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Vol. 14
November Issue
Year 2013
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in Vol. 14 - November Issue - Year 2013
Influence Of Shot Peening On Corrosion Resistance And Residual Stresses Of An Aluminium Alloy ENAW 7075 Under Different State Conditions



Table 1: Chemical composition and mechanical properties of the treated aluminium alloy


Table 2: Shot-peening parameters of the treated aluminium specimens


Fig. 1: Residual stress profiles in quenched state specimens under different SP conditions


Fig. 2: Residual stress profiles in quenched and annealed specimens under different SP conditions


Fig. 3: Cyclic polarisation curves measured in 0.15M NaCl water solution for the quenched state specimen after SP at different Almen intensities


Fig. 4: Cyclic polarisation curves measured in a 0.15M NaCl water solution for specimens in the quenched and precipitation- hardened state (195

Introduction
The surfaces of machine components can be improved by cold surface deformation, which increases fatigue strength and fatigue resistance. Shot peening provides the kinetic energy of hard particles to increase the density of dislocations in the thin surface layer of the material concerned. Shot peening (SP) induces compression residual stresses at the surface, which contribute to an increase in surface-carrying capacity under dynamic loading. Material behaviour after treatment can be additionally estimated by fracture mechanics testing. A weak point of mechanical processes of surface hardening such as shot peening is that the treated surface will be rough and its corrosion resistance will be reduced. The effect of shot peening of machine components, which produces changes in surface topography, microstructure, microhardness and residual stress profiles across the thin surface layers and is frequently complemented by dynamic tests, depends on numerous parameters. Surface treatment efficiency increases with increasing dislocation density in the thin surface layer; the desired surface quality being a surface without damage [1]. The shot-peening process is frequently used in practical applications due to its simplicity, since it is performed at ambient temperature; therefore, residual-stress profiles are easier to manage and there is less variation between them. The effect of shot peening depends on the equipment, media, intensity and coverage used. Because of numerous parameters, the Almen intensity is used in industrial applications to assess the shot-peening efficiency. The Almen intensity can be effectively used to compare the shot peening of surfaces under different surface treatment conditions. In surface treatment, the shot mass flow and workpiece movement velocity are very important since the mean distance between dimples in the workpiece surface occurring during shot impact on the workpiece surface can be determined from both of these variables. Some studies have presented a mathematical model for the calculation of the mean distance between shot impacts [2].

Experimental procedure
Base material specifications
This paper studies the effects of various shot-peening conditions to which the 7075 aluminium alloy in different states was subjected. The alloy designated as ENAW 7075 is in a T651 tempered state. This alloy was precipitation-hardened at a minimum rate of cold deformation between 1 - 3%. The heat treatment process was conducted with homogenization annealing at a temperature of 475ºC, followed by quenching and artificial ageing at a temperature of 130ºC for 12 hours. The chemical composition of the alloy and its mechanical properties are given in Table 1.
Specimens were cut from a rolled plate with a thickness of 10 mm. Cutting was performed with a machine cutter in order to prepare the specimen for metallographic examination. The specimens were cut carefully to avoid overheating the surface and thus to avoid undesirable microstructural changes and the introduction of additional residual stresses into the surface layer. Flat specimens were first subjected to homogenization annealing at a temperature of 475¢XC for 2 hours. This process was followed by quenching. Afterwards, specimens were artificially aged at 195¢XC for 8 hours with the objective of obtaining different numbers and sizes of precipitates. The ageing process was concluded by taking the specimens out of the furnace and leaving them to cool down to ambient temperature.

Shot peening conditions
SP treatment has positive effects on the material surface, causing compressive residual stresses that increase the fatigue strength of the material and prevent the initiation of cracks and the propagation of already existing micro-cracks. An SP-treated surface extends the life cycle of a machine component under the same load. Therefore, the effects of individual treatment parameters need to be known, such as the selection of a treatment medium, particle kinetic energy, and the coverage of traces of individual spheres.
SP treatment was performed using S170 heat treated steel spheres with a diameter of 430 µm and a hardness value of 56 HRc. Aluminium alloy 7075 was treated with two Almen intensities of 12A and 16A, while the degree of coverage was set to 100%. Table 2 illustrates the different parameters of SP treatment of aluminium alloy 7075.

