2011 International Conference on Advanced Materials Engineering
IPCSIT vol.15 (2011) © (2011) IACSIT Press, Singapore
Effect of Shot Peening on High Cycling Fatigue of Al 2024-T4
Y. Fouad and Mostafa M. El Metwally
Faculty of Engineering and Materials Science, German University in Egypt.
Abstract: The present work was aimed at evaluating the effects of shot peening on the high cycle fatigue
performance of the age hardening aircraft alloy Al 2024 at different almen intensities. Shot peening to full
coverage (100%) was performed using spherically conditioned cut wire (SCCW 14) with an average shot size
of 0.36 mm and at almen intensities of 0.1 mmA, 0.2 mmA and 0.3 mmA. After applying the various
mechanical surface treatments, the changes in the surface and near surface layer properties such as microhardness, residual stress-depth profiles and surface roughness were determined. The microhardness, surface
roughness and the residual stresses increased proportionally with the almen intensity. Electropolitically
polished conditions were used as reference in the mechanically surface treated specimens. A significant
improvement was seen in the fatigue performance of the 0.1 mmA.
Keywords: Fatigue, shot peening, Aluminum 2024, surface treatment.
1. Introduction
Mechanical properties such as shot peening and ball burnishing are often applied to light alloys based on
aluminum mostly to improve their high cycle fatigue strengths [1] It is said that 100% coverage in shot
peening is required to achieve full benefit in terms of near surface high dislocation densities, residual
compressive stresses and fatigue performance [1-2] This increase in dislocation densities led to higher
resistances to fatigue crack nucleation while the compressive residual stresses were found to stop microcrack growth [3-5]. In this research study, the effect of shot peening on fatigue performance and
microhardness was studied on Al2024 T4 condition for different almen intensities. Shot peening is surface
treatment frequently subjected to aluminum alloys surface. It has receive extensive attention in research due
it increase the component’s fatigue life [6-8] In many industries, where components often experience
repetitive loading, materials are commonly treated with cold working processes to increase their fatigue
performance. In particular aluminum alloys are shot-peened to improve these characteristics. The process of
shot-peening involves bombarding a surface with hard spherical particles propelled by compressed air or a
centrifugal effect. These properties inhibit short crack growth and increase the fatigue life [9]. Shot-peening
tends to increase the surface roughness which can be detrimental to fatigue performance [10]. It can also
increase the susceptibility to corrosion, but it does not usually affect the tribological performance of
aluminum. The early stages of fatigue crack development are not of course restricted to the growth of small
mode I crack under a tensile cyclic loading. It is in these early stages that faceted and locally divergent crack
paths are often present. Moreover, a compressive loading component is a feature of many service
environments and crack paths are often encountered that are markedly inclined to the loading direction.
Very little experimental work has been reported on this wider area of the influences of compressive
loading and pronounced mixed-mode loading on small crack closure. Even for long cracks, experimental
measurements of closure under mixed-mode loading are relatively underreported [11-13], although mode I
crack closure in the presence of a wide range of loading profiles has received much attention. Despite
additional input from modeling work [14–15], the influence of crack closure on fatigue crack growth during
mixed-mode loading is not well understood. The present work addresses the areas of influence of different
almen intensities on the mechanisms of fatigue life of Al 2024-T4. It describes observations and
measurements of the surface and near surface layer properties such as micro-hardness, residual stress-depth
profiles and surface.
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2. Experimental Work
The age hardening alloy Al 2024 were received as raw rods specimens (11 x 11 x 70) cut specimens
were taken with the long axis parallel to the extrusion direction and were solution heat treated at 495ºC for 1
h followed by water- quenching and naturally aged for 5 days at room temperature (condition T4)
Shot peening was conducted using a gravity induction shot peening machine type OSK-kiefer GmbH. Shot
peening was carried out for 40 seconds using spherically conditioned cut wire (SCCW 14) each at a constant
distance between the nozzle and the specimen of 9 cm . The specimen is rotating continuously during
peening to ensure 100% coverage. Different almen intensities were used to compare the fatigue improvement.
