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Mold-free fs laser shock micro forming and its plastic deformation mechanism
Optics and Lasers in Engineering 67 (2015) 74–82
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Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Mold-free fs laser shock micro forming and its plastic deformation mechanism Yunxia Ye a,b,n, Yayun Feng a, Zuchang Lian a, Yinqun Hua a a b
School of Mechanical Engineering, Jiangsu University, Xuefu Road, Zhenjiang 21203, PR China Jiangsu Provincial Key Laboratory for Science and Technology of Photon Manufacturing, Jiangsu University, Xuefu Road, Zhenjiang 212013, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 16 March 2014 Received in revised form 8 October 2014 Accepted 1 November 2014
Mold-free micro forming using a fs laser was investigated by producing micro pits on pure aluminum foil. The characteristics of the pit profiles, their forming mechanisms, and the influences of some important parameters on the pit profiles were investigated by measuring the profiles and the surface morphologies of the pits. The microstructures of the shocked aluminum foil were observed through transmission electron microscopy (TEM). Pits obtained through fs laser shock forming are composed of two regions: the directly impacted region and the plastically bending region. Diameters of the former strongly depend on laser beam sizes. The plastically bending region has a negative effect on forming precision. Shorter laser pulse width is beneficial for narrowing the range of the plastically bending region and enhancing the forming precision. Using a single-side clamping mode can also narrow the plastically bending region through buffering the local bending. Fs laser-induced microstructures are characteristic of fragmentary short dislocation lines and parallel slip lines, which are the results of the ultrafast and ultrahigh pressure loading. The localization of the fs laser shock forming induced by ultrafast loading can enhance the precision of mold-free forming. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Femtosecond laser Laser shock forming Mold-free forming Plastic deformation Metal foils
1. Introduction With the advancement of micro devices, there is an increasing demand for the development of production technology for micro parts. Cold plastic deformation is a type of near-net-shape forming technology that has sparked many researchers’ interests for its potential application in the fabrication of micro parts. For example, thin-walled parts may be obtained through micro deep drawing or micro punching [1]. Micro gears and other solid parts have been fabricated using micro bulk forming [2]. However, traditional plastic forming methods have many disadvantages hindering their application in micro manufacturing. It is difficult to make a micro mold or die, and, even if that effort is successful, demolding becomes the problem. The spring-back phenomenon is yet another problem that seriously decreases manufacturing accuracy [3,4]. Laser shock forming—a non-thermal laser forming method that has the advantages of non-contact, tool-free, high efficiency, and high precision—has been successfully introduced into micro manufacturing and has been reported as having great importance [5–10]. Generally, there are two categories of laser shock forming: with mold [5–8] and without mold [4,9–10]. For the former, the n Corresponding author at: School of Mechanical Engineering, Jiangsu University, Xuefu Road, Zhenjiang 21203, PR China. Tel.: þ 86 511 88797898; fax: þ86 511 88780241. E-mail address: [email protected] (Y. Ye).
http://dx.doi.org/10.1016/j.optlaseng.2014.11.002 0143-8166/& 2014 Elsevier Ltd. All rights reserved.
maximal horizontal size of the formed microstructure is determined by the mold, and the laser parameters are the main influence on the vertical size. For the latter, the forming process is performed without a mold, so the sizes of the shocked zones are mainly determined by the laser parameters. Laser shock forming without mold is free of all problems resulting from micro molds. In addition, advances in laser technology enables restricting the energy to a small, localized area and inducing highly localized plastic deformations, which may increase the forming precision of the mold-free laser shock forming method. So, mold-free forming technology has great potential as an application in micro manufacturing. Until now, ns lasers have been widely used in laser shock microforming [5–9] .In fact, fs lasers with shorter pulses can induce shock waves with much higher pressure than ns lasers. For direct ablation by fs laser, strong shock wave with a peak pressure of tens to hundreds of GPa can easily be obtained [11–14]. We also quantitatively evaluated the peak pressure of a fs laserinduced shock wave when it was used for confined ablation (where a transparent confining layer is used to confine the expansion of the ablated plasma) and we found that, even with a very low laser fluence, the peak pressure could reach several tens of GPa [15], which is much higher than the dynamic yield strengths of most metals. So, fs laser-induced shock waves have been used in processing materials and fabricating parts [10,15–20]. Compared to a ns laser, a fs laser has unique advantages when
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performing micromachining and micro manufacturing, including higher precision and the lack of a heat-affected zone [21,22]. Besides, due to the ultrashort loading time, the total shock effect induced by a fs laser is not large enough to blow away microparts or burst thin metal targets [19,20]. All these properties make fs lasers very suitable for microforming. In our previous work [10,15], we conducted experiments involving fs laser shock forming. A new confining layer has been used in the fs laser shock process and three-dimensional (3D) microstructures have been produced successfully on metal foil. We have investigated the effects of the pulse width, the impacting times, and the confining layer on the deformation depth. The destroying mechanisms of the confining layer during fs laser shocking also were analyzed. The plastic deformation mechanisms of polycrystalline copper foil that has been shocked using a fs laser were characterized too. This article is a continuation of our previous work. The mold-free fabrication of 3D pits on aluminum foil using fs laser has been investigated. The profile characteristics of the pits and the forming mechanisms were analyzed. We also investigated the effects of important factors, including pulse widths, impacting times, and clamping modes, on these pit profiles. We expect that the results we have found will help promote the use of mold-free fs laser shock forming and enhance the controllability of the forming process. In addition, transmission electron microscopy (TEM) observation has been used to analyze the plastic deformation mechanisms of the aluminum under ultrahigh strain rate and pressure induced by the fs laser.
2. Experiment Rolled pure aluminum foil with a thickness of 20 mm was used as the target. The aluminum foil was annealed at 550 1C for 60 min and then exposed to furnace cooling in a vacuum, so as to decrease the original microstructure defect density in preparation for TEM observation. The hardness of the annealed aluminum foil was measured to be about 44HV0.025. Before performing laser shock, a thin layer of black paint (15 mm) was sprayed on the specimens to act as an absorbent layer. Then, gum water (30 mm) or adhesive tape (30 mm) was coated on top of the absorbent layer to act as a confining layer. The main component of the gum water is polyvinyl alcohol, and its acoustic impedance is 0.16 106 gcm 2 s 1[23]. This kind of confining layer has been used in Ref[10]. Adhesive tape, the main component of which is polypropylene, was another new confining layer used in our experiments[15], because of its higher acoustic impedance (0.19 106 gcm 2 s 1) [24]. An experimental setup similar to Ref [10] was used in our experiments, as shown in Fig. 1 A fs laser with a 800 nm wavelength was used. Its repetition rate can be set for between 1 Hz and 1 kHz; its pulse width between 80fs and 800fs; and its maximum energy can reach 500 mJ. 1 Hz was used to produce the 3D pitting on the metal foil. The laser was focused onto the specimens using a focusing lens with a 1000 mm focal length. By changing the distances between the specimens and the focal point, different laser spot sizes could be obtained on the specimens. The specimens were clamped on one support seat. Rubber gaskets were applied to help clamp the thin foil tightly. In order to investigate the influence of the clamping method on the mold-free shock process, fully clamping and single-side clamping modes were used, as is shown in Fig. 1. Furthermore, on the condition of single-side clamping, we changed the distances d between clamping end and laser impacting point, to obtain more clamping conditions. In this article, the laser spot sizes on the metal targets are about hundreds of micrometers. The hole diameter on the supporting seat is 15 mm, which is much greater than the laser spot sizes, and the hole edge has almost no effect on
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forming macro plastic deformation. Thus, the technique is actually an application of mold-free plastic forming technology. During our experiment, to avoid directly ablating the target surface, we ensured that the absorbent layer was not completely ablated by choosing the appropriate experiment parameters. So, the shock wave launched on the absorbent surface, first transmitted through the remaining absorbent material and then reached the absorbent-target interface. A mismatch between the acoustic impedance of the two materials caused only part of the shock wave energy to penetrate through the interface into the target. This phenomenon would certainly weaken the final shock effect. However, in this article, we will not discuss the influence of this weakening effect, because it exists in all experiments. We do not believe that it will change the relevant common regularities. After the laser shock experiment, the morphologies of the shocked specimens, along with any of the remaining absorbent and confining layers, were observed through a 3D profiler, a Keyence VHX-1000. Then, the specimens were cleaned with acetone to clear away the absorbent and confining layers. The profiles of the laser shocked pits were also observed through the Keyence VHX-1000. TEM was used to observe the microstructures in order to analyze the plastic deformation mechanism of the pure aluminum subjected to fs laser shock. Because the laser spot was only hundreds of micrometers, it was necessary to improve the convenience of conducting TEM by shock scanning some of the specimens through moving the specimens along the x and y axis to obtain the continuously shocked area, as is shown in Fig. 2. The laser repetition rate was tuned to 1 kHz. The moving parameters (speed or feeding distance) along the x and y axis were carefully adjusted to guarantee an overlapping rate of the laser beam. During laser scanning, the laser diameter was about 210 mm. So, the laser scanning speed along the x axis was set to about 100 mm/s and the feeding distance per step along the y axis was set to a laser diameter about 210 mm. Then, disks with the diameter 3 mm were directly cut from these shocked areas on the foil, and thinned to perforating from the specimen back surface, opposite to laser irradiated surface. So most of the laser shocked materials remained. Then, the perforated disks were observed on JEM-2100 operating at 200 kV. The procedure used to prepare the TEM specimen is shown in Fig. 2.
