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Deformation of single and multiple laser peened TC6 titanium alloy
Optics and Laser Technology 100 (2018) 309–316
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Deformation of single and multiple laser peened TC6 titanium alloy A. Umapathi, S. Swaroop ⇑ Surface Modification Laboratory, School of Advanced Sciences, VIT University, Vellore 632014, India
a r t i c l e
i n f o
Article history: Received 3 July 2017 Received in revised form 13 October 2017 Accepted 23 October 2017
Keywords: Laser peening without coating TC6 alloy Residual stress Microhardness Synchrotron radiation Microstructure
a b s t r a c t Laser peening without coating (LPwC) was done on the titanium TC6 alloy at a wavelength of 532 nm using an Nd:YAG laser. The laser power densities of 3, 6 and 9 GW cm2 were used to peen the samples. Samples were also peened multiple times (1, 3 and 5 passes) at 6 GW cm2. Microhardness showed an overall 23% increase from the baseline value. Further, softening of a phase in the bulk was observed above 6 GW cm2 in the samples peened once and above 1 pass in multiply peened samples. A similar trend was observed from the residual stress analysis of the samples. The maximum compressive residual stress was 1780 MPa at a depth of 50 lm at 9 GW cm2. The observed softening of a phase was proposed due to adiabatic heating. Microstructural changes due to adiabatic heating resulting in increased b volume fractions were observed and confirmed by synchrotron radiation measurements. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Titanium based alloys have been extensively used in aerospace applications due to their excellent properties such as high strength, low weight, weldability, toughness and corrosion resistance. These alloys are mainly used to fabricate gas turbines, compressor blades and disks of advanced aero engine components. However, these components continuously deteriorate due to fatigue loading all through their service conditions. The conditions worsen when tensile stresses develop on the surface of the components, which leads to early crack formation and eventual failure. Therefore it is necessary to improve the fatigue life of these components. The fatigue life of the material can be increased by inducing compressive residual stresses (CRS) on the surface and subsurface where cracks initiate, thereby delaying or inhibiting crack propagation. CRS are introduced on the surface and subsurface of the material by surface modification techniques such as shot peening, ultra-shot peening, ball-burnishing, surface mechanical attrition treatment, water jet peening, deep rolling and laser peening (LP) or laser peening without coating (LPwC). As far the peening methods, for the past six decades conventional shot peening has been used to improve the fatigue life of the metallic materials by introducing CRS. Shot peening induces limited CRS in shallow regions. To overcome this problem LP or laser shock peening (LSP) which induces large and deep CRS with very less roughened ⇑ Corresponding author. E-mail addresses: [email protected] (A. Umapathi), [email protected]. in (S. Swaroop). https://doi.org/10.1016/j.optlastec.2017.10.022 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.
surface is used in recent times. The development, background processes, benefits and importance of LP on surface modification was reviewed by Montross et al. [1]. The LP technique has been established as more effective than shot peening [2]. There are two variants of the LP process. The normal process of LP uses a coating (sacrificial coating like black paint, Al foil or vinyl tape [1]) and the other variation that does not use coating is called LP without coating (LPwC) [3–5]. LPwC technique was introduced to the peening community in the reports of Mukai et al. in 1995 [3] and Sano et al. in 1997 and 2006 [4,5]. LPwC takes advantage of high strain rate plastic deformation process induced by high pressure laser shock wave to enhance the mechanical properties of metallic materials. The benefits of LPwC over LP were recently reviewed by us [6]. Additionally, in our laboratory, LPwC was applied to titanium and other alloys resulting in the increase of CRS, fatigue life and corrosion resistance [7–16]. Further, LPwC is reported to have decreased fatigue crack growth rate (FCGR) [17], improvements in cycles in high cycle fatigue (HCF) regime [18–21], stress corrosion cracking (SCC) [22], corrosion resistance [23] and wear resistance [24] in many metallic alloys. All the mentioned properties were mainly influenced by LPwC parameters, such as laser energy, laser power density and pulse density. One additional variant within LP is called multiple peening wherein the same area is peened with different passes or scans [21,25–31]. Multiple peening appears promising because of the propensity to obtain nano grained surface and subsurface of the peened alloys [21,25–31]. In this work near alpha titanium alloy TC6 (widely used in the Chinese aviation industry) is used. There is some limited data on the LP of TC6 [21,25,32]. Further, grain refinement is reported in
