Effect of laser shock processing on fatigue crack growth and fracture toughness of 6061-T6 aluminum alloy

Materials Science and Engineering A 386 (2004) 291–295

Effect of laser shock processing on fatigue crack growth and fracture toughness of 6061-T6 aluminum alloy C. Rubio-Gonz´aleza,∗ , J.L. Oca˜nab , G. Gomez-Rosasa , C. Molpeceresb , M. Paredesa , A. Banderasa , J. Porrob , M. Moralesb a

Centro de Ingenier´ıa y Desarrollo Industrial, Pie de la cuesta No. 702, Desarrollo San Pablo, Quer´etaro, Qro. 76130, M´exico b Departamento de F´ısica Aplicada a la Ingenier´ıa Industrial, E.T.S.I.I. Universidad Polit´ ecnica de Madrid, Spain Received 4 March 2004

Abstract Laser shock processing (LSP) or laser shock peening is a new technique for strengthening metals. This process induces a compressive residual stress field which increases fatigue crack initiation life and reduces fatigue crack growth rate. Specimens of 6061-T6 aluminum alloy are used in this investigation. A convergent lens is used to deliver 1.2 J, 8 ns laser pulses by a Q-switch Nd:YAG laser, operating at 10 Hz. The pulses are focused to a diameter of 1.5 mm onto a water-immersed type aluminum samples. Effect of pulse density in the residual stress field is evaluated. Residual stress distribution as a function of depth is assessed by the hole drilling method. It is observed that the higher the pulse density the larger the zone size with compressive residual stress. Densities of 900, 1350 and 2500 pulses/cm2 with infrared (1064 nm) radiation are used. Pre-cracked compact tension specimens were subjected to LSP process and then tested under cyclic loading with R = 0.1. Fatigue crack growth rate is determined and the effect of LSP process parameters is evaluated. Fatigue crack growth rate is compared in specimens with and without LSP process. In addition fracture toughness is determined in specimens with and without LSP treatment. It is observed that LSP reduces fatigue crack growth and increases fracture toughness in the 6061-T6 aluminum alloy. © 2004 Elsevier B.V. All rights reserved. Keywords: Fatigue test; Laser shock processing; Residual stress

1. Introduction Laser shock processing (LSP) is a new and promising surface treatment technique and 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 aluminum alloys, steels and nickel-based alloys have been demonstrated [1–6]. Effect of power density has already been investigated in [7] but for Ti–6Al–4V alloy. Since laser beams can be easily directed to fatigue-critical areas without masking, LSP technology is expected to be widely applicable for ∗

Corresponding author. Fax: +52-442-2119839. E-mail address: [email protected] (C. Rubio-Gonz´alez).

0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.07.025

improving 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 processing on the fatigue behavior and fracture toughness of 6061-T6 aluminum alloy specimens. Process parameters such as pulse density are varied. The effect of LSP on fatigue crack growth rate, fracture toughness, microhardness, and residual stresses are investigated. In the laser shock processing 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 how the process works 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 vaporizes a thin surface layer of the overlay. High pressure against the surface of the sample causes a shock wave to

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Fig. 1. Pinciple of laser shock processing.

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 [8].

2. Experimental procedure 2.1. Material Plates of 6061-T6 aluminum alloy with thickness of 6.3 mm were machined to obtain the specimens. The T6 condition consists of a solution treatment and natural aging. Its chemical composition was: 0.52 wt.% Si, 0.27 wt.% Fe, 0.13 wt.% Cu, 0.03 wt.% Mn, 0.46 wt.% Mg, 0.011 wt.% Zn, 0.27 wt.% Cr, 0.022 wt.% Ti. Chemical composition was determined using a spark emission spectrometer. The mechanical properties were determined using dog-bone type specimens. Results are shown in Fig. 2. The offset tensile yield stress is 226 MPa, ultimate tensile strength 250 MPa and elastic modulus 59.7 GPa. These values are similar to those reported in literature [9]. The specimens used for residual stress measurement were blocks of 60 mm × 60 mm × 6.3 mm. The specimen used for

Fig. 3. Compact tension test specimens used in the fatigue crack growth tests. Dimensions in millimeter. Specimen thickness is 6 mm.

fatigue crack growth tests were compact tension specimens as illustrated in Fig. 3. All fatigue crack growth tests specimens were machined with the loading axis parallel to the rolling direction (L).Fig. 3 also shows pulse swept direction. 2.2. Laser shock processing The LSP experiments were performed 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 8 ns. 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 into a water bath when they were irradiated. Water was the confined medium. Specimen treated area was 20 mm × 15 mm on both sides. A 2D motion system was used to control specimen position and generate the pulse swept as shown in Fig. 3. Controlling the velocity of the system, the desired pulse density was obtained.

Fig. 2. Stress–strain curve of 6061-T6 aluminum alloy under tensile loading conditions.

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Fig. 4. Micro-hardness profile on the specimen cross-section.

