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Fatigue strength improvement by ultrasonic impact treatment of highly stressed spokes of cast aluminium wheels
International Journal of Fatigue 33 (2011) 513–518
Contents lists available at ScienceDirect
International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue
Fatigue strength improvement by ultrasonic impact treatment of highly stressed spokes of cast aluminium wheels A. Berg-Pollack a, F.-J. Voellmecke b, C.M. Sonsino a,⇑ a b
Fraunhofer-Institute for Structural Durability and System Reliability (LBF), Darmstadt, Germany Borbet Group, Hallenberg-Hesborn, Germany
a r t i c l e
i n f o
Article history: Received 28 June 2010 Accepted 28 September 2010 Available online 13 October 2010 Keywords: Fatigue Constant and variable amplitude loading Cast aluminium Ultrasonic impact treatment Lightweight design
a b s t r a c t This paper reports on the application of the ultrasonic impact treatment on spokes of cast aluminium (AlSi11Mg) wheel spiders and its influence under spectrum loading. After preliminary investigations with specimens removed from the spokes for the adjustment of the treatment parameters, fatigue tests, measurement of residual stresses and metallographic analyses of untreated and treated spokes were performed. The mechanical impacts of the UIT effected a significant change of the microstructure, porosity and hardness. The application of the ultrasonic impact treatment introduced plastic deformations and consequently compressive residual stresses at the surface. The fatigue strength increased significantly compared to the untreated spokes under constant and variable amplitude bending loading. The UIT-technology promises a high potential for improving of the structural durability and safety and the saving of material in the production of cast aluminium wheels. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
2. Material, specimens and testing
Ultrasonic impact treatment (UIT) is a post surface treatment technology aiming at the improvement of the performance of metallic components [1–3] by
2.1. Material and specimens
surface hardening, reduction of porosity (as far as it exists), increase of compressive residual stresses and improvement of surface quality.
This treatment is applied by excitation of hardened needles by ultrasonic impulses and transmission of the energy into the surface, Fig. 1, affecting a depth of nearly up to 2 mm. UIT is widely applied to welded joints and it was also tried on cast aluminium specimens and components (passenger car wheels) for the investigation of its effectiveness. The investigations were carried out with spokes of a passenger car wheel and were accompanied by microstructural analyses, hardness and residual stress measurements.
⇑ Corresponding author. Tel.: +49 6151 705 244. E-mail address: [email protected] (C.M. Sonsino). 0142-1123/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2010.09.017
Figs. 2 and 3 show a five spoke cast aluminium wheel spider. The chemical composition of the no – heat treated alloy AlSi11Mg (EN-AC 44000) and the mechanical properties are compiled in Table 1. These were determined with specimens removed from the middle of the spokes. Prior to the tests with the spokes, the UIT treatment parameters (voltage, feed, velocity, and surface pressure) were adjusted using preliminary fatigue tests with specimens. For confidentiality reasons the details of the parameters are not disclosed. The spokes were manually UIT treated only in the fatigue critical area, in the transition from the hub to the spokes. The wheels were all 3-coat painted (180 °C/15 min) after the UIT treatment. 2.2. Testing The test series performed consisted of constant amplitude loading of untreated and UIT-treated spokes (five for each condition) and spectrum loading of untreated and UIT-treated spokes (again five for each condition), in all 20 tests with four wheel spiders. Each wheel spider was clamped at the hub and the load was introduced as a cantilever bending at the end of one spoke, Fig. 4. After
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A. Berg-Pollack et al. / International Journal of Fatigue 33 (2011) 513–518
• Inducing of vibrations by ultrasonic transducer I
II
I
III
• Transmission of vibrations for controlling the velocity of vibrations
IV
• Impuls of the controller II to the activator III • Impuls of activator III to the surface of workpiece IV • Transformation of the ultrasonic vibrations 1 in mechanical impulses 2 on the surface of the workpiece IV
2
1
Fig. 1. Principle of UIT treatment.
tude distribution with an irregularity factor of I = 0.99 and a sequence length of Ls = 5 104, Fig. 5, was applied [4]. 3. Results 3.1. Hardness and residual stress measurement
Fig. 2. Cast aluminium wheel.
