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Influence of laser parameters in surface texturing of Ti6Al4V and AA2024-T3 alloys
Optics and Lasers in Engineering 103 (2018) 100–109
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Influence of laser parameters in surface texturing of Ti6Al4V and AA2024-T3 alloys J.I. Ahuir-Torres a, M.A. Arenas a, W. Perrie b, J. de Damborenea a,∗ a
Department of Surface Engineering, Corrosion and Durability, Centro Nacional de Investigaciones Metalúrgicas, CENIM-CSIC. Av. Gregorio del Amo 8, 28040 Madrid, Spain b Laser Engineering Group, School of Engineering, University of Liverpool, Brownlow Hill, Liverpool L69 3GQ, United Kingdom
a r t i c l e Keywords: Laser texturing Aluminium alloy Titanium alloy
i n f o
a b s t r a c t Laser texturing can be used for surface modification of metallic alloys in order to improve their properties under service conditions. The generation of textures is determined by the relationship between the laser processing parameters and the physicochemical properties of the alloy to be modified. In the present work the basic mechanism of dimple generation is studied in two alloys of technological interest, titanium alloy Ti6Al4V and aluminium alloy AA2024-T3. Laser treatment was performed using a pulsed solid state Nd: Vanadate (Nd: YVO4 ) laser with a pulse duration of 10 ps, operating at a wavelength of 1064 nm and 5 kHz repetition rate. Dimpled surface geometries were generated through ultrafast laser ablation while varying pulse energy between 1 μJ and 20 μJ/pulse and with pulse numbers from 10 to 200 pulses per spot. In addition, the generation of Laser Induced Periodic Surface Structures (LIPSS) nanostructures in both alloys, as well as the formation of random nanostructures in the impact zones are discussed. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Advanced aluminium and titanium alloys are extensively used in aerospace industry because their excellent compromise between mechanical properties and low density. The most commonly used titanium alloy is the two phase (𝛼 + 𝛽) alloy, Ti6Al4V, offers outstanding strength-to-weight relationship, good mechanical properties and corrosion resistance in a wide range of operational temperatures. On the other hand, high-strength aluminium alloys, specially the AA2024 (Al–Cu–Mg), are an important airframe alloy due to its exceptional strength-to-weight ratio, high damage tolerance and relatively low cost [1,2]. Notwithstanding its properties, these alloys have in common a poor tribological behaviour, mainly due to poor shear strength and low hardness. On the other hand, the growing demand for lightening structures (combination of composites, polymers and metallic alloys) requires the use of adhesives for reduction of other joint technologies. Adhesive bonding of Ti and Al alloys is increasing in importance but in both alloys the surface preparation will plays a crucial role in the success of the adhesive joint. Increasing the contact surface by the formation of surface textures will ensure improved adhesive properties.
∗
Surface texturing is a powerful tool for achieving either topographical or microstructural changes that result in improved material behaviour under service conditions. In this way, the development of surface morphologies could improve the tribological characteristics of the metallic alloys by different mechanisms; e.g., by creating micrometric reservoirs for lubricants or by debris trapping inside the texture. Additionally, surface texturing increases the strength and adhesion capacity provided by the structural adhesives. Different surface texturing methods based on chemical, physical or mechanical processes have been described elsewhere [3]. Among these techniques, lasers have arisen as an important method to modify the surface characteristics of different materials without changing their bulk properties. Laser surface techniques (LST) has been widely used in several advanced technological applications [4-8]. Furthermore, their application on metal alloys allows improving their tribological properties, wettability, and even, the corrosion resistance [3,9,10]. The geometry and topography of the textures generated by laser surface texturing (LST), depends on the interaction that occurs between the laser radiation and the material which in turn is determined by the relationship between the laser process parameters and the physical– chemical material properties [11-14]. For polymers, laser radiation in the ultraviolet range causes photochemical processes. The polymer is
Corresponding author. E-mail address: [email protected] (J. de Damborenea).
https://doi.org/10.1016/j.optlaseng.2017.12.004 Received 14 September 2017; Received in revised form 6 November 2017; Accepted 2 December 2017 0143-8166/© 2017 Elsevier Ltd. All rights reserved.
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Table 1 Chemical composition of AA2024-T3 alloy and Ti6Al4V alloy. AA 2024-T3 (%wt)
The frequency of the laser shot was 5 kHz with pulse energy, Ep, of 1, 2, 6, 8, 10, 12, 14 and 20 μJ. The number of pulses per impact, N, were 10, 25, 50, 100 and 200. SEM analysis was made using field emission gun scanning electron microscopy (FEG-SEM) utilizing a Hitachi S 4800 J instrument equipped with an energy dispersive X-ray (EDX) detector. Finally, the topography of the textures was assessed using a interferometric confocal Sensofar PLμ2300 profilometer.
Ti6Al4V (%wt)
Al
Balance
Ti
Balance
Cu Mg Mn Fe Si Ti Zn Cr
4.300 ± 0.100 1.270 ± 0.040 0.620 0.300 0.160 0.043 ± 0.001 0.039 ± 0.001 0.018 ± 0.001
Al V Fe C N2 O2 H2 –
5.500–6.760 3.500–4.500 0.250 0.080 0.050 0.020 0.013–0.015 –
3. Results and discussion The theoretical laser focal diameter, 2𝜔o(theo) , can be calculated using Eq. (2):
ionised directly because the energy associated with the photons of the laser beam is similar to the ionization energy of the polymer chain [12,15]. In the case of ceramics, the ablation threshold is higher than that of polymers and metals due to their high melting points, and therefore longer high-energy pulses are required [16,17]. In metallic materials, independent of wavelengths, thermal processes occur [12,18]. The complex metal refractive index n(𝜆) = n0 + ik (n0 is the refractive index in air) determines both reflectivity and absorption, where reflectivity R is given by (1), [( )2 ] [( )2 ] 𝑅 = 𝑛0 − 1 − 𝑘2 ∕ 𝑛0 − 1 + 𝑘2 (1)
2𝜔𝑜(𝑡ℎ𝑒𝑜) =
4 ⋅ 𝑓 ⋅ 𝜆 ⋅ 𝑀2 𝜋⋅𝐷
(2)
where f is the focal length of the lens (100 mm), and 𝜆 is the wavelength (1064 nm), M2 the temporal and spatial stability in a TEM00 mode and D the unfocused beam diameter (6 mm). According to (1), the theoretical diameter of the focused beam was 29.3 μm. The Rayleigh length, Zr , which is the maximum distance from the focal point where the fluence remains constant, was calculated by Eq. (3): 𝑍𝑟 =
while the absorption coefficient 𝛼 = 4𝜋k/𝜆 (cm−1 ) where k is the extinction coefficient. Since 𝛼 ∼106 cm−1 in metals, the absorption depth or skin depth ∼1/𝛼 is typically 10–30 nm while heat diffusion depth d √ ∼2 (D𝜏 p ), where D is the diffusivity and 𝜏 p is the temporal pulse length. In metals, 0.1 < D < 1 cm2 /s and this thermal process can produce melting, evaporation and the formation of a plasma from the treated material [14,19,20]. Although LST is a highly localized treatment, the damage zone around the texture is called the heat affected zone (HAZ) ∼d [21,22]. Short pulse lasers (ns), and ultra-short (ps and fs), are usually used in LST because they generate a small HAZ size. However, ultra-short pulse with 𝜏 p ≤ 10 ps reduce HAZ significantly over ns pulses, reducing ablation threshold and minimising melt which increases micro-structuring precision. Also, near ablation threshold, micro and nanostructures appear on metal surfaces called Laser Induced Periodic Surface Structures (LIPSS) which have a different morphology according to the laser parameters and material properties [23,24]. The aim of this study was to determine the influence of processing parameters on the geometry and topography of textures generated by picosecond pulsed laser ablation on the surfaces of two metals alloys, Ti6Al4V alloy and the aluminium alloy 2024-T3.
