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Grain refinement progress of pure titanium during laser shock forming (LSF) and mechanical property characterizations with nanoindentation
Materials Science & Engineering A 564 (2013) 13–21
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Grain refinement progress of pure titanium during laser shock forming (LSF) and mechanical property characterizations with nanoindentation H.X. Liu 1,n, Y. Hu 1, X. Wang, Z.B. Shen, P. Li, C.X. Gu, H. Liu, D.Z. Du, C. Guo School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, People’s Republic of China
a r t i c l e i n f o
abstract
Article history: Received 12 August 2012 Received in revised form 24 November 2012 Accepted 24 November 2012 Available online 2 December 2012
Laser shock forming (LSF) of commercially pure (CP) titanium foil with different levels of laser energies, namely 675 mJ,1200 mJ,1800 mJ, and 2000 mJ has been investigated. Even the microformability of Ti is poor due to its hexagonal close-packed (HCP) structure, Ti exhibits greatly enhanced microformability during LSF, which can be attributed to two mechanisms: (1) changes in the constitution of materials due to high strain rate (up to 106/s), and (2) inertial effects. Failure of workpiece was observed under laser energy of 2000 mJ, and adiabatic shear bands (ASBs), which leads to crack formation, was proposed to account for this failure. Refined microstructure can be produced in the formed sample after LSF. The refinement mechanisms were identified by TEM observations: (1) the onset of twins, (2) development of DTs (dislocation tangles) in original grains and DCs (dislocation cells) formed by DTs, and (3) evolution of DCs into subgrains and high misoriented grains. In addition to this refinement progress, nanograins formed through breakdown and rotation of the elongated subgrains can also be observed. Mechanical properties including surface hardness and elastic modulus were characterized by nanoindentation, and both increased greatly after LSF. Increased surface hardness indicates improved surface properties of formed samples, and enhanced elastic modulus indicates an increased stiffness of the workpiece, providing an evidence for reduced springback of Ti during LSF. The investigation in this paper is believed to lay a solid foundation for the application of LSF. & 2012 Elsevier B.V. All rights reserved.
Keywords: Laser shock forming CP Ti Adiabatic shear bands Grain refinement Nanoindentation
1. Introduction As a newly developed micro fabrication technique, laser shock forming (LSF), which takes the advantages of laser shock peening and high strain rate metal forming [1,2] plays an important role in the electronics productions and micro electro mechanical systems (MEMS). In contrast to the micro metal forming methods existed which have many problems, such as high cost of micro-scale tooling, low speed of processing, and low formability on microscale due to size effect, LSF provides high precision, high repeatability, and dramatically improved formability, and it is also cheap and suitable for forming different materials. In recent years, many exploratory investigations of LSF have been done, these researches are mainly focused on exploring new LSF processes, such as laser shock punching [3,4], investigating the effects of critical parameters including the ratio of the fillet radius to film thickness, the aspect ratio of mold and laser intensities on the deformation behaviors [5], studying the
n Correspondence to: Xuefu Road 301, Jingkou District, Zhenjiang 212013, People’s Republic of China. Tel.: þ86 0511 88797998; fax: þ 86 0511 88780276. E-mail addresses: [email protected], [email protected] (H.X. Liu). 1 Both the authors contributed equally to this work.
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.11.087
forming limits and fracture mode by both numerical and experimental methods [6], as well as measuring the hardness and residual stress on the surface of the workpiece [2,7]. It is well known that the fatigue durability, corrosion, wear resistance, hardness and other mechanical properties and performance of the deformed workpiece are closely related to microstructure in the workpiece after LSF. Furthermore, due to the ultrahigh strain rate (typically 106–107 s 1) in the LSF process, grain refinement is expected to form in the workpiece after LSF and materials with ultrafine grains are found to exhibit many novel properties relative to their coarse-grained counterparts [8,9]. Finally, microformability is grain size dependent. Experimental results showed that amorphous alloys and materials with fine grains have a significant higher microformablity [1], and fine subgrain structure is the major microstructure after LSF, thus the investigation of grain refinement progress in the workpiece can give us an in-depth understanding of the dramatically increased forming limits during LSF process [2,10]so it is of great importance to investigate the microstructure evolution and grain refinement process in the workpiece after LSF. However, with the scope of our knowledge, up to now, the characteristic microstructure and microstructure evolution in the workpiece during LSF process have been rarely reported yet, our aim of this paper is to investigate the microstructure evolution and grain
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refinement process and to study the failure of the workpiece from the standpoint of cracks formation initiated by adiabatic shear band (ASB). Ultrafine grains can be produced in commercially pure titanium by severe plastic deformation methods such as cold rolling (CR) [11], equal channel angular pressing (ECAP) [12], high pressure torsion (HTP) [13] and surface mechanical attrition treatment (SMAT) [14]. Due to its hexagonal close-packed structure, dislocation activities will predominate after the onset of twinning in the grain refinement process of Ti. When slipping or deformed twinning cannot accommodate increasing strain any more, refined grains are formed as a consequence of dynamic recrystallization (DRX), in the same time, adiabatic shear bands (ASBs) accommodating shear instabilities (fully strain-hardened or work hardened regions) characterized by DRX can also be observed. The shear band is one of the most important deformation and failure mechanisms and is considered as a precursor to fracture and fragment [15]. It is well known that titanium displays poor wear resistance and its fatigue performance depends to a large extent on its surface properties. In the same time, due to its hexagonal closepacked (HCP) structure, Ti exhibits limited ductility and has poor formability, the springback of Ti is also large when machining. The LSF process is therefore of considerably technologic importance since it provides greatly enhanced microformability and dramatically improved surface properties of Ti. So in this paper, annealed commercially pure titanium foils of 38 mm in thickness were deformed into mold by LSF. To obtain microstructures at different levels of strain rates and/or different degrees of plastic strains, experiments are conducted at different levels of laser energies. Through the investigation of microstructures on different experimental conditions, the underlying mechanisms of grain refinement process in the workpiece during LSF were revealed by TEM observation and the failure mechanism of workpiece caused by ASBs formation was proposed. Surface hardness and elastic modulus of the workpiece after LSF were characterized by nanoindentation.
2. Experimental 2.1. Forming mechanism Fig. 1(a) shows the schematic of laser shock forming (LSF) process. This progress was realized through laser driven-flyer impacting the workpiece, as demonstrated [4]. When a short and intense laser pulse is irradiated onto the flyer surface, the flyer surface is instantaneously vaporized into a high-temperature and high-pressure plasma. This plasma absorbs the incident laser energy and then acts as working fluid to accelerate the remaining flyer. The remaining thin flyer impacts the workpiece at ultrahigh speed, which induces compressive waves in the workpiece. The flyer also protects the target plate from laser thermal effects.