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TC6 as a consequence of 5 times multiple LSP impacts with absorbent coating, with formation of nanocrystallites distributed uniformly on the surface with grain size in the 30–60 nm range [25]. Additionally, work hardening and enhanced fatigue life were observed. To date there has been no information available on LPwC of TC6 alloy (the focus of the present paper). In this paper we focus on the surface and bulk mechanical properties of the TC6 alloy influenced by different LPwC parameters like power density and multiple impacts. In order to achieve this, a laser wavelength of 532 nm was used at three different power densities (3, 6 and 9 GW cm2) and multiple impacts (1, 3 and 5 passes) were performed at 6 GW cm2. The resulting microhardness, residual stress, microstructural changes and surface roughness were analyzed. Further, the phase analysis of the sample was carried out with synchrotron radiation. 2. Experimental 2.1. Material preparation The titanium TC6 alloy in the form of 5 mm thick cold rolled plates was purchased from Tianjin Haixing Steel Import and Export Co. Ltd., China. Its nominal chemical composition (in wt.%) is 5.5–6 Al, 2–3 Mo, 0.8–2.3 Cr, 0.2–0.7 Fe, 0.15–0.4 Si and Ti (balance). Small samples (15 15 5 mm3) were obtained from as received plate with the help of electric discharge mechanism (EDM) wire cutting. The samples were first solution heat treated at 870 °C for 2 h in air and cooled to room temperature in air followed by ageing at 550 °C for 2 h in air and cooling in air. The samples were further annealed for relieving stress in the inert gas atmosphere at 400 °C for 2 h. Further, samples were polished with silicon carbide sheets ranging from 220 to 3000 grit size and further polished with colloidal silica of particle size 0.04 mm. The samples were chemically etched for 10 s with Keller’s reagent (85 ml of H2O, 5 ml of HNO3 and 10 ml of HF). The resulting microstructure of the sample is shown in Fig. 1, depicting equiaxed grains with predominantly a phase. Tensile tests performed on the samples indicated ultimate tensile strength and percentage elongation at break as 950 MPa and 13.5% respectively. 2.2. Laser peening without coating (LPwC) The LPwC experimental setup is schematically shown in Fig. 2. The sample was fixed on a steel tray (200 200 100 mm3) and
the tray movements in x and y-directions were controlled by two servo motors, operated through a software. The sample was submerged in 10 mm thick water inside the tray. This water worked as a transparent overlay to create the plasma on the sample surface. Experiments were performed with a pulsed Nd:YAG laser system with a pulse duration of 10 ns and at a repetition rate of 10 Hz. Water penetrable frequency doubled 532 nm wavelength (second harmonics) radiation was used. The laser pulse was first reflected at an angle of 45° by a dichroic mirror and the beam was focused using a bi-convex lens of focal length 758 mm. Finally, a 0.8 mm diameter beam was focused on the sample surface. The overlapping rate was constant at 70% (equivalent to a pulse density of 17 pulses mm2). The samples were peened at three different power densities of 3, 6 and 9 GW cm2. Further, multiple peening with 1, 3, and 5 impacts at power density of 6 GW cm2 was also done. 2.3. Microhardness The depth-wise microhardness of the unpeened and peened samples was measured using a MATUZAWA-MMT-X Vickers hardness tester. The polished cross section of samples was used to measure the microhardness with a constant load of 500 g and holding time of 10 s. An average of three measurements was used for each depth. 2.4. Residual stress The residual stress measurement was carried out using an X’pert Pro system (PANalytical, Netherlands). The analysis of the data was performed using the standard X-ray diffraction sin2 w method. A 2 mm diameter beam and X-ray source of Cu-Ka radiation were used. The tilt angle w was varied from 40 to 40° and the lattice plane of (2 0 1) belonging to a phase was used to calculate the stress. The diffraction elastic constants were obtained using the elastic modulus (115 GPa) and Poisson ratio (0.34). The depth wise residual stress was measured by progressive electro polishing to remove layers up to a depth of 500 lm. The resulting stress relaxation was corrected as per the methods of Moore et al. [33] and Fitzpatrick et al. [34]. 2.5. Surface roughness The surface roughness of the unpeened and peened samples was measured using MAHR-GD120 automatic digital surface roughness measurement instrument. A 0.8 mm high pass cut off filter and a scanning length of 5.6 mm along the peened direction was used. For each sample, surface roughness was analyzed in three different places to obtain an average of mean arithmetic roughness (Ra). 2.6. Synchrotron radiation The phase content was analyzed with high energy spatial resolution synchrotron based X-ray diffraction (SR-XRD) measurement, using beamline line BL-11, located at INDUS-2, RRCAT, Indore, India. The diffraction patterns were obtained in an angledispersive X-ray diffraction (AD-XRD) mode with the Bragg angle ranging from 10° to 40°. The beam energy of 20 keV (corresponding to a wavelength of 0.600307 Å) was used. 2.7. Microstructure
Fig. 1. Optical microstructure of TC6 alloy solution heat treated (870 °C, 2 h), air cooled, aged (550 °C, 2 h), air cooled and finally annealed at 400 °C, for 2 h for stress relieving (inert gas atmosphere).
In order to investigate the microstructures before and after LPwC, the cross-sectioned samples were mounted with phenolic powder and prepared for metallographic investigation using
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Fig. 2. Schematic representation of the LPwC processes.
standard polishing and etching processes as detailed in Section 2.1. The cross sectional microstructures were analyzed using a Carl Zeiss optical microscope.
3. Results and discussion 3.1. Microhardness Fig. 3 shows the depth distribution of microhardness of samples after LPwC with three different laser power densities (3, 6 and 9 GW cm2) compared with the unpeened sample. The unpeened sample’s microhardness was around 303 HV, with an error of <1 HV. After LPwC, the maximum microhardness of all treated samples appeared in the near surface region. The microhardness of all treated samples decreased along the depth and finally reached the baseline microhardness of the unpeened sample. The microhardness increase depends upon laser power density, ranging from 7 to 23% with reference to the unpeened sample. Lowest
Fig. 3. Variation of microhardness along the cross section of the unpeened sample and samples peened at 3, 6 and 9 GW cm2.