𝜋 ⋅ 𝜔2𝑜(𝑡ℎ𝑒𝑜) 4 ⋅ 𝑀2 ⋅ 𝜆
(3)
This length was calculated to be ∼ 489 μm, much greater than the z-positioning accuracy which allowed the samples to remain in a wide interval with a nearly constant fluence. Once the samples were placed to the focal plane, laser impacts on both metal alloys will produce a topography defined both by the pulse energy as well as the number of impacts received at a single site, which will in turn determine the temperature reached on the surface and the heating rate at which the material reaches this temperature. In the present study, two types of texture has been observed. Firstly, shallow dots when the treatment is rather superficial with limited ablation depth when pulse numbers ≤10. Secondly, dimples were produced by blind drilling with fixed pulse numbers ≥50 on given surface spots and presented an appreciable depth. Both dots and dimples are the result of material ablation by ultrafast laser-material interaction. The shallow dot is produced by a mechanism known as “normal evaporation" characterized by the formation of a liquid-gas interface; while dimples are generated by the "explosive phase" mechanism. These mechanisms are well described in the literature [25] and both the ablation threshold and the depth are determined by the laser fluence used as well as by the optical and physical-chemical properties of the material [12]. In the case of titanium alloy, the impacts made with 10 pulses, and, energies ≤2 μJ produce shallow dots, Fig. 1a, while the impacts made with a larger number of pulses, ≥25 pulses, generated dimples, even for the smallest energy, 1 μJ, Fig. 1b. Moreover, the impacts generated at energies ≥6 μJ created dimples regardless of the number of pulses, Fig. 1c. The same parameters (energy/pulse numbers) are used in Fig. 2(a–c) in the case of aluminium alloy 2024, again producing dots and dimples, increasing in diameter and depth. Gragossian et al. [26] found similar results when performing treatments on Al using a laser with similar characteristics. In this case, at low irradiances, the material was removed by a mechanism of normal evaporation, while for irradiances higher than 5 J cm− 2 , a superheating occurred on the surface leading to the formation of droplets of molten and vaporised material, due to an explosive mechanism. On the other hand, Willis and Xu [27] using a 25 ps Nd:YAG (1064 nm, pulse repetition rate 10 Hz) on nickel targets, demonstrated by numerical modelling of the process that normal surface evaporation is not the material removal mechanism even though the surface temperature during ablation reaches a value close to the critical temperature. When the threshold fluence is reached, the explosive phase will determine the amount of material removed.
2. Experimental details The alloys used were titanium alloy Ti6Al4V Grade 5 in accordance with the ASTM B 265 standard and aluminium alloy 2024 in T3 condition. Samples of both alloys were cut into pieces of 15 × 15 × 1.8 mm. The composition of the alloys is displayed in Table 1. Samples were mechanically polished up to a mirror-finished using a wet grinding on SiC papers down to 1200 grit and final polishing with a solution to 50% of colloidal silica gel of 0.04 μm in H2 O2 . The surface roughness, Ra , is 15 ± 1 nm. The laser was a solid state Nd:Vanadate (Nd:YVO4 ) laser (High-Q model IC381), with a pulse duration of 10 ps and IR output wavelength of 1064 nm. The beam had high temporal and spatial stability in a TEM00 mode (M2 ≤ 1.32). The laser source generates pulse energies between 0 and 250 μJ with a pulse repetition rate between 5 and 50 kHz. Samples were positioned on a precision X-Y-Z motion table (Aerotech A3200, run under NView MMI software) at the focal plane of a 100 mm focal length f-theta lens. Dimples were generated on the alloy surface in an ambient air environment by applying a known number of laser pulses of specific energy to a single site or ‘spot’. 101
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Fig. 1. SEM images of the laser impacts with 1 μJ and 10 pulses (a) and (d); 10 μJ and 50 pulses (b) and (e); and 20 μJ and 200 pulses (c) and (f) on Ti6Al4V alloy.
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Fig. 2. SEM images of the laser impacts with 1 μJ and 10 pulses (a) and (d); 10 μJ and 50 pulses (b) and (e); and 20 μJ and 200 pulses (c) and (f) on AA2024-T3 alloy.
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Table 2 Chemical composition by EDS (atomic %) in the dimples. Sample
Untextured Center
Rim
Ti6Al4V (%)
1 μJ/10 pulses 10 μJ/50 pulses 20 μJ/200 pulses 1 μJ/10 pulses 10 μJ/50 pulses 20 μJ/200 pulses
AA 2024-T3 (%) O
Ti
Al
V
O
Al
Cu
Mg
– 3.13 6.85 – 10.72 52.32 47.79
85.76 83.11 80.74 85.89 77.95 40.36 43.72
10.06 9.73 9.54 10.28 9.14 4.83 6.83
4.17 4.04 2.78 3.83 2.19 2.45 2.01
– 1.97 3.57 16.06 3.97 8.16 42.55
96.62 93.64 93.38 82.36 92.49 88.93 54.95
2.01 2.66 1.99 1.06 2.41 1.88 1.08
1.37 1.73 1.06 0.52 1.36 1.02 1.42
Fig. 3. Topography (up) and profile (down) of the dimples generated with 20 μJ and 200 pulses on (a) Ti6Al4V and (b) aluminium alloy 2024-T3 obtained by confocal perfilometry.
As the treatment is performed in the absence of a protective atmosphere, a certain degree of surface oxidation can be expected with the increase in energy and number of pulses. In present case, EDS analysis, Table 2, confirmed the presence of oxygen on the treated area, presumable a non-stoichiometric TiO2 or Al2 O3 oxide. The concentration of oxygen was lower in the centre of the dimple than in the dimple border. This could be due to the formation of laser-induced plasma that shields the bottom of the dimple from the environment [28]. In the areas impacted by the laser, it is possible to distinguish the formation of periodic nanometric size structures in both alloys, Fig. 1d–f and Fig. 2d–f. The morphology of these nanostructures changes according to the laser parameters and alloy. Structures known as LIPSS (Laser Induced Periodic Surface Structure) are found for Ti6Al4V alloy, Fig. 2a– c. Its morphology is characterized by a gentle wave shape. In this case,
the regular ripple structure presented a period, Λ, about 1200 nm, close to the laser wavelength, 1064 nm. Literature refers to this type of LIPSS as "low spatial frequency LIPSS" (LSFL) [29,30], which are characterized by a period on the same order of magnitude as the wavelength of the laser used and with direction at right angles to the incident linear polarisation. The formation of such nanostructures has been extensively studied since the early 60s. Although such formations are still under study, it could be accepted that the LSFL type are due to interference between laser radiation and the generated surface scattered wave which produces a non-uniform energy distribution on the molten surface which, eventually, causes both periodic ablation and the self-organization of the surface structure. In the titanium alloy, LIPSS observed in the dots have the same orientation throughout the textured area, Fig. 1a. The orientation was per104
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Fig. 5. Dimple depth versus pulse energy for different number of pulses. (a) Ti6Al4V alloy and (b) aluminium alloy 2024-T3.
