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Article

The Influence of Shot Peening Media on Surface Properties and Fatigue Behaviour of Aluminium Alloy 6082 T6

by
Erik Calvo-García
1,2,*,
Jesús del Val
2,
Antonio Riveiro
1,2,
Sara Valverde-Pérez
1,3,
David Álvarez
1,3,
Manuel Román
4,
César Magdalena
4,
Aida Badaoui
1,
Pablo Pou-Álvarez
2 and
Rafael Comesaña
1,2
1
Materials Engineering, Applied Mechanics and Construction Department, E.E.I., University of Vigo, Lagoas-Marcosende, 36310 Vigo, Spain
2
LaserOn, CINTECX, University of Vigo, Lagoas-Marcosende, 36310 Vigo, Spain
3
ENCOMAT, CINTECX, University of Vigo, Lagoas-Marcosende, 36310 Vigo, Spain
4
CIE Galfor (CIE Automotive Group), P.I. San Cibrao das Viñas, 32901 Ourense, Spain
*
Author to whom correspondence should be addressed.
J. Manuf. Mater. Process. 2024, 8(5), 192; https://doi.org/10.3390/jmmp8050192
Submission received: 29 July 2024 / Revised: 27 August 2024 / Accepted: 28 August 2024 / Published: 3 September 2024
(This article belongs to the Special Issue Deformation and Mechanical Behavior of Metals and Alloys)
Browse Figures
Versions Notes

Abstract

:
Shot peening is generally used to improve the fatigue performance of mechanical components. However, identifying the geometrical and mechanical characteristics of the shots that improve fatigue strength is still a challenging task, as there are many variables involved in the shot peening process. The present work addresses the effect of different shot media on the fatigue behaviour of an aluminium alloy 6082 T6. Four different shot types were used: silica microspheres, alumina shots, aluminium cut wire and zinc cut wire. Axial fatigue tests were carried out to obtain the Wöhler curves corresponding to each shot peening treatment. The surface properties of the shot-peened specimens, such as grain size, hardness, residual stress and roughness were measured to determine their effect on the fatigue results. The fatigue results revealed that silica and zinc shots increased significantly the fatigue life of the alloy, whereas alumina and aluminium shots reduced its fatigue strength. Almen intensities have shown to correlate well with grain refinement and strain hardening. However, better fatigue results were obtained with the shots that generated higher surface compressive residual stresses. It is believed that small and smooth shots are preferable to sharp and irregular ones, regardless of the Almen intensity or surface hardness attained with the latter.

Graphical Abstract

1. Introduction

Aluminium alloys are extensively utilized in multiple industrial areas, such as the automotive, aircraft, naval and construction sectors [1]. The widespread use of aluminium alloys in the production of structural components is due to their high strength-to-weight ratio. Light, strong mechanical components are highly demanded in the industry, as they allow for achieving a significant reduction in carbon dioxide emissions, fuel consumption, resources and general costs [2]. Among the different aluminium alloy families, the 6000 series alloys (Al-Mg-Si) present additional advantages, such as high corrosion resistance and good formability and weldability [3]. These advantages make the 6000 series alloys suitable for the manufacture of extrusions, plates, welded components and aircraft body sheets [4]. The 6082 aluminium alloy is regarded as one of the most commercially available and the hardest of the Al-Mg-Si family [5]. The use of the 6082 alloy is also gaining popularity in the manufacturing of hot forging products for the automotive industry. These products include not only conventional parts of combustion engine vehicles, such as steering knuckles, but also structural parts of hybrid and electric vehicles, such as chassis or crash boxes [6,7,8]. Since most of these products are subjected to cyclic loading during their service life, the study of the fatigue strength of these aluminium alloys is a fundamental issue for the automotive industry. The fatigue properties of the 6082 alloy have been widely studied by many researchers [9,10,11,12,13].
The fatigue behaviour of aluminium alloys can be modified by shot peening. Shot peening is a surface mechanical treatment that induces severe plastic deformation by bombarding the surface of the target material with particles projected at high velocity [14]. The plastic deformation caused by shot peening affects only a thin surface layer of the material. In the affected layer, the number of dislocations increases due to strain hardening, and the grain size decreases. A high surface hardness hinders surface crack initiation and thus improves fatigue strength [15]. The plastic deformation caused by the shots also induces compressive residual stresses at the surface of the treated material, increasing the external load required to cause crack initiation and slowing down crack propagation under cyclic loading [16]. However, an important drawback of the shot peening process is the increase in the surface roughness due to shot indentations. A high roughness may be associated to sharp stress concentrators where a crack is prone to be originated and thus can be detrimental to fatigue behaviour.
In general, the fatigue strengthening effects of a shot peening process are quantified by measuring the compressive residual stresses and hardness values in the strain hardened layer. These strengthening effects depend strongly on the shot peening variables, namely air pressure, shot size, shot material, angle of impingement, peening distance, and peening time (or coverage). The air pressure is closely related to the shot velocity, or in other words, to the kinetic energy transferred to the target material [17]. The use of higher air pressures in the shot peening process tends to widen the residual stress distribution [18]. Larger and harder shots increase the depth of the compressive residual stresses and strain-hardened layer [19,20]. Almen intensity is commonly used to characterize shot peening, considering the effects of air pressure, shot size, shot material, peening distance, and angle of impingement altogether. Coverage is dependent on the peening time and represents the proportion of the target surface deformed by shot impacts. In previous works, it has been demonstrated that higher coverage increases the surface hardness and compressive residual stress values, but does not necessarily improve fatigue life [21,22]. Some researchers have correlated the shot peening variables with the resultant residual stress, hardness or roughness of the treated materials by means of response surface methodology [23,24] or through finite element method simulations [25,26]. Nevertheless, there is still a lack of knowledge about the effect of the shot material on the surface properties and fatigue strength of shot-peened materials.
The objective of the present work is to evaluate the fatigue performance of the aluminium alloy 6082 T6 after different shot peening treatments. Four types of commercially available shots were used: silica microspheres, alumina particles, aluminium cut wire and zinc cut wire. For each shot peening treatment, the microstructure of the alloy was analysed at the surface to determine whether grain refinement occurred or not. Nanohardness and residual stress measurements were carried out in the strain-hardened layers to characterize the fatigue strengthening mechanisms. The topography of each surface finish was also examined, as it presents a significant influence on the fatigue results. Aluminium specimens were subjected to axial fatigue tests to obtain the Wöhler diagram for each surface treatment. Finally, fracture surface analysis was carried out to identify the crack initiation sites in the fatigue tests.