Results and discussion
Residual stresses

Residual stresses were measured using a relaxation hole-drilling method in accordance with ASTM E 837-01E, a CEA-06-062-UM measuring rosette, and a Vishay RS-200 drilling device with a pneumatic turbine for achieving high drilling speeds.
The residual stress variation in treated specimens was determined using an integral method and an H-drill program package. The integral method provides a separate evaluation of residual stress at each increment of depth. Thus, its spatial resolution is the highest of all the methods and is the method of choice when measuring rapidly varying residual stresses, such as shot peening.
Figure 1 shows a comparison of the minimum principal residual stress profiles for the specimens in quench state, then the SP at 100% coverage, and two different Almen intensities of 12A and 16A.
From the residual stress variation,  it can be inferred that the values of the minimum residual stresses in the specimens at the near surface amount to around -90 MPa at 16A and around -150 MPa at 12A. The minimum principal compressive residual stresses of the SP- treated specimen with 12A reaches a maximal value of -230 MPa at a depth of 250 µm and then slowly rises towards the surface, where at depth 550 µm it remains almost constant until the end of the measured depth with a value of -235 MPa. The specimen treated with a 16A Almen intensity shows almost the same gradient as the previous one, where the residual stresses from the near surface go down to a maximal value of -380 MPa at a depth of 280 µm and then rise to the same values as the specimen treated with an Almen intensity 12A.
Figure 2 shows a comparison of the minimum principal residual stress profiles for the specimens at an ageing temperature 195¢XC and 100% coverage after SP at intensities of 12A and 16A. From the residual stress profile, it can be inferred that the values of the compressive residual stress of the specimen peened at Almen intensity 12A goes down from the initial value of -100 MPa near the surface to a maximal residual stress of -125 MPa at a depth of 350 µm,  and then remains almost constant at greater depths, i.e. 965 µm with a value of -240 MPa.
By increasing the intensity to 16A, the residual stress of the surface has a lower value, i.e. -60 MPa, but then goes to a higher compressive residual stress at a depth of 350 µm with a value of -196 MPa. After having reached the highest value, the residual stress profile is directed towards the tensile area with a small gradient and reaches -80 MPa at the end of the measuring depth, i.e. 965 µm.
It is important to note that residual stress profiles obtained at greater depths are still of a compressive nature. All measured values are in the range of tensile strength of the alloy. Similar residual stress profiles of the aluminium alloys after SP are also found in the technical literature on other aluminium alloys.