Fig. 4. Diameter of the dimples versus pulse energy for different number of pulses. For (a) Ti6Al4V alloy and (b) aluminium alloy 2024-T3.
creasing amount of molten material which is greater at higher energy and exposure. Similarly, nanostructures observed on the treated surfaces of the aluminium alloy also depend on the laser processed parameters, Fig. 2a–d. However, LIPSS structures are much less apparent on this alloy as Aluminium has a low melting point and long electron–phonon coupling time, around 50 ps [32]. Unlike the titanium alloy, the formation of random nanostructures in the centre of impact is observed, regardless laser parameters used, due to melting. When the number of impacts was <10, the surface acquired a random nanoroughness that substantially increased the absorption of radiation compared to that for an untreated surface. It seems that when the number of irradiations is high enough, the molten material may cause an explosive phase due to superheating processes, resulting in the morphology type shown in Fig. 2a. This type of morphology would be consistent with the above explanation about the formation of nanospheres. Vorobyev [33] found similar structures on a platinum surface when treated with a femtosecond laser operating at 66 fs pulses at a 1 kHz repetition rate with 𝜆 of 800 nm. As the number of impacts on aluminium alloy increases, LIPSS begin to appear with the same morphology as those found in the titanium alloy, although they are mainly located on the edge of the dimple, Fig. 2e and f.
pendicular to the laser beam polarization. However, when increasing the fluence and the number of shots, LIPSS on the dimple border presented a different orientation than the LIPSS at the centre, Fig. 1b. Finally, at the highest energy and pulses (20 μJ, 200), the central part of the dimple—and even in the walls—showed a blurring of the ripples structure, due to an excess of molten material that erases the LIPSS, generating a smooth surface as seen in Fig. 1c. In the LIPSS generated at low energy and short number of pulses (1 μJ, 10), Fig. 1d, it is possible to distinguish nanospheres of about 500 nm in diameter. Its formation is due to the rapid cooling of microdroplets of molten material, avoiding the hydrodynamic expansions that deform the spherical structure of the droplet. Vorobyev and Guo [31] working with a femtosecond laser on pure titanium observed that part of the molten material is also deposited on the surface of the material as droplets of 20 nm in diameter. The difference in size of the nanospheres described in that paper and those found in the present study could be due to the different experimental conditions employed in each work. Oh the other hand, at a higher energy, 10 μJ, and number of pulses, 50 pulses, the small droplets of molten material are joined to form small columnar structures, Fig. 1e. The probability that the droplets join during the redeposition of the material increases with in105
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Fig. 7. Square diameter of the dimples versus fluence for different number of pulses. a) Ti6Al4V alloy and b) aluminium alloy 2024-T3.
Fig. 6. Ablation volume versus pulse energy for different number of pulses. (a) Ti6Al4V alloy and (b) aluminium alloy 2024-T3.
hibited a similar shape to the Gaussian beam. The depth of dimples on the aluminium alloy (≈20 μm) is greater than on the titanium alloy (≈6 μm). This could be due to the different thermal conductivities of both materials, 237.0 W/m K for pure aluminium and 22 W/m K for pure titanium. The diameter of the dimples varied with the laser parameters and the metallic alloys, Fig. 4. The diameter increased with increasing energy and number of pulses for both alloys, exhibiting dimples with larger diameter on titanium than on aluminium alloy. Similarly, the depth of the dimples also increased with the increase in energy and number of pulses on both alloys, Fig. 5. The depth of dimples on the aluminium alloys is greater than on the titanium alloy for the same texturing conditions. Finally, the volume of ablated materials, measured by confocal profilometry, was higher for the AA2024 than for Ti6Al4V, Fig. 6. In a Gaussian laser beam, the fluence peak, 𝜑, and pulse energy, Ep , are directly related by expression (5) [40,41]:
Finally, it is worth highlighting that the LIPSS on the titanium alloy are better defined than those on the aluminium alloy. The definition of LIPSS is correlated with the electron–phonon coupling energy coefficient of the material, so that the higher the coefficient, the more pronounced the LIPSS [34]. As titanium has a higher coupling energy coefficient, 1.50 × 1018 W/K m3 , than the aluminium, 2.45 × 1017 W/ K m3 , LIPSS on the titanium alloy irradiated surfaces are observed more clearly than on aluminium alloy. Laser ablation by short or ultra-short pulses is a complex process, although well studied in the literature [35–37]. The laser used in present study has a pulse length of 10 ps, that can be considered as an ultrashort pulse length, with the so-called two-temperature model to induce material removal process [38,39]. It is remarkable, however, the higher ablation rate/pulse in AA2024 than in Ti6Al4V. The physical properties of the alloys should be also taken into account as, for instance, the higher reflectivity of Al than Ti or even that melt material expulsion by expanding plasma with pulse numbers are higher in Al due to its lower melting point. However, even if these assumptions were true, the ablated volume is nearly four times bigger for the Ti6Al4V than for the Al2024 alloy. This apparent contradiction may be explained as follows. Fig. 3 picture the profile of the dimples fabricated at 20 μJ and 200 pulses for both alloys. Dimples on the titanium alloy presented nearly vertical walls, while those produced on the aluminium alloy ex-
𝜑=
2 ⋅ 𝐸𝑝 𝜔20 ⋅ 𝜋
(5)
𝜔0 being the beam radius. Additionally, the damage threshold fluence, or ablation threshold, 𝜑th( N ) , for a given number of pulses (N) is related to the diameter of the dimple, D, according to Eq. (6) [42]: 𝐷2 = 2𝜔20 ln 𝜑 − 2𝜔20 ln 𝜑𝑡ℎ(𝑁 )
(6)
Figure 7 shows the graphical representation of Eq. (6) for each alloy. The diameter of the focused beam, 2𝜔o , obtained from the slope of the 106
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Fig. 9. Depth of the dimples per applied pulse, L, versus pulse energy for both alloys.
graph is 24 ± 2 μm, a value similar to the theoretically calculated value, 29.4 μm. The threshold fluence for pulse laser ablation, 𝜑th , changes according to the number of pulses [43]: 𝜑th (𝑁 ) =
2𝐸𝑝(th(𝑁 )) 𝜋𝜔20
Ti6Al4V alloy. Accordingly, polished titanium absorbs a greater amount of radiation of the first pulse than the polished aluminium surface and, therefore, the energy needed to ablate aluminium is greater than that for titanium for a single pulse. Similar values of 𝜑th(1) have been reported in [45] for a 10 ps Nd:VAN laser, 0.24 J/cm2 for titanium and 0.37 J/cm2 for aluminium. The dependence of ablation rate, L, of the alloys tested here on the laser fluence,𝜑, is depicted in Fig. 9a for 100 pulses. The effective penetration depth of the laser, 𝓁, follows a logarithmic dependence with the fluence determined by Eq. (9), [45]:
(7)
where Ep (th( N )) is the threshold pulse energy needed for the ablation of the material. Fig. 8a shows the decrease of the ablation threshold with the increase of the number of pulses for both alloys. For a single-shot ablation, it will mainly depend on the physical properties of the alloy. In contrast, for multiple pulses, the ablation threshold will decrease Eq. (8) [44]: ( ) log 𝑁 ⋅ 𝜑th(𝑁 ) = log 𝜑th(1) + 𝑆 ⋅ log 𝑁 (8)
𝐿 = 𝓁 ⋅ ln 𝜑 − 𝓁 ⋅ ln 𝜑𝑡ℎ
(9)
where 𝜑th is the threshold fluence, Fig. 9b. In the simplest approach, 𝓁 can be reduced to 1/𝛼 which is the optical penetration depth, in the order of 10 nm for a wide range of metals. According to this equation, if 𝜑 ≤ 𝜑th , the material ablated will be little. When the 𝜑 > 𝜑th , the ablation depth will increase. This dependence varied with the number of pulses although showing a similar lineal trend. Only for 10 and 25 pulses and the highest fluence there is a deviation from this behaviour. For the experimental conditions used, the effective penetration depth, 𝓁, was of 5.5 nm and of 23.9 nm for Ti6Al4V alloys and AA2024-T3 alloy respectively. These values are