Finally, the workpiece deform into the mold and LSF process is completed. 2.2. Experimental preparation and procedures In this work, the experiments were done using a short pulse Nd-YAG laser with Gaussian distribution beam (pulse duration:7 ns, wavelength:1064 nm, maximum pulse energy:2 J).The laser pulse is conducted to the interaction area by means of a series of reflecting mirrors and a focusing lens (f¼10 cm). Fig. 1(b) shows the layout of the experiment. Laser shock forming (LSF) experiments were performed on CP Ti foils of 38 mm in thickness. Fig. 2 shows the 2D and 3D shape of the mold which was machined by micro-milling cutter on printed circuit board, and the diameter and the depth of the mold are about 2126 mm and 953 mm, respectively. Before the experiments, the samples were cut into squares and anhydrous alcohol was adopted to clean the surface of the workpiece. Aluminum (99% purity) foil with the thickness of 20 mm was used as the ablative medium and replaced after each experiment, and the height of the flying cavity is 50 mm. The confining medium (k9 glass of 40 mm in diameter and 1.69 mm in thickness) is placed on the ablative medium and firmly clamped against the ablative medium with a holder. Experiments were conducted at various laser energies, namely, 675 mJ, 1200 mJ and 1800 mJ, 2000 mJ. 2.3. 3D shape characterization and micro structural characterization An Axio CSM 700 mot type True color confocal microscope for materials microscopy with scanning stage was used to observe the 3D shape of mold and the workpiece after LSF at various laser energies. The initial structure of the CP Ti foil is observed by optical microscopy (OM), before observation, the samples were etched at room temperature in a solution of 2 ml HF, 3 ml H2O2 and 96 ml distilled water. The microstructures of workpiece after LSF were characterized by using a JEM-2010 transmission electron microscope (TEM) operated at a voltage of 200 kV. Thin foils for TEM observations can be prepared by cutting a piece of the workpiece of 3 mm in diameter with the treated zone in the middle and then dimpling and ion thining the workpiece to perforation at room temperature directly as the workpiece is only 38 mm in thickness. 2.4. Mechanical property characterizations Nanoindentaion has been widely used to characterize the micro-scale mechanical properties including surface hardness and elastic modulus. The mechanical properties are achieved through the history of the variation of the indentation load p as a function of the depth of the penetration provided by the indentation. In this paper, the mechanical properties of 38 mmthick CP Ti foil after LSF with laser energy of 1200 mJ were performed by nano-indentation technique on a Nano Indenter XP,
Fig. 1. (a) Schematic diagram of laser shock forming, and (b) the layout of the experiment.
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Fig. 2. (a) 2D shape of the mold, and (b) 3D shape of the mold.
Hystron Corporation, USA by using a Berkovich diamond indenter. The hardness and the elastic modulus of the specimens are determined by the ‘Oliver-PHARR’ method [16] and calculated automatically in the proprietary software (Triboscan) of the nanoindentation. The total depth of the penetration is controlled to be less than 5% of the film thickness, thus the contribution by the elastic deformation of the substrate should be negligible [17].
3. Results and discussion 3.1. 3D shape characterization To quantitatively characterize the deformation, an Axio CSM 700 mot type True color confocal microscope for materials microscopy with scanning stage was used to profile deformed regions under different laser energy levels. Fig. 3 shows the crosssectional measurement and 3D plot of the formed sample with laser pulse energies of 675 mJ, 1200 mJ, 1800 mJ. As seen, formed samples with good geometry are obtained with laser energy ranging from 625 mJ to 1800 mJ, indicating that an occurrence of strong plastic deformation in the process. Meanwhile, the cross-sectional measurements show a smooth profile, which indicates that materials flow into the die homogeneously during the shock forming process. Note the formed sample is round with a diameter of 2 mm, which is corresponding to the die diameter, as shown in Fig. 2. Fig. 4 shows a failure workpiece with crack formation in the center when laser energy reaches 2000 mJ. 3.2. Enhanced microformability of Ti foil during LSF It can be observed from the measurements in Fig. 3 that the deformation depth increases with the enhancement of laser energy. The deformation depths of the formed samples are 110 mm, 139 mm, 233 mm with laser pulse energies of 625 mJ, 1200 mJ and 1800 mJ, respectively. This is understandable because when the laser energy increases, the ablation of the absorbent coating becomes more efficient, and stronger plasma can be generated. After that, a more powerful shock wave pressure is induced and propagates into the metal target. As a result, the deformation depth accelerates since more forming energy is available [18]. While the CP Ti foil usually exhibits limited ductility and has poor formability at room temperature because of its hexagonal
close-packed (HCP) structure, it is interesting to note that CP Ti foil can obtain so large deformation in the shock forming process. The deformation depth at pulse energy¼ 1800 mJ can reach as deep as 233 mm in the experiments. The generation of large plastic deformation of CP Ti foil may be attributed to two mechanisms. Firstly, in micro-scale laser shock forming, the strain rate can reach up to 107 s 1, which causes changes in the constitution of materials and contributes to the improved micro formability. The effect of strain rate sensitivity on the formability is examined by means of the finite element simulation of deep drawing [19], the numerical simulations show that the strain rate sensitivity has a bearing on the good formability. The flow stress and strain rate sensitivity increase with strain rate, which helps to improve micro formability. Secondly, inertial effects are also responsible for the increased ductility at high speed forming. Inertial forces diffuse deformation throughout a specimen leading to stabilization against neck growth [20].Besides the two mechanisms mentioned above, the as-annealed initial structure of Ti foil can also contribute to the microformability because annealing can eliminate the internal stresses and crystal internal defects. 3.3. Failure mechanism of workpiece during LSF Fig. 4 illustrates the failure of the formed sample and crack formation emerges in the center of the workpiece, which is in contrary to failures of micro deep drawn cups in which ductile failure occurs around the pouch corner [21]. The reasons lie in that the spatial profile of the laser beam is non-uniform over the entire area of the spot and the shock wave pressure obeys Gaussian spatial distribution [22]. As the workpiece flow into the die, high strain localization zones formed in the center where shock wave pressure is higher. At strain localization zones, cracks were formed by the nucleation growth, and coalescence of submicro and microvoids [23], which further leads to the failure of the formed sample. The failure of forming progress is somewhat like a pin penetrates through the workpiece at high speed. It is well known that the failure mechanisms for materials deformed at different strain rate regimes are different, so the fracture in LDF process has its own features. Shear localization, which manifests itself in adiabatic shear bands (ASBs), is an important mode of deformation and occurs quite frequently in a variety of materials during dynamic loading or severe plastic
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Fig. 3. Measurements of the formed sample after LSF under different levels of laser energy: (a) 2D shape with laser energy of 675 mJ, (b) the corresponding 3D shape to (a), (c) 2D shape with laser energy of 1200 mJ, (d) the corresponding 3D shape to (c), (e) 2D shape with laser energy of 1800 mJ, and (f) the corresponding 3D shape to (e).
deformation (SPD) applications. These include penetration [24], impact cratering [25], ballistic plugging [26], metalworking operations such as punching and machining [27], explosive forming and welding and a variety of dynamic processing phenomena. Laser shock forming, which features high strain rate (up to 107 s 1), is a dynamic loading mode and can introduce extreme plastic deformation in the workpiece. In the same time, titanium and its alloys are particularly susceptible to shear localization at high strain and strain rate due to its properties of low heat conductivity and high adiabatic shearing sensitivity [28]. So ASBs are expected to occur in the workpiece during laser shock forming process when laser energy is larger enough. The formation of ASBs requires extremely large shear strain (g), and the mechanism of dynamic recrystallization (DRX) suggested by Meyers et al. [29] is responsible for this large strain.
