are stretched along the moving direction of the mandrel and compressed perpendicular to it. Furthermore, the grain boundaries in this region become more circuitous as compared with the WCE sample (Fig.2b). The compressed grains and circuitous grain boundaries can decrease the tendency for crack initiation and propagation and improve the fatigue property. 2.2 Microhardness The Vickers hardness measured near the hole of CE and WCE specimens are presented in Fig.3. The hardness near the surface of CE is much higher than that of WCE, and the width of hardened region is about 150 μm, which is consistent with the thickness of plastic deformation layer (Fig.2c). It is expected that the hardened layer will improve the abrasive resistance of lugs and prolong the fatigue life. 2.3 Residual stress The crack initiation is the most important in high cycle fatigue. For small unnotched samples, the initiation life for a 250 μm crack can even occupy more than 90% of the total
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a
c
b
100 µm Fig.2
Optical microstructure of original material (a), near the hole of WCE specimens (b), and near the hole of CE specimens (c)
CE WCE
180 176 172 168 164 160 156
N f =C( max max0 )
0 50 100 150 200 Distance From Sureface of Hole, d/m
Vickers hardness profile along the Y axis (Fig. 1b) from the hole wall
life[9]. However, because of the stress concentration, crack initiation life for notched samples may be shortened evidently. Hence, the increase of crack initiation life by strengthening the stress concentration region is the most feasible method to improve the total fatigue life. The residual stress near the hole of lugs is important since the applied stress is typically high during the service process. High compressive residual stress delays the micro-crack formation by countering applied tensile stress at the surface. Fig.4 shows the distribution of residual stress along the Y axis of the CE specimen. It can be seen that the compressive residual stress is distributed around the hole. The stress near the hole is －80 MPa, which is expected to counter the applied tensile stress significantly, especially at the low stress levels. The compressive residual stresses increase first and then decrease with the increase of the distance to the surface of hole, and the highest compressive residual stress is 220 MPa. Within the tested region of 400 μm, the compressive stress is higher than 80 MPa. It has been reported that the initial crack length is between 250 and 500 μm for notched samples [9,10]. Therefore, the compressive residual stress layer is expected to delay the micro-crack formation.
2.4 Fatigue property The S-N curve of the WCE and CE specimens are shown in Fig5. The three-parameter power-law equation (2) is used
( m)
(2) This fit assumes power-law dependence of maximum stress (σ max) and cycles to failure (Nf) [13]. σmax0 represents maximum stress of cyclic load that can be applied to the material without causing fatigue failure. The fitting results, listed in Table 1, show that the CE lugs have higher fatigue limit than WCE ones. The gains in life are then evaluated by the life improvement factor (LIF): 0 Residual Stress, r/MPa
Fig.3
184
-40 -80 -120 -160 -200 -240 0 100 200 300 Distance from Hole Edge, d/m
Fig.4
400
Distribution of residual stress along the Y axis of CE specimen
Maximum Stress, max/MPa
Vickers Hardness, HV/×10 MPa
to fit the measured data [11,12].
160 120 80 40 10
Fig.5
CE WCE
4
5
10 10 Number of Cycles to Failure, Nf
6
S-N curves showing the effect of CE on the high cycle fatigue property
of
straight
lugs
(arrows
indicate
specimens without failure, and lines show 50% fracture probability)
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Wen Shizhen et al. / Rare Metal Materials and Engineering, 2015, 44(10): 2358-2362
LIF=
N CE N WCE
(3)
which is defined as the ratio of the geometric mean lifetimes for the two conditions at a given stress level. The results indicate that the life of CE lugs is much higher than that of WCE lugs at all stress levels, especially at lower stress levels. For instance, the value of LIF is 1.7 at 100 MPa, higher than 2.7 at 50 MPa. These results also show that cold expansion is an effective method to improve the fatigue strength of 7A85 alloy straight lugs. The CE specimens exhibit fatigue strength of 100 MPa after 10 5 cycle, while for the WCE specimens, it is 80 MPa. That is to say, the fatigue strength is increased by 25%. At 5×105 cycle, the fatigue strength is increased by 25%. It is also worth to be mentioned that because the compressive residual stress reduces the surface sensitivity to the flaws and defects, CE decreases the scattering of the tested fatigue data.
