Surface modification by oil jet peening in Al alloys, AA6063-T6 and AA6061-T4: Part 2: Surface morphology, erosion, and mass loss

doi:10.1016/j.apsusc.2005.12.164

Applied Surface Science

Volume 253, Issue 2, 15 November 2006, Pages 997-1005

https://doi.org/10.1016/j.apsusc.2005.12.164 Get rights and content

Abstract

Oil jet peening is a new surface treatment technique that can be potentially applied to impart compressive residual stresses in metal parts. The effect of oil jet pressure on the surface integrity and texture of metals are discussed. The surface morphology, mass loss rate, indentation, and erosion are reported. With increasing stand-off distance, the size of indents significantly decreases and reduces the average roughness in the both specimens. Results are also compared with other mechanical surface treatment process such as shot peening, laser shock peening, and water jet peening.

Introduction

Surface morphology plays an important role in the performance of machine parts that experience cyclic loading. Under fatigue loading cracks always nucleate from the free surface. Cracks nucleate at positions where the plastic strain concentration are high such as notches, dimples, dents, or any other source [1], [2], [3]. Presence of a compressive residual stress layers prevents crack initiation and growth and therefore has a beneficial effect on the fatigue life. High surface roughness generates local stress concentration and accelerates crack initiation [4], [5]. Fatigue limit strength was found improved by about 50% due to the presence of compressive residual stress on the surface [6]. However, fatigue limit strength decreases by about 35% due to the roughening due to shot peening in aluminum alloy 7075 [6]. Surface roughness also affects the wear rate under sliding conditions [7].

Shot peening generally results in a rough surface with a large increase in the average surface roughness, R_a, and peak-to-valley roughness, R_max. This is detrimental to wear and fatigue [4], [8]. Further increase in fatigue limit is possible by secondary polishing after shot peening [9], [10], [11]. In practice, it is not advisable to remove more than 10% of depth of compressive stress induced by peening. For wear resistance applications, removal of the roughened surface is necessary. However, due to the presence of thin shot peened compressive residual stress layer, removal of the rough surface also produces a significant decrease in the compressive residual stress layer thickness. Shot peening also leaves undesired features such as surface lapping, folds, etc., on treated aluminum surface [12]. In shot peening, as the shots used are in the range of 0.18–6 mm [11], solid shot cannot always penetrate the most inaccessible areas of components such as gear tooth, fillets, joints, and notches. This causes inadequate coverage and limits the depth of the compressive residual stress achievable.

Many investigations on the changes in surface morphology of laser shock processed materials have been performed using scanning electron microscope (SEM) and roughness measurements [7], [13], [14]. When no protective laser-absorbent coating was used on the substrate, the laser shock processing causes a severe surface melting and vaporization, particularly in aluminum [15], [16]. Water jet peening has the same surface deformation effect as conventional shot peening [17], [18]. Water jet peened surface exhibits increased crack initiation life because of an improved coverage and surface finish. The large water pressure results in reduced compressive residual stress, increase in surface roughness (pit like surface), and erosion [17], [19]. However, water jet peening studies have shown that during cyclic loading, the induced compressive residual stress helps to reduce the mean stress set by the external force and inhibit the slip within the thin layer [19].

Material erosion during water jet peening is an issue that limits the practical application [17], [19]. Peening using a high viscosity oil will reduce the erosion problems encountered in water jet peening. Pai and Hargreaves [20] conducted erosion experiments on various metals including aluminum alloy, AA5005, using hydraulic fluids. Authors have reported that an increasing viscosity of the oil led to a decrease in the rate of growth and collapse of bubble and hence reduced erosion. Talks and Moreton [21] summarized that cavitation erosion rate of different hydraulic fluids against steels was: water > w/o emulsion > water–glycol > invert emulsion > mineral oil. Tsujino et al. [22] also found with a different hydraulic fluids the order of erosion rate to be: water > heavy water base fluid > water–glycol > phosphate ester > mineral oil. The oil jet peening is expected to reduce the erosion compared to water jet and lead to a smoother surface.

