1. Introduction

Surfaces of machine components can be improved by hardening techniques, such as laser and shot peening, which increases fatigue strength and fatigue resistance. Laser Peening (LP) is an innovative surface treatment, with which mostly a Q-switched Nd:YAG laser with short pulses of maximum 50 ns and with a power density, in the pulse peak, of as much as several tens of GW/cm2 are used. LP is based on plasma generation at the moment of the interaction of laser beam with a workpiece material, which produces shock impact waves, which, in turn, produce elastoplastic shifts of atomic planes in the material. Zhang et al. [1] applied LP to different types of steels, aluminium alloys, and titanium alloys. LP produces shocks of motive quantity, which produce considerable densification of dislocations and generation of compressive residual stresses of high gradient [2]. Conventional Shot Peening (SP) provides kinetic energy of hard particles that induce compressive residual stresses, 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 SP, is that the surface treated will be rough and its corrosion resistance will reduce [3]. The effect of SP of machine components depends on numerous parameters that produce changes in surface topography, microstructure, in variations of hardness and residual stresses and are frequently complemented by dynamic tests. Herzog et al [4] described the SP process and optimum parameters, which ensure surface-layer properties chosen in advance. They proved that a distribution of compression residual stresses could not be presented clearly by Almen intensity alone; it is necessary to also include other parameters influencing the residual-stress variation. The process is frequently used in practical applications due to its practicality and simplicity since it is performed at ambient temperature; therefore, residual-stress variations are easier to manage and they have less determined variation.

2. Experimental procedure

2.1 Base material specifications

For a comparison of the effect of LP and SP aluminium alloy ENAW 6082 was chosen. Alloy was in the precipitation-hardened state T-651. The material was subjected to preliminary homogenization at a temperature of 540°C, then quenched to ambient temperature, subjected to tensile loading with a 2% strain, and subjected to artificial ageing at a temperature of 160°C for 10 hours.

2.2 Peening conditions

Laser and shot peening has favourable effects due to the generation of compression residual stresses, which prevent the initiation of new cracks and propagation of existing microcracks [5]. LP was preformed with a Q-switched Nd:YAG laser with a wavelength of 1064 nm and a power density of 10.75 GW/cm2. A treatment intensity of 900 pulses/cm2, equivalent to a final cumulative fluence of 1.71 kJ/cm2 was chosen, with laser pulse duration time of 10 ns being uniform with a repetition rate of 10 Hz. At the LP surface sweep, the specimen was clamped in a movable computer-aided x-y table and submerged in water, with the overlapping pitch among individual pulses equal to 0.33 mm. SP was preformed with S170 steel shots of the same diameter with a particle hardness of 56 HRC. The chosen operating pressure was 1.6 bar, mass flow  =1.5 kg/min and arc height 0.31 mmA.

3. Results

3.1 Surface roughness

Surface roughness indicates a coincidental deviation of an actual surface profile from an ideal one. At the surface, there are frequently various defects and damages for which there are no specified procedures for the assessment with an optical or electronic microscope.

For the comparison and analysis of specimen surface roughness, the mean arithmetic roughness Ra and mean roughness depth Rz at a measuring length lm of 8 mm was chosen. Measurements were made with a profile meter Surtronic 3+, product of Taylor/Hobson Pneumo, using a Gaussian filter, cut-off 0.8 mm. All the roughness measurements made were performed in different directions at the specimen edge and in the middle. Thus, two measurements were made at the specimen edge in the longitudinal direction and two of them in the transverse direction. The remaining two measurements were made in the centre of the hardened specimen; one in the longitudinal direction and one in the transverse direction.

Figure 1 shows topographic images of the surface that confirmed an extreme dependence of treatment conditions to the final surface roughness. From a comparison of the surface topographies images, it can be inferred that the specimen surfaces after LP and SP differ because of the difference in crater sizes. After LP treatment with the pulse density of 900 pulse/cm2, the size of the surface craters occurring ranges between 50 μm and 100 μm. After SP, the craters are almost greater by a factor of 2 with crater diameters ranging between 90 μm and 170 μm. From the results of surface roughness measurements, it can be inferred that an increase of waviness and in turn, of surface roughness, is established both after LP and SP treatment. Specimens after SP, hardened with the operating pressure p = 1.6 bar and the particle mass flow, i.e. 1.5 kg/min had the roughness values Ra = 6.38 mm and Rz = 31.75 mm. Specimens which were treated with LP were more favorable and had lower roughness values, i.e. Ra = 4.88 mm and Rz = 25.83 mm. Lower roughness is indeed desired, since it means lower friction characteristics of the components part in various applications. The increase in surface roughness after LP is a consequence of numerous laser-beam interactions with the specimen surface and cumulative action of shock waves that in turn, produce microplastic shifts, first of all in the surface layer [2]. The results obtained in the surface roughness measurements after SP are in accordance with the expectations after hardening of soft materials [6].

3.2 Microhardness

The microhardness prior to and after LP and SP of aluminium alloy AlSi1MgMn was measured using the Vickers method with a load of 200g (HV0,2). As at least ten measurements are required to establish a microhardness profile in the hardened layer, a suitable load was chosen in order to provide a sufficient number of measurements and microhardness variation. The hardening results in Fig. 2 indicate that the material microhardness increased for both LP and SP techniques, where the average hardness of alloy ENAW 6082 in the soft state amounts to approximately 89 HV0.2.