2.5 Fractography The fractures for each testing condition were examined to investigate the crack initiation. All the cracks initiate from the smallest cross section around the hole. The WCE specimens only exhibit surface crack initiation, while surface or near-surface initiation is found in the CE specimens. The crack initiations of the WCE specimens distributed along the hole wall, but for all the CE specimens, the crack initiated in one corner of hole wall (Fig.6f), which is attributed to the compressive residual stress in the specimen gradually increased from the entrance to the exit of oversized mandrel during CE process [14-16]. These results indicate that CE can affect the location of the crack initiation. C. Giummarra reported that because surface flaws and defects were sensitive to stress, multiple cracks often initiated at micro-cracks of the anodized layer [17]. In the present work, multiple crack initiations were found in the WCE specimens. As shown in Fig.6d four crack initiations were observed in the WCE specimen tested at 100 MPa. But only single crack initiation was found in the CE specimen. These results indicate that CE effectively reduces the possibility of crack initiation. A number of fatigue striations are observed in the stable crack propagation region of both CE and WCE specimens, shown in Fig.7. Striation is the typical feature for ductile alloys. Each striation is created by a single cycle of fatigue loading [18,19] , that is to say, the width (a) of striation indicates the speed of crack propagation (da/dN). The width of the fatigue striation increases with the propagation direction of cracks in both CE and WCE conditions. But the striations on WCE fractures are wider than those on CE fractures at the same load condition and crack length. This result indicates the CE can retard the crack propagation at the stable crack propagation stage and improve the fatigue life of the lugs.
a
Hole wall
b
Hole wall
c
Hole wall
Crack initiation
2 mm d 1 mm e
Crack initiation
1 mm f
Crack initiation
1 mm Fig.6
Typical SEM images of fracture surface: (a) WCE, 100 MPa, 46 900 cycle; (b) WCE, 70 MPa, 290 700 cycle; (c) CE, 60 MPa, 642 300 cycles specimens (the high magnification images near the hole wall of Fig.6d for Fig.6a, Fig.6e for Fig.6b, Fig.6f for Fig.6c)
Table 1
Slope exponents (m), constants C and σmax0 used to fit
the S-N data Specimen m CE 2.11 WCE 2.65
C 7.87×108 1.18×1010
σmax0/MPa 28.51 3.74×10-13
a
b
c
d
e
f
2 µm Fig.7
Fatigue striations in stable crack propagation region of WCE lug, σmax=60 MPa, Nf=124 700 cycle: (a) a= 5.28 mm, da/dN =0.12 μm; (b) a= 7.7 mm, da/dN= 0.53 μm; (c) a= 9.9 mm, da/dN= 0.71 μm. Fatigue striations in stable crack propagation region of CE lug, σmax=60 MPa, Nf=642 300 cycle: (d) a= 5.8 mm, da/dN=0.12 μm; (e) a=7.2 mm, da/dN= 0.18 μm; (f) a= 10.4 mm, da/dN= 0.27 μm
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3
Conclusions
1) A plastic deformation layer of about 150 μm in depth will be formed after cold expansion. The microhardness of cold expansion (CE) specimens is higher than that of specimens without cold expansion (WCE) in the plastic deformation layer. And a compressive residual stress at least 80 MPa can be introduced by cold expansion in the near-surface of the hole. 2) The fatigue life and strength are improved significantly by the cold expansion. At a high stress of 100 MPa, CE improves the fatigue life by a factor of 1.7 compared with the WCE lugs. And at a low stress of 50 MPa, the factor is higher than 2.7. At 1×10 5 cycles, the strength is increased by 25% and at 5×10 5 cycles, the fatigue strength is increased by 39%. 3) CE affects the location and amount of crack initiations. At the same load and crack length, the striations of WCE fractures are wider than those of CE.
