Introduction

Laser shock peening (LSP) is a surface treatment technique that has been shown to be effective in improving the fatigue properties of a number of metals and alloys. Potential applications are directed to aerospace and automotive industries. The beneficial effects of LSP on static, cyclic, fretting fatigue and stress corrosion performance of aluminium alloys, steels and nickel-based alloys have been demonstrated [1-5]. Since laser beams can be easily directed to the fatigue critical areas without masking, LSP technology is expected to be widely applicable for enhancing the fatigue properties of metals and alloys, particularly those that show a positive response to shot peening.

The objective of this work is to examine the effect of laser shock peening on the fatigue behaviour of 6061-T6 aluminium alloy (AA) specimens. Process parameters such as pulse density are varied. The effect of LSP on fatigue crack growth rate, microhardness, and residual stress are investigated.

In laser shock peening of metals, the sample is either completely immersed in water or in air. The laser pulse is then focused onto the sample. The schematic of the process in water is shown in Fig. 1. When the laser beam is directed onto the surface to be treated, it passes through the transparent overlay and strikes the sample. It immediately vaporises a thin surface layer of the overlay. High pressure against the surface of the sample causes a shock wave to propagate into the material. The plastic deformation caused by the shock wave produces the compressive residual stresses at the surface of the sample. Laser pulse may come directly from the laser apparatus or may be delivered using an optical fiber [6].

Materials and Experimental Procedures

Plates of 6061-T6 AA with 6.3 mm thickness were subjected to the corresponding solution treatment and natural aging (T6 condition). Chemical composition was determined using a spark emission spectrometer (Table 1).

The specimens used for the residual stress measurements were coupons of 60x60x6.3 mm. Compact tension specimens for fatigue crack growth tests were used (prepared according to ASTM E-647 [8]). See Fig. 2. All these latter specimens were machined with the loading axis parallel to the rolling direction (L). In Fig. 2 the pulse swept direction is also shown.

The LSP experiments were carried out using a Q switched Nd:YAG laser operating at 10 Hz with a wave length of 1064 nm and the FWHM of the pulses was 8ns. A convergent lens is used to deliver 1.2 J. Spot diameter was 1.5 mm. Three pulse densities were used: 900, 1350 and 2500 pulses/cm2. Specimens were submerged in a water bath as they were irradiated. Water was the confined medium. Specimen treated area was 20mm x 15 mm both sides of the specimen. A 2D motion system was used to control the specimen position and generate the swept pulse. Controlling the velocity of the system, the desired pulse density was reached.

Microhardness measurements were undertaken with a 50g load and 10s holding time. Residual stress distribution was determined by the hole drilling method according to ASTM E837 standard [9]. Strain gauge rosettes EA-13-062RE-120 along with a RS-200 milling guide from Measurements Group were used.

Fatigue crack growth tests were carried out on a MTS 810 servo-hydraulic system at room temperature in air. Load ratio (R= ?min/?max) was maintained at R= 0.1. It was help a frequency of 20 Hz with a sin wave form during the experiment. Two specimen groups with 900, 1350 and 2500 pul/cm2 were formed. One specimen on each group was tested to maximum load of 3000N and another to 5000N. Crack lengths were measured at a magnification of x10 using a CCD camera. Stress intensity factor KI due to external load P was determined by using the equation [8, 10]: Formula can not be printed for internet use.

Results and Discussion

Microhardness distribution in depth is shown in Fig. 3. It is clear from the plot that the higher the pulse density the deeper the hardness. However, these hardness values still remain beneath those reported for the controlled shot peening process on high strength aluminium alloys [11]. This latter is  indicative of a negligible strain hardening.

Residual stress distributions as a function of depth are depicted in Fig. 4. Analogously, it is observed that the higher the pulse density the larger the residual stresses S2. From the residual stress profiles, it is also observed that there is an effect of the pulse swept direction, i.e. compressive stress component to the swept direction is much higher than that parallel to such direction.

Fig. 5  shows the fatigue crack growth rates. A comparison of results without and with LSP treatment is illustrated. As the pulse density is increased, fatigue crack growth rate slows down for ?K1 constant. It is clear that da/dN values are lower than for the pristine material. These results show the effect of the compressive residual stresses field induced on the surfaces of the treated specimens. Fitting the experimental results to the well known Paris rule below it is observed (see Table 2) that C increases as pulse density increases and m decreases when pulse density is also increased. Formula can not be printed for internet use.

Concluding Remarks

Laser shock peening is an effective surface treatment method to enhance fatigue properties of 6061-T6 aluminium alloy. This latter can mainly be attributed to the compressive residual stresses which  are acting as decelerators of the crack growth as shown by the experimental results. Fatigue crack growth was quantified through the determination of Paris law parameters for stress ratio R = 0.1. On the other hand, this surface treatment was found to cause some increase in hardness but not as substantial as controlled shot peening process. Therefore, it may be assumed that strain hardening is nearly negligible.

References

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J. Solis-Romero (PhD)
E-mail: josesolis@itesm.mx

C. Rubio-González (PhD)
E-mail: crubio@cidesi.mx

M. Paredes-Guillén (MSc)

Affiliations
Instituto Tecnológico de Tlalnepantla
(Edo. de Mexico)

Centro de Ingeniería y Desarrollo Industrial (Queretáro, México)