microhardness (324 ± 2 HV) and a shallow depth of work hardened region (1000 lm) were observed in sample peened at 3 GW cm2 compared with samples peened at 6 and 9 GW cm2. With the increase in power density (3–6 GW cm2), microhardness increased significantly to 365 ± 6 HV, along with the increase in work hardened depth to 1500 lm. Increase in hardness and work hardened depth are attributed to increased shock wave pressures corresponding to increased power densities during LPwC. However, increasing power density does not always improve microhardness and work hardened depth as the hardness profiles saturate after 6 GW cm2. A similar effect on TiAl titanium alloy was also observed by Qiao et al. [35]. They reported that microhardness significantly increased up to 3.4 GW cm2 laser power density and with further increase in laser power density, microhardness saturated at 5.5 GW cm2. We also found such effects in our earlier studies with the titanium alloy Ti-2.5Cu [7] wherein it was observed that increasing the overlap percentage rate does not necessarily increase the work hardening behaviour in a linear fashion due to dominating softening mechanisms. This phenomenon was also determined to be dependent on the wavelength of the laser used [8], with the softening mechanisms setting in at lower wavelengths (532 nm compared to 1064 nm, for example). The same explanations can be extended to the present study with the hindsight that the effect of increasing overlap rate and power density are the same, that is, it increases the laser beam-material interaction. This trend can then be expected to be extrapolated to the case of multiple peening, as that also would increase the laser beam-material interaction, as in the following discussion. The micro hardness measurements are average of the microhardness of both phases a and b. Later in the discussion (Section 3.4) we also report presence of b phase after peening. Therefore the question of phases involved in softening will be taken up in the next section and Section 3.4. Fig. 4 depicts the microhardness variation along the depth of an unpeened sample and samples treated by LPwC with three different multiple LPwC impacts (1, 3 and 5) at 6 GW cm2. Single impact LPwC sample shows maximum microhardness and larger work hardened depth. However, beyond single impact the microhardness and work hardened depth decreased at surface and in the bulk of the sample, as expected and discussed above. In
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Fig. 4. Variation of microhardness along the cross section of the unpeened sample and samples peened multiple times at 6 GW cm2.
contrast to the present study no softening effect was observed by Nie et al. [21] in TC6, with multiple peening up to 10 impacts at 4.24 GW cm2, with absorbent coating. They observed microhardness increase up to 5 impacts and for 10 impacts microhardness reached maximum with no further significant increase. The apparent contrast in results can be explained through the difference in laser power densities and wavelength of the source, as they used lower power density of 5 GW cm2 at 1064 nm compared with present study (6 GW cm2 at 532 nm). 3.2. Residual stress The variation in residual stress along depth is shown in Fig. 5 for the laser power densities of 3, 6 and 9 GW cm2 along with the unpeened material. The residual stress of unpeened sample shows relatively small CRS (about 120 MPa on the average) throughout the depth. After LPwC, in all cases, the magnitude of residual stresses increases with increasing laser power density, as the laser shock wave pressure increases. The maximum residual stresses of 1110, 1696 and 1780 MPa are about 50 lm below the surface for the laser power densities of 3, 6, and 9 GW cm2 respectively. Decrease in CRS at the surface with increasing laser power density is evidently observed. The surface residual stresses of 721, 635, and 509 MPa were obtained for laser power densi-
Fig. 5. Variation of residual stress along the cross section of the unpeened sample and samples peened at 3, 6 and 9 GW cm2.
ties of 3, 6, and 9 GW cm2 respectively. At the surface, thermal effects (thermal softening) and stress relaxation were observed because of the direct interaction of laser beam and material surface. Similar kinds of results at the surface were seen in our previous studies [7,8] and also from the other reports in the literature [20,35–38]. Such results are usually attributed to local melting and re-solidification. In contrast to this, no melting or resolidification was noticed in this study, as evident from the microstructures presented in Section 3.5. On the other hand, this effect was not observed in the same material investigated by Nie et al. [21] perhaps as they used aluminium foil coating on the surface of the sample and it was protecting the surface from thermal effects, resulting in maximum CRS at the surface. Nevertheless, in the present study, deeper inside the material, residual stress increases with laser power density. Beyond 50 mm beneath the surface, the CRS drop drastically, finally reaching closer to the tensile residual stress region at 500 lm depth from the surface. Similar to present trend results were observed in previous report on TiAl alloy [35] where also beyond the laser power density of 6 GW cm2, (that is, at 5.5 GW cm2) there is no increase in CRS (in whole of the profile). This is similar to the results of hardness and hence an indication of softening. Similar to the hardness case, it is also expected here that softening would set in if multiple peening is employed. It is important to note here that the softening is only of the a phase, as the (2 0 1) peak of the a phase was used in the analysis. In the XRD technique used for residual stress measurement, sufficiently intense b peak at higher Bragg angles could not be clearly discerned. Further, in hindsight we can infer that as the softening trend depicted by both the hardness and residual stress data are similar, the contribution from a phase to overall micro hardness should be predominant compared to the b phase. The residual stress along the depth before and after LPwC with multiple impacts (1, 3 and 5 impacts) done at 6 GW cm2 as shown in Fig. 6. It is evident that the residual stress decreased in the bulk of the material. However, in contrast, softening effect was not observed in the same material (TC6 alloy) earlier reported by Nie et al. [21]. The resolution of this apparent contrast and the softening behaviour observed in the present study is exactly same as in the hardness case (Section 3.1). 3.3. Surface roughness The surface roughness of TC6 alloy samples before and after LPwC with three different laser power densities (3, 6 and 9 GW
Fig. 6. Variation of residual stress along the cross section of the unpeened sample and samples peened multiple times at 6 GW cm2.
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cm2) and after multiple peening are presented in Table 1. It can be seen that the surface roughness increases with an increase in laser power density. However, the surface roughness increase is significant from unpeened to the 3 GW cm2 case (0.013 to 0.168 mm) while it is only marginal among the peened cases (with a maximum of 0.1952 mm at 5 impacts of multiple peening). Also presented in Table 1 is the percentage increase in roughness, which falls within the range of 12–14%, when compared with the unpeened sample. From these results it is clear that although there is an increase in roughness after peening, the final roughness is not dependent on the power densities or the number of passes (in multiple peening). Many other reports confirmed the surface roughness increase in different materials after LPwC, increasing with multiple impacts, laser pulse energy or higher power density and higher overlapping percentage or higher pulse density [5,9,39,40]. Apart from this usual trend, in our earlier reports on Ti-2.5 Cu alloy [7,8], we observed that surface roughness depends not only overlapping rate (equivalent to power density in terms of mechanism) but also on the laser wavelength. Consequently we observed [7,8] smaller surface roughness in samples peened with 532 nm laser wavelength compared with 1064 nm. This is primarily due to less absorption of the 532 nm radiation compared to 1064 nm radiation in water, corresponding to a larger dissipation of energy in the latter case leading to increased damage at the surface, which is observed as increased roughness. As 532 nm radiation is used in the present study, the roughness among peened samples is not significant.