They considered a rotational mechanism to replace the migrational mechanism to explain the dynamic recrystallization in the short time during impact. The temperature that results in thermal recovery or recrystallization processes in metals is generally T ¼ 0:40:5 T m
ð1Þ
where Tm is the melting point, the temperature rise within the shear band, caused by the plastic work hardening produced during shear band formation, may be calculated using [30] Z e c DT ¼ sde ð2Þ rC p 0 where c is the fraction of plastic deformation work converted to heat, here c ¼ 0.9; r is the density of the material; and C P is specific
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Some deformed twins can also be seen in the initial structure but twin density is relatively low. Fig. 6 shows the TEM observation of the initial structure of the CP Ti foil. Because the CP Ti is as-annealed, few dislocations could be found in the entire grains. Some grains have stacking faults in their interior and the grain size is relatively large, which is corresponding to the optical observation.
Fig. 4. Measurement of a failure workpiece with laser energy of 2000 mJ.
3.4.2. TEM observation of workpiece after LSF with laser energy of 675 mJ. Transmission electron microscope examination of the sample deformed under laser energy of 675 mJ shows that the microstructure differs considerably from the initial structure of the CP Ti foil. As shown in Fig. 7, multiple twins are deformed within a coarse grain, which may initiate from stress concentrations at grain boundaries or junctions. Some deformed twins terminate in the grain, whereas some reach the boundary. The twin boundaries are parallel to each other and the lengths of the twins are a few micrometers and the widths measure tens of the nanometers. No twinning intersections can be observed in our investigation partly
Fig. 5. Optical observation of the initial structure of Ti. Fig. 6. TEM observation of the initial structure of Ti.
heat capacity. When the temperature rise induced by the strain in the workpiece during LSF reaches T, DRX will happen. ASB formation is considered as a precursor to fracture and fragmentation [15] and one of most important failure mechanisms. As illustrated in Fig. 4, when laser energy reached 2 J, shear localization zone may be first formed at the bottom of the workpiece due to high shear strain, the plastic strain then produced huge temperature rise, when the rise reaches 0.4– 0.5 Tm, DRX may happen. It can be argued that shear bands or shear band regimes, characterized by, or composed of very fine DRX grains can account for all forms of extreme deformation, especially large plastic strain (true strains 43)and high strain rate(4103) [31]. As the deformation strain increases, micro-voids nucleate, grow and coarsen in the shear band, eventually link together to form cracks, finally the workpiece fails. Further investigation needs to be done to verify this progress.
3.4. Micro-structural observations 3.4.1. OM and TEM observations of the initial structure of Ti foil Fig. 5 shows the initial structure of the CP Ti foil. It can be seen that the grain size is relatively large, measuring about 50–70 mm.
Fig. 7. TEM observation showing deformation twins.
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due to that the workpiece was deformed under only one laser pulse impact [32]. Different dislocation characteristics can be induced by shock wave which propagates into the workpiece by flyer impact. Fig. 8(a) shows a lot of dislocation pile-ups in or near the grain boundaries generated by plastic deformation. The dislocations arranged in networks and tangles are complex, which is consistent with the observation in a-titanium by SMAT [14]. Fig. 8(b) is the higher magnifications of zone A in Fig. 8(a). Fig. 8(b) shows that dislocation loops were visible inside the grain, which indicates that the dislocations were cross-slip and left behind the dislocation loop in the grain. Fig. 9(a) shows dislocation tangles are tangled with each other in a complex way, and the corresponding selected area electron
diffraction shows obvious misorientations, which indicates that dislocation structures may be arranged in DCs.
3.4.3. TEM observation of workpiece after LSF with laser energy of 1200 mJ. Subgrains (number 1–6 inserted are six subgrains) with width and length of micrometer scale are observed as shown in Fig. 10. Fine lamellae may be divided into finer blocks. It is reasonable to believe that some of these subboundaries may be developed from dislocation tangles (DTs) by accumulation and annihilation of more dislocations. DTs are believed to result from dislocation accumulation and rearrangement for minimizing the total energy state. High-density dislocations arraying in tangles are
Fig. 8. TEM image showing dislocation activities: (a) a lot of dislocation pile-ups near the boundary, and (b) is the higher magnifications of zone A in (a).
Fig. 9. (a) TEM image showing dislocation cells formed by tangles, and (b) the corresponding selected area electron diffraction.
Fig. 10. TEM image showing subgrains: (a) lamellae-like subgrains (number 1–3 inserted), and (b) block-like subgrains (number 4–6 inserted).
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also observed in association with dislocation cells. By absorbing of more and more dislocations, subboundary will transform into boundary with high misorientations with increasing strain. It can be noted that the microstructure characteristics vary a lot from place to place due to the complex stress and strain distribution of the formed sample after laser shock forming. Fig. 11 shows that the microstructure around the bottom of the workpiece is composed of elongated subgrains arraying in the direction of the workpiece flowing into the die. The lengths of the elongated subgrains measure hundreds of nanometers and the widths are about 80–150 nm. These grains are elongated under tensile stress during laser shock forming. Then these elongated subgrains will break down and rotate into nanograins with high angles, as shown in Fig. 12. The grain size measures about 60–100 nm, and a lot of pile-ups can be seen in the grains. Fig. 13 shows the HRTEM image of the nanocrystalline grains. Several nano-sized crystallites (nanocrytallites a, b, c) with different misorientations were clearly identified.
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3.5.1. The onset of twins To undergo homogeneous plastic deformation in polycrystalline materials, five independent slip systems are necessary. However, only four independent slip systems are available in titanium, so twinning is necessary in order to maintain the deformation
3.5. Grain refinement mechanisms in the workpiece during LSF Based on the micro-structural features observed in various regions of the workpiece with different levels of strains during LSF under laser energies of 675 mJ and 1200 mJ, it can be concluded that the following changes occur in the microstructure evolution of titanium during laser shock forming process:(1) the onset of twins; (2) development of DTs in original grains and dislocation cells (DCs) formed by DTs; and (3) evolution of DCs into subgrains and high misoriented grains.
Fig. 13. HRTEM image of the nanograins around the bottom of the workpiece.
Fig. 11. (a) TEM image showing elongated subgrains, and (b) the corresponding selected area electron diffraction.