2. Materials and Methods

The material used in this work is an aluminium alloy 6082. The chemical composition of this material in weight percentage is within the ranges stablished by EN 573-3 [27], as it is shown in Table 1. This aluminium alloy was subjected to a heat treatment T6, which means that was artificially aged. The mechanical properties of this 6082 T6 alloy are presented in Table 2. Moreover, the grain structure of this aluminium alloy is shown in Figure 1. It is observed that this alloy is comprised of equiaxed grains. The size of these grains follows a log-normal distribution with an average value of 11.0 µm and a standard deviation of 3.3 µm. The microstructure revealed a high concentration of the β-Mg2Si intermetallic phase, with round shapes and sizes in the range of 0.5 to 5 µm. More elongated and thin α-Al12(Fe,Mn)3Si intermetallics were identified along the grain boundaries as well. Such phases have been widely reported in the literature for the 6082 alloy [28,29].
Shot peening treatments were applied to the 6082 T6 aluminium alloy using different types of shots. In all these treatments, the shots were projected under an air pressure of 8 bar, an angle of impingement of 90°, a peening distance of 10 cm between the nozzle and the target and a peening time of 240 s. The four types of shots used were silica microspheres (SiO2), alumina particles (Al2O3), 0.8 mm aluminium cut wire (Al) and 0.8 mm zinc cut wire (Zn). These shots are commercially available, and they were selected in order to analyse potential differences in the resultant surface properties and fatigue strength due to the shot material or size. Images of these shots are displayed in Figure 2. For a certain shot peening treatment, Almen intensity is defined as the arc height of a standard Almen strip at the saturation time, and the saturation time corresponds to the time when, if doubled, the arc height increases by only 10%. Thus, Almen intensities of each shot peening treatment were measured according to SAE J442 and J443 [30,31]. The main physical properties of these shots, along with the corresponding Almen intensities are presented in Table 3.
The grain structure of the aluminium alloy 6082 T6 was studied to observe any potential grain refinement in the surface layer after the shot peening treatments. Samples of this alloy were cut perpendicularly to the shot-peened surfaces. Next, they were ground using 320 silicon carbide sandpaper, mirror-polished with 9 µm and 3 µm diamond suspensions and finished with 50 nm colloidal silica. The aluminium samples were etched for 5 min in a solution of 10 g NaOH in 100 mL water heated up to 70 °C to reveal the grain boundaries. The observation of the resultant grains after each shot peening treatment was performed with an Olympus GX-51 optical microscope, and the grain sizes were measured using software ImageJ (1.52i). A total of 100 grains were measured within the first 50 µm depth for each surface treatment, and their mean size and standard deviation were calculated.
The hardness profiles of the plastically deformed layers after each shot peening process were acquired through nanohardness measurements. These measurements were performed with a MTS Nanoindenter NanoXP (MTS Systems Corporation, Eden Prairie, MN, USA) using the Continuous Stiffness Measurement mode and an indentation depth of 500 nm. The hardness profile was obtained up to 800 µm far from the shot-peened surfaces.
Surface residual stress measurements were performed via XRD analysis according to the UNE-EN 15305 standard [32]. These measurements were obtained with Cr (Kα 2.219 Å) radiation by means of the sin2 ψ method on the {311} planes of the aluminium alloy. A total of 13 different tilt angles between −40° and 40° were applied. The diffraction elastic constants were ½ S2 = 19.57 × 10−6 MPa−1 and S1 = −5.16 × 10−6 MPa−1.
An optical profilometer Filmetrics Profilm3D (Filmetrics Inc., San Diego, CA, USA) was used to obtain scans of the different surface finishes obtained after shot peening. For each type of shot, images of the surface finishes of the shot-peened fatigue specimens were acquired, and their respective areal surface roughness (Sa) values were obtained.
Axial fatigue tests were performed according to the ISO 1099 standard [33]. The specimens used on fatigue tests were machined according to the dimensions shown in Figure 3. After the turning process, these specimens were subjected to grinding and longitudinal polishing operations to obtain a surface roughness below Ra = 0.2 µm and ensure the absence of scratches. A servohydraulic dynamic test Walter-Bai LFV-25 machine (Walter+Bai AG, Löhningen, Switzerland) was used to carry out the fatigue tests. All fatigue tests were load-controlled, applying a sinusoidal load profile between zero and a maximum value at a frequency of 10 Hz, which means that the tested specimens were subjected only to tension throughout the fatigue tests. The stress ratio R = 0 was chosen for comparative purposes, as it is a common reference test condition for obtaining the fatigue properties of materials. The maximum force values were selected so that the alloy could be analysed in the high-cycle fatigue (HCF) domain. One specimen was tested for each maximum stress and surface treatment. The first fatigue test for each surface treatment was performed by applying a maximum axial load of 6250 N, and subsequent tests were carried out by decreasing this load by 250 N until reaching the run-out at each condition. The run-out was established at 107 cycles. The fatigue results were fitted to Basquin’s exponential law to calculate the Wöhler curve for each shot peening treatment.
The fracture surfaces of the specimens that failed in the fatigue tests were analysed at high magnification. A Nikon SMZ-1000 optical microscope (Nikon Corporation, Tokyo, Japan) and a JEOL JSM-6010-LA (JEOL Ltd., Tokyo, Japan) scanning electron microscope (SEM) were used to identify the crack initiation sites and failure mechanisms.