where 𝜑th(1) is the ablation threshold fluence for a single pulse and S is the incubation coefficient, Fig. 8b. The S value for both alloys is <1 (0.59 for Ti6Al4V alloy and 0.67 for AA2024-T3 alloy), as can be deduced from the slope of the plots. On the other hand, the ablation threshold fluence for a single shot, 𝜑th(1) , is less for titanium alloy, 0.45 J/cm2 , than for aluminium alloy, 0.53 J/cm2 . It would be related to differences in the reflectivity of the material. When the first impact pulse occurs, the polished surface of AA2024-T3 alloy has a reflectivity of 97% while it is only 57% for the 107
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smaller than those obtained by Cheng et al [45] for fluencies <3 J/cm2 , 39 nm for Ti and 42 nm for Al, although for a single pulse. As it has been mentioned before, literature reported that material ablation takes place by different mechanisms depending on the pulse duration, the fluence and the physicochemical properties of the material. In the picosecond lasers, the laser energy is absorbed by the free electrons of the metal surface and, due to the electron-phonon coupling, this energy is transferred to the crystal lattice [46]. If the pulse duration were less than the time of electron–phonon coupling, a solid direct transformation to vapour or to plasma occurs. However, if the electron–phonon coupling time were less than the pulse duration, the laser–material interaction is heat diffusion limited and, therefore, a temperature gradient which results in a thermal diffusion of a given length occurs. Finally, in order to corroborate the experimental results of the effective penetration length of the laser, the thermal diffusion length has been estimated. The laser used had pulse duration of 10 ps; as the electronphonon coupling time of titanium and aluminium are 5.8 ps and 4.3 ps, respectively, the effective penetration depth of the laser, 𝓁, matches the thermal diffusion length and can be calculated according to: √ 𝓁 ≈𝜁 𝐷⋅𝜏 (10)
Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.optlaseng.2017.12.004. References [1] Peters M, Kumpfert J, Ward ChH, Leyens C. Titanium alloys for aerospace applications. Adv Eng Mater 2003;5(6):419–27. [2] Dursun T, Soutis C. Recent developments in advanced aircraft aluminium alloys. Mater Des 2014;56:862–71. [3] Etsion I. State of the art in laser surface texturing. J Tribol 2005;127(1):248–53. [4] Wahab JA, Ghazali MJ, Yusoff WMW, Sajuri Z. Enhancing material performance through laser surface texturing: a review. Trans IMF 2016;94(4):193–8. [5] Kontermann S, Gimpel T, Baumanna AL, Guenther M, Schade W. Laser processed black silicon for photovoltaic applications. Energy Procedia 2012;27:390–5. [6] Sher M-J, Winkler MT, Mazur E. Pulsed-laser hyperdoping and surface texturing for photovoltaics. MRS Bull 2011;36(6):439–45. [7] Zang Z, Zeng X, Du J, Wang M, Tang X. Femtosecond laser direct writing of microholes on roughened ZnO for output power enhancement of InGaN light-emitting diodes. Opt Lett 2016;41(15):3463–6. [8] Günther D, Scharnweber D, Hess R, Wolf-Brandstetter C, Grosse Holthaus M, Lasagni AFVilar R, editor. High precision patterning of biomaterials using the direct laser interference patterning technology. Laser surface modification of biomaterials: techniques and applications 2016:3–28 Chapter 1. [9] Zhou J, Shen H, Pan Y, Ding X. Experimental study on laser microstructures using long pulse. Opt Lasers Eng 2016;78:113–20. [10] Vorobyev AY, Guo C. Direct femtosecond laser surface nano/microstructuring and its applications. Laser Photonics Rev 2013;7(3):385–407. [11] Majumdar JD, Manna I, editors. Laser-assisted fabrication of materials, Chapter 1 Springer series in materials science 161. Berlin, Heidelberg: Springer-Verlag; 2013. doi:10.1007/978-3-642-28359-8_1. [12] Wu B, Özel T. Micro-lase processing. In: Koc M, özel T, editors. Micro-manufacturing: design and manufacturing of micro-products. New Jersey; 2011. p. 159–95. [13] Cabalin L, Laserna J. Experimental determination of laser induced breakdown thresholds of metals under nanosecond Q-switched laser operation. Spectrochim Acta Part B: Atom Spectrosc 1998;53(5):723–30. [14] Lopez J, Faucon M, Devillard R, Zaouter Y, Honninger C, Mottay E. Parameters of influence in surface ablation and texturing of metals using high-power ultrafast laser. J Laser Micro Nanoeng 2015;10(1):1. [15] Lorazo P, Lewis LJ, Meunier M. Short-pulse laser ablation of solids: from phase explosion to fragmentation. Phys Rev Lett 2003;91(22):225502. [16] Hendow ST, Ozkan AM, Oliveira M, Mottay E. Pulsed fiber laser texturing of metals, silicon, and ceramics: comparison of surface structural changes induced by ns and fs pulses. In: Proc. SPIE 8243, laser applications in microelectronic and optoelectronic manufacturing (LAMOM) XVII; (16 February 2012 82430P. [17] Qian B, Shen Z. Laser sintering of ceramics. J Asian Ceram Soc 2013;1:315–21. [18] Majumdar JD, Manna I. Laser processing of materials. Sadhana 2003;28(3-4):495–562. [19] Brown MS, Arnold CB. Fundamentals of laser-material interaction and application to multiscale surface modification. In: Laser precision microfabrication. Springer; 2010. p. 93–4. [20] Yip W, Cheung N. Analysis of aluminium alloys by resonance-enhanced laser-induced breakdown spectroscopy: how the beam profile of the ablation laser and the energy of the dye laser affect analytical performance. Spectrochim Acta Part B: Atom Spectrosc 2009;64(4):315–22. [21] Cheng J, Liu CS, Shang S, Liu D, Perrie W, Dearden G. A review of ultrafast laser materials micromachining. Opt Laser Technol 2013;46:88–102. [22] Lewis LJ, Perez D. Theory and simulation of laser ablation–from basic mechanisms to applications. In: Laser precision microfabrication. Springer; 2010. p. 35–61. [23] Tanvir-Ahmmed KM, Ling EJY, Servio P, Kietzig AM. Introducing a new optimization tool for femtosecond laser-induced surface texturing on titanium, stainless steel, aluminium and copper. Opt Lasers Eng 2015;66:258–68. [24] Mellor L, Edwardson SP, Perrie W, Dearden G, Watkins K. Surface plasmon polaritons for micro and nano-texturing of metal surfaces. In: Proc. ICALEO; 2009. [25] Arnaldo D. del Corro Ph.D. thesis. University of Twente; 2014. [26] Gragossian A, Tavassoli SH, Shokri B. Laser ablation of aluminium from normal evaporation to phase explosion. J Appl Phys 2009;105:103304. [27] Willis DA, Xu X. Heat transfer and phase change during picosecond laser ablation of nickel. Int J Heat Mass Trans 2002;45:3911–18. [28] György E, Pérez del Pino A, Serra P, Morenza JL. Chemical composition of dome-shaped structures grown on titanium by multi-pulse Nd: YAG laser irradiation. Appl Surf Sci 2004;222(1):415–22. [29] Bonse J, Krüger J, Höhm S, Rosenfeld A. Femtosecond laser-induced periodic surface structures. J Laser Appl 2012;24(4):042006. [30] Bonse J, Höhm S, Kirner SV, Rosenfeld A, Krüger J. Laser-induced periodic surface structures—a scientific evergreen. IEEE J Select Top Quant Electron 2017;23:9000615 Paper. [31] Vorobyev A, Guo C. Femtosecond laser structuring of titanium implants. Appl Surf Sci 2007;253:7272–80. [32] Perrie W, Gill M, Robinson G, Fox P, O’Neill W. Femtosecond laser micro-structuring of aluminium under helium. Appl Surf Sci 2004;230:50–9. [33] Vorobyev A, Guo C. Effects of nanostructure-covered femtosecond laser-induced periodic surface structures on optical absorptance of metals. Appl Phys A 2007;86:321– 324.
where 𝜁 is a geometric constant that depends on the geometry of the √ system, and it has a value on the order of the unit, (often 2); D is the thermal diffusivity of the material, and 𝜏 can be simplified as the duration of the laser pulse. Thermal diffusivity is 0.900 cm2 / s for aluminium and 0.029 cm2 /s for titanium; therefore a thermal length diffusion of 5.4 nm for the Ti6Al4V and 30.0 nm for the AA2024T-3 is obtained. These values are in agreement with the experimentally obtained (5.5 nm for the titanium alloy and 23.9 nm for the aluminium alloy), allowing to explain the differences in dimple depths between AA2024-T3 and Ti6Al4V at same processing energies. 4. Conclusions Two different textures due to material ablation by ultra-short lasermaterial interaction are produced on Ti6Al4V and aluminium alloy 2024-T3 the for the laser experimental used (pulse energy -between 1 and 20 μJ—and number of pulses per impact—from 10 to 200): •
•
Shallow dots for numbers of pulses <10. The treatment is rather superficial with limited ablation depth. Dimples with an appreciable depth produced by blind drilling with fixed pulsed numbers >50 on a particular place on the surface.