6
Materials and Engineering [J], 2007, 36(4): 597 (in Chinese) 7
Luong H, Hill M R. Materials Science and Engineering A[J],
1974, 6: 275 Chakherlou T N, Shakouri M, Aghdam A B et al. Materials &
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Glinka G, Stephens R I. ASTM, STP[J], 1983, 791: 427
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Rateick R G, Binkowski T C, Boray B C. Journal of Materials
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Chaussumier M, Mabru C, Shahzad M et al. International Fan Juan, Li Fuguo, Li Jiang et al. Rare Metal Materials and Engineering [J], 2012, 41(6): 978 (in Chinese)
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Fu Huimin, Gao Zhentong. Acta Aeronautica et Astronautica Sinica [J], 1988, 9: 339 (in Chinese)
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MS Handbook-MIL-HDBK-5D: Metallic Materials and Elements for Aerospace Vehicle Structures[M]. Washington: US Department of Defense, 1984
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Yanishevsky M, Li G, Shi G et al. Engineering Failure Analysis[J], 2013, 30: 74
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Shi G, Yanishevsky M, Li G. Aging Aircraft 2007 Conference [C]. Montana: American Institute of Aeronautics and Astronautics, 2007
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Mann J Y, Sparrow J G, Beaver P W. International Journal of Fatigue[J], 1989, 11: 214
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Giummarra C, Zonker H R. ICAF 2005 Proceedings[C]. New York: ACM, 2005
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Furukawa K, Murakami Y, Nishida S. International Journal
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Bowles C Q, Broek D. International Journal of Fracture
of Fatigue[J], 1998, 20: 509
Journal of Fatigue[J], 2012, 48: 205 4
Weibull W. Fatigue Testing and Analysis of Results[M]. London: Pergamon Press,1961: 37
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Science Letters[J], 1996, 15: 1321 3
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References 1
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Mechanics[J], 1972, 8: 75
Gao Yukui. Rare Metal Materials and Engineering[J], 2004, 33(9): 1000 (in Chinese)
2362
ARTICLE
Effect of Cold Expansion on High Cycle Fatigue of 7A85 Aluminum Alloy Straight Lugs Wen Shizhen,
Liu Cuiyun,
Wu Ruolin,
Ma Chaoli
Key Laboratory of Aerospace Advanced Materials and Performance of Ministry of Education, Beihang University, Beijing 100191, China
Abstract: The cold expansion method was proposed to improve the high cycle fatigue (HCF) property of 7A85 aluminum alloy straight lugs. The microstructure, the microhardness and the residual stress near the hole wall of cold expanded lugs were investigated. The results indicate that a plastic deformation layer with about 150 μm in thickness is formed on the surface of the hole wall. Within the plastic deformation layer, the microhardness and the compressive residual stress are high, and the grains are stretched along the moving direction of the mandrel and compressed perpendicular to it. The fatigue life and fatigue strength were found to be improved significantly by cold expansion (CE). The fatigue crack initiation and propagation of cold expansion specimens were analyzed compared with those of the specimens without cold expansion (WCE). The mechanism of the improved fatigue property for cold expanded 7A85 aluminum alloy straight lugs was discussed. Key words: cold expansion; 7A85 Al alloy; lugs; high cycle fatigue (HCF)
Weight saving and reliability are two primary goals in aircraft. The high specific strength and fatigue property of materials are favorable to structural design. Al-Zn-Mg-Cu alloys (7xxx aluminum alloy) are a good choice for structural materials in aerospace due to their low density, excellent stress corrosion resistance and fracture toughness. Recently, a new high strength thick plate 7xxx aluminum alloy named 7A85, with a similar composition with 7085, has been developed in China. 7A85 alloy shows excellent hardenability besides the advantages above for 7xxx aluminum alloy. If its fatigue property can also meet design requirement, it will be a perfect candidate for spar and joint structure manufacture of aircraft. There are some constraints which may reduce the fatigue property of 7A85 structure. To improve wear and corrosion resistance for aluminum alloy components in the aircraft industry, anodization is an essential technology [1,2]. But Chaussumier et al [3] pointed out that multiple cracks may initiate from the surface flaws and defects caused by anodization, and the fatigue property will be reduced