3. Results

3.1. Grain Size

The grain structures at the surface of the 6082 T6 aluminium alloy subjected to different shot peening treatments are shown in Figure 4. The grains remained equiaxed after the shot peening treatments. The results of these grain size measurements are summarized in Table 4. All shot peening treatments led to some degree of grain refinement, which has already been observed in previous research works [34,35]. The severe plastic deformation caused by the shot peening processes leads to dislocation slip and deformation twinning in metallic materials. As the plastic deformation increases, the dislocation slip phenomenon eventually forms dislocation cells and subgrains [36,37]. In this research, the cut wire shots presented the least effect on grain size, whereas the ceramic particles caused more grain refinement. This fact can be attributed to the higher hardness of the ceramic shots, which consequently led to higher plastic deformation of the target material. In general, it was noted that the higher the Almen intensity, the smaller the average grain size obtained at the surface of the treated samples. No significant changes in the intermetallic phase concentrations were observed after the shot peening treatments.

3.2. Nanohardness

The results of nanohardness measurements are shown in Figure 5. The hardness of the non-treated aluminium alloy remained constant throughout its depth, while all shot peening treatments led to a significant enhancement in hardness at the surface. In all the treatments, hardness decreases with depth. Beyond 800 µm depth, the strain hardening effect is assumed to be negligible. The greatest increase in hardness was achieved with alumina particles, followed by silica microspheres, whereas this effect was less noticeable with cut wire shots. This fact was consistent with the grain refinement observations and suggested that the decrease in grain size and strain hardening tends to be more intense when using higher Almen intensities, as observed in Figure 6. It is well known that surface hardness can be increased when grain refinement takes place, and the dislocation density increases, as predicted by the Hall–Petch effect and Taylor strengthening. Such phenomena are boosted by the plastic deformation attained through the shot peening processes and becomes more intense at the surface layer of the treated material [36,37]. The evolution of hardness with depth, as obtained through these shot peening treatments, is in accordance with other research works in the literature [38,39,40].

3.3. Residual Stresses

Residual stresses at the surface of the different treatments are shown in Figure 7. Unlike nanohardness or grain refinement measurements, the surface residual stress values obtained through XRD were not sorted by Almen intensity. The highest compressive residual stress at the surface was attained with silica microspheres, followed by zinc cut wire. However, the alumina shots decreased the compressive residual stresses at the surface with respect to the non-treated samples. Aluminium cut wire shots seemed to be inefficient at inducing compressive residual stresses at the surface of the samples.