3.4. Synchrotron radiation Phase analysis was carried out with diffraction patterns from synchrotron radiation, as this offers three major advantages compared to conventional X-rays: high-density flux, high energy tunability of wavelength and high spatial resolution. The diffraction pattern for the unpeened samples and samples peened with laser power density of 3, 6 and 9 GW cm2 are depicted in Fig. 7. It is clear that the unpeened and peened samples predominantly consist a phase and b phase (at relatively lesser volume fraction). In the case of LPwC at 3 GW cm2, the sample showed increase in b (1 1 0) volume fraction and decreased in a (1 0 1) volume fraction. Further increasing of laser power density from 3 to 9 GW cm2, with this increase of the b (1 1 0) phase volume fraction in the material, an additional new crystalline b (2 0 0) phase peak appeared and increased proportionally with laser power density along with significant decrease in peak intensity of a (1 0 1). This indicates that a to b phase transformation occurred in the material. Similar trend can be observed in the case of multiple peened samples, as depicted in Fig. 8. Generally, phase transformation (a to b) occurs in titanium alloys due to increase in temperature above b transus temperature. In our earlier study [8] we had proposed that the increase in temperature in LPwC occurs due to adiabatic heating, resulting from the frictional dissipation of dislocations in the
Fig. 7. Synchrotron diffraction pattern for the unpeened and samples peened with different power densities (synchrotron energy: 20 keV).
Table 1 Surface roughness before and after peening with different power densities (single peening) and multiple peening at 6 GW cm2. Peening condition (power density)
Average surface roughness (Ra) (lm)
Unpeened 3 GW cm2 6 GW cm2 9 GW cm2 6 GW cm2/3 Times 6 GW cm2/5 Times
0.0133 0.1688 0.1746 0.1882 0.1751 0.1952
Ra RaðUnpeenedÞ
(%) (rounded off
to nearest integer) – 12 13 14 13 14
Fig. 8. Synchrotron diffraction pattern for the unpeened and samples peened multiple times at 6 GW cm2 (synchrotron energy: 20 keV).
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Fig. 9. Cross-section optical microstructures of (a) unpeened sample and samples peened at power densities of (b) 3 GW cm2, (c) 6 GW cm2 and (d) 9 GW cm2.
Fig. 10. Cross-section optical microstructures of (a) unpeened sample and samples peened multiple times with (b) 1 pass, (c) 3 passes and (d) 5 passes at 6 GW cm2.
A. Umapathi, S. Swaroop / Optics and Laser Technology 100 (2018) 309–316 Table 2 Grain size measurements before and after peening with different power densities (single peening) and multiple peening at 6 GW cm2. Peening condition (power density)
Grain size (mm)
Error (mm)
Unpeened 3 GW cm2 6 GW cm2 9 GW cm2 6 GW cm2/3 Times 6 GW cm2/5 Times
6.9 7.5 6.5 7.1 6.6 6.8
0.1 0.3 0.1 0.3 0.2 0.2
bulk of the material. Further, with 532 nm radiation that is not absorbed in water, more shock wave energy transfer to the bulk of the material can be expected to occur, leading to increased dislocation density and hence increasing the chances of adiabatic heating. Moreover, severe plastic deformation increases with increase in laser power density, causing increase in dislocation density, as reflected in the peak shifts of deformed material compared unpeened material. These higher angle peak shifts are well correlated with residual stresses as discussed in Section 3.2. This is an indication of increasing CRS in the material [41]. In contrast to the present study, in the same material phase transformation was not observed by Nie et al. [21], because of three reasons. First is the variation in laser power density, as they used 5 GW cm2 compared present study very less (6–9 GW cm2). Second, they used sacrificial layer coating as aluminium foil, unlike the present study as this would decrease the shock wave pressure. Third, they peened the samples with a laser wavelength of 1064 nm, while in the present study water penetrable 532 nm radiation was used, as laser wavelength influences the deformation in titanium alloy and significant variation in results were noticed as a function of wavelength [8]. It should be noted that all the three enumerated reasons (high power density, no coating on the surface and lesser wavelength of radiation) increase the chances of adiabatic heating leading to harder b phase formation. Further, increasing the number of passes (in multiple peening) also increases the chance of adiabatic heating, due to increased dislocation activity. Interestingly, LPwC induced transformation of softer to harder phase is not uncommon even in athermal cases. For example, LPwC induced harder martensite phase transformation (athermal) in an originally austenitic AISI321 steel was also observed by Karthik et al. [12]. Further, phase transformation was also supported by microstructural observations as detailed in next section. The detection of harder b phase may appear contradictory to the claim of softening in the bulk discussed in Sections 3.1 and 3.2 on hardness and residual stress. However, it is important to note the softening is with respect to a phase alone and the contribution to residual stress from b phase is difficult to evaluate due to insufficient intensity of b peaks at higher angles.