3. Results and discussions 3.1. The typical profiles of the pits obtained through mold-free FS laser shock Fig. 3 presents the typical 3D profiles of the pits. The main process parameters were: laser energy 500 μJ per pulse, beam diameter 400 μm, and pulse duration 340fs. Gum water was used as the confining layer in this experiment. During the experiment, specimens were fully clamped as shown in Fig. 1 and specimens were impacted two times on the same point to increase the pit depths. In this experiment, the energy density was set to about 0.4 J/cm2. Due to ultrashort pulse duration, the power density can still reach about 1.17 1012 W/cm2. So, following the procedures in Ref [15], the peak pressure is evaluated about 35.2 GPa (the acoustic impedance of the aluminum is about 1.5 106 g cm 2 s 1 [25]). With such a strong shock wave, obvious plastic deformations have been observed in our experiment. Fig. 3(a) is a typical central profile line of the pits. It is found that the entrance diameter of the pit is much bigger than the laser beam size after mold-free laser shock process. We analyze that the pit is composed of two regions. One is directly produced by fs laser induced shock wave, which is located at the central of pit. Around the pit edge, there is the plastically bending region because shock pressure induces the bending moment. These two regions can be
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Fig. 1. Schematic illustration of the experimental setup.
Fig. 2. Schematic for preparing the TEM specimens.
distinguished clearly in some other typical profile lines, as shown in Fig. 3(b). In Fig. 3(b), the pecked lines represent the slopes of the local curves. It can be distinguished clearly that curvature mutations exist on the profile line, as indicated by the two arrows. The deep region within the two mutation regions is approximately equal to the laser beam diameter of 400 μm. So, we confirm that the central deep region is the region directly impacted by the fs laser-induced shock wave and that the edge region found outside the mutation regions is the result of plastic bending. During the laser shock forming, if the bending direction of the edge region matches the central deep region well, then the two regions will connect smoothly, as seen in Fig. 3(a). Otherwise, obvious curvature mutations can be observed on the profile line, as shown in Fig. 3(b). The above analysis can be further verified in Fig. 4. Fig. 4 presents the corresponding surface morphologies of the shocked specimen shown in Fig. 3(b). Fig. 4(a) is the 3D morphology of the specimen before cleaning away the confining and absorbent layers and Fig. 4(b) is the 3D morphology of the specimen after cleaning the specimen with acetone. The surface morphology of unclean specimen is found to be composed of four regions: I, II, III, and IV.
Zone I, with a size equivalent to the laser beam diameter, is the result of the direct laser impact. Zone II is the edge zone caused by plastic bending. In this region, obvious interference fringes of equal thickness can be seen, indicating the small distance between the confining layer and the specimen. Zone IV is a completely unaffected zone, in which the confining layer and absorbent layer bond well. In zone III, the morphology is seriously different from Zone IV. However, there also has no obvious interference fringe just like zone II. We deduce that it is caused by elastic deformation between plastic bending zone II and unaffected zone IV. In region III, the confining layer has stripped from absorbent layer due to the elastic deformation of the metal foil during the fs laser shock process, but there is no obvious distance between them because of the elastic recovery that occurred after the shock wave disappeared. In Fig. 4(b), the central deep region and the transition region can also be distinguished by their color differences. The pit forming mechanism is demonstrated in Fig. 5. During the fs laser shock, the laser energy stopped before the plasma expanded significantly, which made the fs laser-induced plasma expansion limited. The duration of the fs laser-induced shock wave is about ps
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Fig. 3. Typical profile lines of the produced 3D pits without obvious curvature mutation points (a) and with obvious curvature mutation points (b).
Fig. 4. Morphologies of the shocked specimen corresponding to Fig. 3(b), unclean (a) and cleaned (b).
order of magnitude [26]. Such an ultrafast loading made the directly-affected region highly localized within the laser spot. So, the central, deeply drawn region has a size nearly equivalent with the laser beam diameter. Simultaneously, the bending moment produced the deformed transition region, which de-bonded the
confining and absorbent layers. After the shock wave disappeared, the plastically bending region retained its de-bonding, while the elastic part recovered. It must be mentioned that we also observed obvious mutation regions on some pit profile lines when using adhesive tape as the confining layer, as is shown in Fig. 6(a), which is similar to Fig. 3(b). However, the surface morphology of the unclean specimen that used adhesive tape as the confining layer has some differences from that of Fig. 4(a). As shown in Fig. 6(b), only three zones can be identified. The laser parameters were: pulse energy 400 mJ, laser beam 210 mm, pulse duration 340fs, fully clamped, and two times of impact. Zone I is the direct impact region. Zone II is the plastically bending region, where obvious sputtering traces of the absorbent layer (black paint) can be observed below the adhesive tape (indicated within the rectangular) due to the laser impact. The sputtering traces of black paint further reveal the separation between the confining layer and the metal target. Zone IV is the unaffected region. However, we cannot find an obvious elastic bending region just like the zone III in Fig. 4(a). Furthermore, interference fringes of equal thickness have not been observed in the plastically bending region. We believe the semi-liquid mucus on the adhesive tape must be responsible for these differences. On one hand, the semi-liquid mucus greatly reduces the reflection ability of the air gap surface, which eventually weakens the interference effect in the plastic bending region. Besides, the adhesion force due to the semi-liquid mucus is not conducive for the creation of a wide elastic bending region. In any case, mold-free fs laser shock forming produced 3D pits that are composed of a directly shocked region (central deep region) and plastically bending region (edge transition region). Compared to plastic forming with a mold, the plastically bending region may decrease the controllability of the shock process and influence the forming precision. Next, the influences of the pulse duration, the specimen clamping mode, and the impacting times on the plastically bending region will be investigated. 3.2. Influence of laser pulse durations on pit profiles Fig. 7 shows the profiles of pits obtained using fs lasers set at different pulse durations. The laser energy was about 350 mJ and the laser beam size 210 mm. Adhesive tape acted as the confining
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Fig. 5. Pit forming mechanism of mold-free fs laser shock forming.