3.4. Surface Roughness

The average surface roughness values, Sa, obtained with each shot peening treatment are presented in Table 5. As the ceramic particles were harder than the metallic ones, the effect of the former ones in the surface roughness was more evident. The roughness obtained with the alumina particles was substantially higher than that obtained with the other shots, as these particles presented the highest hardness and widest size distribution. Silica microspheres also affected the surface finish considerably due to their high hardness. Nevertheless, the effect of silica microspheres on surface roughness was more moderate than the effect of alumina shots, probably because of the smaller size and lower hardness of silica. Interestingly, the zinc cut wire particles led to rougher surface finishes compared to the aluminium cut wire ones despite the higher hardness and Almen intensity of the latter. The explanation for this phenomenon could be based on the higher density of zinc, which would have provided higher kinetic energy to zinc cut wire shots and thus attained greater plastic deformation in the shot peening process. The effect of aluminium cut wire particles in surface roughness was negligible.
The surface topography of all treatments performed on fatigue specimens can be observed in Figure 8. All surface finishes are presented following the same scale bar given in Figure 8 for the sake of comparison. The non-treated specimens presented a wavy profile characteristic of a machining process, and their roughness was very low. The roughest profile was obtained with alumina particles, showing sharp deep irregular indentations, whereas the silica microspheres left small spherical dimples distributed homogeneously across the surface. Specimens shot-peened with the aluminium cut wire still presented the machining grooves of the non-treated specimens, though modest plastic deformation could be noticed. The effect of zinc shots on the surface finish of the aluminium alloy was more visible than the effect of aluminium shots, as the former caused the grooves of the non-treated specimens to vanish.

3.5. Fatigue Strength

The results of the axial fatigue tests are shown in Figure 9, where the maximum stress values are plotted as a function of the cycles to failure on a semi-logarithmic scale for each surface treatment. All points fell within the HCF domain (104–107 cycles) for the applied load levels. For each surface treatment, the Wöhler curves were calculated using regression analysis, fitting the data to Basquin’s exponential law according to the following expression (1):
σ m a x = a   N b ,
where σ m a x is the maximum applied stress and N is the number of cycles to failure. The coefficients obtained in the regression analysis, a and b, along with the corresponding coefficient of determination, R2, are summarized in Table 6. These Wöhler curves are also displayed in Figure 9.
The non-treated specimens presented a fatigue strength of 268 MPa at 107 cycles. The Wöhler curve of the non-treated alloy was the steepest one, with a high goodness of fit, although there was a knee point where the slope changed abruptly, and the material exhibited an apparent fatigue limit. The position of this knee point may vary with the size of the defects in the material [41].
Regarding the shot-peened specimens, the best fatigue results were obtained with the silica microspheres, as this Wöhler curve presented the lowest slope and the highest fatigue strength at 107 cycles. At some stress amplitudes, the fatigue lives of the specimens treated with silica were extended by a factor greater than 10. Such enhancement in the fatigue life could have been related to the high surface compressive residual stress obtained with silica microspheres compared to the other treatments, as well as their small size and sphericity, which did not cause severe stress concentrations at the surfaces of the specimens. The zinc cut wire shots were also successful in improving the fatigue life of the aluminium alloy. The slope of this Wöhler curve was high compared to the others, but there was a pronounced knee point that caused a sudden change in trend and led to an improvement in the fatigue strength at 107 cycles. Specimens treated with zinc increased their surface compressive residual stresses in spite of the extremely low Almen intensity of this treatment. On the other hand, the surface treatment carried out with the aluminium cut wire seemed to cause little improvement in the fatigue life of the aluminium alloy and even a slight reduction in fatigue strength at 107 cycles. The alumina particles were the shots which led to the worst fatigue results, decreasing substantially the fatigue life of aluminium specimens at lower stress amplitudes. Some reasons to this phenomenon could have been the excessive roughness of these specimens and the sharpness of alumina shots, which may have caused severe stress concentrations that advanced fatigue failure.
The cycles to failure were more scattered in the specimens treated with shots that worsened the fatigue strength of the material. The coefficients of determination of the Wöhler curves obtained from the specimens treated with silica microspheres (0.958) or zinc cut wire (0.993) showed a very high goodness of fit, whereas those of the specimens shot-peened with alumina particles (0.816) or aluminium cut wire (0.866) were slightly lower. The low coefficient of determination of the specimens treated with alumina particles could have been the result of the wide distribution of the size of these particles, thus leading to a wide distribution of surface defects.