In Part I, the compressive residual stress profile and hardening due to oil jet peened are investigated [23]. This article describes the surface morphology, coverage, mass loss rate, and erosion in oil jet peened aluminum alloys, AA6063-T6 and AA6061-T4. Surface profilometry and optical microscope were used for characterizing the surface texture and topography.

Section snippets

Test materials and experimental procedure

Rectangular specimens machined to the dimensions 20 mm × 30 mm surface area and 4 mm thick were cleaned using an ultrasonic cleaner in acetone before peening. Peening was carried out at stand-off distances (SODs), 25 and 40 mm with the oil pressure of 50 MPa. The transverse nozzle velocity is kept constant (1.5 mm/s). The angle of attack is 90° and the pitch is 0.25 mm. The nozzle used has a throat diameter of 0.25 mm. In order to identify the essential feature of oil jet peening, the unpeened and oil

Surface morphology

Fig. 1 shows the micrographs of unpeened and oil jet peened surfaces of AA6063-T6 specimens peened at various stand-off distances. A large number of small indents and a few large indents were observed on the peened surface of AA6063-T6 specimens. The indents formed on the peened surface are due to impact of oil jet droplets. During peening, each drop of oil jet generates a large impulse pressure, P_imp, and shear stresses and causes a severe surface plastic deformation. The size of indent

Conclusions

In oil jet peened AA6063-T6 and AA6061-T4, the indent diameter, visual coverage, roughness, and depth of indent decreases with increasing stand-off distances. No erosion cavities were observed in the high yield strength AA6063-T6 alloy. Severe plastic deformation and erosion cavities were observed in the low yield strength of AA6061-T4. The developed oil jet peening is capable of producing 100% coverage in a single pass. The size of indents formed is lower than that formed in conventional shot

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Water cavitation jet peening (WCP) uses cavitation caused by the shear layer created by two concentric co-flowing jets with large velocity difference to introduce compressive residual stresses in the surface layers of metal components subjected to fatigue loading or a corrosive environment. Mass loss and surface alteration in WCP have been shown to be minimal compared to other mechanical surface enhancement techniques, such as shot peening (SP). This paper investigates the effect of concentric jet velocities in cavitation jet peening in a co-flow configuration on cavitation intensity and peening performance, which are characterized by accelerated erosion on Al 1100-O and Al 7075-T6 and a strip curvature test on Al 7075-T6. Accelerated erosion tests reveal that cavitation intensity and associated erosion (measured by mass loss) are greatly affected by the combination of the inner (V_in) and outer (V_out) jet velocities and the normalized standoff distance (s_n). Two characteristic erosion patterns are found depending on the relative magnitudes of the jet velocities: one that is focused at the jet center (termed center regime) and another that is concentrated in the surrounding annular region (termed ring regime). Erosion tests on Al 1100-O and Al 7075-T6 give unexpectedly different results in terms of the maximum mass loss as a function of the jet velocities and standoff distance. When compared to strip curvature tests, it is found that the accelerated erosion tests on Al 1100-O do not capture the influence of inner jet velocity V_in and imply misleading trends with regard to outer flow velocity V_out. Erosion and curvature tests on Al 7075-T6 are found to be in good agreement and therefore are believed to be better suited to identify the optimum process conditions in WCP. Notwithstanding the higher mass loss density values observed in the center regime, the resultant strip curvature is found to be higher in the ring regime for a higher inner jet velocity V_in, potentially leading to higher and deeper compressive residual stresses.
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Surface modification by oil jet peening in Al alloys, AA6063-T6 and AA6061-T4: Part 2: Surface morphology, erosion, and mass loss

Abstract

Introduction

Section snippets

Test materials and experimental procedure

Surface morphology

Conclusions

Eng. Fract. Mech.

Int. J. Fatigue

Mater. Sci. Eng.

Mater. Sci. Eng.

Int. J. Machine Tools Technol.

Int. J. Fatigue

Opt. Lasers Eng.

Mater. Lett.

Mater. Sci. Eng.

Ann. CIRP

Wear

Eng. Fract. Mech.

J. Mater. Process. Tech.

Wear

Wear

Int. J. Fatigue

Mater. Sci. Eng.

Appl. Phys. Lett.