substantially. On the other hand, for the joint structure, for example lugs, the holes are often drilled into plates in order to accommodate the fasteners. Fatigue cracks tend to initiate from the hole because of stress concentrations, and the fatigue property is quite different from the unnotched specimens. Therefore, not only the high performance of material is required, but also special treatment is used to improve the reliability of the joint component [4-6]. It is well accepted that a compressive residual stress near the surface of fatigue weak area is beneficial to improve the fatigue property, and various surface treatment techniques have been used to introduce the compressive residual stress. Cold expansion (CE), having been used in aeronautical industry for more than 40 years [7,8], can evidently enhance the fatigue life of fastener joints with the benefit of no addition of extra material and cost saving. But it is difficult to improve the fatigue property by deformation strengthening if the strength of material is high. For high strength aluminum alloy, it is not obvious whether cold expansion can significantly decrease the influence of crack
Received date: October, 14, 2014 Foundation item: National Key Basic Research Development Program of China (“973” Program) (2012CB619503) Corresponding author: Wen Shizhen, Master, Key Laboratory of Aerospace Advanced Materials and Performance of Ministry of Education, School of Materials Science and Engineering, Beihang University, Beijing 100191, P. R. China, Tel: 0086-10-82339772, E-mail: [email protected] Copyright © 2015, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
2358
Wen Shizhen et al. / Rare Metal Materials and Engineering, 2015, 44(10): 2358-2362
initiation induced by anodization and resist stress concentrations of joint structures [2]. In this paper, we studied the effect of cold expansion on the high cycle fatigue (HCF) property of 7A85 lugs.
1
Experiment
7A85 forged Al alloy was provided by the Southwest Aluminum (Group) Co., Ltd, China. The chemical composition of this alloy (wt%) is Zn 7.64, Mg 1.41, Cu 1.57, Zr 1.00, Fe 0.56, Si 0.013, Ti 0.54 and Al Bal. The straight lugs were firstly machined from the forged alloy. A part of straight lugs were then processed by cold expansion (CE), in which an oversized mandrel was forced through the holes of the lugs, and the holes were expanded. The degree of cold expansion (DCE) can be calculated as follows: D - d0 (1) DCE= 100% d0 where, D represents the diameter of the oversized mandrel and d0 is the diameter of the hole before CE. In this study, the diameter of the oversized mandrel is 20 mm, and the DCE is 0.77%. All the specimens were anodic oxidized after being machined into the dimension shown in Fig.1a. S amp l es f o r mi cro s t ru ct u re an d mi cro h a rd n es s measurements were cut from the sections near the hole, shown in Fig.1b. Polishing was performed with SiC sand paper and electrochemical etched in a 5vol% HBF4 solution. a
155
10
45
Φ20
b Cross-section selected to be tested and observed
Y ZX
Fig.1
Shape of the lug specimen: (a) planar graph (all dimensions in mm) and (b) 3D sketch of 1/4 lug
The hardness profile along the Y axis was measured by FM-800 microhardness tester. The applied load and holding time were 0.098 N and 10s, respectively. The residual stress along the Y axis induced by CE was measured layer by layer with Stresstech X3000 X-ray Stress Analyzer. Fatigue tests of the straight lugs were carried out using a conventional high frequency fatigue testing machine under a sinusoidal wave load. The stress ratio (R=σ min/σ max) was 0.06, and the frequency was from 150 Hz to 170 Hz. Six stress levels were selected, and three specimens at least were tested for each stress level. The tests were not finalized until the specimens fractured or the cycle number reached 1 ×106. After fatigue test, the fractures were cut by an electrical discharging machine and cleaned with acetone for fractographic analysis. The fracture surfaces were examined with optical microscopy and scanning electron microscope (SEM).