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presented in the previous section about the formation of b phase resulting from adiabatic heating. The microstructure of multiple peened samples at 6 GW cm2 are presented in Fig. 10. Similar trends in increase of b phase in the bulk can be observed from the figure, indicating the increased chances of adiabatic heating with increase in number of passes. The grain sizes were measured from the micrographs and are presented in Table 2. From the table it can be observed that there is no any significant variation in the grain sizes. 4. Conclusions Laser peening without coating was performed on a TC6 alloy with laser power density of 3, 6 and 9 GW cm2 and multiple times at 6 GW cm2 with laser pulses of wavelength of 532 nm, pulse duration of 10 ns and overlapping percentage of 70%. Following conclusions could be drawn out of the study. 1. Microhardness data showed that the hardness variations were within 23% of the baseline hardness (for unpeened sample). Saturation in hardness and softening (in the bulk) with single peening (beyond 6 GW cm2) and in multiple peening (from 3 times onwards at 6 GW cm2) was observed. 2. Residual stress analysis showed largest CRS around 1780 MPa at a depth of 50 lm at 9 GW cm2. Softening of a phase in the bulk of the samples was confirmed in residual stress data also and it was proposed due to adiabatic heating. 3. Surface roughness in all peened samples was found to have increase within 12–14% compared to the peened samples. The increase in surface roughness was found to be insignificantly affected by power density and/or multiple peening. 4. Synchrotron radiation diffraction studies revealed that a to b phase transformation was noticed on peened specimens (due to adiabatic heating) and it increased with increase in power density. This was confirmed with microstructures after LPwC.
Acknowledgment We thank DST-SERB, India (Grant No. SB/S3/ME/36/2013) for the financial support, VIT University for the infrastructure and constant support throughout the project and National Facility of OIM and Texture at IIT-Bombay for the residual stress measurements. The Centre for Advanced Material Processing and Testing at VIT University (funded by DST-FIST) is acknowledged for assistance in mechanical testing. RRCAT, Indore, India is acknowledged for the Synchrotron facility (AD-XRD, BL-11). References
3.5. Microstructure Fig. 9(a) represents unpeened specimen and (b), (c), and (d) represents the samples peened with laser power density of 3, 6, and 9 GW cm2 respectively. All the micrographs are taken in the cross section. The unpeened microstructure shows predominantly aphase (grey area) and very less b phase (dark area). After LPwC, the b phase volume fraction increases with increasing laser power density. The sample peened with a power density of 3 GW cm2 indicates small increase in dark region (b phase) and corresponding b phase volume fraction. In the same way, 6 and 9 GW cm2 peened samples exhibit substantial increase of the dark area, as confirmed by SR-XRD patterns shown in Figs. 7 and 8. There is a good correlation of phase information between microstructure and SR-XRD patterns. This is also consistent with the arguments
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Deformation of single and multiple laser peened TC6 titanium alloy A. Umapathi, S. Swaroop ⇑ Surface Modification Laboratory, School of Advanced Sciences, VIT University, Vellore 632014, India
a r t i c l e
i n f o
Article history: Received 3 July 2017 Received in revised form 13 October 2017 Accepted 23 October 2017
Keywords: Laser peening without coating TC6 alloy Residual stress Microhardness Synchrotron radiation Microstructure
a b s t r a c t Laser peening without coating (LPwC) was done on the titanium TC6 alloy at a wavelength of 532 nm using an Nd:YAG laser. The laser power densities of 3, 6 and 9 GW cm2 were used to peen the samples. Samples were also peened multiple times (1, 3 and 5 passes) at 6 GW cm2. Microhardness showed an overall 23% increase from the baseline value. Further, softening of a phase in the bulk was observed above 6 GW cm2 in the samples peened once and above 1 pass in multiply peened samples. A similar trend was observed from the residual stress analysis of the samples. The maximum compressive residual stress was 1780 MPa at a depth of 50 lm at 9 GW cm2. The observed softening of a phase was proposed due to adiabatic heating. Microstructural changes due to adiabatic heating resulting in increased b volume fractions were observed and confirmed by synchrotron radiation measurements. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Titanium based alloys have been extensively used in aerospace applications due to their excellent properties such as high strength, low weight, weldability, toughness and corrosion resistance. These alloys are mainly used to fabricate gas turbines, compressor blades and disks of advanced aero engine components. However, these components continuously deteriorate due to fatigue loading all through their service conditions. The conditions worsen when tensile stresses develop on the surface of the components, which leads to early crack formation and eventual failure. Therefore it is necessary to improve the fatigue life of these components. The fatigue life of the material can be increased by inducing compressive residual stresses (CRS) on the surface and subsurface where cracks initiate, thereby delaying or inhibiting crack propagation. CRS are introduced on the surface and subsurface of the material by surface modification techniques such as shot peening, ultra-shot peening, ball-burnishing, surface mechanical attrition treatment, water jet peening, deep rolling and laser peening (LP) or laser peening without coating (LPwC). As far the peening methods, for the past six decades conventional shot peening has been used to improve the fatigue life of the metallic materials by introducing CRS. Shot peening induces limited CRS in shallow regions. To overcome this problem LP or laser shock peening (LSP) which induces large and deep CRS with very less roughened ⇑ Corresponding author. E-mail addresses: [email protected] (A. Umapathi), [email protected]. in (S. Swaroop). https://doi.org/10.1016/j.optlastec.2017.10.022 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.