By the UIT treatment the hardness is increased from about 70 HV 0.05 to 100 HV 0.05 within a depth of 1.2 mm, Fig. 6, and if present the porosity is reduced due to the plastic deformation. Also the surface is smoothed, Fig. 7. The results of the residual stress measurements on a spoke, obtained by cutting out of a cube (10 mm 10 mm 10 mm) with a strain gauge rosette, Fig. 8, in five steps are displayed in Fig. 9. Compressive residual stresses of about 20 MPa for the as cast state and 100 MPa for the treated state are measured, Fig. 9. 3.2. Fatigue tests and cumulative damage
the failures of one spoke, the hub was turned by 72° for testing the next spoke, in all five tests per wheel spider. The constant amplitude tests were carried out with a load ratio of R = 1 under a frequency of f = 10 s1, and the spectrum loading under a load ratio of the maximum load of R = 0.25 for the inclusion of the preloading of a wheel. The testing frequency of these tests was f = 10–15 s1. For the spectrum loading a Gaussian ampli-
Primarily due to the pronounced increase of the compressive residual stresses a significant improvement of the fatigue performance of the spokes is rendered, Fig. 10. Under constant amplitude loading before the knee point of the Woehler-curve an improvement of fatigue life by a factor of about six and an increase of fatigue strength by 26% resulted. Under spectrum loading the
Fig. 3. Dimensions of the cast aluminium wheel 8J 18H2 with mounted spider.
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A. Berg-Pollack et al. / International Journal of Fatigue 33 (2011) 513–518 Table 1 Data of the cast aluminium alloy AlSi11Mg (EN-AC 44000). (a) Chemical composition Weight%
Si 10.78
Cu 0.001
Fe 0.096
Zn 0.005
Mn 0.006
Ti 0.110
Mg 0.306
Sr 0.0317
(b) Mechanical properties AlSi11Mg
Rm in MPa 182
Rp0,2 in MPa 106
a. 60 kN-test rig
A5 in % 5.8
Hardness in HB 54
b. Load introduction
Fig. 4. (a) 60 kN test rig and (b) load introduction.
Fig. 5. Applied load sequence.
observed increase of fatigue life is also about six times, but due to the shallower course of the Gassner-curves the fatigue strength increase only by 21% corresponding to almost the same grade of strength increase as under constant amplitude loading. This indicates that despite of the higher load levels under spectrum loading, the compressive residual stresses are not so much affected as is the case with shot-peened or surface rolled components [5] due to plasticity effects. The location of the fatigue failures is shown for two tests with not treated spokes in Fig. 11. At the failure location local stresses were also determined by strain gauges. These are displayed on the right axis of Fig. 10. The UIT treatment does not change the failure location. Beside the experiments the Gassner-lines were determined also by cumulative damage calculations. For this the slopes of the Woehler-lines were modified after the knee points at Nk = 1 106 cycles by k0 = 2k 2 according to Haibach [6] for accounting the damaging influence of small amplitudes and the fatigue lives were calculated by the theoretical damage sum Dth = 1.0. Mean stress effects by transformation of amplitudes to R = 1 were
Fig. 6. Hardness distribution for as cast and UIT treated states.
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A. Berg-Pollack et al. / International Journal of Fatigue 33 (2011) 513–518
a. As cast
b.UITtreated
Fig. 7. Influence of the UIT treatment on surface roughness: (a) As cast and (b) UIT treated.
1.0 for other parameters of treatment or other load sequences lower real damage sums are not excluded [4]. The microstructural analysis of the surface revealed some microcracks before fatigue testing of treated spokes. Their avoidance by optimization of the UIT parameters may certainly result in a better performance than obtained in this investigation.
4. Conclusions and outlook
Fig. 8. Spoke with applied 0°/90°-strain gauge rosette for residual stress measurements.
considered by the mean-stress sensitivity of M = 0.4 [6,7]. The comparison of experimental and calculated Gassner-lines, Fig. 12, result the real damage sums Dreal ¼ N exp =N calc ðDth ¼ 1:0Þ. They amount between Dreal = 1.5 for the as cast and Dreal = 1.0, for the UIT-treated spokes. Even if the damage sums are near to the theoretical value
The investigations carried out reveal that under both constant and variable amplitude loading a significant performance increase was obtained. The fatigue strength increase of 21–26% does not yet seem to be the possible optimum due to the observed microcracks on the surface of the treated spokes. However, the adjustment to more suitable parameters may lead to a further improvement. In this context it should also be mentioned that the UIT treatment of the spokes was performed manually. Under industrial production conditions this would be carried out automatically and assure a controlled constant treatment quality. The increase of fatigue strength by UIT can also be used for a reduction of the cross sections of the spokes, if the fatigue life of
Fig. 9. Influence of the UIT treatment on residual stresses: (a) As cast and (b) UIT treated.