Additionally, in the treated areas, it was possible to distinguish the formation of nanometric size structures in both alloys. Structures known as low spatial frequency LIPSS (Laser Induced Periodic Surface Structure) are found for Ti6Al4V alloy with a period of 1200 nm, close to the laser wavelength, 1064 nm. In the case of the aluminium alloy, it is observed the formation of random nanostructures in the centre of impact regardless laser parameters used. LIPSS appeared only located on the edge of the dimples when the number of pulses increased. The effective penetration depth of the laser was of 5.5 nm and of 23.9 nm for Ti6Al4V alloy and AA2024-T3 alloy respectively. These results fully agreed with those found for the thermal diffusion length, estimated from the values of the thermal diffusivity of each alloy and the duration of the laser pulse. Acknowledgements This work was supported by the Spanish Ministry of Science and Innovation [SMOTI MAT2009-13751] and the Spanish Ministry of Economy and Competitiveness [MAT2013-48224-C2-1-R-MUNSUTI]. Authors also acknowledge to Professor G. Dearden from the Laser Engineering Group, University of Liverpool, for his support for this study. 108
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[34] Lin Z, Zhigilei LV, Celli V. Electron-phonon coupling and electron heat capacity of metals under conditions of strong electron-phonon nonequilibrium. Phys Rev B 2008;77:075133. [35] Momma C, Chichkov BN, Nolte S, von Alvensleben F, Tiinnermann A, Welling H. Short-pulse laser ablation of solid targets. Opt Commun 1996;129:134–42. [36] Anisimov SI, Luk’yanchuk BS. Selected problems of laser ablation theory. Phys-Uspekhi 2002;45:293–324. [37] Rethfeld B, Ivanov DS, Garcia ME, Anisimov SI. Modelling ultrafast laser ablation. J Phys D: Appl Phys 2017;50:193001 (39pp). [38] Hamad AH. Effects of different laser pulse regimes (nanosecond, picosecond and femtosecond) on the ablation of materials for production of nanoparticles in liquid solution. High energy and short pulse lasers. Viskup R, editor. InTech; 2016. Chapter 12. [39] Leitz KH, Redlingshöfer B, Reg Y, Otto A, Schmidt M. Metal ablation with short and ultrashort laser pulses. Physics Procedia 2011;12:230–8. [40] Mannion PT, Magee J, Coyne E, O’Connor GM, Glynn TJ. The effect of damage accumulation behaviour on ablation thresholds and damage morphology in ultrafast laser micro-machining of common metals in air. Appl Surf Sci 2004;233:275–87.
[41] Kalsoom UI, Bashir S, Ali N, Rafique MS, Husinsky W, Nathala CSR. Effect of fluence and ambient environment on the surface and structural modification of femtosecond laser irradiated. Chin Phys B 2015;25:018101. [42] Stašića J, Gakovića B, Perrie W, Watkins K, Petrović S, Trtica M. Surface texturing of the carbon steel AISI 1045 using femtosecond laser in single pulse and scanning regime. Appl Surf Sci 2011;258:290–6. [43] Byskov-Nielsen J, Savolainen JM, Christensen MS, Balling P. Ultra-short pulse laser ablation of metals: threshold fluence, incubation coefficient and ablation rates. Appl Phys A 2010;101:97–101. [44] Eichstädt J, Römer G, Huis in‘t Veld A. Determination of irradiation parameters for laser-induced periodic surface structures. Appl Surf Sci 2013;264:79–87. [45] Cheng J, Perrie W, Sharp M, Edwardson SP, Semaltianos NG, Dearden G. Single-pulse drilling study on Au, Al and Ti alloy by using a picosecond laser. Appl Phys A 2009;95(3):739–46. [46] Mishra S, Yadava V. Laser beam micromachining (LBMM)—a review. Opt Lasers in Eng 2015;73:89–122.
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Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Influence of laser parameters in surface texturing of Ti6Al4V and AA2024-T3 alloys J.I. Ahuir-Torres a, M.A. Arenas a, W. Perrie b, J. de Damborenea a,∗ a
Department of Surface Engineering, Corrosion and Durability, Centro Nacional de Investigaciones Metalúrgicas, CENIM-CSIC. Av. Gregorio del Amo 8, 28040 Madrid, Spain b Laser Engineering Group, School of Engineering, University of Liverpool, Brownlow Hill, Liverpool L69 3GQ, United Kingdom
a r t i c l e Keywords: Laser texturing Aluminium alloy Titanium alloy
i n f o
a b s t r a c t Laser texturing can be used for surface modification of metallic alloys in order to improve their properties under service conditions. The generation of textures is determined by the relationship between the laser processing parameters and the physicochemical properties of the alloy to be modified. In the present work the basic mechanism of dimple generation is studied in two alloys of technological interest, titanium alloy Ti6Al4V and aluminium alloy AA2024-T3. Laser treatment was performed using a pulsed solid state Nd: Vanadate (Nd: YVO4 ) laser with a pulse duration of 10 ps, operating at a wavelength of 1064 nm and 5 kHz repetition rate. Dimpled surface geometries were generated through ultrafast laser ablation while varying pulse energy between 1 μJ and 20 μJ/pulse and with pulse numbers from 10 to 200 pulses per spot. In addition, the generation of Laser Induced Periodic Surface Structures (LIPSS) nanostructures in both alloys, as well as the formation of random nanostructures in the impact zones are discussed. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Advanced aluminium and titanium alloys are extensively used in aerospace industry because their excellent compromise between mechanical properties and low density. The most commonly used titanium alloy is the two phase (𝛼 + 𝛽) alloy, Ti6Al4V, offers outstanding strength-to-weight relationship, good mechanical properties and corrosion resistance in a wide range of operational temperatures. On the other hand, high-strength aluminium alloys, specially the AA2024 (Al–Cu–Mg), are an important airframe alloy due to its exceptional strength-to-weight ratio, high damage tolerance and relatively low cost [1,2]. Notwithstanding its properties, these alloys have in common a poor tribological behaviour, mainly due to poor shear strength and low hardness. On the other hand, the growing demand for lightening structures (combination of composites, polymers and metallic alloys) requires the use of adhesives for reduction of other joint technologies. Adhesive bonding of Ti and Al alloys is increasing in importance but in both alloys the surface preparation will plays a crucial role in the success of the adhesive joint. Increasing the contact surface by the formation of surface textures will ensure improved adhesive properties.