2
Results and Discussion
2.1 Microstructure Optical microstructures of the samples after etching are shown in Fig.2. They show that the microstructure near the hole of the sample without cold expansion (WCE, Fig.2b) is almost the same as the original microstructure (Fig.2a). After the CE process (Fig.2c), the microstructure near the surface region of the hole is much different from that of the original region. Because of the compression stress and shear stress caused by the oversized mandrel during the CE, the microstructure on the surface of the hole is deformed due to the mandrel movement. It can be found that a plastic deformation layer with about 150 μm in thickness is formed in the CE sample (Fig.2c). Within the plastic deformation layer, the grains are stretched along the moving direction of the mandrel and compressed perpendicular to it. Furthermore, the grain boundaries in this region become more circuitous as compared with the WCE sample (Fig.2b). The compressed grains and circuitous grain boundaries can decrease the tendency for crack initiation and propagation and improve the fatigue property. 2.2 Microhardness The Vickers hardness measured near the hole of CE and WCE specimens are presented in Fig.3. The hardness near the surface of CE is much higher than that of WCE, and the width of hardened region is about 150 μm, which is consistent with the thickness of plastic deformation layer (Fig.2c). It is expected that the hardened layer will improve the abrasive resistance of lugs and prolong the fatigue life. 2.3 Residual stress The crack initiation is the most important in high cycle fatigue. For small unnotched samples, the initiation life for a 250 μm crack can even occupy more than 90% of the total
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Wen Shizhen et al. / Rare Metal Materials and Engineering, 2015, 44(10): 2358-2362
a
c
b
100 µm Fig.2
Optical microstructure of original material (a), near the hole of WCE specimens (b), and near the hole of CE specimens (c)
CE WCE
180 176 172 168 164 160 156
N f =C( max max0 )
0 50 100 150 200 Distance From Sureface of Hole, d/m
Vickers hardness profile along the Y axis (Fig. 1b) from the hole wall
life[9]. However, because of the stress concentration, crack initiation life for notched samples may be shortened evidently. Hence, the increase of crack initiation life by strengthening the stress concentration region is the most feasible method to improve the total fatigue life. The residual stress near the hole of lugs is important since the applied stress is typically high during the service process. High compressive residual stress delays the micro-crack formation by countering applied tensile stress at the surface. Fig.4 shows the distribution of residual stress along the Y axis of the CE specimen. It can be seen that the compressive residual stress is distributed around the hole. The stress near the hole is －80 MPa, which is expected to counter the applied tensile stress significantly, especially at the low stress levels. The compressive residual stresses increase first and then decrease with the increase of the distance to the surface of hole, and the highest compressive residual stress is 220 MPa. Within the tested region of 400 μm, the compressive stress is higher than 80 MPa. It has been reported that the initial crack length is between 250 and 500 μm for notched samples [9,10]. Therefore, the compressive residual stress layer is expected to delay the micro-crack formation.
2.4 Fatigue property The S-N curve of the WCE and CE specimens are shown in Fig5. The three-parameter power-law equation (2) is used
( m)
(2) This fit assumes power-law dependence of maximum stress (σ max) and cycles to failure (Nf) [13]. σmax0 represents maximum stress of cyclic load that can be applied to the material without causing fatigue failure. The fitting results, listed in Table 1, show that the CE lugs have higher fatigue limit than WCE ones. The gains in life are then evaluated by the life improvement factor (LIF): 0 Residual Stress, r/MPa
Fig.3
184
-40 -80 -120 -160 -200 -240 0 100 200 300 Distance from Hole Edge, d/m
Fig.4
400
Distribution of residual stress along the Y axis of CE specimen
Maximum Stress, max/MPa
Vickers Hardness, HV/×10 MPa
to fit the measured data [11,12].
160 120 80 40 10
Fig.5
CE WCE
4
5
10 10 Number of Cycles to Failure, Nf
6
S-N curves showing the effect of CE on the high cycle fatigue property
of
straight
lugs
(arrows
indicate
specimens without failure, and lines show 50% fracture probability)
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Wen Shizhen et al. / Rare Metal Materials and Engineering, 2015, 44(10): 2358-2362
LIF=
N CE N WCE
(3)
which is defined as the ratio of the geometric mean lifetimes for the two conditions at a given stress level. The results indicate that the life of CE lugs is much higher than that of WCE lugs at all stress levels, especially at lower stress levels. For instance, the value of LIF is 1.7 at 100 MPa, higher than 2.7 at 50 MPa. These results also show that cold expansion is an effective method to improve the fatigue strength of 7A85 alloy straight lugs. The CE specimens exhibit fatigue strength of 100 MPa after 10 5 cycle, while for the WCE specimens, it is 80 MPa. That is to say, the fatigue strength is increased by 25%. At 5×105 cycle, the fatigue strength is increased by 25%. It is also worth to be mentioned that because the compressive residual stress reduces the surface sensitivity to the flaws and defects, CE decreases the scattering of the tested fatigue data.