Fig. 6. Laser shocked specimen with adhesive tape as confining layer, pit profile (a) and unclean surface morphology (b).
layer. The specimens were fully clamped and were impacted two times per point. In Fig. 7, the laser beam radius of 105 mm has been marked at the profile line.
It is obvious that the range outside of the directly shocked region widens considerably with an increase in pulse duration (150fs: 61.5 μm, 340fs: 116.5 μm,and 500fs: 148.5 μm). We determine that
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Fig. 7. Pits produced with a fs laser set at different pulse durations: 500fs (a), 340fs (b), and 150fs (c).
the longer pulse duration presents more time for the surrounding material to react to the ultrafast shock loading, because the common plastic deformation modes, such as the dislocation slip and twin, need time to complete. As a result, a wider range of the material became plastically deformed under the longer shock loading. On the contrary, for the shorter pulse durations, the plastic deformation outside of the laser beam was only slightly deformed. Besides, with the increase of the pulse duration, the depth of the central direct shock region increased a little, but not obvious (150fs: 8.6 μm, 340fs: 9.8 μm, and 500fs: 11 μm), while the whole pit depth increased obviously (150fs: 13 μm, 340fs: 29 μm,and 500fs: 27 μm), which means that the increase in the whole pit depth is mainly contributed from plastic bending, not from direct laser shock. So, for the sake of forming precision during mold-free laser shock forming, shortening the pulse duration is preferred for enhancing the forming precision and promoting the controllability of the forming range without sacrificing the deformation depth of the directly shocked region. However, it is worth noting that too short pulse duration brings more risks for destroying the confining layers through nonlinear absorption during the fs laser shock forming[10], so the appropriate pulse width should be determined by carefully considering the ablation thresholds of the confining layer. 3.3. Influence of impacting times on pit profiles Since the entrance diameter of the central deep region is strongly dependent on the laser beam size, we tried to impact the same point of the metal foil more times to increase the pit depth and keep the entrance diameter of the central deep region nearly unchanged. Fig. 8 gives the results of the experiment. The
pulse width used is 340fs. The other experimental parameters are the same as those reported in Section 3.2. We found that the sizes of the pits only increased a little at the first stage (from one time to two times), then quickly evolved to maintain one certain value. That is to say, increasing the impacting times did not increase the pit depth efficiently. This result was out of our expectations. We analyze that there are two possible reasons: (1) After the first laser impact, the adhesive tape (the confining layer) had been stripped from the specimen and the air gap between the confining layer and the specimen seriously weakened the confining effect during the later shock; (2) the absorbent layer had been ablated in the first impact and the plasma resource produced during the later shock process decreased accordingly, which also weakened the shock effect. So, timely updating of the confining and absorbent layers may be necessary for efficiently improving the shock effects of multiple impacts. 3.4. Influence of clamping modes on pit profiles During our experiment, we used two clamping modes: fully clamping and single-side clamping at one end, as shown in Fig. 1. When using single-side clamping, the distance d between the laser interacting point and the clamping end has been changed to obtain different clamping conditions. The other experimental parameters are also the same as reported in Section 3.2. Fig. 9 presents the pit profile sizes varying with the distances d and the results obtained using a fully clamping mode. It is found that clamping mode has little effect on the depth of the central deep regions (direct shock regions), which further proves that the fs laser-induced shock loading is highly localized and so ultrafast that the materials out of the laser beam have no obvious effect on
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Fig. 8. Influence of the impacting times on pit profiles.
Fig. 9. Influence of the clamping modes on pit profiles.
directly shocked regions. However, the clamping modes greatly influenced the plastic bending regions. As distance d increased, the plastically bending regions narrowed and the depth of them decreased. The whole pit depths also decreased accordingly. This phenomenon is reasonable, because when singe-side clamping is used, the specimen is just like a cantilever beam. The larger the distance, the bigger the bending moment, which will cause the entire metal foil specimen to experience elastic bend during the fs laser shock process. The entire bend of the metal foil buffered the shock effect and weakened the local plastic bending effect. So increasing the distance between the laser interacting point and the clamping end appropriately will be a beneficial way to narrow the edge of the bending region and will improve the precision of the mold-free fs laser shock forming.
4. The plastic deformation mechanism of aluminum foil under fs laser shock forming Fs laser shock forming is characteristic of an ultrahigh pressure and an ultrahigh strain rate. The strain rate can reach up to 108 s 1 [26] and the peak pressure of a fs laser-induced shock wave can reach tens to hundreds of GPa [11–15,27]. In our experiment, the peak pressure can reach several tens of GPa. Under these extreme conditions, how does aluminum react to the shock wave? Besides, as we have analyzed above, the materials outside the laser beam have little effect on the central deep regions during fs laser shock,
which is also the external manifestation of the internal plastic deformation mechanisms. TEM was used to study the internal plastic deformation mechanisms of aluminum under fs laser shock forming. The laser energy was 350 mJ, laser beam size 210 mm, pulse duration 340fs, and the confining layer was adhesive tape. The TEM specimen preparation was reported above. Fig. 10 gives the unique microstructures of the shocked pure aluminum after the fs laser shock, such as the high density of fragmentary dislocation lines shown in Fig. 10(a) and the parallel straight slip lines shown in Fig. 10(b), (c), and (d). In the specimen, there exists large amount of fragmentary, short dislocation lines with a similar “L” shape, which is one of the most important phenomena. The forming mechanism for this kind of dislocation morphology is analyzed as follows. The ultrahigh strain rate and ultrahigh pressure loading mean the local region has a high degree of stress concentration and a high degree of distortion energy, which produce high density dislocation sources. On the other hand, the plastic deformation needs time to complete [28]. So, during the fs laser shock processing, the dislocations originated from sources have no enough time to slip forward due to the ultrashort loading time. As a result, high density of short dislocation lines were observed. Furthermore, under certain applied stresses in the same grain, the dislocation lines slipped in the same modes and along the same direction. Then, “L-shaped” fragmentary dislocation lines with similar profiles were observed, as shown in Fig. 10(a). So, from the phenomenon that dislocations in shocked regions have no enough time to extend, we further
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Fig. 10. The typical microstructures obtained through fs laser shock.
confirm that the fs laser shock forming is highly localized in the direct shock regions. As long as the plastic bending regions are controlled by choosing the appropriate technique parameters, high precision can be realized during mold-free fs laser shock forming. The second typical microstructure produced by fs laser shocking is profuse parallel straight slip lines in some regions. Fig. 10(b) shows that the straight lines begin to form along one direction and Fig. 10(c) shows two sets of parallel straight lines beginning to form. In Fig. 10(d), it is obvious that there have two sets of parallel dislocation slip lines. As is well known that, in aluminum, it is difficult for perfect dislocations to decomposed to partial dislocations because of its high stacking faults energy(SFE). So, dislocation cross slipping is the main plastic deformation mechanism. Dislocation tangles, dislocation walls, and dislocation cells are the most common microstructures in aluminum. However, in our experiments, lots of parallel straight dislocation lines have been observed. We attribute this phenomenon to the ultrahigh strain rate and ultrahigh pressure induced by the fs laser.
5. Conclusions 3D pits have been produced through fs laser microforming without a mold. The profile characteristics and its forming mechanisms have been investigated. The influences of some important technique parameters on the pit profiles were analyzed. TEM has been used to observe the microstructures of the fs laser shocked aluminum. The main conclusions are as follows: (1) The 3D pits obtained through mold-free fs laser shock microforming are composed of two regions: a central deep region and an edge transition region. Fs laser direct shock produces the central deep region and the bending moment produces the edge transition region.