A. Berg-Pollack et al. / International Journal of Fatigue 33 (2011) 513–518
517
Fig. 10. Fatigue life curves of as cast and UIT treated aluminium spokes.
the untreated component is sufficient and therefore maintained. So, this treatment technology is able to realize lightweight design. Acknowledgements The authors acknowledge Dr. E.S. Statnikov (in memoriam) and L. Tehini from Applied Ultrasonics, Birmingham, Alabama, USA for the ultrasonic impact treatment of the wheel spokes and their kind and valuable recommendations. Mrs. A. Till (LBF) is acknowledged for the metallographical analyses and their interpretation. References
Fig. 11. Fatigue failure locations.
[1] Statnikov ES, Muktepavel VO. Technology of ultrasound impact treatment as a means of improving the reliability and endurance of welded metal structures. Weld Int 2003;17(9):741–4. [2] Haagensen PJ, Statnikov ES, Lopez-Martinez L. Introductory fatigue tests on welded joints in high strength steel and aluminium improved by various methods including ultrasonic impact treatment (UIT). IIW Doc. XIII-1748-98; 1998. [3] Maddox SJ, Hopkin GR, Holy A, Moura Branco CA, Infante V, Baptista R, et al. Improving the fatigue performance of welded stainless steels. Report no. EUR 22809 EN. Information and Communication Unit European Commission, Brussels; 2007.
Fig. 12. Experimental and calculated fatigue life curves.
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A. Berg-Pollack et al. / International Journal of Fatigue 33 (2011) 513–518
[4] Sonsino CM. Fatigue testing under variable amplitude loading. Int J Fatigue 2007;29:1080–9. [5] Sonsino CM. Effects on lifetime under spectrum loading. MP Mater Test 2010;52(7–8):440–51. [6] Haibach E. Betriebsfestigkeit – Verfahren und Daten zur Bauteilberechnung VDI-Verlag, Düsseldorf; 1989.
[7] Sonsino CM, Berg-Pollack A, Grubisic V. Structural durability proof of automotive aluminium safety components – Present state of the art SAEpaper no. 2005-01-0800. Society of Automotive Engineers, Detroit/Michigan/ USA; 2005.
Contents lists available at ScienceDirect
International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue
Fatigue strength improvement by ultrasonic impact treatment of highly stressed spokes of cast aluminium wheels A. Berg-Pollack a, F.-J. Voellmecke b, C.M. Sonsino a,⇑ a b
Fraunhofer-Institute for Structural Durability and System Reliability (LBF), Darmstadt, Germany Borbet Group, Hallenberg-Hesborn, Germany
a r t i c l e
i n f o
Article history: Received 28 June 2010 Accepted 28 September 2010 Available online 13 October 2010 Keywords: Fatigue Constant and variable amplitude loading Cast aluminium Ultrasonic impact treatment Lightweight design
a b s t r a c t This paper reports on the application of the ultrasonic impact treatment on spokes of cast aluminium (AlSi11Mg) wheel spiders and its influence under spectrum loading. After preliminary investigations with specimens removed from the spokes for the adjustment of the treatment parameters, fatigue tests, measurement of residual stresses and metallographic analyses of untreated and treated spokes were performed. The mechanical impacts of the UIT effected a significant change of the microstructure, porosity and hardness. The application of the ultrasonic impact treatment introduced plastic deformations and consequently compressive residual stresses at the surface. The fatigue strength increased significantly compared to the untreated spokes under constant and variable amplitude bending loading. The UIT-technology promises a high potential for improving of the structural durability and safety and the saving of material in the production of cast aluminium wheels. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
2. Material, specimens and testing
Ultrasonic impact treatment (UIT) is a post surface treatment technology aiming at the improvement of the performance of metallic components [1–3] by
2.1. Material and specimens
surface hardening, reduction of porosity (as far as it exists), increase of compressive residual stresses and improvement of surface quality.
This treatment is applied by excitation of hardened needles by ultrasonic impulses and transmission of the energy into the surface, Fig. 1, affecting a depth of nearly up to 2 mm. UIT is widely applied to welded joints and it was also tried on cast aluminium specimens and components (passenger car wheels) for the investigation of its effectiveness. The investigations were carried out with spokes of a passenger car wheel and were accompanied by microstructural analyses, hardness and residual stress measurements.