Table (1) indicates the various almen intensities, time and pressure used.
Table 1: Various almen intensities, time and pressure for Al 2024-T4 alloy.
Peening
pressure
0.5 bar
2 bar
5 bar
Almen intensity
Peening medium
Distance from nozzle
Coverage
Time
0.1 mmA
0.19 mmA
0.3 mmA
SCCW 14
SCCW 14
SCCW 14
9 cm
9 cm
9 cm
100 %
100 %
100 %
40 s
40 s
40 s
The surface roughness was measured by a profilometer. The residual stress were investigated using the
incremental hole drilling method technique where the residual strains are calculated first using a 1.9 mm
diameter drill . The induced strains are measured every 10 μm. These strains are then converted to stresses by
a series of equations using the macroscopic Young’s modulus of E=70 GPa and a Poisson’s ratio of 0.34.
The microhardness depth profile was measured using a Struers Duramin Härteprüfer with a Vickers hardness
pyramidal diamond indenter for a dwell time of 15 seconds. Fatigue tests were performed on hour glass
shaped specimens with a gage diameter of 6 mm in rotating beam loading (R= -1) in air at frequency 50HZ.
Specimens were electropolished and taken as reference .Roughly 100 μm were removed from the as
machined and mechanically pre-polished to remove any machining effect that can affect the results while the
rest of the specimens were mechanically polished before shot peening.
3. Results and Discussion.
Microstructure for the shot peened aluminum specimens was observed for different almen intensities in
the longitudinal direction Fig. (1). It was clearly observed that the regions of plastic deformation in the high
almen intensities (0.2 mmA and 0.3 mmA) are much greater than that of the 0.1 mmA which is also
explained by the microhardness depth profile.
As expected, the roughness increases gradually with increasing the almen intensity of the shot peening
process as in Table (2). The high roughness of the 0.3mmA has a great influence on the fatigue performance.
Table 2: Roughness values for Al 2024-T4 at different Almen intensities.
Condition
EP
SP 0.1 mmA
SP 0.2 mmA
SP 0.3 mmA
RA
0.08
1.51
2.76
3.42
As observed in the microstructure investigation, the microhardness was increasing proportionally with
the almen intensity of shot peening. Because of the amount of cold working and plastic deformation the
microhardness was increased.
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A
B
C
D
E
F
Fig. 1: Microstructure and section topographies of peened AL 2024 at different Almen intensities (A, B near surface)
SP 0.1 mmA, (C, D near surface) SP 0.2 mmA and ( E,F near surface) SP 0.3 mmA.
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Fig. 2: Microhardness-depth profile of peened Al 2024-T4.
As expected the higher almen intensity gave us the higher residual stress profiles which will increase the
fatigue performance Fig. 2.
Fig. 3: Residual stress-depth profile of peened Al 2024-T4.
In Fig. 3 After shot peening the fatigue performance was increased clearly in the 0.1 mmA almen
intensity (250 MPA) and then the 0.2 mmA but no further improvement in the fatigue performance was
observed in the 0.3 mmA(300 MPA) Fig. 4 although the amount of residual stress was higher but this can be
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interpreted from the high roughness so that the crack nucleation will be easier or may it is because of some
surface damage before testing.
Fig. 4: S-N curves of Al 2024-T4 alloy after shot peening
4. Conclusions
In this study, the effect of shot peening on fatigue performance was studied on Al2024 T4 and the
following results were obtained:
1. A significant improvement was seen in the fatigue performance of the 0.1 mmA almen intensity,
while improvement in the fatigue performance of the 0.2 and 0.3 mmA almen intensities were
equally because of the high surface roughness of the 0.3 mmA.
2. The microhardness, surface roughness and the residual stresses increased proportionally with the
almen intensity.
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