(2) The entrance diameter of the central deep region seriously depends on the laser beam diameter. The pulse durations greatly influence the transition regions. As the pulse duration increases, the width of the transition region increases also, which is not favorable for enhancing the forming precision of the mold-free forming process. (3) Clamping modes have obvious effects on the profiles of the 3D pits. The full clamping method widens the transition region and the single-side clamping mode lessens the range of the transition region, because the complete bending of the metal foil buffers the shock effect and then weakens the local bending in the pit. (4) For fs laser shock, dislocation is still the main plastic deformation mechanism of aluminum. Ultrafast loading and ultrahigh pressure produce high density dislocation sources. These dislocations do not have enough time to slip and grow longer. So, fragmentary short dislocation lines are one of the important dislocation morphology. Another important dislocation morphology is the parallel dislocation slip lines, which is difficult to be observed in aluminum because of its high SFE. We determine that this is also the result of the ultrahigh strain rate of the plastic deformation. (5) Fs laser shock forming is highly localized in the directly shocked regions. As long as the plastic bending regions are controlled through choosing appropriate technique parameters, high precision can be realized during mold-free fs laser shock forming. Another problem we must emphasize is the influence of absorbenttarget energy coupling. Until now, most researchers (including us) have ignored the influence of absorbent-target energy coupling. In fact, during laser shock processing, some absorbent material must be left on the target surface on many occasions to avoid directly ablating the target surface. That is to say, the shock wave transmits through the multilayer material into the target. Dramatic differences between acoustic impedances of the absorbent material and the target will
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weaken the final shock effect. This is one issue we really must address in our future research involving laser shock microforming, because the thickness of the absorbent material is on the same order of magnitude as the target. On these occasions, some influences of the absorbent material cannot be ignored.
Acknowledgements This work is supported by the National Natural Science Foundation of China(No.51205172),Key Technology R&D Program of Jiangsu Province(No.BE2013097), and Six Talent Summit Foundation of Jiangsu Province(No. 2010-JXQC069) References [1] Gau JT, Teegala S, Huang KM, Hsiao TJ, Lin BT. Using micro deep drawing with ironing stages to form stainless steel 304 micro cups. J Manuf Process 2013;15:298–305. [2] Wang D, Shi TL, Pan J, Liao GL, Tang ZR, Liu L. Finite element simulation and experimental investigation of forming micro-gear with Zr-Cu-Ni-Al bulk metallic glass. J Mater Process Tech 2010;210:684–8. [3] Masuzawa T. State of the art of micromachining. CIRP Ann-Manuf Techn 2000;49:473–88. [4] Nagarajan B, Castagne S. Wang ZK.Mold-free fabrication of 3D microfeatures using laser-induced shock pressure. Appl Surf Sci 2013;268:529–34. [5] Zheng C, Sun S, Ji Z, Wang W, Liu J. Numerical simulation and experimentation of micro scale laser bulge forming. Int J Mach Tool Manu 2010;50:1048–56. [6] Di JK, Zhou M, Li J, Li C, Zhang W, Amoako G. Micro-punching process based on spallation delamination induced by laser driven-flyer. Appl Surf Sci 2012;258:2339–43. [7] Zheng C, Sun S, Ji Z, Wang W. Effect of laser energy on the deformation behavior in microscale laser bulge forming. Appl Surf Sci 2010;257:1589–95. [8] Schulze Niehoff H, Vollertsen F. Non-thermal laser strech-forming. Adv Mater Res 2005;6-8:433–40. [9] Ocaña JL, Morales M, Porro JA, García-Ballesteros JJ, Correa C. Laser shock microforming of thin metal sheets with ns lasers. Phys Procedia 2011;12:201–6. [10] Ye YX, Feng YY, Hua XJ, Lian ZC. Experimental research on laser shock forming metal foils with femtosecond laser. Appl Surf Sci 2013;285B:600–6.
[11] Cuq-Lelandais JP, Boustie M, Berthe L, De Rességuier T, Combis P, Colombier JP, et al. Spallation generated by femtosecond laser driven shocks in thin metallic targets. J Phys D Appl Phys 2009;42:65402–11. [12] Huang L, Yang YQ, Wang YH, Zheng ZR, Su WH. Measurement of transit time for femtosecond-laser-driven shock wave through aluminium films by ultrafast microscopy. J Phys D Appl Phys 2009;42 (045502-1- 6). [13] Ionin AA, Kudryashov SI, Makarov SV, Seleznev LV, Sinitsyn DV. Generation and detection of superstrong shock waves during ablation of an aluminum surface by intense femtosecond laser pulses. JETP Lett 2011;94:34–8. [14] Inogamov NA, Khokhlov V, Petrov Y, Anisimov S, Zhakhovsky VV, Demaske BJ, et al. Ultrashort elastic and plastic shockwaves in aluminum. AIP Conf Proc 2012;1426:909–12. [15] Ye YX, Feng YY, Lian ZC, Hua YQ. Plastic deformation mechanism of polycrystalline copper foil shocked with femtosecond laser. Appl Surf Sci 2014;309:240–9. [16] Nakano H, Miyauti S, Butani N, Shibayanagi T, Tsukamoto M, Abe N. Femtosecond laser peening of stainless steel. J Laser Micro/Nanoen 2009;4:35–8. [17] Lee D. Feasibility study on laser microwelding and laser shock peening using femtosecond laser pulses [Ph.D. thesis]. University of Michigen; 2008. [18] Wu B, Tao S, Lei S. Numerical modeling of laser shock peening with femtosecond laser pulsesand comparisons to experiments. Appl Surf Sci 2010;256:4376–82. [19] Sagisaka Y.Micro forming using shock wave generated by femtosecond laser irradiation. Gyeongju, Korea. Proceedings of the nineth ICTP; 2008. p. 1878-83. [20] Sagisaka Y, Kamiya M, Matsuda M, Ohta Y. Thin-sheet-metal bending by laser peen forming with femtosecond laser. J Mater Process Tech 2010;210:2304–9. [21] Chichkov BN, Momma C, Nolte S. Femtosecond,picosecond and nanosecond laser ablation of solids. Appl Phys A 1996;63:109–15. [22] Zhu X, Villeneuve DM, Naumov AY, Nikumb S, Corkum PB. Experimental study of drilling sub-10 mm holes in thin metal foils with femtosecond laser pulses. Appl Surf Sci 1999;152:138–48. [23] Fromageau J, Brusseau E, Vray D, Gimenez G, Delachartre P. Characterization of PVA cryogel for intravascular ultrasound elasticity imaging. IEEE Trans Ultrason Ferroelectr Freq Control 2003;50:1318–24. [24] Lochab J, Singh VR. Acoustic behavior of plastics for medical applications. Indian J Pure Appl Phys 2004;42:595–9. [25] Menezes V, Kumar S, Takayama K. Shock wave driven liquid microjets for drug delivery. J Appl Phys 2009;106 (086102-1-3). [26] Tamura H, Kohama T, Kondo K, Yoshida M. Femtosecond-laser-induced spallation in aluminum. J Appl Phys 2001;89:3520–2. [27] Sano T, Mori H, Sakata O, Ohmura E, Miyamoto I, Hirose A, et al. Femtosecond laser driven shock synthesis of the high pressure phase of iron. Appl Surf Sci 2005;247:571–6. [28] Demaske BJ, Zhakhovsky VV, White CT, Oleynik II. Evolution of metastable elastic shockwaves in nickel. AIP Conf Proc 2012;1426:1303–6.