Fig. 12. (a) TEM image of the nanograins with a lot of dislocations pile-ups in the grain, and (b) the corresponding selected area electron diffraction.
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compatibility [33]. Twinning could not accommodate a huge amount of deformation due to the fact the atomic displacement by twinning is less than one inter-atomic distance [34]. Thus dislocation will occur after the onset of twinning with increasing strain. These are in contrary to TEM observation of FCC metals, such as Cu [35], during dynamic loading on these metals, to reach the local stress for deformation twinning to be activated, sufficient strain hardening (dislocation density) is needed, so slip takes place prior to deformation twinning [36]. In the present study, as shown in Fig. 7, deformation twins occur near the grain boundary and dislocations are also observed near the twin boundary. The twinning can convert a large number of grains into a twin orientation and make dislocation slip again by the activating of other favorably oriented slip systems, thus the deformation of Ti could proceed continuously. 3.5.2. Development of DTs and dislocation cells (DCs) formed by DTs In order to accommodate plastic strain in polycrystalline materials, after the onset of twinning, various dislocation activities are normally motivated, including sliding, accumulation, interaction, tangling, and spatial rearrangement in the regions where the strain is higher, then the dislocation activities will predominate. Fig. 8 shows dislocation lines and dislocations arranged in networks near the grain boundary where the dislocations are hindered. A huge amount of dislocation tangles can be seen everywhere and are tangled with each other in a complex way, as shown in Fig. 9. Development of DTs is for the purpose of reducing the total free energy of a high density of dislocations. The dislocation cell dimensions (L) formed in the coarse grains are basically a function of the acting shear stress (t) by L ¼ 10 Gb=t (G is the shear modulus and b is the burgers vector) [37]. Apparently, with increasing shear stress, the dislocation cell size shrinks. 3.5.3. Evolution of DCs into subgrains and High misoriented grains At a certain strain level, dislocation annihilation and rearrangement occur in DCs will transform DCs into subgrains with small misorientations for minimizing the total system energy, as shown in Fig. 10. When laser induced strain in the workpiece is high enough, subgrains will transform into fine grains with high misorientations as a result of DRX by the rotation of grains. Grain refinement progress will proceed on a finer scale until dislocation multiplication rate is balanced by the annihilation rate, the increase of strains could not reduce the grain size any more. In addition to the grain refinement mechanism mentioned above, nanograins formed by breakdown and then rotation of the elongated subgrains can also be observed close to the center of the workpiece during LSF with laser energy of 1200 mJ, as shown in Figs. 11 and 12. When the strain rate is low, dislocations have enough time to proliferation, migration, climb or cross slip, ultimately, subgrains are formed. Whereas around the center of the workpiece where the laser energy is higher and the strain rate is relatively high, the accumulation and annihilation of dislocations are not enough, in order to retard the plastic strain in nanoseconds, nanograins, as shown in Fig. 13, are formed through the five stages: (a) randomly distributed dislocations; (b) elongated dislocation cell formation (i.e. dynamic recovery); (c) elongated subgrain formation; (d) initial break-up of elongated subgrains; and (e) recrystallized microstructure [38]. 3.6. Mechanical property characterizations with nanoindentation Fig. 14 shows the distributions of the hardness and elastic modulus on the sample surface after LSF. The indentations are made along the center line of the formed sample from the edge to the center. It is obvious that the hardness after LSF increases
Fig. 14. The profiles of the surface nanohardness and elastic modulus of the samples after LSF.
Fig. 15. Comparison of P h curves of nanoindentation in different regions of the samples after LSF.
significantly and the hardness at the center measures 6–7 times higher than that at the edge of the formed sample due to that the spatial profile of the laser beam obeys Gaussian spatial distribution and laser energy at the center is higher. It can also be explained from the point of the microstructural evolution that the microstructures at the edge are merely dislocation activities in large grains, including dislocation lines, dislocation tangles, while in the center, subgrains and nanograins are formed. These are also consistent with a well-known empirical relationship such as the Hall–Petch relation between average grain/subgrain size and the hardness or yielding strength [39,40]: H ¼ Ho þ kH d
1=2
ð3Þ
where H is the hardness, Ho is a constant related to the frictional stress resisting the motion of gliding dislocations or the internal back stress, kH is the Hall–Petch slope, and d is the average grain size. On the other hand, the elastic modulus of the formed sample after LSF is also increased greatly from the edge to the center, as shown in Fig. 14. Fig. 15 shows the load curves around 500 mm from the center and center of the sample after LSF. It can be seen that the elastic recovery of the two load curves is relatively high by comparing with other metals, such as Cu [1], stainless steel [41], which indicates that the elastic modulus of Ti is relatively small.
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The values of surface nanohardness around 500 mm from the center and in the center of the sample after LSF are 4.74 GPa and 8.79 GPa, and the values of elastic modulus are 10.27 GPa and 23 GPa, both two increased greatly. Although the elastic modulus is an intrinsic material property and fundamentally related to atomic bonding, the elastic modulus can also be changed by some treatment technologies, such as laser shock peening (LSP) [41]. It is well known that the increment of elastic modulus is favorable in enhancing the stiffness of the workpiece, and the elastic modulus is relative to the springback of metal when machining. So it can be concluded that the elastic modulus of Ti is greatly enhanced during LSF, which may be an evidence of reduced springback of Ti during LSF.
4. Conclusions 1. LSF of CP Ti foils with different levels of laser energies, namely 675, 1200, 1800, 2000 mJ, has been investigated. Ti has poor microformability due to its hexagonal close-packed (HCP) structure whereas LSF of Ti exhibits greatly enhanced microformability which can be attributed to two mechanisms: (1) changes in the constitution of materials due to high strain rate (up to 107/s), and (2) inertial effects. Failure of workpiece was observed under laser energy of 2000 mJ, shear bands (ASBs) leading to crack formation was proposed to account for this ductile failure. 2. It has been shown in this study that refined microstructures can be produced in the formed sample after LSF. The refinement mechanisms can be identified by TEM observation: (1) the onset of twins; (2) development of DTs (dislocation tangles) in original grains and DCs (dislocation cells) formed by DTs; (3) evolution of DCs into subgrains and high misoriented grains. In addition to this refinement progress, nanograins formed through breakdown and rotation of the elongated subgrains can also be observed. 3. Mechanical properties including surface hardness and elastic modulus were characterized by nanoindentation. (1). The peak of surface hardness at the center of the workpiece of CP Ti after LSF has been increased by 6 times. (2). The elastic modulus was also increased after LSF which provides an evidence for the enhanced microformability and reduced springback of Ti in the LSF.