⇑ Corresponding author. Tel.: +49 6151 705 244. E-mail address: [email protected] (C.M. Sonsino). 0142-1123/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2010.09.017
Figs. 2 and 3 show a five spoke cast aluminium wheel spider. The chemical composition of the no – heat treated alloy AlSi11Mg (EN-AC 44000) and the mechanical properties are compiled in Table 1. These were determined with specimens removed from the middle of the spokes. Prior to the tests with the spokes, the UIT treatment parameters (voltage, feed, velocity, and surface pressure) were adjusted using preliminary fatigue tests with specimens. For confidentiality reasons the details of the parameters are not disclosed. The spokes were manually UIT treated only in the fatigue critical area, in the transition from the hub to the spokes. The wheels were all 3-coat painted (180 °C/15 min) after the UIT treatment. 2.2. Testing The test series performed consisted of constant amplitude loading of untreated and UIT-treated spokes (five for each condition) and spectrum loading of untreated and UIT-treated spokes (again five for each condition), in all 20 tests with four wheel spiders. Each wheel spider was clamped at the hub and the load was introduced as a cantilever bending at the end of one spoke, Fig. 4. After
514
A. Berg-Pollack et al. / International Journal of Fatigue 33 (2011) 513–518
• Inducing of vibrations by ultrasonic transducer I
II
I
III
• Transmission of vibrations for controlling the velocity of vibrations
IV
• Impuls of the controller II to the activator III • Impuls of activator III to the surface of workpiece IV • Transformation of the ultrasonic vibrations 1 in mechanical impulses 2 on the surface of the workpiece IV
2
1
Fig. 1. Principle of UIT treatment.
tude distribution with an irregularity factor of I = 0.99 and a sequence length of Ls = 5 104, Fig. 5, was applied [4]. 3. Results 3.1. Hardness and residual stress measurement
Fig. 2. Cast aluminium wheel.
By the UIT treatment the hardness is increased from about 70 HV 0.05 to 100 HV 0.05 within a depth of 1.2 mm, Fig. 6, and if present the porosity is reduced due to the plastic deformation. Also the surface is smoothed, Fig. 7. The results of the residual stress measurements on a spoke, obtained by cutting out of a cube (10 mm 10 mm 10 mm) with a strain gauge rosette, Fig. 8, in five steps are displayed in Fig. 9. Compressive residual stresses of about 20 MPa for the as cast state and 100 MPa for the treated state are measured, Fig. 9. 3.2. Fatigue tests and cumulative damage
the failures of one spoke, the hub was turned by 72° for testing the next spoke, in all five tests per wheel spider. The constant amplitude tests were carried out with a load ratio of R = 1 under a frequency of f = 10 s1, and the spectrum loading under a load ratio of the maximum load of R = 0.25 for the inclusion of the preloading of a wheel. The testing frequency of these tests was f = 10–15 s1. For the spectrum loading a Gaussian ampli-
Primarily due to the pronounced increase of the compressive residual stresses a significant improvement of the fatigue performance of the spokes is rendered, Fig. 10. Under constant amplitude loading before the knee point of the Woehler-curve an improvement of fatigue life by a factor of about six and an increase of fatigue strength by 26% resulted. Under spectrum loading the
Fig. 3. Dimensions of the cast aluminium wheel 8J 18H2 with mounted spider.
515
A. Berg-Pollack et al. / International Journal of Fatigue 33 (2011) 513–518 Table 1 Data of the cast aluminium alloy AlSi11Mg (EN-AC 44000). (a) Chemical composition Weight%
Si 10.78
Cu 0.001
Fe 0.096
Zn 0.005
Mn 0.006
Ti 0.110
Mg 0.306
Sr 0.0317
(b) Mechanical properties AlSi11Mg
Rm in MPa 182
Rp0,2 in MPa 106
a. 60 kN-test rig
A5 in % 5.8
Hardness in HB 54
b. Load introduction
Fig. 4. (a) 60 kN test rig and (b) load introduction.