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Mold-free fs laser shock micro forming and its plastic deformation mechanism Yunxia Ye a,b,n, Yayun Feng a, Zuchang Lian a, Yinqun Hua a a b
School of Mechanical Engineering, Jiangsu University, Xuefu Road, Zhenjiang 21203, PR China Jiangsu Provincial Key Laboratory for Science and Technology of Photon Manufacturing, Jiangsu University, Xuefu Road, Zhenjiang 212013, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 16 March 2014 Received in revised form 8 October 2014 Accepted 1 November 2014
Mold-free micro forming using a fs laser was investigated by producing micro pits on pure aluminum foil. The characteristics of the pit profiles, their forming mechanisms, and the influences of some important parameters on the pit profiles were investigated by measuring the profiles and the surface morphologies of the pits. The microstructures of the shocked aluminum foil were observed through transmission electron microscopy (TEM). Pits obtained through fs laser shock forming are composed of two regions: the directly impacted region and the plastically bending region. Diameters of the former strongly depend on laser beam sizes. The plastically bending region has a negative effect on forming precision. Shorter laser pulse width is beneficial for narrowing the range of the plastically bending region and enhancing the forming precision. Using a single-side clamping mode can also narrow the plastically bending region through buffering the local bending. Fs laser-induced microstructures are characteristic of fragmentary short dislocation lines and parallel slip lines, which are the results of the ultrafast and ultrahigh pressure loading. The localization of the fs laser shock forming induced by ultrafast loading can enhance the precision of mold-free forming. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Femtosecond laser Laser shock forming Mold-free forming Plastic deformation Metal foils
1. Introduction With the advancement of micro devices, there is an increasing demand for the development of production technology for micro parts. Cold plastic deformation is a type of near-net-shape forming technology that has sparked many researchers’ interests for its potential application in the fabrication of micro parts. For example, thin-walled parts may be obtained through micro deep drawing or micro punching [1]. Micro gears and other solid parts have been fabricated using micro bulk forming [2]. However, traditional plastic forming methods have many disadvantages hindering their application in micro manufacturing. It is difficult to make a micro mold or die, and, even if that effort is successful, demolding becomes the problem. The spring-back phenomenon is yet another problem that seriously decreases manufacturing accuracy [3,4]. Laser shock forming—a non-thermal laser forming method that has the advantages of non-contact, tool-free, high efficiency, and high precision—has been successfully introduced into micro manufacturing and has been reported as having great importance [5–10]. Generally, there are two categories of laser shock forming: with mold [5–8] and without mold [4,9–10]. For the former, the n Corresponding author at: School of Mechanical Engineering, Jiangsu University, Xuefu Road, Zhenjiang 21203, PR China. Tel.: þ 86 511 88797898; fax: þ86 511 88780241. E-mail address: [email protected] (Y. Ye).
http://dx.doi.org/10.1016/j.optlaseng.2014.11.002 0143-8166/& 2014 Elsevier Ltd. All rights reserved.
maximal horizontal size of the formed microstructure is determined by the mold, and the laser parameters are the main influence on the vertical size. For the latter, the forming process is performed without a mold, so the sizes of the shocked zones are mainly determined by the laser parameters. Laser shock forming without mold is free of all problems resulting from micro molds. In addition, advances in laser technology enables restricting the energy to a small, localized area and inducing highly localized plastic deformations, which may increase the forming precision of the mold-free laser shock forming method. So, mold-free forming technology has great potential as an application in micro manufacturing. Until now, ns lasers have been widely used in laser shock microforming [5–9] .In fact, fs lasers with shorter pulses can induce shock waves with much higher pressure than ns lasers. For direct ablation by fs laser, strong shock wave with a peak pressure of tens to hundreds of GPa can easily be obtained [11–14]. We also quantitatively evaluated the peak pressure of a fs laserinduced shock wave when it was used for confined ablation (where a transparent confining layer is used to confine the expansion of the ablated plasma) and we found that, even with a very low laser fluence, the peak pressure could reach several tens of GPa [15], which is much higher than the dynamic yield strengths of most metals. So, fs laser-induced shock waves have been used in processing materials and fabricating parts [10,15–20]. Compared to a ns laser, a fs laser has unique advantages when
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performing micromachining and micro manufacturing, including higher precision and the lack of a heat-affected zone [21,22]. Besides, due to the ultrashort loading time, the total shock effect induced by a fs laser is not large enough to blow away microparts or burst thin metal targets [19,20]. All these properties make fs lasers very suitable for microforming. In our previous work [10,15], we conducted experiments involving fs laser shock forming. A new confining layer has been used in the fs laser shock process and three-dimensional (3D) microstructures have been produced successfully on metal foil. We have investigated the effects of the pulse width, the impacting times, and the confining layer on the deformation depth. The destroying mechanisms of the confining layer during fs laser shocking also were analyzed. The plastic deformation mechanisms of polycrystalline copper foil that has been shocked using a fs laser were characterized too. This article is a continuation of our previous work. The mold-free fabrication of 3D pits on aluminum foil using fs laser has been investigated. The profile characteristics of the pits and the forming mechanisms were analyzed. We also investigated the effects of important factors, including pulse widths, impacting times, and clamping modes, on these pit profiles. We expect that the results we have found will help promote the use of mold-free fs laser shock forming and enhance the controllability of the forming process. In addition, transmission electron microscopy (TEM) observation has been used to analyze the plastic deformation mechanisms of the aluminum under ultrahigh strain rate and pressure induced by the fs laser.
2. Experiment Rolled pure aluminum foil with a thickness of 20 mm was used as the target. The aluminum foil was annealed at 550 1C for 60 min and then exposed to furnace cooling in a vacuum, so as to decrease the original microstructure defect density in preparation for TEM observation. The hardness of the annealed aluminum foil was measured to be about 44HV0.025. Before performing laser shock, a thin layer of black paint (15 mm) was sprayed on the specimens to act as an absorbent layer. Then, gum water (30 mm) or adhesive tape (30 mm) was coated on top of the absorbent layer to act as a confining layer. The main component of the gum water is polyvinyl alcohol, and its acoustic impedance is 0.16 106 gcm 2 s 1[23]. This kind of confining layer has been used in Ref[10]. Adhesive tape, the main component of which is polypropylene, was another new confining layer used in our experiments[15], because of its higher acoustic impedance (0.19 106 gcm 2 s 1) [24]. An experimental setup similar to Ref [10] was used in our experiments, as shown in Fig. 1 A fs laser with a 800 nm wavelength was used. Its repetition rate can be set for between 1 Hz and 1 kHz; its pulse width between 80fs and 800fs; and its maximum energy can reach 500 mJ. 1 Hz was used to produce the 3D pitting on the metal foil. The laser was focused onto the specimens using a focusing lens with a 1000 mm focal length. By changing the distances between the specimens and the focal point, different laser spot sizes could be obtained on the specimens. The specimens were clamped on one support seat. Rubber gaskets were applied to help clamp the thin foil tightly. In order to investigate the influence of the clamping method on the mold-free shock process, fully clamping and single-side clamping modes were used, as is shown in Fig. 1. Furthermore, on the condition of single-side clamping, we changed the distances d between clamping end and laser impacting point, to obtain more clamping conditions. In this article, the laser spot sizes on the metal targets are about hundreds of micrometers. The hole diameter on the supporting seat is 15 mm, which is much greater than the laser spot sizes, and the hole edge has almost no effect on
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forming macro plastic deformation. Thus, the technique is actually an application of mold-free plastic forming technology. During our experiment, to avoid directly ablating the target surface, we ensured that the absorbent layer was not completely ablated by choosing the appropriate experiment parameters. So, the shock wave launched on the absorbent surface, first transmitted through the remaining absorbent material and then reached the absorbent-target interface. A mismatch between the acoustic impedance of the two materials caused only part of the shock wave energy to penetrate through the interface into the target. This phenomenon would certainly weaken the final shock effect. However, in this article, we will not discuss the influence of this weakening effect, because it exists in all experiments. We do not believe that it will change the relevant common regularities. After the laser shock experiment, the morphologies of the shocked specimens, along with any of the remaining absorbent and confining layers, were observed through a 3D profiler, a Keyence VHX-1000. Then, the specimens were cleaned with acetone to clear away the absorbent and confining layers. The profiles of the laser shocked pits were also observed through the Keyence VHX-1000. TEM was used to observe the microstructures in order to analyze the plastic deformation mechanism of the pure aluminum subjected to fs laser shock. Because the laser spot was only hundreds of micrometers, it was necessary to improve the convenience of conducting TEM by shock scanning some of the specimens through moving the specimens along the x and y axis to obtain the continuously shocked area, as is shown in Fig. 2. The laser repetition rate was tuned to 1 kHz. The moving parameters (speed or feeding distance) along the x and y axis were carefully adjusted to guarantee an overlapping rate of the laser beam. During laser scanning, the laser diameter was about 210 mm. So, the laser scanning speed along the x axis was set to about 100 mm/s and the feeding distance per step along the y axis was set to a laser diameter about 210 mm. Then, disks with the diameter 3 mm were directly cut from these shocked areas on the foil, and thinned to perforating from the specimen back surface, opposite to laser irradiated surface. So most of the laser shocked materials remained. Then, the perforated disks were observed on JEM-2100 operating at 200 kV. The procedure used to prepare the TEM specimen is shown in Fig. 2.