∗
Surface texturing is a powerful tool for achieving either topographical or microstructural changes that result in improved material behaviour under service conditions. In this way, the development of surface morphologies could improve the tribological characteristics of the metallic alloys by different mechanisms; e.g., by creating micrometric reservoirs for lubricants or by debris trapping inside the texture. Additionally, surface texturing increases the strength and adhesion capacity provided by the structural adhesives. Different surface texturing methods based on chemical, physical or mechanical processes have been described elsewhere [3]. Among these techniques, lasers have arisen as an important method to modify the surface characteristics of different materials without changing their bulk properties. Laser surface techniques (LST) has been widely used in several advanced technological applications [4-8]. Furthermore, their application on metal alloys allows improving their tribological properties, wettability, and even, the corrosion resistance [3,9,10]. The geometry and topography of the textures generated by laser surface texturing (LST), depends on the interaction that occurs between the laser radiation and the material which in turn is determined by the relationship between the laser process parameters and the physical– chemical material properties [11-14]. For polymers, laser radiation in the ultraviolet range causes photochemical processes. The polymer is
Corresponding author. E-mail address: [email protected] (J. de Damborenea).
https://doi.org/10.1016/j.optlaseng.2017.12.004 Received 14 September 2017; Received in revised form 6 November 2017; Accepted 2 December 2017 0143-8166/© 2017 Elsevier Ltd. All rights reserved.
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Table 1 Chemical composition of AA2024-T3 alloy and Ti6Al4V alloy. AA 2024-T3 (%wt)
The frequency of the laser shot was 5 kHz with pulse energy, Ep, of 1, 2, 6, 8, 10, 12, 14 and 20 μJ. The number of pulses per impact, N, were 10, 25, 50, 100 and 200. SEM analysis was made using field emission gun scanning electron microscopy (FEG-SEM) utilizing a Hitachi S 4800 J instrument equipped with an energy dispersive X-ray (EDX) detector. Finally, the topography of the textures was assessed using a interferometric confocal Sensofar PLμ2300 profilometer.
Ti6Al4V (%wt)
Al
Balance
Ti
Balance
Cu Mg Mn Fe Si Ti Zn Cr
4.300 ± 0.100 1.270 ± 0.040 0.620 0.300 0.160 0.043 ± 0.001 0.039 ± 0.001 0.018 ± 0.001
Al V Fe C N2 O2 H2 –
5.500–6.760 3.500–4.500 0.250 0.080 0.050 0.020 0.013–0.015 –
3. Results and discussion The theoretical laser focal diameter, 2𝜔o(theo) , can be calculated using Eq. (2):
ionised directly because the energy associated with the photons of the laser beam is similar to the ionization energy of the polymer chain [12,15]. In the case of ceramics, the ablation threshold is higher than that of polymers and metals due to their high melting points, and therefore longer high-energy pulses are required [16,17]. In metallic materials, independent of wavelengths, thermal processes occur [12,18]. The complex metal refractive index n(𝜆) = n0 + ik (n0 is the refractive index in air) determines both reflectivity and absorption, where reflectivity R is given by (1), [( )2 ] [( )2 ] 𝑅 = 𝑛0 − 1 − 𝑘2 ∕ 𝑛0 − 1 + 𝑘2 (1)
2𝜔𝑜(𝑡ℎ𝑒𝑜) =
4 ⋅ 𝑓 ⋅ 𝜆 ⋅ 𝑀2 𝜋⋅𝐷
(2)
where f is the focal length of the lens (100 mm), and 𝜆 is the wavelength (1064 nm), M2 the temporal and spatial stability in a TEM00 mode and D the unfocused beam diameter (6 mm). According to (1), the theoretical diameter of the focused beam was 29.3 μm. The Rayleigh length, Zr , which is the maximum distance from the focal point where the fluence remains constant, was calculated by Eq. (3): 𝑍𝑟 =
while the absorption coefficient 𝛼 = 4𝜋k/𝜆 (cm−1 ) where k is the extinction coefficient. Since 𝛼 ∼106 cm−1 in metals, the absorption depth or skin depth ∼1/𝛼 is typically 10–30 nm while heat diffusion depth d √ ∼2 (D𝜏 p ), where D is the diffusivity and 𝜏 p is the temporal pulse length. In metals, 0.1 < D < 1 cm2 /s and this thermal process can produce melting, evaporation and the formation of a plasma from the treated material [14,19,20]. Although LST is a highly localized treatment, the damage zone around the texture is called the heat affected zone (HAZ) ∼d [21,22]. Short pulse lasers (ns), and ultra-short (ps and fs), are usually used in LST because they generate a small HAZ size. However, ultra-short pulse with 𝜏 p ≤ 10 ps reduce HAZ significantly over ns pulses, reducing ablation threshold and minimising melt which increases micro-structuring precision. Also, near ablation threshold, micro and nanostructures appear on metal surfaces called Laser Induced Periodic Surface Structures (LIPSS) which have a different morphology according to the laser parameters and material properties [23,24]. The aim of this study was to determine the influence of processing parameters on the geometry and topography of textures generated by picosecond pulsed laser ablation on the surfaces of two metals alloys, Ti6Al4V alloy and the aluminium alloy 2024-T3.
𝜋 ⋅ 𝜔2𝑜(𝑡ℎ𝑒𝑜) 4 ⋅ 𝑀2 ⋅ 𝜆
(3)
This length was calculated to be ∼ 489 μm, much greater than the z-positioning accuracy which allowed the samples to remain in a wide interval with a nearly constant fluence. Once the samples were placed to the focal plane, laser impacts on both metal alloys will produce a topography defined both by the pulse energy as well as the number of impacts received at a single site, which will in turn determine the temperature reached on the surface and the heating rate at which the material reaches this temperature. In the present study, two types of texture has been observed. Firstly, shallow dots when the treatment is rather superficial with limited ablation depth when pulse numbers ≤10. Secondly, dimples were produced by blind drilling with fixed pulse numbers ≥50 on given surface spots and presented an appreciable depth. Both dots and dimples are the result of material ablation by ultrafast laser-material interaction. The shallow dot is produced by a mechanism known as “normal evaporation" characterized by the formation of a liquid-gas interface; while dimples are generated by the "explosive phase" mechanism. These mechanisms are well described in the literature [25] and both the ablation threshold and the depth are determined by the laser fluence used as well as by the optical and physical-chemical properties of the material [12]. In the case of titanium alloy, the impacts made with 10 pulses, and, energies ≤2 μJ produce shallow dots, Fig. 1a, while the impacts made with a larger number of pulses, ≥25 pulses, generated dimples, even for the smallest energy, 1 μJ, Fig. 1b. Moreover, the impacts generated at energies ≥6 μJ created dimples regardless of the number of pulses, Fig. 1c. The same parameters (energy/pulse numbers) are used in Fig. 2(a–c) in the case of aluminium alloy 2024, again producing dots and dimples, increasing in diameter and depth. Gragossian et al. [26] found similar results when performing treatments on Al using a laser with similar characteristics. In this case, at low irradiances, the material was removed by a mechanism of normal evaporation, while for irradiances higher than 5 J cm− 2 , a superheating occurred on the surface leading to the formation of droplets of molten and vaporised material, due to an explosive mechanism. On the other hand, Willis and Xu [27] using a 25 ps Nd:YAG (1064 nm, pulse repetition rate 10 Hz) on nickel targets, demonstrated by numerical modelling of the process that normal surface evaporation is not the material removal mechanism even though the surface temperature during ablation reaches a value close to the critical temperature. When the threshold fluence is reached, the explosive phase will determine the amount of material removed.
2. Experimental details The alloys used were titanium alloy Ti6Al4V Grade 5 in accordance with the ASTM B 265 standard and aluminium alloy 2024 in T3 condition. Samples of both alloys were cut into pieces of 15 × 15 × 1.8 mm. The composition of the alloys is displayed in Table 1. Samples were mechanically polished up to a mirror-finished using a wet grinding on SiC papers down to 1200 grit and final polishing with a solution to 50% of colloidal silica gel of 0.04 μm in H2 O2 . The surface roughness, Ra , is 15 ± 1 nm. The laser was a solid state Nd:Vanadate (Nd:YVO4 ) laser (High-Q model IC381), with a pulse duration of 10 ps and IR output wavelength of 1064 nm. The beam had high temporal and spatial stability in a TEM00 mode (M2 ≤ 1.32). The laser source generates pulse energies between 0 and 250 μJ with a pulse repetition rate between 5 and 50 kHz. Samples were positioned on a precision X-Y-Z motion table (Aerotech A3200, run under NView MMI software) at the focal plane of a 100 mm focal length f-theta lens. Dimples were generated on the alloy surface in an ambient air environment by applying a known number of laser pulses of specific energy to a single site or ‘spot’. 101
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Fig. 1. SEM images of the laser impacts with 1 μJ and 10 pulses (a) and (d); 10 μJ and 50 pulses (b) and (e); and 20 μJ and 200 pulses (c) and (f) on Ti6Al4V alloy.