3.6. Fracture Surface Analysis

Images of the specimens that failed in the fatigue tests were acquired using an optical microscope to obtain a general view of the fatigue fracture features of the aluminium alloy. A fracture surface of the non-treated alloy is shown in Figure 10, where the arrows indicate the crack initiation site. Two zones were clearly distinguished: the fatigue crack propagation area and the final fracture zone. The fatigue crack propagation area was smooth, plane, and perpendicular to the applied axial load, whereas the final fracture zone was rougher and deviated at a significant angle from the plane of the crack propagation area due to the high ductility of the alloy.
Analogous features can be observed in the shot-peened specimens in Figure 11. In all cases, the marks that were present in the crack propagation area suggested that crack initiation took place close to the surface of the specimens, as indicated by the arrows. The specimens shot-peened with alumina particles (Figure 11b) and aluminium cut wire (Figure 11c) showed different crack propagation areas at different planes, which means that different cracks appeared at several locations due to the presence of surface defects. This observation was in accordance with the fact that these treatments are the ones which led to the worst fatigue results.
Higher magnification images were acquired with the SEM in order to identify the crack initiation sites. In Figure 12a, the crack initiation site of a non-treated aluminium specimen is shown. It is clear that crack initiation took place at the surface of the specimen, as indicated by the arrow. Once the crack originated, it propagated in a transgranular manner, leaving marks throughout the fracture surface that are oriented to the crack initiation site, as illustrated in Figure 12b.
The fracture surfaces of the shot-peened aluminium specimens are shown in Figure 13, where the crack initiation sites are indicated by the arrows. All these specimens were subjected to the same fatigue loading conditions for the sake of comparison. In Figure 13a, the plastically deformed layer due to the impacts of silica shots is clearly observed. The orientation of the marks in the fracture surface suggested that crack initiation took place a few microns below the surface of the specimen. Subsurface crack initiation explained the better fatigue performance of this treatment, as stress concentrations below the surface are usually lower. Regarding the specimen treated with alumina particles in Figure 13b, a considerable dimple was present, which caused crack initiation. This observation unequivocally confirmed the introduction of severe stress concentrations by alumina shots, which led to a worse performance in fatigue tests. In Figure 13c, the fracture surface of the specimen shot-peened with aluminium cut wire is shown. In this case, the crack initiation site was located at a shallow dimple at the surface of the material. As for the specimen surface treated with zinc cut wire in Figure 13d, it seemed that the crack was originated at the subsurface, as there was a discontinuity in the crack propagation features few microns below the surface. This observation would explain the higher fatigue lives achieved with the zinc shots.
The induction of compressive residual stresses and strain hardening effect by the shot peening process affects the surface layer of the material. These phenomena may hinder crack initiation at the surface and displace the critical stress concentration zone to the interior [42]. Since shot peening is a treatment that only affects the surface layer of the material, no significant modifications in the crack propagation mechanisms between different treatments were expected. The effect of shot peening on the fatigue life of materials in the HCF domain was mainly due to the delay in crack initiation.