surface is used in recent times. The development, background processes, benefits and importance of LP on surface modification was reviewed by Montross et al. [1]. The LP technique has been established as more effective than shot peening [2]. There are two variants of the LP process. The normal process of LP uses a coating (sacrificial coating like black paint, Al foil or vinyl tape [1]) and the other variation that does not use coating is called LP without coating (LPwC) [3–5]. LPwC technique was introduced to the peening community in the reports of Mukai et al. in 1995 [3] and Sano et al. in 1997 and 2006 [4,5]. LPwC takes advantage of high strain rate plastic deformation process induced by high pressure laser shock wave to enhance the mechanical properties of metallic materials. The benefits of LPwC over LP were recently reviewed by us [6]. Additionally, in our laboratory, LPwC was applied to titanium and other alloys resulting in the increase of CRS, fatigue life and corrosion resistance [7–16]. Further, LPwC is reported to have decreased fatigue crack growth rate (FCGR) [17], improvements in cycles in high cycle fatigue (HCF) regime [18–21], stress corrosion cracking (SCC) [22], corrosion resistance [23] and wear resistance [24] in many metallic alloys. All the mentioned properties were mainly influenced by LPwC parameters, such as laser energy, laser power density and pulse density. One additional variant within LP is called multiple peening wherein the same area is peened with different passes or scans [21,25–31]. Multiple peening appears promising because of the propensity to obtain nano grained surface and subsurface of the peened alloys [21,25–31]. In this work near alpha titanium alloy TC6 (widely used in the Chinese aviation industry) is used. There is some limited data on the LP of TC6 [21,25,32]. Further, grain refinement is reported in
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TC6 as a consequence of 5 times multiple LSP impacts with absorbent coating, with formation of nanocrystallites distributed uniformly on the surface with grain size in the 30–60 nm range [25]. Additionally, work hardening and enhanced fatigue life were observed. To date there has been no information available on LPwC of TC6 alloy (the focus of the present paper). In this paper we focus on the surface and bulk mechanical properties of the TC6 alloy influenced by different LPwC parameters like power density and multiple impacts. In order to achieve this, a laser wavelength of 532 nm was used at three different power densities (3, 6 and 9 GW cm2) and multiple impacts (1, 3 and 5 passes) were performed at 6 GW cm2. The resulting microhardness, residual stress, microstructural changes and surface roughness were analyzed. Further, the phase analysis of the sample was carried out with synchrotron radiation. 2. Experimental 2.1. Material preparation The titanium TC6 alloy in the form of 5 mm thick cold rolled plates was purchased from Tianjin Haixing Steel Import and Export Co. Ltd., China. Its nominal chemical composition (in wt.%) is 5.5–6 Al, 2–3 Mo, 0.8–2.3 Cr, 0.2–0.7 Fe, 0.15–0.4 Si and Ti (balance). Small samples (15 15 5 mm3) were obtained from as received plate with the help of electric discharge mechanism (EDM) wire cutting. The samples were first solution heat treated at 870 °C for 2 h in air and cooled to room temperature in air followed by ageing at 550 °C for 2 h in air and cooling in air. The samples were further annealed for relieving stress in the inert gas atmosphere at 400 °C for 2 h. Further, samples were polished with silicon carbide sheets ranging from 220 to 3000 grit size and further polished with colloidal silica of particle size 0.04 mm. The samples were chemically etched for 10 s with Keller’s reagent (85 ml of H2O, 5 ml of HNO3 and 10 ml of HF). The resulting microstructure of the sample is shown in Fig. 1, depicting equiaxed grains with predominantly a phase. Tensile tests performed on the samples indicated ultimate tensile strength and percentage elongation at break as 950 MPa and 13.5% respectively. 2.2. Laser peening without coating (LPwC) The LPwC experimental setup is schematically shown in Fig. 2. The sample was fixed on a steel tray (200 200 100 mm3) and
the tray movements in x and y-directions were controlled by two servo motors, operated through a software. The sample was submerged in 10 mm thick water inside the tray. This water worked as a transparent overlay to create the plasma on the sample surface. Experiments were performed with a pulsed Nd:YAG laser system with a pulse duration of 10 ns and at a repetition rate of 10 Hz. Water penetrable frequency doubled 532 nm wavelength (second harmonics) radiation was used. The laser pulse was first reflected at an angle of 45° by a dichroic mirror and the beam was focused using a bi-convex lens of focal length 758 mm. Finally, a 0.8 mm diameter beam was focused on the sample surface. The overlapping rate was constant at 70% (equivalent to a pulse density of 17 pulses mm2). The samples were peened at three different power densities of 3, 6 and 9 GW cm2. Further, multiple peening with 1, 3, and 5 impacts at power density of 6 GW cm2 was also done. 2.3. Microhardness The depth-wise microhardness of the unpeened and peened samples was measured using a MATUZAWA-MMT-X Vickers hardness tester. The polished cross section of samples was used to measure the microhardness with a constant load of 500 g and holding time of 10 s. An average of three measurements was used for each depth. 2.4. Residual stress The residual stress measurement was carried out using an X’pert Pro system (PANalytical, Netherlands). The analysis of the data was performed using the standard X-ray diffraction sin2 w method. A 2 mm diameter beam and X-ray source of Cu-Ka radiation were used. The tilt angle w was varied from 40 to 40° and the lattice plane of (2 0 1) belonging to a phase was used to calculate the stress. The diffraction elastic constants were obtained using the elastic modulus (115 GPa) and Poisson ratio (0.34). The depth wise residual stress was measured by progressive electro polishing to remove layers up to a depth of 500 lm. The resulting stress relaxation was corrected as per the methods of Moore et al. [33] and Fitzpatrick et al. [34]. 2.5. Surface roughness The surface roughness of the unpeened and peened samples was measured using MAHR-GD120 automatic digital surface roughness measurement instrument. A 0.8 mm high pass cut off filter and a scanning length of 5.6 mm along the peened direction was used. For each sample, surface roughness was analyzed in three different places to obtain an average of mean arithmetic roughness (Ra). 2.6. Synchrotron radiation The phase content was analyzed with high energy spatial resolution synchrotron based X-ray diffraction (SR-XRD) measurement, using beamline line BL-11, located at INDUS-2, RRCAT, Indore, India. The diffraction patterns were obtained in an angledispersive X-ray diffraction (AD-XRD) mode with the Bragg angle ranging from 10° to 40°. The beam energy of 20 keV (corresponding to a wavelength of 0.600307 Å) was used. 2.7. Microstructure
Fig. 1. Optical microstructure of TC6 alloy solution heat treated (870 °C, 2 h), air cooled, aged (550 °C, 2 h), air cooled and finally annealed at 400 °C, for 2 h for stress relieving (inert gas atmosphere).