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Fig. 2. SEM images of the laser impacts with 1 μJ and 10 pulses (a) and (d); 10 μJ and 50 pulses (b) and (e); and 20 μJ and 200 pulses (c) and (f) on AA2024-T3 alloy.
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Table 2 Chemical composition by EDS (atomic %) in the dimples. Sample
Untextured Center
Rim
Ti6Al4V (%)
1 μJ/10 pulses 10 μJ/50 pulses 20 μJ/200 pulses 1 μJ/10 pulses 10 μJ/50 pulses 20 μJ/200 pulses
AA 2024-T3 (%) O
Ti
Al
V
O
Al
Cu
Mg
– 3.13 6.85 – 10.72 52.32 47.79
85.76 83.11 80.74 85.89 77.95 40.36 43.72
10.06 9.73 9.54 10.28 9.14 4.83 6.83
4.17 4.04 2.78 3.83 2.19 2.45 2.01
– 1.97 3.57 16.06 3.97 8.16 42.55
96.62 93.64 93.38 82.36 92.49 88.93 54.95
2.01 2.66 1.99 1.06 2.41 1.88 1.08
1.37 1.73 1.06 0.52 1.36 1.02 1.42
Fig. 3. Topography (up) and profile (down) of the dimples generated with 20 μJ and 200 pulses on (a) Ti6Al4V and (b) aluminium alloy 2024-T3 obtained by confocal perfilometry.
As the treatment is performed in the absence of a protective atmosphere, a certain degree of surface oxidation can be expected with the increase in energy and number of pulses. In present case, EDS analysis, Table 2, confirmed the presence of oxygen on the treated area, presumable a non-stoichiometric TiO2 or Al2 O3 oxide. The concentration of oxygen was lower in the centre of the dimple than in the dimple border. This could be due to the formation of laser-induced plasma that shields the bottom of the dimple from the environment [28]. In the areas impacted by the laser, it is possible to distinguish the formation of periodic nanometric size structures in both alloys, Fig. 1d–f and Fig. 2d–f. The morphology of these nanostructures changes according to the laser parameters and alloy. Structures known as LIPSS (Laser Induced Periodic Surface Structure) are found for Ti6Al4V alloy, Fig. 2a– c. Its morphology is characterized by a gentle wave shape. In this case,
the regular ripple structure presented a period, Λ, about 1200 nm, close to the laser wavelength, 1064 nm. Literature refers to this type of LIPSS as "low spatial frequency LIPSS" (LSFL) [29,30], which are characterized by a period on the same order of magnitude as the wavelength of the laser used and with direction at right angles to the incident linear polarisation. The formation of such nanostructures has been extensively studied since the early 60s. Although such formations are still under study, it could be accepted that the LSFL type are due to interference between laser radiation and the generated surface scattered wave which produces a non-uniform energy distribution on the molten surface which, eventually, causes both periodic ablation and the self-organization of the surface structure. In the titanium alloy, LIPSS observed in the dots have the same orientation throughout the textured area, Fig. 1a. The orientation was per104
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Fig. 5. Dimple depth versus pulse energy for different number of pulses. (a) Ti6Al4V alloy and (b) aluminium alloy 2024-T3.
Fig. 4. Diameter of the dimples versus pulse energy for different number of pulses. For (a) Ti6Al4V alloy and (b) aluminium alloy 2024-T3.
creasing amount of molten material which is greater at higher energy and exposure. Similarly, nanostructures observed on the treated surfaces of the aluminium alloy also depend on the laser processed parameters, Fig. 2a–d. However, LIPSS structures are much less apparent on this alloy as Aluminium has a low melting point and long electron–phonon coupling time, around 50 ps [32]. Unlike the titanium alloy, the formation of random nanostructures in the centre of impact is observed, regardless laser parameters used, due to melting. When the number of impacts was <10, the surface acquired a random nanoroughness that substantially increased the absorption of radiation compared to that for an untreated surface. It seems that when the number of irradiations is high enough, the molten material may cause an explosive phase due to superheating processes, resulting in the morphology type shown in Fig. 2a. This type of morphology would be consistent with the above explanation about the formation of nanospheres. Vorobyev [33] found similar structures on a platinum surface when treated with a femtosecond laser operating at 66 fs pulses at a 1 kHz repetition rate with 𝜆 of 800 nm. As the number of impacts on aluminium alloy increases, LIPSS begin to appear with the same morphology as those found in the titanium alloy, although they are mainly located on the edge of the dimple, Fig. 2e and f.
pendicular to the laser beam polarization. However, when increasing the fluence and the number of shots, LIPSS on the dimple border presented a different orientation than the LIPSS at the centre, Fig. 1b. Finally, at the highest energy and pulses (20 μJ, 200), the central part of the dimple—and even in the walls—showed a blurring of the ripples structure, due to an excess of molten material that erases the LIPSS, generating a smooth surface as seen in Fig. 1c. In the LIPSS generated at low energy and short number of pulses (1 μJ, 10), Fig. 1d, it is possible to distinguish nanospheres of about 500 nm in diameter. Its formation is due to the rapid cooling of microdroplets of molten material, avoiding the hydrodynamic expansions that deform the spherical structure of the droplet. Vorobyev and Guo [31] working with a femtosecond laser on pure titanium observed that part of the molten material is also deposited on the surface of the material as droplets of 20 nm in diameter. The difference in size of the nanospheres described in that paper and those found in the present study could be due to the different experimental conditions employed in each work. Oh the other hand, at a higher energy, 10 μJ, and number of pulses, 50 pulses, the small droplets of molten material are joined to form small columnar structures, Fig. 1e. The probability that the droplets join during the redeposition of the material increases with in105
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Fig. 6. Ablation volume versus pulse energy for different number of pulses. (a) Ti6Al4V alloy and (b) aluminium alloy 2024-T3.