Corrosion resistance
Corrosion resistance of the aluminium alloy was tested using potentiodynamic polarisation tests in 0.15M NaCl water solution at pH=6.6. In the presence of chloride ions, the protective property of a passive/oxide film at the surface of specimens made of aluminium alloys is drastically reduced, which results in strong corrosion damage. The research results showed that at these points,  oxygen is driven out by the chloride ions, which results in small surface pits. This type of corrosion is called pitting corrosion [3].
Potentiostatic polarisation tests were performed using a Voltalab PGZ100 device and CEC/TH corrosion cells, both products of Radiometer Analytical. The data were established with a scan rate of the potential of 1mV/s, where the electrode potential was being increased up to -100 mVSCE. The sweep from the anode to cathode direction was reversed at a limited threshold value of 1 mA/cm2.
All cyclic polarisation (CP) measurements were carried out after a 1.5 h stabilisation with an open circuit potential. From the CP curves, it can be seen that in the forward scan, which started at about 100 mV more negative than the specimen free corrosion potential, a partial passivation occurred. Afterwards, a sharp increase in anodic current due to stable pit growth or pit propagation is observed [4].
Figure 3 shows the cyclic polarisation curves for the aluminium alloy in the quenched state measured after SP with two different intensities, 12A and 16A. The results show that the specimen treated with an intensity of 12A has a lower free corrosion potential Ecorr value of −788 mV than the specimen treated with an intensity of 16A with its value of −811 mV. A comparison of the corrosion current densities Icorr was performed with a Tafel analysis. The specimen treated with an intensity of 12A had a corrosion current density value of 0.0192 mA/cm2, which was 1.7 times higher than the specimen treated with an intensity of 16A, whose Icorr was value 0.0115 mA/cm2. The increase in the corrosion current density indicated a higher pit growth rate. The increased surface roughness of the SP treated specimens and the possible remains of the steel medium at the aluminium surface could have influenced the electrochemical properties, although the treated surface was chemically cleaned. In general, the larger the area of the hysteresis loop, the greater the susceptibility of the material to pitting corrosion.
Figure 4 shows the cyclic polarisation curves for aluminium alloy in the quenched and precipitation-hardened state at 195¢XC, measured after SP with intensities of 12A and 16A. It is noted that the Ecorr of the treated aluminium alloys did not change significantly for either SP conditions and that the 16A intensity corresponds to a slightly higher corrosion potential with a value of -779 mV. This is consistent with the notion that the bulk potential is largely dictated by the bulk composition of the alloy and typically does not depend on the microstructure. Because neither the heat treatment temperature nor time alters the bulk composition, the corresponding Ecorr was more or less unchanged. The corrosion current density Icorr of the SP specimens treated with an intensity of 12A was calculated to be 0.0184 mA/cm2 and for 16A 0.012 l mA/cm2.
A comparison of the corrosion current densities Icorr was performed with a Tafel analysis where it is noted that the specimen treated with an Almen intensity of 12A has a 1.5-fold higher corrosion current density and this is reflected in a higher pit growth rate.

Conclusion
This paper describes a study conducted on the corrosion behaviour of AA7075 under different state conditions, immersed in aqueous solutions of NaCl. To determine the effects of pitting corrosion on the fatigue properties of the SP treated aluminium alloy 7075 in different states, a series of tests were performed. Based on the current corrosion data, all collected values indicate that the worst corrosion resistance is found in specimens treated with an Almen intensity 12A, which corresponds to increased surface roughness of the SP-treated specimens. The results of the cyclic polarisation test indicate extensive localised corrosion attack in the tested chloride solution. After potential Esw at which the potential reached a limited threshold value of 1 mA/cm2, hysteresis was observed in the reversal scan, intersecting the forward scan in the cathodic region. A wider hysteresis loop and, consequently, a more negative Eprot are obtained for the aluminium alloy in the quenched state, indicating the most severe pitting propagation. Some authors have proposed that pitting propagation does not stop but continues at a decreasing rate until Eprot is reached, where pits or crevices repassivate, i.e. stop propagating the corrosion process. Other authors have proposed that the pit transition potential Eptp corresponds to the condition of complete repassivation for small pits, but that it is indicative of surface repassivation for deeper pits, which requires further potential depression at Eprot [5].

References
[1] V. Schulze. Modern Mechanical Surface Treatment: States, Stability, Effects, WILEY-VCH Verlag Gmbh & Co. KGaA, 2005.
[2] R. Clausen, J. Stangenberg. Roughness of shot-peened surfaces-definition and measurement, Proceedings of the 7th Int. Conference on Shot peening, 1999, 69-77.
[3] ASM Metals Handbooks, Volume 13: ¡¥Corrosion¡¦, ASM INTERNATIONAL, 2004, USA.
[4] U. Trdan, S. Žagar, J. Grum, J.L. Ocaña. Comparison of surface integrity and corrosion resistance of laser peened and shot peened ENAW 6082 alluminium alloy, Int. J. Struct. Integr. 2, 2011, 9¡V21.
[5] M. Finšgar, I. Milošev. Corrosion behaviour of stainless steels in aqueous solutions of methanesulfonic acid, Corros. Sci. 52, 2010, 2430¡V2438.




Author: Sebastjan Žagar BSc. & Prof. Dr. Janez Grum
Faculty of Mechanical Engineering
1000 Ljubljana, Aškerčeva 6, Slovenia
Tel. +386.1.477.12.03, Fax +386.1.477.12.25
E-mail: sebastjan.zagar@fs.uni-lj.si
E-mail: janez.grum@fs.uni-lj.si