Acknowledgments The authors acknowledge the National Natural Science Foundation of China (Grant no. 51175235) and the National Natural Science Foundation of Jiangsu province (Grant no. BK2012712), and Supports from Photonic Manufacturing and Science Technology Center of Jiangsu University, Industrial Center of Jiangsu University and the
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Grain refinement progress of pure titanium during laser shock forming (LSF) and mechanical property characterizations with nanoindentation H.X. Liu 1,n, Y. Hu 1, X. Wang, Z.B. Shen, P. Li, C.X. Gu, H. Liu, D.Z. Du, C. Guo School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, People’s Republic of China
a r t i c l e i n f o
abstract
Article history: Received 12 August 2012 Received in revised form 24 November 2012 Accepted 24 November 2012 Available online 2 December 2012
Laser shock forming (LSF) of commercially pure (CP) titanium foil with different levels of laser energies, namely 675 mJ,1200 mJ,1800 mJ, and 2000 mJ has been investigated. Even the microformability of Ti is poor due to its hexagonal close-packed (HCP) structure, Ti exhibits greatly enhanced microformability during LSF, which can be attributed to two mechanisms: (1) changes in the constitution of materials due to high strain rate (up to 106/s), and (2) inertial effects. Failure of workpiece was observed under laser energy of 2000 mJ, and adiabatic shear bands (ASBs), which leads to crack formation, was proposed to account for this failure. Refined microstructure can be produced in the formed sample after LSF. The refinement mechanisms were identified by TEM observations: (1) the onset of twins, (2) development of DTs (dislocation tangles) in original grains and DCs (dislocation cells) formed by DTs, and (3) evolution of DCs into subgrains and high misoriented grains. In addition to this refinement progress, nanograins formed through breakdown and rotation of the elongated subgrains can also be observed. Mechanical properties including surface hardness and elastic modulus were characterized by nanoindentation, and both increased greatly after LSF. Increased surface hardness indicates improved surface properties of formed samples, and enhanced elastic modulus indicates an increased stiffness of the workpiece, providing an evidence for reduced springback of Ti during LSF. The investigation in this paper is believed to lay a solid foundation for the application of LSF. & 2012 Elsevier B.V. All rights reserved.
Keywords: Laser shock forming CP Ti Adiabatic shear bands Grain refinement Nanoindentation
1. Introduction As a newly developed micro fabrication technique, laser shock forming (LSF), which takes the advantages of laser shock peening and high strain rate metal forming [1,2] plays an important role in the electronics productions and micro electro mechanical systems (MEMS). In contrast to the micro metal forming methods existed which have many problems, such as high cost of micro-scale tooling, low speed of processing, and low formability on microscale due to size effect, LSF provides high precision, high repeatability, and dramatically improved formability, and it is also cheap and suitable for forming different materials. In recent years, many exploratory investigations of LSF have been done, these researches are mainly focused on exploring new LSF processes, such as laser shock punching [3,4], investigating the effects of critical parameters including the ratio of the fillet radius to film thickness, the aspect ratio of mold and laser intensities on the deformation behaviors [5], studying the
n Correspondence to: Xuefu Road 301, Jingkou District, Zhenjiang 212013, People’s Republic of China. Tel.: þ86 0511 88797998; fax: þ 86 0511 88780276. E-mail addresses: [email protected], [email protected] (H.X. Liu). 1 Both the authors contributed equally to this work.
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.11.087
forming limits and fracture mode by both numerical and experimental methods [6], as well as measuring the hardness and residual stress on the surface of the workpiece [2,7]. It is well known that the fatigue durability, corrosion, wear resistance, hardness and other mechanical properties and performance of the deformed workpiece are closely related to microstructure in the workpiece after LSF. Furthermore, due to the ultrahigh strain rate (typically 106–107 s 1) in the LSF process, grain refinement is expected to form in the workpiece after LSF and materials with ultrafine grains are found to exhibit many novel properties relative to their coarse-grained counterparts [8,9]. Finally, microformability is grain size dependent. Experimental results showed that amorphous alloys and materials with fine grains have a significant higher microformablity [1], and fine subgrain structure is the major microstructure after LSF, thus the investigation of grain refinement progress in the workpiece can give us an in-depth understanding of the dramatically increased forming limits during LSF process [2,10]so it is of great importance to investigate the microstructure evolution and grain refinement process in the workpiece after LSF. However, with the scope of our knowledge, up to now, the characteristic microstructure and microstructure evolution in the workpiece during LSF process have been rarely reported yet, our aim of this paper is to investigate the microstructure evolution and grain
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refinement process and to study the failure of the workpiece from the standpoint of cracks formation initiated by adiabatic shear band (ASB). Ultrafine grains can be produced in commercially pure titanium by severe plastic deformation methods such as cold rolling (CR) [11], equal channel angular pressing (ECAP) [12], high pressure torsion (HTP) [13] and surface mechanical attrition treatment (SMAT) [14]. Due to its hexagonal close-packed structure, dislocation activities will predominate after the onset of twinning in the grain refinement process of Ti. When slipping or deformed twinning cannot accommodate increasing strain any more, refined grains are formed as a consequence of dynamic recrystallization (DRX), in the same time, adiabatic shear bands (ASBs) accommodating shear instabilities (fully strain-hardened or work hardened regions) characterized by DRX can also be observed. The shear band is one of the most important deformation and failure mechanisms and is considered as a precursor to fracture and fragment [15]. It is well known that titanium displays poor wear resistance and its fatigue performance depends to a large extent on its surface properties. In the same time, due to its hexagonal closepacked (HCP) structure, Ti exhibits limited ductility and has poor formability, the springback of Ti is also large when machining. The LSF process is therefore of considerably technologic importance since it provides greatly enhanced microformability and dramatically improved surface properties of Ti. So in this paper, annealed commercially pure titanium foils of 38 mm in thickness were deformed into mold by LSF. To obtain microstructures at different levels of strain rates and/or different degrees of plastic strains, experiments are conducted at different levels of laser energies. Through the investigation of microstructures on different experimental conditions, the underlying mechanisms of grain refinement process in the workpiece during LSF were revealed by TEM observation and the failure mechanism of workpiece caused by ASBs formation was proposed. Surface hardness and elastic modulus of the workpiece after LSF were characterized by nanoindentation.
2. Experimental 2.1. Forming mechanism Fig. 1(a) shows the schematic of laser shock forming (LSF) process. This progress was realized through laser driven-flyer impacting the workpiece, as demonstrated [4]. When a short and intense laser pulse is irradiated onto the flyer surface, the flyer surface is instantaneously vaporized into a high-temperature and high-pressure plasma. This plasma absorbs the incident laser energy and then acts as working fluid to accelerate the remaining flyer. The remaining thin flyer impacts the workpiece at ultrahigh speed, which induces compressive waves in the workpiece. The flyer also protects the target plate from laser thermal effects.