2.5 Fractography The fractures for each testing condition were examined to investigate the crack initiation. All the cracks initiate from the smallest cross section around the hole. The WCE specimens only exhibit surface crack initiation, while surface or near-surface initiation is found in the CE specimens. The crack initiations of the WCE specimens distributed along the hole wall, but for all the CE specimens, the crack initiated in one corner of hole wall (Fig.6f), which is attributed to the compressive residual stress in the specimen gradually increased from the entrance to the exit of oversized mandrel during CE process [14-16]. These results indicate that CE can affect the location of the crack initiation. C. Giummarra reported that because surface flaws and defects were sensitive to stress, multiple cracks often initiated at micro-cracks of the anodized layer [17]. In the present work, multiple crack initiations were found in the WCE specimens. As shown in Fig.6d four crack initiations were observed in the WCE specimen tested at 100 MPa. But only single crack initiation was found in the CE specimen. These results indicate that CE effectively reduces the possibility of crack initiation. A number of fatigue striations are observed in the stable crack propagation region of both CE and WCE specimens, shown in Fig.7. Striation is the typical feature for ductile alloys. Each striation is created by a single cycle of fatigue loading [18,19] , that is to say, the width (a) of striation indicates the speed of crack propagation (da/dN). The width of the fatigue striation increases with the propagation direction of cracks in both CE and WCE conditions. But the striations on WCE fractures are wider than those on CE fractures at the same load condition and crack length. This result indicates the CE can retard the crack propagation at the stable crack propagation stage and improve the fatigue life of the lugs.
a
Hole wall
b
Hole wall
c
Hole wall
Crack initiation
2 mm d 1 mm e
Crack initiation
1 mm f
Crack initiation
1 mm Fig.6
Typical SEM images of fracture surface: (a) WCE, 100 MPa, 46 900 cycle; (b) WCE, 70 MPa, 290 700 cycle; (c) CE, 60 MPa, 642 300 cycles specimens (the high magnification images near the hole wall of Fig.6d for Fig.6a, Fig.6e for Fig.6b, Fig.6f for Fig.6c)
Table 1
Slope exponents (m), constants C and σmax0 used to fit
the S-N data Specimen m CE 2.11 WCE 2.65
C 7.87×108 1.18×1010
σmax0/MPa 28.51 3.74×10-13
a
b
c
d
e
f
2 µm Fig.7
Fatigue striations in stable crack propagation region of WCE lug, σmax=60 MPa, Nf=124 700 cycle: (a) a= 5.28 mm, da/dN =0.12 μm; (b) a= 7.7 mm, da/dN= 0.53 μm; (c) a= 9.9 mm, da/dN= 0.71 μm. Fatigue striations in stable crack propagation region of CE lug, σmax=60 MPa, Nf=642 300 cycle: (d) a= 5.8 mm, da/dN=0.12 μm; (e) a=7.2 mm, da/dN= 0.18 μm; (f) a= 10.4 mm, da/dN= 0.27 μm
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3
Conclusions
1) A plastic deformation layer of about 150 μm in depth will be formed after cold expansion. The microhardness of cold expansion (CE) specimens is higher than that of specimens without cold expansion (WCE) in the plastic deformation layer. And a compressive residual stress at least 80 MPa can be introduced by cold expansion in the near-surface of the hole. 2) The fatigue life and strength are improved significantly by the cold expansion. At a high stress of 100 MPa, CE improves the fatigue life by a factor of 1.7 compared with the WCE lugs. And at a low stress of 50 MPa, the factor is higher than 2.7. At 1×10 5 cycles, the strength is increased by 25% and at 5×10 5 cycles, the fatigue strength is increased by 39%. 3) CE affects the location and amount of crack initiations. At the same load and crack length, the striations of WCE fractures are wider than those of CE.
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