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Vol. 7
July Issue
Year 2006
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Science Update


in Vol. 7 - July Issue - Year 2006
Fatigue Strength Improvements in Various Aluminum Alloys after Shot Peening



Author: Prof. Lothar Wagner


Author: Dipl.-Ing. Tomasz Ludian


Table 1: Compositions (wt. %) of the various alloys


Table 2: Extrusion process parameters


Table 3: Mechanical properties


Fig. 1: Microhardness-depth profiles


Fig. 2: Residual stress-depth profiles


a) Al 6082-T4


b) Al 2024-T4


c) Al 7075-T73
Fig. 3: S-N curves comparing conditions EP and SP

Abstract: The effect of shot peening on the rotating beam fatigue strength of age-hardened 6082, 2024 and 7075 aluminum covering a wide range of yield stresses has been investigated. The observed improvements range from about 10% to almost 60% and are not simply related to yield stress or shot-peening induced residual compressive stresses. Preliminary analysis suggests that the work hardening behavior in these alloys predominate. Further work is needed to understand the different response of age-hardened aluminum alloys to shot peening.

Experimental

The age-hardening aluminum alloys Al 6082, Al 2024 and Al 7075 were received as cylindrical extrusions from Otto Fuchs Metallwerke. Chemical compositions of the various alloys are listed in Table 1. The process parameters during extrusion are given in Table 2. Al 2024 and Al 6082 were given a natural temper T4 while Al 7075 was artificially aged to a T73 temper. Details of the heat treatments utilized are given in [1]. The tensile properties (? = 10-3 s-1) in the longitudinal direction are listed in Table 3 and indicate that the lowest yield stress was observed in Al 6082-T4 (220 MPa), the highest in Al 7075-T73 (480 MPa) with Al 2024-T4 (450 MPa) in between. As expected, ductility values were highest in Al 6082-T4 followed by Al 2024-T4 and Al 7075-T73. The work hardening capability, as expressed as UTS - ?y, is highest in Al 2024-T4 (170 MPa) followed by Al 6082-T4 (130 MPa). The lowest value was observed in Al 7075-T73 (70 MPa).

Shot peening was done using a gravity system with spherically conditioned cut wire shot (SCCW14) having an average size of 0.36 mm. Since previous work had shown that incomplete coverage can decrease the fatigue performance of aluminum alloys [2], peening was done to full coverage using an Almen intensity of 0.20 mmA.
Microhardness and residual stress depth profiles were determined to characterize the changes in surface layer properties caused by the shot peening process [3, 4]. The residual stresses were evaluated by incremental hole drilling [5].
Fatigue tests were performed on hour-glass shaped specimens with a gage diameter of 3.6 mm in rotating beam loading (R = -1) at a frequency of 60 Hz in lab air. Before shot peening, all specimens were electropolished (100 ?m being removal from the surface) to exclude any machining effects that might mask the fatigue results. The electrolytically polished condition (EP) was taken as the baseline for comparison to the shot peened condition (SP).

Surface properties

Microhardness-depth profiles of the various alloys as affected by shot peening are illustrated in Fig. 1. The observed surface layer strengthening in Al 6082-T4 and Al 2024-T4 was much greater than in Al 7075-T73, this result being related to the lower work hardening response of the latter (Table 3).

Residual stress-depth profiles are shown in Fig. 2. Compressive residual stresses were observed with a pronounced maxima below the surface. The highest compressive residual stresses were measured for Al 2024-T4 followed by Al 7075-T73 and Al 6082-T4. The comparatively low level of residual compressive stresses measured on the high-strength Al 7075-T73 is again thought to be related to the low work hardening capability, UTS – ?y = 70 MPa, of this alloy (Table 3).

Fatigue performance

The S-N curves comparing the EP and SP conditions for the various alloys are shown in Fig. 3. The 107 cycles fatigue strengths of the baseline conditions (EP) for Al 6082-T4, Al 2024-T4 and Al 7075-T73, are 135 MPa, 225 MPa and 150 MPa, respectively. Comparison with the tensile properties (Table 3), indicate that the 107 cycles fatigue strengths do not correllate with either yield or tensile strength. Indeed 7075-T73 had the highest yield strength among the alloys, while its fatigue strength was low, 150 MPa (Fig. 3c). After shot peening, the fatigue strengths of the various alloys increased by 50 MPa, 25 MPa and 85 MPa for Al 6082-T4, Al 2024-T4 and Al 7075-T73, respectively (Fig. 3). Interestingly, the peening-induced improvement of the 107 cycles fatigue strength of Al 7075-T73 was the highest observed, although the life improvement at intermediate and high stress amplitudes was the lowest among the various alloys (Fig. 3). Presumably, the latter is related to cyclic softening of the slightly over-aged T73 condition in this alloy, this leading to a substantial cyclic relaxation of the as-peened residual compressive stresses in the finite life regime [6].

Acknowledgements

Thanks are due to Dr. M. Hilpert of Otto Fuchs Metallwerke, Meinerzhagen, Germany for providing the Aluminum Alloys. The experimental assistance of K. Piec is gratefully acknowledged.

References

1.ASM Handbook, Vol.4, Heat Treating, 1991, 847.
2.T. Ludian and L. Wagner, Coverage effects in shot peening of Al 2024-T4, Shot Peening (V. Schulze and A. Niku Lari, eds.) 2005, 296.
3.V. Schulze, Modern Mechanical Surface Treatment, Wiley-VCH, 2006
4.S. Baiker, Shot Peening, MFN Publishing House, 2006
5.T. Schwarz and T. Kockelmann, VDI Report 940 (1992) 99.
6.T. Ludian and L. Wagner, Effect of Age-Hardening Condition on HCF Performance of Mechanicaly Surface Treated Al2024, Fatigue and Fracture of Traditional and Advanced Materials, (L. Shaw, J. Larsen and P. Liaw, eds.), TMS (in press).




Author: Lothar Wagner & Tomasz Ludian

For Information:
Clausthal University of Technology
Institute of Materials Science and Engineering
Agricolastrasse 6, Germany
Tel: +49.5323.72 2770, Fax: +49.5323.72 2766

Authors:
Prof. Lothar Wagner             
Dipl.-Ing. Tomasz Ludian
E-Mail: lothar.wagner@tu-clausthal.de
E-Mail: tomasz.ludian@tu-clausthal.de