In order to investigate the microstructures before and after LPwC, the cross-sectioned samples were mounted with phenolic powder and prepared for metallographic investigation using
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Fig. 2. Schematic representation of the LPwC processes.
standard polishing and etching processes as detailed in Section 2.1. The cross sectional microstructures were analyzed using a Carl Zeiss optical microscope.
3. Results and discussion 3.1. Microhardness Fig. 3 shows the depth distribution of microhardness of samples after LPwC with three different laser power densities (3, 6 and 9 GW cm2) compared with the unpeened sample. The unpeened sample’s microhardness was around 303 HV, with an error of <1 HV. After LPwC, the maximum microhardness of all treated samples appeared in the near surface region. The microhardness of all treated samples decreased along the depth and finally reached the baseline microhardness of the unpeened sample. The microhardness increase depends upon laser power density, ranging from 7 to 23% with reference to the unpeened sample. Lowest
Fig. 3. Variation of microhardness along the cross section of the unpeened sample and samples peened at 3, 6 and 9 GW cm2.
microhardness (324 ± 2 HV) and a shallow depth of work hardened region (1000 lm) were observed in sample peened at 3 GW cm2 compared with samples peened at 6 and 9 GW cm2. With the increase in power density (3–6 GW cm2), microhardness increased significantly to 365 ± 6 HV, along with the increase in work hardened depth to 1500 lm. Increase in hardness and work hardened depth are attributed to increased shock wave pressures corresponding to increased power densities during LPwC. However, increasing power density does not always improve microhardness and work hardened depth as the hardness profiles saturate after 6 GW cm2. A similar effect on TiAl titanium alloy was also observed by Qiao et al. [35]. They reported that microhardness significantly increased up to 3.4 GW cm2 laser power density and with further increase in laser power density, microhardness saturated at 5.5 GW cm2. We also found such effects in our earlier studies with the titanium alloy Ti-2.5Cu [7] wherein it was observed that increasing the overlap percentage rate does not necessarily increase the work hardening behaviour in a linear fashion due to dominating softening mechanisms. This phenomenon was also determined to be dependent on the wavelength of the laser used [8], with the softening mechanisms setting in at lower wavelengths (532 nm compared to 1064 nm, for example). The same explanations can be extended to the present study with the hindsight that the effect of increasing overlap rate and power density are the same, that is, it increases the laser beam-material interaction. This trend can then be expected to be extrapolated to the case of multiple peening, as that also would increase the laser beam-material interaction, as in the following discussion. The micro hardness measurements are average of the microhardness of both phases a and b. Later in the discussion (Section 3.4) we also report presence of b phase after peening. Therefore the question of phases involved in softening will be taken up in the next section and Section 3.4. Fig. 4 depicts the microhardness variation along the depth of an unpeened sample and samples treated by LPwC with three different multiple LPwC impacts (1, 3 and 5) at 6 GW cm2. Single impact LPwC sample shows maximum microhardness and larger work hardened depth. However, beyond single impact the microhardness and work hardened depth decreased at surface and in the bulk of the sample, as expected and discussed above. In
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Fig. 4. Variation of microhardness along the cross section of the unpeened sample and samples peened multiple times at 6 GW cm2.
contrast to the present study no softening effect was observed by Nie et al. [21] in TC6, with multiple peening up to 10 impacts at 4.24 GW cm2, with absorbent coating. They observed microhardness increase up to 5 impacts and for 10 impacts microhardness reached maximum with no further significant increase. The apparent contrast in results can be explained through the difference in laser power densities and wavelength of the source, as they used lower power density of 5 GW cm2 at 1064 nm compared with present study (6 GW cm2 at 532 nm). 3.2. Residual stress The variation in residual stress along depth is shown in Fig. 5 for the laser power densities of 3, 6 and 9 GW cm2 along with the unpeened material. The residual stress of unpeened sample shows relatively small CRS (about 120 MPa on the average) throughout the depth. After LPwC, in all cases, the magnitude of residual stresses increases with increasing laser power density, as the laser shock wave pressure increases. The maximum residual stresses of 1110, 1696 and 1780 MPa are about 50 lm below the surface for the laser power densities of 3, 6, and 9 GW cm2 respectively. Decrease in CRS at the surface with increasing laser power density is evidently observed. The surface residual stresses of 721, 635, and 509 MPa were obtained for laser power densi-
Fig. 5. Variation of residual stress along the cross section of the unpeened sample and samples peened at 3, 6 and 9 GW cm2.