Finally, the workpiece deform into the mold and LSF process is completed. 2.2. Experimental preparation and procedures In this work, the experiments were done using a short pulse Nd-YAG laser with Gaussian distribution beam (pulse duration:7 ns, wavelength:1064 nm, maximum pulse energy:2 J).The laser pulse is conducted to the interaction area by means of a series of reflecting mirrors and a focusing lens (f¼10 cm). Fig. 1(b) shows the layout of the experiment. Laser shock forming (LSF) experiments were performed on CP Ti foils of 38 mm in thickness. Fig. 2 shows the 2D and 3D shape of the mold which was machined by micro-milling cutter on printed circuit board, and the diameter and the depth of the mold are about 2126 mm and 953 mm, respectively. Before the experiments, the samples were cut into squares and anhydrous alcohol was adopted to clean the surface of the workpiece. Aluminum (99% purity) foil with the thickness of 20 mm was used as the ablative medium and replaced after each experiment, and the height of the flying cavity is 50 mm. The confining medium (k9 glass of 40 mm in diameter and 1.69 mm in thickness) is placed on the ablative medium and firmly clamped against the ablative medium with a holder. Experiments were conducted at various laser energies, namely, 675 mJ, 1200 mJ and 1800 mJ, 2000 mJ. 2.3. 3D shape characterization and micro structural characterization An Axio CSM 700 mot type True color confocal microscope for materials microscopy with scanning stage was used to observe the 3D shape of mold and the workpiece after LSF at various laser energies. The initial structure of the CP Ti foil is observed by optical microscopy (OM), before observation, the samples were etched at room temperature in a solution of 2 ml HF, 3 ml H2O2 and 96 ml distilled water. The microstructures of workpiece after LSF were characterized by using a JEM-2010 transmission electron microscope (TEM) operated at a voltage of 200 kV. Thin foils for TEM observations can be prepared by cutting a piece of the workpiece of 3 mm in diameter with the treated zone in the middle and then dimpling and ion thining the workpiece to perforation at room temperature directly as the workpiece is only 38 mm in thickness. 2.4. Mechanical property characterizations Nanoindentaion has been widely used to characterize the micro-scale mechanical properties including surface hardness and elastic modulus. The mechanical properties are achieved through the history of the variation of the indentation load p as a function of the depth of the penetration provided by the indentation. In this paper, the mechanical properties of 38 mmthick CP Ti foil after LSF with laser energy of 1200 mJ were performed by nano-indentation technique on a Nano Indenter XP,
Fig. 1. (a) Schematic diagram of laser shock forming, and (b) the layout of the experiment.
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Fig. 2. (a) 2D shape of the mold, and (b) 3D shape of the mold.
Hystron Corporation, USA by using a Berkovich diamond indenter. The hardness and the elastic modulus of the specimens are determined by the ‘Oliver-PHARR’ method [16] and calculated automatically in the proprietary software (Triboscan) of the nanoindentation. The total depth of the penetration is controlled to be less than 5% of the film thickness, thus the contribution by the elastic deformation of the substrate should be negligible [17].
3. Results and discussion 3.1. 3D shape characterization To quantitatively characterize the deformation, an Axio CSM 700 mot type True color confocal microscope for materials microscopy with scanning stage was used to profile deformed regions under different laser energy levels. Fig. 3 shows the crosssectional measurement and 3D plot of the formed sample with laser pulse energies of 675 mJ, 1200 mJ, 1800 mJ. As seen, formed samples with good geometry are obtained with laser energy ranging from 625 mJ to 1800 mJ, indicating that an occurrence of strong plastic deformation in the process. Meanwhile, the cross-sectional measurements show a smooth profile, which indicates that materials flow into the die homogeneously during the shock forming process. Note the formed sample is round with a diameter of 2 mm, which is corresponding to the die diameter, as shown in Fig. 2. Fig. 4 shows a failure workpiece with crack formation in the center when laser energy reaches 2000 mJ. 3.2. Enhanced microformability of Ti foil during LSF It can be observed from the measurements in Fig. 3 that the deformation depth increases with the enhancement of laser energy. The deformation depths of the formed samples are 110 mm, 139 mm, 233 mm with laser pulse energies of 625 mJ, 1200 mJ and 1800 mJ, respectively. This is understandable because when the laser energy increases, the ablation of the absorbent coating becomes more efficient, and stronger plasma can be generated. After that, a more powerful shock wave pressure is induced and propagates into the metal target. As a result, the deformation depth accelerates since more forming energy is available [18]. While the CP Ti foil usually exhibits limited ductility and has poor formability at room temperature because of its hexagonal
close-packed (HCP) structure, it is interesting to note that CP Ti foil can obtain so large deformation in the shock forming process. The deformation depth at pulse energy¼ 1800 mJ can reach as deep as 233 mm in the experiments. The generation of large plastic deformation of CP Ti foil may be attributed to two mechanisms. Firstly, in micro-scale laser shock forming, the strain rate can reach up to 107 s 1, which causes changes in the constitution of materials and contributes to the improved micro formability. The effect of strain rate sensitivity on the formability is examined by means of the finite element simulation of deep drawing [19], the numerical simulations show that the strain rate sensitivity has a bearing on the good formability. The flow stress and strain rate sensitivity increase with strain rate, which helps to improve micro formability. Secondly, inertial effects are also responsible for the increased ductility at high speed forming. Inertial forces diffuse deformation throughout a specimen leading to stabilization against neck growth [20].Besides the two mechanisms mentioned above, the as-annealed initial structure of Ti foil can also contribute to the microformability because annealing can eliminate the internal stresses and crystal internal defects. 3.3. Failure mechanism of workpiece during LSF Fig. 4 illustrates the failure of the formed sample and crack formation emerges in the center of the workpiece, which is in contrary to failures of micro deep drawn cups in which ductile failure occurs around the pouch corner [21]. The reasons lie in that the spatial profile of the laser beam is non-uniform over the entire area of the spot and the shock wave pressure obeys Gaussian spatial distribution [22]. As the workpiece flow into the die, high strain localization zones formed in the center where shock wave pressure is higher. At strain localization zones, cracks were formed by the nucleation growth, and coalescence of submicro and microvoids [23], which further leads to the failure of the formed sample. The failure of forming progress is somewhat like a pin penetrates through the workpiece at high speed. It is well known that the failure mechanisms for materials deformed at different strain rate regimes are different, so the fracture in LDF process has its own features. Shear localization, which manifests itself in adiabatic shear bands (ASBs), is an important mode of deformation and occurs quite frequently in a variety of materials during dynamic loading or severe plastic
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Fig. 3. Measurements of the formed sample after LSF under different levels of laser energy: (a) 2D shape with laser energy of 675 mJ, (b) the corresponding 3D shape to (a), (c) 2D shape with laser energy of 1200 mJ, (d) the corresponding 3D shape to (c), (e) 2D shape with laser energy of 1800 mJ, and (f) the corresponding 3D shape to (e).
deformation (SPD) applications. These include penetration [24], impact cratering [25], ballistic plugging [26], metalworking operations such as punching and machining [27], explosive forming and welding and a variety of dynamic processing phenomena. Laser shock forming, which features high strain rate (up to 107 s 1), is a dynamic loading mode and can introduce extreme plastic deformation in the workpiece. In the same time, titanium and its alloys are particularly susceptible to shear localization at high strain and strain rate due to its properties of low heat conductivity and high adiabatic shearing sensitivity [28]. So ASBs are expected to occur in the workpiece during laser shock forming process when laser energy is larger enough. The formation of ASBs requires extremely large shear strain (g), and the mechanism of dynamic recrystallization (DRX) suggested by Meyers et al. [29] is responsible for this large strain.