hibited a similar shape to the Gaussian beam. The depth of dimples on the aluminium alloy (≈20 μm) is greater than on the titanium alloy (≈6 μm). This could be due to the different thermal conductivities of both materials, 237.0 W/m K for pure aluminium and 22 W/m K for pure titanium. The diameter of the dimples varied with the laser parameters and the metallic alloys, Fig. 4. The diameter increased with increasing energy and number of pulses for both alloys, exhibiting dimples with larger diameter on titanium than on aluminium alloy. Similarly, the depth of the dimples also increased with the increase in energy and number of pulses on both alloys, Fig. 5. The depth of dimples on the aluminium alloys is greater than on the titanium alloy for the same texturing conditions. Finally, the volume of ablated materials, measured by confocal profilometry, was higher for the AA2024 than for Ti6Al4V, Fig. 6. In a Gaussian laser beam, the fluence peak, 𝜑, and pulse energy, Ep , are directly related by expression (5) [40,41]:
Finally, it is worth highlighting that the LIPSS on the titanium alloy are better defined than those on the aluminium alloy. The definition of LIPSS is correlated with the electron–phonon coupling energy coefficient of the material, so that the higher the coefficient, the more pronounced the LIPSS [34]. As titanium has a higher coupling energy coefficient, 1.50 × 1018 W/K m3 , than the aluminium, 2.45 × 1017 W/ K m3 , LIPSS on the titanium alloy irradiated surfaces are observed more clearly than on aluminium alloy. Laser ablation by short or ultra-short pulses is a complex process, although well studied in the literature [35–37]. The laser used in present study has a pulse length of 10 ps, that can be considered as an ultrashort pulse length, with the so-called two-temperature model to induce material removal process [38,39]. It is remarkable, however, the higher ablation rate/pulse in AA2024 than in Ti6Al4V. The physical properties of the alloys should be also taken into account as, for instance, the higher reflectivity of Al than Ti or even that melt material expulsion by expanding plasma with pulse numbers are higher in Al due to its lower melting point. However, even if these assumptions were true, the ablated volume is nearly four times bigger for the Ti6Al4V than for the Al2024 alloy. This apparent contradiction may be explained as follows. Fig. 3 picture the profile of the dimples fabricated at 20 μJ and 200 pulses for both alloys. Dimples on the titanium alloy presented nearly vertical walls, while those produced on the aluminium alloy ex-
𝜑=
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𝜔0 being the beam radius. Additionally, the damage threshold fluence, or ablation threshold, 𝜑th( N ) , for a given number of pulses (N) is related to the diameter of the dimple, D, according to Eq. (6) [42]: 𝐷2 = 2𝜔20 ln 𝜑 − 2𝜔20 ln 𝜑𝑡ℎ(𝑁 )
(6)
Figure 7 shows the graphical representation of Eq. (6) for each alloy. The diameter of the focused beam, 2𝜔o , obtained from the slope of the 106
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Fig. 9. Depth of the dimples per applied pulse, L, versus pulse energy for both alloys.
graph is 24 ± 2 μm, a value similar to the theoretically calculated value, 29.4 μm. The threshold fluence for pulse laser ablation, 𝜑th , changes according to the number of pulses [43]: 𝜑th (𝑁 ) =
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Ti6Al4V alloy. Accordingly, polished titanium absorbs a greater amount of radiation of the first pulse than the polished aluminium surface and, therefore, the energy needed to ablate aluminium is greater than that for titanium for a single pulse. Similar values of 𝜑th(1) have been reported in [45] for a 10 ps Nd:VAN laser, 0.24 J/cm2 for titanium and 0.37 J/cm2 for aluminium. The dependence of ablation rate, L, of the alloys tested here on the laser fluence,𝜑, is depicted in Fig. 9a for 100 pulses. The effective penetration depth of the laser, 𝓁, follows a logarithmic dependence with the fluence determined by Eq. (9), [45]:
(7)
where Ep (th( N )) is the threshold pulse energy needed for the ablation of the material. Fig. 8a shows the decrease of the ablation threshold with the increase of the number of pulses for both alloys. For a single-shot ablation, it will mainly depend on the physical properties of the alloy. In contrast, for multiple pulses, the ablation threshold will decrease Eq. (8) [44]: ( ) log 𝑁 ⋅ 𝜑th(𝑁 ) = log 𝜑th(1) + 𝑆 ⋅ log 𝑁 (8)
𝐿 = 𝓁 ⋅ ln 𝜑 − 𝓁 ⋅ ln 𝜑𝑡ℎ
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where 𝜑th is the threshold fluence, Fig. 9b. In the simplest approach, 𝓁 can be reduced to 1/𝛼 which is the optical penetration depth, in the order of 10 nm for a wide range of metals. According to this equation, if 𝜑 ≤ 𝜑th , the material ablated will be little. When the 𝜑 > 𝜑th , the ablation depth will increase. This dependence varied with the number of pulses although showing a similar lineal trend. Only for 10 and 25 pulses and the highest fluence there is a deviation from this behaviour. For the experimental conditions used, the effective penetration depth, 𝓁, was of 5.5 nm and of 23.9 nm for Ti6Al4V alloys and AA2024-T3 alloy respectively. These values are
where 𝜑th(1) is the ablation threshold fluence for a single pulse and S is the incubation coefficient, Fig. 8b. The S value for both alloys is <1 (0.59 for Ti6Al4V alloy and 0.67 for AA2024-T3 alloy), as can be deduced from the slope of the plots. On the other hand, the ablation threshold fluence for a single shot, 𝜑th(1) , is less for titanium alloy, 0.45 J/cm2 , than for aluminium alloy, 0.53 J/cm2 . It would be related to differences in the reflectivity of the material. When the first impact pulse occurs, the polished surface of AA2024-T3 alloy has a reflectivity of 97% while it is only 57% for the 107
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smaller than those obtained by Cheng et al [45] for fluencies <3 J/cm2 , 39 nm for Ti and 42 nm for Al, although for a single pulse. As it has been mentioned before, literature reported that material ablation takes place by different mechanisms depending on the pulse duration, the fluence and the physicochemical properties of the material. In the picosecond lasers, the laser energy is absorbed by the free electrons of the metal surface and, due to the electron-phonon coupling, this energy is transferred to the crystal lattice [46]. If the pulse duration were less than the time of electron–phonon coupling, a solid direct transformation to vapour or to plasma occurs. However, if the electron–phonon coupling time were less than the pulse duration, the laser–material interaction is heat diffusion limited and, therefore, a temperature gradient which results in a thermal diffusion of a given length occurs. Finally, in order to corroborate the experimental results of the effective penetration length of the laser, the thermal diffusion length has been estimated. The laser used had pulse duration of 10 ps; as the electronphonon coupling time of titanium and aluminium are 5.8 ps and 4.3 ps, respectively, the effective penetration depth of the laser, 𝓁, matches the thermal diffusion length and can be calculated according to: √ 𝓁 ≈𝜁 𝐷⋅𝜏 (10)
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where 𝜁 is a geometric constant that depends on the geometry of the √ system, and it has a value on the order of the unit, (often 2); D is the thermal diffusivity of the material, and 𝜏 can be simplified as the duration of the laser pulse. Thermal diffusivity is 0.900 cm2 / s for aluminium and 0.029 cm2 /s for titanium; therefore a thermal length diffusion of 5.4 nm for the Ti6Al4V and 30.0 nm for the AA2024T-3 is obtained. These values are in agreement with the experimentally obtained (5.5 nm for the titanium alloy and 23.9 nm for the aluminium alloy), allowing to explain the differences in dimple depths between AA2024-T3 and Ti6Al4V at same processing energies. 4. Conclusions Two different textures due to material ablation by ultra-short lasermaterial interaction are produced on Ti6Al4V and aluminium alloy 2024-T3 the for the laser experimental used (pulse energy -between 1 and 20 μJ—and number of pulses per impact—from 10 to 200): •
•
Shallow dots for numbers of pulses <10. The treatment is rather superficial with limited ablation depth. Dimples with an appreciable depth produced by blind drilling with fixed pulsed numbers >50 on a particular place on the surface.
Additionally, in the treated areas, it was possible to distinguish the formation of nanometric size structures in both alloys. Structures known as low spatial frequency LIPSS (Laser Induced Periodic Surface Structure) are found for Ti6Al4V alloy with a period of 1200 nm, close to the laser wavelength, 1064 nm. In the case of the aluminium alloy, it is observed the formation of random nanostructures in the centre of impact regardless laser parameters used. LIPSS appeared only located on the edge of the dimples when the number of pulses increased. The effective penetration depth of the laser was of 5.5 nm and of 23.9 nm for Ti6Al4V alloy and AA2024-T3 alloy respectively. These results fully agreed with those found for the thermal diffusion length, estimated from the values of the thermal diffusivity of each alloy and the duration of the laser pulse. Acknowledgements This work was supported by the Spanish Ministry of Science and Innovation [SMOTI MAT2009-13751] and the Spanish Ministry of Economy and Competitiveness [MAT2013-48224-C2-1-R-MUNSUTI]. Authors also acknowledge to Professor G. Dearden from the Laser Engineering Group, University of Liverpool, for his support for this study. 108
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