Fig. 5. Applied load sequence.
observed increase of fatigue life is also about six times, but due to the shallower course of the Gassner-curves the fatigue strength increase only by 21% corresponding to almost the same grade of strength increase as under constant amplitude loading. This indicates that despite of the higher load levels under spectrum loading, the compressive residual stresses are not so much affected as is the case with shot-peened or surface rolled components [5] due to plasticity effects. The location of the fatigue failures is shown for two tests with not treated spokes in Fig. 11. At the failure location local stresses were also determined by strain gauges. These are displayed on the right axis of Fig. 10. The UIT treatment does not change the failure location. Beside the experiments the Gassner-lines were determined also by cumulative damage calculations. For this the slopes of the Woehler-lines were modified after the knee points at Nk = 1 106 cycles by k0 = 2k 2 according to Haibach [6] for accounting the damaging influence of small amplitudes and the fatigue lives were calculated by the theoretical damage sum Dth = 1.0. Mean stress effects by transformation of amplitudes to R = 1 were
Fig. 6. Hardness distribution for as cast and UIT treated states.
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A. Berg-Pollack et al. / International Journal of Fatigue 33 (2011) 513–518
a. As cast
b.UITtreated
Fig. 7. Influence of the UIT treatment on surface roughness: (a) As cast and (b) UIT treated.
1.0 for other parameters of treatment or other load sequences lower real damage sums are not excluded [4]. The microstructural analysis of the surface revealed some microcracks before fatigue testing of treated spokes. Their avoidance by optimization of the UIT parameters may certainly result in a better performance than obtained in this investigation.
4. Conclusions and outlook
Fig. 8. Spoke with applied 0°/90°-strain gauge rosette for residual stress measurements.
considered by the mean-stress sensitivity of M = 0.4 [6,7]. The comparison of experimental and calculated Gassner-lines, Fig. 12, result the real damage sums Dreal ¼ N exp =N calc ðDth ¼ 1:0Þ. They amount between Dreal = 1.5 for the as cast and Dreal = 1.0, for the UIT-treated spokes. Even if the damage sums are near to the theoretical value
The investigations carried out reveal that under both constant and variable amplitude loading a significant performance increase was obtained. The fatigue strength increase of 21–26% does not yet seem to be the possible optimum due to the observed microcracks on the surface of the treated spokes. However, the adjustment to more suitable parameters may lead to a further improvement. In this context it should also be mentioned that the UIT treatment of the spokes was performed manually. Under industrial production conditions this would be carried out automatically and assure a controlled constant treatment quality. The increase of fatigue strength by UIT can also be used for a reduction of the cross sections of the spokes, if the fatigue life of
Fig. 9. Influence of the UIT treatment on residual stresses: (a) As cast and (b) UIT treated.
A. Berg-Pollack et al. / International Journal of Fatigue 33 (2011) 513–518
517
Fig. 10. Fatigue life curves of as cast and UIT treated aluminium spokes.
the untreated component is sufficient and therefore maintained. So, this treatment technology is able to realize lightweight design. Acknowledgements The authors acknowledge Dr. E.S. Statnikov (in memoriam) and L. Tehini from Applied Ultrasonics, Birmingham, Alabama, USA for the ultrasonic impact treatment of the wheel spokes and their kind and valuable recommendations. Mrs. A. Till (LBF) is acknowledged for the metallographical analyses and their interpretation. References
Fig. 11. Fatigue failure locations.
[1] Statnikov ES, Muktepavel VO. Technology of ultrasound impact treatment as a means of improving the reliability and endurance of welded metal structures. Weld Int 2003;17(9):741–4. [2] Haagensen PJ, Statnikov ES, Lopez-Martinez L. Introductory fatigue tests on welded joints in high strength steel and aluminium improved by various methods including ultrasonic impact treatment (UIT). IIW Doc. XIII-1748-98; 1998. [3] Maddox SJ, Hopkin GR, Holy A, Moura Branco CA, Infante V, Baptista R, et al. Improving the fatigue performance of welded stainless steels. Report no. EUR 22809 EN. Information and Communication Unit European Commission, Brussels; 2007.
Fig. 12. Experimental and calculated fatigue life curves.
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A. Berg-Pollack et al. / International Journal of Fatigue 33 (2011) 513–518
[4] Sonsino CM. Fatigue testing under variable amplitude loading. Int J Fatigue 2007;29:1080–9. [5] Sonsino CM. Effects on lifetime under spectrum loading. MP Mater Test 2010;52(7–8):440–51. [6] Haibach E. Betriebsfestigkeit – Verfahren und Daten zur Bauteilberechnung VDI-Verlag, Düsseldorf; 1989.
[7] Sonsino CM, Berg-Pollack A, Grubisic V. Structural durability proof of automotive aluminium safety components – Present state of the art SAEpaper no. 2005-01-0800. Society of Automotive Engineers, Detroit/Michigan/ USA; 2005.