3. Results and discussions 3.1. The typical profiles of the pits obtained through mold-free FS laser shock Fig. 3 presents the typical 3D profiles of the pits. The main process parameters were: laser energy 500 μJ per pulse, beam diameter 400 μm, and pulse duration 340fs. Gum water was used as the confining layer in this experiment. During the experiment, specimens were fully clamped as shown in Fig. 1 and specimens were impacted two times on the same point to increase the pit depths. In this experiment, the energy density was set to about 0.4 J/cm2. Due to ultrashort pulse duration, the power density can still reach about 1.17 1012 W/cm2. So, following the procedures in Ref [15], the peak pressure is evaluated about 35.2 GPa (the acoustic impedance of the aluminum is about 1.5 106 g cm 2 s 1 [25]). With such a strong shock wave, obvious plastic deformations have been observed in our experiment. Fig. 3(a) is a typical central profile line of the pits. It is found that the entrance diameter of the pit is much bigger than the laser beam size after mold-free laser shock process. We analyze that the pit is composed of two regions. One is directly produced by fs laser induced shock wave, which is located at the central of pit. Around the pit edge, there is the plastically bending region because shock pressure induces the bending moment. These two regions can be
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Fig. 1. Schematic illustration of the experimental setup.
Fig. 2. Schematic for preparing the TEM specimens.
distinguished clearly in some other typical profile lines, as shown in Fig. 3(b). In Fig. 3(b), the pecked lines represent the slopes of the local curves. It can be distinguished clearly that curvature mutations exist on the profile line, as indicated by the two arrows. The deep region within the two mutation regions is approximately equal to the laser beam diameter of 400 μm. So, we confirm that the central deep region is the region directly impacted by the fs laser-induced shock wave and that the edge region found outside the mutation regions is the result of plastic bending. During the laser shock forming, if the bending direction of the edge region matches the central deep region well, then the two regions will connect smoothly, as seen in Fig. 3(a). Otherwise, obvious curvature mutations can be observed on the profile line, as shown in Fig. 3(b). The above analysis can be further verified in Fig. 4. Fig. 4 presents the corresponding surface morphologies of the shocked specimen shown in Fig. 3(b). Fig. 4(a) is the 3D morphology of the specimen before cleaning away the confining and absorbent layers and Fig. 4(b) is the 3D morphology of the specimen after cleaning the specimen with acetone. The surface morphology of unclean specimen is found to be composed of four regions: I, II, III, and IV.
Zone I, with a size equivalent to the laser beam diameter, is the result of the direct laser impact. Zone II is the edge zone caused by plastic bending. In this region, obvious interference fringes of equal thickness can be seen, indicating the small distance between the confining layer and the specimen. Zone IV is a completely unaffected zone, in which the confining layer and absorbent layer bond well. In zone III, the morphology is seriously different from Zone IV. However, there also has no obvious interference fringe just like zone II. We deduce that it is caused by elastic deformation between plastic bending zone II and unaffected zone IV. In region III, the confining layer has stripped from absorbent layer due to the elastic deformation of the metal foil during the fs laser shock process, but there is no obvious distance between them because of the elastic recovery that occurred after the shock wave disappeared. In Fig. 4(b), the central deep region and the transition region can also be distinguished by their color differences. The pit forming mechanism is demonstrated in Fig. 5. During the fs laser shock, the laser energy stopped before the plasma expanded significantly, which made the fs laser-induced plasma expansion limited. The duration of the fs laser-induced shock wave is about ps
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Fig. 3. Typical profile lines of the produced 3D pits without obvious curvature mutation points (a) and with obvious curvature mutation points (b).
Fig. 4. Morphologies of the shocked specimen corresponding to Fig. 3(b), unclean (a) and cleaned (b).
order of magnitude [26]. Such an ultrafast loading made the directly-affected region highly localized within the laser spot. So, the central, deeply drawn region has a size nearly equivalent with the laser beam diameter. Simultaneously, the bending moment produced the deformed transition region, which de-bonded the
confining and absorbent layers. After the shock wave disappeared, the plastically bending region retained its de-bonding, while the elastic part recovered. It must be mentioned that we also observed obvious mutation regions on some pit profile lines when using adhesive tape as the confining layer, as is shown in Fig. 6(a), which is similar to Fig. 3(b). However, the surface morphology of the unclean specimen that used adhesive tape as the confining layer has some differences from that of Fig. 4(a). As shown in Fig. 6(b), only three zones can be identified. The laser parameters were: pulse energy 400 mJ, laser beam 210 mm, pulse duration 340fs, fully clamped, and two times of impact. Zone I is the direct impact region. Zone II is the plastically bending region, where obvious sputtering traces of the absorbent layer (black paint) can be observed below the adhesive tape (indicated within the rectangular) due to the laser impact. The sputtering traces of black paint further reveal the separation between the confining layer and the metal target. Zone IV is the unaffected region. However, we cannot find an obvious elastic bending region just like the zone III in Fig. 4(a). Furthermore, interference fringes of equal thickness have not been observed in the plastically bending region. We believe the semi-liquid mucus on the adhesive tape must be responsible for these differences. On one hand, the semi-liquid mucus greatly reduces the reflection ability of the air gap surface, which eventually weakens the interference effect in the plastic bending region. Besides, the adhesion force due to the semi-liquid mucus is not conducive for the creation of a wide elastic bending region. In any case, mold-free fs laser shock forming produced 3D pits that are composed of a directly shocked region (central deep region) and plastically bending region (edge transition region). Compared to plastic forming with a mold, the plastically bending region may decrease the controllability of the shock process and influence the forming precision. Next, the influences of the pulse duration, the specimen clamping mode, and the impacting times on the plastically bending region will be investigated. 3.2. Influence of laser pulse durations on pit profiles Fig. 7 shows the profiles of pits obtained using fs lasers set at different pulse durations. The laser energy was about 350 mJ and the laser beam size 210 mm. Adhesive tape acted as the confining
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Fig. 5. Pit forming mechanism of mold-free fs laser shock forming.