The highest increase of micro-hardness was established after LP and it amounted to 119 HV0.2. In comparison, the treated material with SP had the maximum micro-hardness value of 116 HV0.2. From the micro-hardness profile, it was also observed that after LP the hardened depth was higher than after SP, with the hardened depth of 230 μm. After SP, the material micro-hardness was almost the same as in the initial state in the depth of 230 μm, whereas after LP, the micro-hardness values were higher even at the depth of 340 μm. From the hardness variation it can be therefore inferred that the hardened-layer depth does depend on the peening conditions, whereas after LP, it is more favorable, with higher-hardened depth.

3.3 Analysis of residual stresses by drilling a blind hole

Residual stresses were measured using a relaxation hole-drilling method in accordance with ASTM E 837-01E [7], a measuring rosettes CEA-06-062-UM, and a drilling device Vishay RS-200 with a pneumatic turbine for achieving high drilling speeds. The residual-stress variation in the LP- and SP-treated specimens was determined using an integral method and a program package H-drill. The integral method provides a separate evaluation of residual stress within each increment of depth. Thus, the spatial resolution is the highest of all the methods and is the method of choice when measuring rapidly varying residual stresses, such as laser and shot peening [8].

Figure 3 shows a comparison of the variations of the minimum principal residual stresses for the specimen in the initial state, after SP and after LP. From the residual-stress variation, it can be inferred that the values of the minimum residual stresses in the specimens in the initial state are ideal since they amount to around 0 MPa. Such a variation confirms that the heat treatment chosen (T-651) is adequate for the initial material state. The minimum principal compressive residual stresses of the LP-treated specimens are just beneath the surface in the compressive range and achieve maximum values of -242 MPa, which is in the range of tensile strength of the alloy. Similar residual-stress variations of the aluminium alloys after SP are found also in the technical literature on other aluminium alloys [9]. The minimum principal residual stresses after SP were also of a compressive character in the entire surface layer, but they were rather lower than after LP. The maximum value of compressive residual stress was equal to -113 MPa at the depth of 0.45 mm.

3.4 Pitting corrosion analysis

Corrosion resistance of the aluminium alloy was tested with potentiodynamic polarisation tests in a 3.5% water solution of NaCl. In the presence of chloride ions, of which action is very deteriorating, the protective property of a passive/oxide film at the surface of specimens made of aluminium alloys drastically reduces, which results in strong corrosion damages. The research results showed that at these points, oxygen is driven out by chloride ions, which results in small surface pits. Such a type of corrosion is called pitting corrosion [10]. The potentiostatic polarisation tests were performed with device Voltalab 21 and corrosion cells CEC/TH, products of Radiometer Analytical. The data were established with a scan rate of potential of 10 mV/s, where the electrode potential was being increased up to -500 mVSCE.

Figure 4 shows the potentiodynamic polarisation curves of aluminium-alloy measured prior to and after LP. From the variation of polarisation curves, it can be inferred that after laser peening the pitting potential (Epitt) will increase too. The LP specimens treated with 900 pulses/cm2 showed an increase in the pitting potential of 62mV in comparison with the same material in the as-delivered state. For an additional confirmation of improved corrosion resistance after LP, the specimens were verified also with the SEM microscope. From the SEM surface images it can be assessed that with both aluminium alloys the largest number of corrosion damages (pits) at the specimen surfaces occurs in the as-delivered state. The macrosection images of the specimen surfaces, which were subjected to a preliminary LP, confirm that the number of pits will reduce.

4. Conclusions

The paper treated comparison of laser and shot peening of aluminium alloy ENAW 6082-T651. The research results indicate that LP treatment in comparison to the traditional SP treatment produces higher and more favorable microhardness variation in the thin sub-surface layer. The hardened depth obtained with SP amounted to around 230 μm, but with LP treatment, the higher micro-hardness value was achieved even at the depth of 340 μm. In addition, after SP, higher surface roughness is obtained and SP may also damage the surface with bigger surface craters and possible microcracks, whilst surface finish after LP showed better characteristics, with higher dimensional accuracy. The compressive principal residual stresses obtained and their gradient in the thin surface layer confirmed an exceptionally favourable variation after LP treatment. The principal compressive residual stresses of the LP-treated specimens achieved maximum values of -242 MPa. After SP, the maximum value of compressive residual stress was equal to -113 MPa, which is almost two times lower than after LP treatment. The favourable variation of the principal residual stresses indicates higher material resistance to damages under dynamic loading of LP-treated machine components. Corrosion testing confirmed a pitting corrosion attack in the as-delivered state and in the treated state. The intensity of pitting corrosion attack, however, decreases with the LP-treated specimens. An increase in pitting potential after LP was also confirmed.

Acknowledgement

The authors acknowledge the financial support from the state budget by the Slovenian Research Agency (Programme No. P2-0270).

References

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E-mail: sebastjan.zagar@fs.uni-lj.si
uroš.trdan@fs.uni-lj.si,
janez.grum@fs.uni-lj.si

University of Ljubljana
Faculty of Mechanical Engineering
1000 Ljubljana, Slovenia