2.3. Characterization of the effects induced by LSP Micro-hardness measurement was made with 50 g load and 10 s hold time. Residual stress distribution was determined by the hole drilling method according to the ASTM standard E837 [10]. Strain gage rosettes EA-13-062RE-120 along with a RS-200 Milling Guide from Measurements Group were used.

a CCD camera. A fatigue pre-crack 5 mm long (from notch tip) was growth on each specimen before LSP treatment. Stress intensity factor KI due to external load P was determined using the following equation [11,12], KI =

a 2 + (a/W)
0.886 + 4.64 W B W (1 − (a/W))3/2  a 3  a 4   a 2 + 14.72 − 5.60 −13.32 W W W P √

(1)

2.4. Fatigue crack growth test Fatigue crack growth tests were performed on a MTS 810 servo-hydraulic system at room temperature in the air. Load ratio R = Pmin /Pmax was maintained at R = 0.1. Frequency of 20 Hz with a sine wave form was used in the experiments. Two specimen groups with 900, 1350 and 2500 pulses/cm2 pulse densities were formed. One specimen on each group was tested to maximum load of 3000 N and another to 5000 N. Crack lengths were measured at a magnification of 10× using

2.5. Fracture toughness Compact tension test specimens with and without LSP treatment were used to measure fracture toughness according to ASTM standard E399 [13]. A fatigue pre-crack to give a crack length of 27 mm was done on each specimen. Densities of 900, 1350 and 2500 pulses/cm2 were considered for the treated specimens.

Fig. 5. Residual stress distribution induced by LSP.

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3. Results and discussion

Table 1 Parameters of the fatigue crack growth rate model

Micro-hardness profiles of the specimen cross section are shown in Fig. 4. Note first that far from the surface, on the untreated material, hardness takes a constant value of 83 HV. On surface, hardness reaches a larger value than that reached on the untreated material. Surface micro-hardness values are 95.8, 94.4 HV, for pulse densities of 900 and 2500 pulses/cm2 , respectively. Residual stress distributions as a function of depth are shown in Fig. 5. It is observed that the higher the pulse density the larger the compressive residual stress S2 , which is perpendicular to pulse swept direction. Residual stress component S1 is parallel to pulse swept direction. From residual stress distribution plot, it seems that there is an effect of pulse swept direction, i.e., compressive stress component perpendicular to swept direction is much higher than that parallel to that direction. It has been noted [6] that if the metallic surface is first coated with material opaque to laser light (typically a black paint) only that thin layer is vaporized, thus thermal effects on the metallic substrate are suppressed. Hence mechanical effects of the interaction of a local laser-induced stress wave with a metal depend mainly on whether the target is or is not covered. With no overlay, thermal effects dominate: the heated zone is compressively plastified by the surrounding material during its dilatation. Tensile stresses occur after cooling. In light of this explanation, residual stress distribution shown in Fig. 5 can be interpreted. Aluminum specimens used on the experiments were not covered during LSP treatment, and thermal effects dominate on surface and on thin layer 0.2–0.4 mm thick. Therefore pure mechanical effect (i.e., residual stress) is observed principally inside the material at depths greater than 0.4 mm. Below

Pulse density (pulses/cm2 ) No LSP 900 1350 2500

C

m 10−13

4× 8 × 10−13 2 × 10−11 3 × 10−10

7.664 6.818 5.733 4.723

this distance, residual stresses reach the maximum absolute value. Fig. 6 shows the fatigue crack growth rates for the aluminum alloy. A comparison of results without and with LSP treatment is illustrated. Solid lines are the least square fitting curves. First, note that as pulse density is increased, fatigue crack growth rate da/dN, decreases for KI constant. This is true only for K values above 20 MPa(m)1/2 approximately. Second, da/dN values are lower for samples with LSP treatment than for samples without the treatment on fatigue crack growth, which is derived from the compressive residual stress field induced on the surfaces of the specimens. Fitting the experimental results with the well known Paris rule da = ( K)m dN

(2)

it is observed (see Table 1) that C increases as pulse density increases and m decreases increasing pulse density. To determine fracture toughness the load–displacement curve was registered for each test. These curves are shown in Fig. 7(a). Note that load–displacement curve for 1350 pulses/cm2 illustrated in Fig. 7(a), has a fast dropping after the displacement of 2 mm, compared with the other curves. Tests were performed under load control, therefore this behavior shows a faster growth due to plastic collapse. According to the ASTM standard E399 [11], a PQ load value

Fig. 6. Fatigue crack growth rates for 6061-T6 aluminum alloy without LSP and with LSP treatment with different pulse densities.

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Fig. 7. (a) Load–displacement curves to determine fracture toughness. (b) Determination of PQ . Table 2 Effect of LSP on fracture toughness Pulse density (pulses/cm2 )

Load PQ (kN)

Fracture toughness (MPa(m)1/2 )

Without LSP 900 1350 2500

4.61 4.78 5.24 5.32

34.04 35.29 38.69 39.28

is determined from the intersection of the load–displacement curve with a straight line of 95% of the slope in the linear region. Such an intersection is shown in Fig. 7(b) for the specimen with no LSP treatment. Values of PQ are used in Eq. (1) to calculate fracture toughness. Table 2 shows a summary of these calculations. Note that increasing pulse densities the fracture toughness is increased as well. With 2500 pulses/cm2 we get an increment of 15% in fracture toughness compared with the untreated material result. Because the thickness of the specimens used in determination of fracture toughness is small, these specimens do not satisfy plane strain conditions. However the purpose of the test is to evaluate qualitatively the effect of LSP on this aluminum alloy.

4. Conclusions It has been demonstrated that the laser shock processing (LSP) is an effective surface treatment technique to improve fatigue properties of 6061-T6 aluminum alloy. This is due to the residual stress field induced on surface. It has been shown that increasing the pulse density, fatigue crack growth rate is reduced for values of K greater than 20 MPa(m)1/2 .

Fatigue crack growth rate has been quantified through the determination of Paris rule parameters for loading rate R = 0.1. Fracture toughness has been determined for specimens with different pulse densities. It has been shown that LSP treatment improves this mechanical property.

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