They considered a rotational mechanism to replace the migrational mechanism to explain the dynamic recrystallization in the short time during impact. The temperature that results in thermal recovery or recrystallization processes in metals is generally T ¼ 0:40:5 T m
ð1Þ
where Tm is the melting point, the temperature rise within the shear band, caused by the plastic work hardening produced during shear band formation, may be calculated using [30] Z e c DT ¼ sde ð2Þ rC p 0 where c is the fraction of plastic deformation work converted to heat, here c ¼ 0.9; r is the density of the material; and C P is specific
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Some deformed twins can also be seen in the initial structure but twin density is relatively low. Fig. 6 shows the TEM observation of the initial structure of the CP Ti foil. Because the CP Ti is as-annealed, few dislocations could be found in the entire grains. Some grains have stacking faults in their interior and the grain size is relatively large, which is corresponding to the optical observation.
Fig. 4. Measurement of a failure workpiece with laser energy of 2000 mJ.
3.4.2. TEM observation of workpiece after LSF with laser energy of 675 mJ. Transmission electron microscope examination of the sample deformed under laser energy of 675 mJ shows that the microstructure differs considerably from the initial structure of the CP Ti foil. As shown in Fig. 7, multiple twins are deformed within a coarse grain, which may initiate from stress concentrations at grain boundaries or junctions. Some deformed twins terminate in the grain, whereas some reach the boundary. The twin boundaries are parallel to each other and the lengths of the twins are a few micrometers and the widths measure tens of the nanometers. No twinning intersections can be observed in our investigation partly
Fig. 5. Optical observation of the initial structure of Ti. Fig. 6. TEM observation of the initial structure of Ti.
heat capacity. When the temperature rise induced by the strain in the workpiece during LSF reaches T, DRX will happen. ASB formation is considered as a precursor to fracture and fragmentation [15] and one of most important failure mechanisms. As illustrated in Fig. 4, when laser energy reached 2 J, shear localization zone may be first formed at the bottom of the workpiece due to high shear strain, the plastic strain then produced huge temperature rise, when the rise reaches 0.4– 0.5 Tm, DRX may happen. It can be argued that shear bands or shear band regimes, characterized by, or composed of very fine DRX grains can account for all forms of extreme deformation, especially large plastic strain (true strains 43)and high strain rate(4103) [31]. As the deformation strain increases, micro-voids nucleate, grow and coarsen in the shear band, eventually link together to form cracks, finally the workpiece fails. Further investigation needs to be done to verify this progress.
3.4. Micro-structural observations 3.4.1. OM and TEM observations of the initial structure of Ti foil Fig. 5 shows the initial structure of the CP Ti foil. It can be seen that the grain size is relatively large, measuring about 50–70 mm.
Fig. 7. TEM observation showing deformation twins.
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due to that the workpiece was deformed under only one laser pulse impact [32]. Different dislocation characteristics can be induced by shock wave which propagates into the workpiece by flyer impact. Fig. 8(a) shows a lot of dislocation pile-ups in or near the grain boundaries generated by plastic deformation. The dislocations arranged in networks and tangles are complex, which is consistent with the observation in a-titanium by SMAT [14]. Fig. 8(b) is the higher magnifications of zone A in Fig. 8(a). Fig. 8(b) shows that dislocation loops were visible inside the grain, which indicates that the dislocations were cross-slip and left behind the dislocation loop in the grain. Fig. 9(a) shows dislocation tangles are tangled with each other in a complex way, and the corresponding selected area electron
diffraction shows obvious misorientations, which indicates that dislocation structures may be arranged in DCs.
3.4.3. TEM observation of workpiece after LSF with laser energy of 1200 mJ. Subgrains (number 1–6 inserted are six subgrains) with width and length of micrometer scale are observed as shown in Fig. 10. Fine lamellae may be divided into finer blocks. It is reasonable to believe that some of these subboundaries may be developed from dislocation tangles (DTs) by accumulation and annihilation of more dislocations. DTs are believed to result from dislocation accumulation and rearrangement for minimizing the total energy state. High-density dislocations arraying in tangles are
Fig. 8. TEM image showing dislocation activities: (a) a lot of dislocation pile-ups near the boundary, and (b) is the higher magnifications of zone A in (a).
Fig. 9. (a) TEM image showing dislocation cells formed by tangles, and (b) the corresponding selected area electron diffraction.
Fig. 10. TEM image showing subgrains: (a) lamellae-like subgrains (number 1–3 inserted), and (b) block-like subgrains (number 4–6 inserted).
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also observed in association with dislocation cells. By absorbing of more and more dislocations, subboundary will transform into boundary with high misorientations with increasing strain. It can be noted that the microstructure characteristics vary a lot from place to place due to the complex stress and strain distribution of the formed sample after laser shock forming. Fig. 11 shows that the microstructure around the bottom of the workpiece is composed of elongated subgrains arraying in the direction of the workpiece flowing into the die. The lengths of the elongated subgrains measure hundreds of nanometers and the widths are about 80–150 nm. These grains are elongated under tensile stress during laser shock forming. Then these elongated subgrains will break down and rotate into nanograins with high angles, as shown in Fig. 12. The grain size measures about 60–100 nm, and a lot of pile-ups can be seen in the grains. Fig. 13 shows the HRTEM image of the nanocrystalline grains. Several nano-sized crystallites (nanocrytallites a, b, c) with different misorientations were clearly identified.
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3.5.1. The onset of twins To undergo homogeneous plastic deformation in polycrystalline materials, five independent slip systems are necessary. However, only four independent slip systems are available in titanium, so twinning is necessary in order to maintain the deformation
3.5. Grain refinement mechanisms in the workpiece during LSF Based on the micro-structural features observed in various regions of the workpiece with different levels of strains during LSF under laser energies of 675 mJ and 1200 mJ, it can be concluded that the following changes occur in the microstructure evolution of titanium during laser shock forming process:(1) the onset of twins; (2) development of DTs in original grains and dislocation cells (DCs) formed by DTs; and (3) evolution of DCs into subgrains and high misoriented grains.
Fig. 13. HRTEM image of the nanograins around the bottom of the workpiece.
Fig. 11. (a) TEM image showing elongated subgrains, and (b) the corresponding selected area electron diffraction.
Fig. 12. (a) TEM image of the nanograins with a lot of dislocations pile-ups in the grain, and (b) the corresponding selected area electron diffraction.