4. Discussion

This work has addressed the fatigue behaviour and strengthening mechanisms of 6082 T6 aluminium alloy specimens subjected to shot peening with four types of commercially available shots. Shot peening parameters, such as the air pressure, the peening distance, the angle of impingement or peening time were kept constant in all treatments. However, these shots presented different physical properties, sizes, and shapes, which consequently led to different Almen intensities for each treatment. The shots utilized in this work, sorted by Almen intensity, were alumina, silica microspheres, aluminium cut wire and zinc cut wire.
The ceramic shots caused more significant strain hardening effects than the metallic shots. Notwithstanding these effects, the use of alumina shots decreased the surface compressive residual stress and thus the fatigue strength of the aluminium alloy. Previous works have reported that too high Almen intensities may be detrimental to fatigue behaviour [43,44,45]. Furthermore, it was observed that the sharpness of alumina shots had great influence on the shape of the indentations left in the target material, which may have affected the stress concentration level at the surface of the treated specimens [46]. Fracture surface analysis corroborated that the crack initiation of the specimens treated with alumina shots took place at deep indentations and at several planes simultaneously. Moreover, the fact that these specimens presented lower surface compressive residual stresses than the non-treated ones suggested that the alumina shots caused severe surface damage or even microcracks, besides the plastic deformation. It should also be pointed out that the specimens peened with alumina shots generated the Wöhler curve with the highest scatter. This observation can be linked to the fact that alumina shots presented wider size distribution and more irregular shapes than the other shots used in this study. On the other hand, the silica microspheres were successful in improving the fatigue life of the aluminium alloy. Unlike the alumina shots, the silica microspheres were small and spherical, so the dimples in the treated surface did not cause severe stress concentrations despite the increase in the surface roughness. Furthermore, silica shots induced the highest compressive residual stresses at the surface of the treated specimens. Many research works have demonstrated that the use of finer particles in the shot peening process can lead to better results in terms of improving the fatigue life of components [47,48].
The comparison between aluminium and zinc cut wire shots is of special interest since these two types of shots presented the same size and geometry. Although the Almen intensity and surface hardness attained with aluminium shots were more significant than those obtained with zinc, the effect of the aluminium shots on surface roughness was negligible. In contrast, zinc cut wire reached higher surface compressive residual stress and roughness. Although the zinc shots presented less hardness compared to the aluminium ones, the latter had less density, which could have affected the kinetic energy transferred to the target material during the shot peening process. The zinc cut wire particles were successful in enhancing the fatigue life of the aluminium alloy in spite of the extremely low Almen intensity of this treatment, whereas the effect of aluminium cut wire on the fatigue life of the 6082 T6 alloy was marginal, and the fatigue strength at 107 cycles was to some extent reduced with the latter. As there is no foundation to believe that the aluminium shots were detrimental to fatigue behaviour; the slight reduction in fatigue strength in specimens treated with aluminium cut wire shots is probably attributed to the natural variability in fatigue life results rather than to a negative effect of these shots.
In general, it was observed that the harder the shot used in the shot peening treatment, the higher the Almen intensity was. Every shot peening treatment presented some effect on surface grain size, nanohardness, residual stresses and surface integrity. The grain size measurements of the shot-peened specimens, although not significantly different from the non-treated material ones, were consistent with the Almen intensity values. The strain hardening effect measured by means of nanoindentation was also in good agreement with the grain refinement observations, and Almen intensities were highly correlated with the increase in nanohardness in shot-peened specimens.
Nevertheless, the fatigue strengths obtained in this study for each surface treatment were more correlated with the surface compressive residual stresses induced by each type of shot rather than with their Almen intensities. Since Almen intensity quantifies peening intensity by measuring the deflections of shot-peened strips, Almen intensity would correlate better with the resultant internal force caused by the whole stress distribution rather than with the sole values of the residual stress induced at the surface. As most specimens fail due to surface crack initiation in the HCF domain, it is the residual stress values obtained in the vicinity of the surface that will determine the resistance to crack initiation under cyclic loading of the treated parts.
It is well known that surface integrity has an important effect on fatigue life as well [49]. Not only did the hardness of the shots influence the surface finish of the target material but the shape and density of the shots also had a remarkable effect. The ceramic shots presented a more significant effect on roughness compared to the metallic ones, as the hardness values of the ceramic shots were substantially higher. The alumina shots induced much rougher surface profiles due to their massive hardness, their big size and their sharpness. As a result, the fracture surface analysis revealed that the poor surface integrity caused by the alumina shots led to excessive stress concentrations, thus advancing crack initiation. The effect of the silica microspheres on surface roughness was significant but not as noticeable as the effect of alumina, and the surface finish attained with the former was smooth. Therefore, the effect of the compressive residual stresses obtained with silica overcame the negative effect of the increase in roughness. As for the metallic shots, even though the aluminium ones were harder than zinc, the latter presented higher density, so the aluminium cut wire barely altered the roughness of the target material, whereas the zinc cut wire notably increased surface roughness. Then again, the beneficial effect of compressive residual stresses attained with zinc resulted in higher fatigue lives than the ones obtained with aluminium, since the aluminium cut wire barely modified the surface residual stress.
Shot peening is widely used to expand the service life of aluminium components subjected to cyclic loading, especially in key industrial sectors such as the automotive or aircraft industries. However, the fatigue strength of industrial components depends on a wide variety of factors, which include surface hardness, residual stresses and surface integrity. As shot peening treatments can modify all of them, it is a challenging task to correlate all these properties with fatigue strength. This work has proven that the choice of the shots utilized to perform a shot peening treatment in an industrial component can lead to a wide range of fatigue results and surface strengthening effects. An optimal improvement of the service life of industrial components will have a strong effect on the maintenance and repair costs of many companies. Hence, this research has aimed to emphasize the effect of the shot medium on the surface strengthening mechanisms and fatigue behaviour of aluminium alloys, in an attempt to help future research works to design optimal shot peening treatments.