Fig. 6. Laser shocked specimen with adhesive tape as confining layer, pit profile (a) and unclean surface morphology (b).
layer. The specimens were fully clamped and were impacted two times per point. In Fig. 7, the laser beam radius of 105 mm has been marked at the profile line.
It is obvious that the range outside of the directly shocked region widens considerably with an increase in pulse duration (150fs: 61.5 μm, 340fs: 116.5 μm,and 500fs: 148.5 μm). We determine that
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Fig. 7. Pits produced with a fs laser set at different pulse durations: 500fs (a), 340fs (b), and 150fs (c).
the longer pulse duration presents more time for the surrounding material to react to the ultrafast shock loading, because the common plastic deformation modes, such as the dislocation slip and twin, need time to complete. As a result, a wider range of the material became plastically deformed under the longer shock loading. On the contrary, for the shorter pulse durations, the plastic deformation outside of the laser beam was only slightly deformed. Besides, with the increase of the pulse duration, the depth of the central direct shock region increased a little, but not obvious (150fs: 8.6 μm, 340fs: 9.8 μm, and 500fs: 11 μm), while the whole pit depth increased obviously (150fs: 13 μm, 340fs: 29 μm,and 500fs: 27 μm), which means that the increase in the whole pit depth is mainly contributed from plastic bending, not from direct laser shock. So, for the sake of forming precision during mold-free laser shock forming, shortening the pulse duration is preferred for enhancing the forming precision and promoting the controllability of the forming range without sacrificing the deformation depth of the directly shocked region. However, it is worth noting that too short pulse duration brings more risks for destroying the confining layers through nonlinear absorption during the fs laser shock forming[10], so the appropriate pulse width should be determined by carefully considering the ablation thresholds of the confining layer. 3.3. Influence of impacting times on pit profiles Since the entrance diameter of the central deep region is strongly dependent on the laser beam size, we tried to impact the same point of the metal foil more times to increase the pit depth and keep the entrance diameter of the central deep region nearly unchanged. Fig. 8 gives the results of the experiment. The
pulse width used is 340fs. The other experimental parameters are the same as those reported in Section 3.2. We found that the sizes of the pits only increased a little at the first stage (from one time to two times), then quickly evolved to maintain one certain value. That is to say, increasing the impacting times did not increase the pit depth efficiently. This result was out of our expectations. We analyze that there are two possible reasons: (1) After the first laser impact, the adhesive tape (the confining layer) had been stripped from the specimen and the air gap between the confining layer and the specimen seriously weakened the confining effect during the later shock; (2) the absorbent layer had been ablated in the first impact and the plasma resource produced during the later shock process decreased accordingly, which also weakened the shock effect. So, timely updating of the confining and absorbent layers may be necessary for efficiently improving the shock effects of multiple impacts. 3.4. Influence of clamping modes on pit profiles During our experiment, we used two clamping modes: fully clamping and single-side clamping at one end, as shown in Fig. 1. When using single-side clamping, the distance d between the laser interacting point and the clamping end has been changed to obtain different clamping conditions. The other experimental parameters are also the same as reported in Section 3.2. Fig. 9 presents the pit profile sizes varying with the distances d and the results obtained using a fully clamping mode. It is found that clamping mode has little effect on the depth of the central deep regions (direct shock regions), which further proves that the fs laser-induced shock loading is highly localized and so ultrafast that the materials out of the laser beam have no obvious effect on
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Fig. 8. Influence of the impacting times on pit profiles.
Fig. 9. Influence of the clamping modes on pit profiles.
directly shocked regions. However, the clamping modes greatly influenced the plastic bending regions. As distance d increased, the plastically bending regions narrowed and the depth of them decreased. The whole pit depths also decreased accordingly. This phenomenon is reasonable, because when singe-side clamping is used, the specimen is just like a cantilever beam. The larger the distance, the bigger the bending moment, which will cause the entire metal foil specimen to experience elastic bend during the fs laser shock process. The entire bend of the metal foil buffered the shock effect and weakened the local plastic bending effect. So increasing the distance between the laser interacting point and the clamping end appropriately will be a beneficial way to narrow the edge of the bending region and will improve the precision of the mold-free fs laser shock forming.
4. The plastic deformation mechanism of aluminum foil under fs laser shock forming Fs laser shock forming is characteristic of an ultrahigh pressure and an ultrahigh strain rate. The strain rate can reach up to 108 s 1 [26] and the peak pressure of a fs laser-induced shock wave can reach tens to hundreds of GPa [11–15,27]. In our experiment, the peak pressure can reach several tens of GPa. Under these extreme conditions, how does aluminum react to the shock wave? Besides, as we have analyzed above, the materials outside the laser beam have little effect on the central deep regions during fs laser shock,
which is also the external manifestation of the internal plastic deformation mechanisms. TEM was used to study the internal plastic deformation mechanisms of aluminum under fs laser shock forming. The laser energy was 350 mJ, laser beam size 210 mm, pulse duration 340fs, and the confining layer was adhesive tape. The TEM specimen preparation was reported above. Fig. 10 gives the unique microstructures of the shocked pure aluminum after the fs laser shock, such as the high density of fragmentary dislocation lines shown in Fig. 10(a) and the parallel straight slip lines shown in Fig. 10(b), (c), and (d). In the specimen, there exists large amount of fragmentary, short dislocation lines with a similar “L” shape, which is one of the most important phenomena. The forming mechanism for this kind of dislocation morphology is analyzed as follows. The ultrahigh strain rate and ultrahigh pressure loading mean the local region has a high degree of stress concentration and a high degree of distortion energy, which produce high density dislocation sources. On the other hand, the plastic deformation needs time to complete [28]. So, during the fs laser shock processing, the dislocations originated from sources have no enough time to slip forward due to the ultrashort loading time. As a result, high density of short dislocation lines were observed. Furthermore, under certain applied stresses in the same grain, the dislocation lines slipped in the same modes and along the same direction. Then, “L-shaped” fragmentary dislocation lines with similar profiles were observed, as shown in Fig. 10(a). So, from the phenomenon that dislocations in shocked regions have no enough time to extend, we further
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Fig. 10. The typical microstructures obtained through fs laser shock.
confirm that the fs laser shock forming is highly localized in the direct shock regions. As long as the plastic bending regions are controlled by choosing the appropriate technique parameters, high precision can be realized during mold-free fs laser shock forming. The second typical microstructure produced by fs laser shocking is profuse parallel straight slip lines in some regions. Fig. 10(b) shows that the straight lines begin to form along one direction and Fig. 10(c) shows two sets of parallel straight lines beginning to form. In Fig. 10(d), it is obvious that there have two sets of parallel dislocation slip lines. As is well known that, in aluminum, it is difficult for perfect dislocations to decomposed to partial dislocations because of its high stacking faults energy(SFE). So, dislocation cross slipping is the main plastic deformation mechanism. Dislocation tangles, dislocation walls, and dislocation cells are the most common microstructures in aluminum. However, in our experiments, lots of parallel straight dislocation lines have been observed. We attribute this phenomenon to the ultrahigh strain rate and ultrahigh pressure induced by the fs laser.
5. Conclusions 3D pits have been produced through fs laser microforming without a mold. The profile characteristics and its forming mechanisms have been investigated. The influences of some important technique parameters on the pit profiles were analyzed. TEM has been used to observe the microstructures of the fs laser shocked aluminum. The main conclusions are as follows: (1) The 3D pits obtained through mold-free fs laser shock microforming are composed of two regions: a central deep region and an edge transition region. Fs laser direct shock produces the central deep region and the bending moment produces the edge transition region.