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compatibility [33]. Twinning could not accommodate a huge amount of deformation due to the fact the atomic displacement by twinning is less than one inter-atomic distance [34]. Thus dislocation will occur after the onset of twinning with increasing strain. These are in contrary to TEM observation of FCC metals, such as Cu [35], during dynamic loading on these metals, to reach the local stress for deformation twinning to be activated, sufficient strain hardening (dislocation density) is needed, so slip takes place prior to deformation twinning [36]. In the present study, as shown in Fig. 7, deformation twins occur near the grain boundary and dislocations are also observed near the twin boundary. The twinning can convert a large number of grains into a twin orientation and make dislocation slip again by the activating of other favorably oriented slip systems, thus the deformation of Ti could proceed continuously. 3.5.2. Development of DTs and dislocation cells (DCs) formed by DTs In order to accommodate plastic strain in polycrystalline materials, after the onset of twinning, various dislocation activities are normally motivated, including sliding, accumulation, interaction, tangling, and spatial rearrangement in the regions where the strain is higher, then the dislocation activities will predominate. Fig. 8 shows dislocation lines and dislocations arranged in networks near the grain boundary where the dislocations are hindered. A huge amount of dislocation tangles can be seen everywhere and are tangled with each other in a complex way, as shown in Fig. 9. Development of DTs is for the purpose of reducing the total free energy of a high density of dislocations. The dislocation cell dimensions (L) formed in the coarse grains are basically a function of the acting shear stress (t) by L ¼ 10 Gb=t (G is the shear modulus and b is the burgers vector) [37]. Apparently, with increasing shear stress, the dislocation cell size shrinks. 3.5.3. Evolution of DCs into subgrains and High misoriented grains At a certain strain level, dislocation annihilation and rearrangement occur in DCs will transform DCs into subgrains with small misorientations for minimizing the total system energy, as shown in Fig. 10. When laser induced strain in the workpiece is high enough, subgrains will transform into fine grains with high misorientations as a result of DRX by the rotation of grains. Grain refinement progress will proceed on a finer scale until dislocation multiplication rate is balanced by the annihilation rate, the increase of strains could not reduce the grain size any more. In addition to the grain refinement mechanism mentioned above, nanograins formed by breakdown and then rotation of the elongated subgrains can also be observed close to the center of the workpiece during LSF with laser energy of 1200 mJ, as shown in Figs. 11 and 12. When the strain rate is low, dislocations have enough time to proliferation, migration, climb or cross slip, ultimately, subgrains are formed. Whereas around the center of the workpiece where the laser energy is higher and the strain rate is relatively high, the accumulation and annihilation of dislocations are not enough, in order to retard the plastic strain in nanoseconds, nanograins, as shown in Fig. 13, are formed through the five stages: (a) randomly distributed dislocations; (b) elongated dislocation cell formation (i.e. dynamic recovery); (c) elongated subgrain formation; (d) initial break-up of elongated subgrains; and (e) recrystallized microstructure [38]. 3.6. Mechanical property characterizations with nanoindentation Fig. 14 shows the distributions of the hardness and elastic modulus on the sample surface after LSF. The indentations are made along the center line of the formed sample from the edge to the center. It is obvious that the hardness after LSF increases
Fig. 14. The profiles of the surface nanohardness and elastic modulus of the samples after LSF.
Fig. 15. Comparison of P h curves of nanoindentation in different regions of the samples after LSF.
significantly and the hardness at the center measures 6–7 times higher than that at the edge of the formed sample due to that the spatial profile of the laser beam obeys Gaussian spatial distribution and laser energy at the center is higher. It can also be explained from the point of the microstructural evolution that the microstructures at the edge are merely dislocation activities in large grains, including dislocation lines, dislocation tangles, while in the center, subgrains and nanograins are formed. These are also consistent with a well-known empirical relationship such as the Hall–Petch relation between average grain/subgrain size and the hardness or yielding strength [39,40]: H ¼ Ho þ kH d
1=2
ð3Þ
where H is the hardness, Ho is a constant related to the frictional stress resisting the motion of gliding dislocations or the internal back stress, kH is the Hall–Petch slope, and d is the average grain size. On the other hand, the elastic modulus of the formed sample after LSF is also increased greatly from the edge to the center, as shown in Fig. 14. Fig. 15 shows the load curves around 500 mm from the center and center of the sample after LSF. It can be seen that the elastic recovery of the two load curves is relatively high by comparing with other metals, such as Cu [1], stainless steel [41], which indicates that the elastic modulus of Ti is relatively small.
H.X. Liu et al. / Materials Science & Engineering A 564 (2013) 13–21
The values of surface nanohardness around 500 mm from the center and in the center of the sample after LSF are 4.74 GPa and 8.79 GPa, and the values of elastic modulus are 10.27 GPa and 23 GPa, both two increased greatly. Although the elastic modulus is an intrinsic material property and fundamentally related to atomic bonding, the elastic modulus can also be changed by some treatment technologies, such as laser shock peening (LSP) [41]. It is well known that the increment of elastic modulus is favorable in enhancing the stiffness of the workpiece, and the elastic modulus is relative to the springback of metal when machining. So it can be concluded that the elastic modulus of Ti is greatly enhanced during LSF, which may be an evidence of reduced springback of Ti during LSF.
4. Conclusions 1. LSF of CP Ti foils with different levels of laser energies, namely 675, 1200, 1800, 2000 mJ, has been investigated. Ti has poor microformability due to its hexagonal close-packed (HCP) structure whereas LSF of Ti exhibits greatly enhanced microformability which can be attributed to two mechanisms: (1) changes in the constitution of materials due to high strain rate (up to 107/s), and (2) inertial effects. Failure of workpiece was observed under laser energy of 2000 mJ, shear bands (ASBs) leading to crack formation was proposed to account for this ductile failure. 2. It has been shown in this study that refined microstructures can be produced in the formed sample after LSF. The refinement mechanisms can be identified by TEM observation: (1) the onset of twins; (2) development of DTs (dislocation tangles) in original grains and DCs (dislocation cells) formed by DTs; (3) evolution of DCs into subgrains and high misoriented grains. In addition to this refinement progress, nanograins formed through breakdown and rotation of the elongated subgrains can also be observed. 3. Mechanical properties including surface hardness and elastic modulus were characterized by nanoindentation. (1). The peak of surface hardness at the center of the workpiece of CP Ti after LSF has been increased by 6 times. (2). The elastic modulus was also increased after LSF which provides an evidence for the enhanced microformability and reduced springback of Ti in the LSF.
Acknowledgments The authors acknowledge the National Natural Science Foundation of China (Grant no. 51175235) and the National Natural Science Foundation of Jiangsu province (Grant no. BK2012712), and Supports from Photonic Manufacturing and Science Technology Center of Jiangsu University, Industrial Center of Jiangsu University and the
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