5. Conclusions

This study investigated the fatigue behaviour of a 6082 T6 aluminium alloy subjected to shot peening treatments with four types of particles. The particles used in this study were silica microspheres, alumina particles, aluminium cut wire and zinc cut wire. The main surface effects after shot peening were analysed, including grain refinement, strain hardening, surface residual stress and roughness, in order to better understand the fatigue results. The most important findings of this research are summarised below:
  • Higher Almen intensities were obtained with harder shots in all cases.
  • The surface hardness and the grain refinement of the shot-peened material presented high correlation with the Almen intensity.
  • The surface roughness of the shot-peened specimens depended not only on the hardness of the shot but also on its density.
  • The best fatigue behaviour was obtained with silica microspheres, as these shots led to the highest compressive residual stress at the surface and shifted crack initiation to the interior of the specimens.
  • Alumina particles caused a significant decrease in the fatigue strength due to severe surface damage, despite showing the highest surface hardness. It is also believed that the wide size distribution of alumina shots could have increased the scatter of this Wöhler curve.
  • The fatigue results obtained hereby are consistent with the surface compressive residual stresses, rather than with Almen intensity, strain hardening or surface integrity.

Author Contributions

Conceptualization, A.R. and R.C.; methodology, E.C.-G. and S.V.-P.; software, P.P.-Á.; validation, J.d.V., D.Á. and A.B.; formal analysis, E.C.-G.; investigation, E.C.-G. and R.C.; resources, M.R. and C.M.; data curation, S.V.-P. and P.P.-Á.; writing—original draft preparation, E.C.-G.; writing—review and editing, A.R. and R.C.; visualization, J.d.V.; supervision, A.B.; project administration, A.R., D.Á. and R.C.; funding acquisition, M.R. and C.M. All authors have read and agreed to the published version of the manuscript.

Funding

This work was partially supported by the by the “European Union—NextGenerationEU”, by the Government of Spain—Spanish Center for Industrial Technological Development—CDTI [project number SOLDALAC IDI-20221122 and IDI-20221123], the Spanish Ministry of Universities CAS21/00454 grant and by Xunta de Galicia (ED431C 2023/25 and ED431C 2019/23).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

The authors thank the technical staff of CACTI (University of Vigo) for their help with the sample characterization.