(2) The entrance diameter of the central deep region seriously depends on the laser beam diameter. The pulse durations greatly influence the transition regions. As the pulse duration increases, the width of the transition region increases also, which is not favorable for enhancing the forming precision of the mold-free forming process. (3) Clamping modes have obvious effects on the profiles of the 3D pits. The full clamping method widens the transition region and the single-side clamping mode lessens the range of the transition region, because the complete bending of the metal foil buffers the shock effect and then weakens the local bending in the pit. (4) For fs laser shock, dislocation is still the main plastic deformation mechanism of aluminum. Ultrafast loading and ultrahigh pressure produce high density dislocation sources. These dislocations do not have enough time to slip and grow longer. So, fragmentary short dislocation lines are one of the important dislocation morphology. Another important dislocation morphology is the parallel dislocation slip lines, which is difficult to be observed in aluminum because of its high SFE. We determine that this is also the result of the ultrahigh strain rate of the plastic deformation. (5) Fs laser shock forming is highly localized in the directly shocked regions. As long as the plastic bending regions are controlled through choosing appropriate technique parameters, high precision can be realized during mold-free fs laser shock forming. Another problem we must emphasize is the influence of absorbenttarget energy coupling. Until now, most researchers (including us) have ignored the influence of absorbent-target energy coupling. In fact, during laser shock processing, some absorbent material must be left on the target surface on many occasions to avoid directly ablating the target surface. That is to say, the shock wave transmits through the multilayer material into the target. Dramatic differences between acoustic impedances of the absorbent material and the target will
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weaken the final shock effect. This is one issue we really must address in our future research involving laser shock microforming, because the thickness of the absorbent material is on the same order of magnitude as the target. On these occasions, some influences of the absorbent material cannot be ignored.
Acknowledgements This work is supported by the National Natural Science Foundation of China(No.51205172),Key Technology R&D Program of Jiangsu Province(No.BE2013097), and Six Talent Summit Foundation of Jiangsu Province(No. 2010-JXQC069) References [1] Gau JT, Teegala S, Huang KM, Hsiao TJ, Lin BT. Using micro deep drawing with ironing stages to form stainless steel 304 micro cups. J Manuf Process 2013;15:298–305. [2] Wang D, Shi TL, Pan J, Liao GL, Tang ZR, Liu L. Finite element simulation and experimental investigation of forming micro-gear with Zr-Cu-Ni-Al bulk metallic glass. J Mater Process Tech 2010;210:684–8. [3] Masuzawa T. State of the art of micromachining. CIRP Ann-Manuf Techn 2000;49:473–88. [4] Nagarajan B, Castagne S. Wang ZK.Mold-free fabrication of 3D microfeatures using laser-induced shock pressure. Appl Surf Sci 2013;268:529–34. [5] Zheng C, Sun S, Ji Z, Wang W, Liu J. Numerical simulation and experimentation of micro scale laser bulge forming. Int J Mach Tool Manu 2010;50:1048–56. [6] Di JK, Zhou M, Li J, Li C, Zhang W, Amoako G. Micro-punching process based on spallation delamination induced by laser driven-flyer. Appl Surf Sci 2012;258:2339–43. [7] Zheng C, Sun S, Ji Z, Wang W. Effect of laser energy on the deformation behavior in microscale laser bulge forming. Appl Surf Sci 2010;257:1589–95. [8] Schulze Niehoff H, Vollertsen F. Non-thermal laser strech-forming. Adv Mater Res 2005;6-8:433–40. [9] Ocaña JL, Morales M, Porro JA, García-Ballesteros JJ, Correa C. Laser shock microforming of thin metal sheets with ns lasers. Phys Procedia 2011;12:201–6. [10] Ye YX, Feng YY, Hua XJ, Lian ZC. Experimental research on laser shock forming metal foils with femtosecond laser. Appl Surf Sci 2013;285B:600–6.
[11] Cuq-Lelandais JP, Boustie M, Berthe L, De Rességuier T, Combis P, Colombier JP, et al. Spallation generated by femtosecond laser driven shocks in thin metallic targets. J Phys D Appl Phys 2009;42:65402–11. [12] Huang L, Yang YQ, Wang YH, Zheng ZR, Su WH. Measurement of transit time for femtosecond-laser-driven shock wave through aluminium films by ultrafast microscopy. J Phys D Appl Phys 2009;42 (045502-1- 6). [13] Ionin AA, Kudryashov SI, Makarov SV, Seleznev LV, Sinitsyn DV. Generation and detection of superstrong shock waves during ablation of an aluminum surface by intense femtosecond laser pulses. JETP Lett 2011;94:34–8. [14] Inogamov NA, Khokhlov V, Petrov Y, Anisimov S, Zhakhovsky VV, Demaske BJ, et al. Ultrashort elastic and plastic shockwaves in aluminum. AIP Conf Proc 2012;1426:909–12. [15] Ye YX, Feng YY, Lian ZC, Hua YQ. Plastic deformation mechanism of polycrystalline copper foil shocked with femtosecond laser. Appl Surf Sci 2014;309:240–9. [16] Nakano H, Miyauti S, Butani N, Shibayanagi T, Tsukamoto M, Abe N. Femtosecond laser peening of stainless steel. J Laser Micro/Nanoen 2009;4:35–8. [17] Lee D. Feasibility study on laser microwelding and laser shock peening using femtosecond laser pulses [Ph.D. thesis]. University of Michigen; 2008. [18] Wu B, Tao S, Lei S. Numerical modeling of laser shock peening with femtosecond laser pulsesand comparisons to experiments. Appl Surf Sci 2010;256:4376–82. [19] Sagisaka Y.Micro forming using shock wave generated by femtosecond laser irradiation. Gyeongju, Korea. Proceedings of the nineth ICTP; 2008. p. 1878-83. [20] Sagisaka Y, Kamiya M, Matsuda M, Ohta Y. Thin-sheet-metal bending by laser peen forming with femtosecond laser. J Mater Process Tech 2010;210:2304–9. [21] Chichkov BN, Momma C, Nolte S. Femtosecond,picosecond and nanosecond laser ablation of solids. Appl Phys A 1996;63:109–15. [22] Zhu X, Villeneuve DM, Naumov AY, Nikumb S, Corkum PB. Experimental study of drilling sub-10 mm holes in thin metal foils with femtosecond laser pulses. Appl Surf Sci 1999;152:138–48. [23] Fromageau J, Brusseau E, Vray D, Gimenez G, Delachartre P. Characterization of PVA cryogel for intravascular ultrasound elasticity imaging. IEEE Trans Ultrason Ferroelectr Freq Control 2003;50:1318–24. [24] Lochab J, Singh VR. Acoustic behavior of plastics for medical applications. Indian J Pure Appl Phys 2004;42:595–9. [25] Menezes V, Kumar S, Takayama K. Shock wave driven liquid microjets for drug delivery. J Appl Phys 2009;106 (086102-1-3). [26] Tamura H, Kohama T, Kondo K, Yoshida M. Femtosecond-laser-induced spallation in aluminum. J Appl Phys 2001;89:3520–2. [27] Sano T, Mori H, Sakata O, Ohmura E, Miyamoto I, Hirose A, et al. Femtosecond laser driven shock synthesis of the high pressure phase of iron. Appl Surf Sci 2005;247:571–6. [28] Demaske BJ, Zhakhovsky VV, White CT, Oleynik II. Evolution of metastable elastic shockwaves in nickel. AIP Conf Proc 2012;1426:1303–6.