Conflicts of Interest

Author Manuel Román and Author César Magdalena were employed by CIE Galfor S.A. The remaining authors declare that the research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Grain structure of the non-treated aluminium alloy 6082 T6.
Figure 1. Grain structure of the non-treated aluminium alloy 6082 T6.
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Figure 2. Particles used in the shot peening process: (a) silica microspheres, (b) alumina, (c) aluminium cut wire and (d) zinc cut wire.
Figure 2. Particles used in the shot peening process: (a) silica microspheres, (b) alumina, (c) aluminium cut wire and (d) zinc cut wire.
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Figure 3. Dimensions and requirements of the fatigue test specimens.
Figure 3. Dimensions and requirements of the fatigue test specimens.
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Figure 4. Grain structures at the surface of the aluminium alloy 6082 T6 after shot peening treatments with (a) silica microspheres, (b) alumina, (c) aluminium cut wire and (d) zinc cut wire.
Figure 4. Grain structures at the surface of the aluminium alloy 6082 T6 after shot peening treatments with (a) silica microspheres, (b) alumina, (c) aluminium cut wire and (d) zinc cut wire.
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Figure 5. Nanohardness measurements as a function of depth for different shot peening treatments.
Figure 5. Nanohardness measurements as a function of depth for different shot peening treatments.
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Figure 6. Effect of Almen intensity on (a) grain refinement and (b) surface nanohardness increase.
Figure 6. Effect of Almen intensity on (a) grain refinement and (b) surface nanohardness increase.
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Figure 7. Surface residual stresses for different shot peening treatments.
Figure 7. Surface residual stresses for different shot peening treatments.
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Figure 8. Surface topography of aluminium alloy 6082 T6 specimens (a) with no surface treatment and shot-peened with (b) silica microspheres, (c) alumina particles, (d) aluminium cut wire and (e) zinc cut wire.
Figure 8. Surface topography of aluminium alloy 6082 T6 specimens (a) with no surface treatment and shot-peened with (b) silica microspheres, (c) alumina particles, (d) aluminium cut wire and (e) zinc cut wire.
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Figure 9. Wöhler diagrams of the 6082 T6 alloy subjected to different shot peening treatments.
Figure 9. Wöhler diagrams of the 6082 T6 alloy subjected to different shot peening treatments.
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Figure 10. Fracture surface (front and lateral) of a non-treated specimen of 6082 aluminium alloy tested to a maximum stress of 305 MPa.
Figure 10. Fracture surface (front and lateral) of a non-treated specimen of 6082 aluminium alloy tested to a maximum stress of 305 MPa.
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Figure 11. Fracture surfaces (front and lateral) of the 6082 aluminium alloy shot-peened with (a) silica microspheres, (b) alumina particles, (c) aluminium cut wire and (d) zinc cut wire, all tested to a maximum stress of 305 MPa.
Figure 11. Fracture surfaces (front and lateral) of the 6082 aluminium alloy shot-peened with (a) silica microspheres, (b) alumina particles, (c) aluminium cut wire and (d) zinc cut wire, all tested to a maximum stress of 305 MPa.
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Figure 12. Fatigue crack (a) initiation and (b) propagation of a non-treated specimen of 6082 aluminium alloy tested to a maximum stress of 305 MPa.
Figure 12. Fatigue crack (a) initiation and (b) propagation of a non-treated specimen of 6082 aluminium alloy tested to a maximum stress of 305 MPa.
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Figure 13. Fatigue crack initiation sites of 6082 aluminium alloy specimens shot-peened with (a) silica microspheres, (b) alumina particles, (c) aluminium cut wire and (d) zinc cut wire, all tested to a maximum stress of 305 MPa.
Figure 13. Fatigue crack initiation sites of 6082 aluminium alloy specimens shot-peened with (a) silica microspheres, (b) alumina particles, (c) aluminium cut wire and (d) zinc cut wire, all tested to a maximum stress of 305 MPa.
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Table 1. Chemical composition of the 6082 alloy (weight %).
Table 1. Chemical composition of the 6082 alloy (weight %).
ElementMgSiMnFeCuCrZnOtherAl
Content min.0.600.700.40-----Bal.
Content max.1.201.301.000.500.100.250.200.10Bal.
Content0.751.090.500.200.020.050.020.0997.28
Table 2. Mechanical properties of the 6082 alloy.
Table 2. Mechanical properties of the 6082 alloy.
PropertyYield StrengthTensile StrengthDuctilityHardness
Value361 ± 11 MPa383 ± 11 MPa5.3 ± 0.1%1.7 ± 0.1 GPa
Table 3. Properties of the particles used in the shot peening process and Almen intensities.
Table 3. Properties of the particles used in the shot peening process and Almen intensities.
MaterialShapeSize
(mm)		Hardness
(HV)		Density
(g/cm3)		Almen Int. (mm)
Silica (SiO2)Spheres0.15 ± 0.05500–6002.3–2.60.24 A
Alumina (Al2O3)Irregular0.8 ± 0.2>20003.90.31 A
Aluminium (Al)Cut wire~0.890–1302.70.19 N
Zinc (Zn)Cut wire~0.830–407.20.03 N
Table 4. Average grain size of the aluminium alloy 6082 T6 after shot peening treatments.
Table 4. Average grain size of the aluminium alloy 6082 T6 after shot peening treatments.
Shot TypeNoneSilica Alumina Aluminium Zinc
Grain size (µm)11.0 ± 3.39.9 ± 2.88.7 ± 3.710.3 ± 3.110.3 ± 4.4
Table 5. Surface roughness values of specimens subjected to different shot peening treatments.
Table 5. Surface roughness values of specimens subjected to different shot peening treatments.
Shot TypeNoneSilica Alumina Aluminium Zinc
Sa (µm)0.98 ± 0.291.91 ± 0.064.51 ± 0.460.97 ± 0.201.59 ± 0.42
Table 6. Coefficients of the regression analysis for each surface treatment.
Table 6. Coefficients of the regression analysis for each surface treatment.
Shot Typea (MPa)b (-)R2
None1129−0.1100.980
Silica (SiO2)471−0.0290.958
Alumina (Al2O3)602−0.0560.816
Aluminium (Al)609−0.0580.866
Zinc (Zn)787−0.0730.993
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Calvo-García, E.; del Val, J.; Riveiro, A.; Valverde-Pérez, S.; Álvarez, D.; Román, M.; Magdalena, C.; Badaoui, A.; Pou-Álvarez, P.; Comesaña, R. The Influence of Shot Peening Media on Surface Properties and Fatigue Behaviour of Aluminium Alloy 6082 T6. J. Manuf. Mater. Process. 2024, 8, 192. https://doi.org/10.3390/jmmp8050192

AMA Style

Calvo-García E, del Val J, Riveiro A, Valverde-Pérez S, Álvarez D, Román M, Magdalena C, Badaoui A, Pou-Álvarez P, Comesaña R. The Influence of Shot Peening Media on Surface Properties and Fatigue Behaviour of Aluminium Alloy 6082 T6. Journal of Manufacturing and Materials Processing. 2024; 8(5):192. https://doi.org/10.3390/jmmp8050192

Chicago/Turabian Style

Calvo-García, Erik, Jesús del Val, Antonio Riveiro, Sara Valverde-Pérez, David Álvarez, Manuel Román, César Magdalena, Aida Badaoui, Pablo Pou-Álvarez, and Rafael Comesaña. 2024. "The Influence of Shot Peening Media on Surface Properties and Fatigue Behaviour of Aluminium Alloy 6082 T6" Journal of Manufacturing and Materials Processing 8, no. 5: 192. https://doi.org/10.3390/jmmp8050192

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