ties of 3, 6, and 9 GW cm2 respectively. At the surface, thermal effects (thermal softening) and stress relaxation were observed because of the direct interaction of laser beam and material surface. Similar kinds of results at the surface were seen in our previous studies [7,8] and also from the other reports in the literature [20,35–38]. Such results are usually attributed to local melting and re-solidification. In contrast to this, no melting or resolidification was noticed in this study, as evident from the microstructures presented in Section 3.5. On the other hand, this effect was not observed in the same material investigated by Nie et al. [21] perhaps as they used aluminium foil coating on the surface of the sample and it was protecting the surface from thermal effects, resulting in maximum CRS at the surface. Nevertheless, in the present study, deeper inside the material, residual stress increases with laser power density. Beyond 50 mm beneath the surface, the CRS drop drastically, finally reaching closer to the tensile residual stress region at 500 lm depth from the surface. Similar to present trend results were observed in previous report on TiAl alloy [35] where also beyond the laser power density of 6 GW cm2, (that is, at 5.5 GW cm2) there is no increase in CRS (in whole of the profile). This is similar to the results of hardness and hence an indication of softening. Similar to the hardness case, it is also expected here that softening would set in if multiple peening is employed. It is important to note here that the softening is only of the a phase, as the (2 0 1) peak of the a phase was used in the analysis. In the XRD technique used for residual stress measurement, sufficiently intense b peak at higher Bragg angles could not be clearly discerned. Further, in hindsight we can infer that as the softening trend depicted by both the hardness and residual stress data are similar, the contribution from a phase to overall micro hardness should be predominant compared to the b phase. The residual stress along the depth before and after LPwC with multiple impacts (1, 3 and 5 impacts) done at 6 GW cm2 as shown in Fig. 6. It is evident that the residual stress decreased in the bulk of the material. However, in contrast, softening effect was not observed in the same material (TC6 alloy) earlier reported by Nie et al. [21]. The resolution of this apparent contrast and the softening behaviour observed in the present study is exactly same as in the hardness case (Section 3.1). 3.3. Surface roughness The surface roughness of TC6 alloy samples before and after LPwC with three different laser power densities (3, 6 and 9 GW
Fig. 6. Variation of residual stress along the cross section of the unpeened sample and samples peened multiple times at 6 GW cm2.
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cm2) and after multiple peening are presented in Table 1. It can be seen that the surface roughness increases with an increase in laser power density. However, the surface roughness increase is significant from unpeened to the 3 GW cm2 case (0.013 to 0.168 mm) while it is only marginal among the peened cases (with a maximum of 0.1952 mm at 5 impacts of multiple peening). Also presented in Table 1 is the percentage increase in roughness, which falls within the range of 12–14%, when compared with the unpeened sample. From these results it is clear that although there is an increase in roughness after peening, the final roughness is not dependent on the power densities or the number of passes (in multiple peening). Many other reports confirmed the surface roughness increase in different materials after LPwC, increasing with multiple impacts, laser pulse energy or higher power density and higher overlapping percentage or higher pulse density [5,9,39,40]. Apart from this usual trend, in our earlier reports on Ti-2.5 Cu alloy [7,8], we observed that surface roughness depends not only overlapping rate (equivalent to power density in terms of mechanism) but also on the laser wavelength. Consequently we observed [7,8] smaller surface roughness in samples peened with 532 nm laser wavelength compared with 1064 nm. This is primarily due to less absorption of the 532 nm radiation compared to 1064 nm radiation in water, corresponding to a larger dissipation of energy in the latter case leading to increased damage at the surface, which is observed as increased roughness. As 532 nm radiation is used in the present study, the roughness among peened samples is not significant.
3.4. Synchrotron radiation Phase analysis was carried out with diffraction patterns from synchrotron radiation, as this offers three major advantages compared to conventional X-rays: high-density flux, high energy tunability of wavelength and high spatial resolution. The diffraction pattern for the unpeened samples and samples peened with laser power density of 3, 6 and 9 GW cm2 are depicted in Fig. 7. It is clear that the unpeened and peened samples predominantly consist a phase and b phase (at relatively lesser volume fraction). In the case of LPwC at 3 GW cm2, the sample showed increase in b (1 1 0) volume fraction and decreased in a (1 0 1) volume fraction. Further increasing of laser power density from 3 to 9 GW cm2, with this increase of the b (1 1 0) phase volume fraction in the material, an additional new crystalline b (2 0 0) phase peak appeared and increased proportionally with laser power density along with significant decrease in peak intensity of a (1 0 1). This indicates that a to b phase transformation occurred in the material. Similar trend can be observed in the case of multiple peened samples, as depicted in Fig. 8. Generally, phase transformation (a to b) occurs in titanium alloys due to increase in temperature above b transus temperature. In our earlier study [8] we had proposed that the increase in temperature in LPwC occurs due to adiabatic heating, resulting from the frictional dissipation of dislocations in the
Fig. 7. Synchrotron diffraction pattern for the unpeened and samples peened with different power densities (synchrotron energy: 20 keV).
Table 1 Surface roughness before and after peening with different power densities (single peening) and multiple peening at 6 GW cm2. Peening condition (power density)
Average surface roughness (Ra) (lm)
Unpeened 3 GW cm2 6 GW cm2 9 GW cm2 6 GW cm2/3 Times 6 GW cm2/5 Times
0.0133 0.1688 0.1746 0.1882 0.1751 0.1952
Ra RaðUnpeenedÞ
(%) (rounded off
to nearest integer) – 12 13 14 13 14
Fig. 8. Synchrotron diffraction pattern for the unpeened and samples peened multiple times at 6 GW cm2 (synchrotron energy: 20 keV).
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Fig. 9. Cross-section optical microstructures of (a) unpeened sample and samples peened at power densities of (b) 3 GW cm2, (c) 6 GW cm2 and (d) 9 GW cm2.
Fig. 10. Cross-section optical microstructures of (a) unpeened sample and samples peened multiple times with (b) 1 pass, (c) 3 passes and (d) 5 passes at 6 GW cm2.
A. Umapathi, S. Swaroop / Optics and Laser Technology 100 (2018) 309–316 Table 2 Grain size measurements before and after peening with different power densities (single peening) and multiple peening at 6 GW cm2. Peening condition (power density)
Grain size (mm)
Error (mm)
Unpeened 3 GW cm2 6 GW cm2 9 GW cm2 6 GW cm2/3 Times 6 GW cm2/5 Times
6.9 7.5 6.5 7.1 6.6 6.8
0.1 0.3 0.1 0.3 0.2 0.2
bulk of the material. Further, with 532 nm radiation that is not absorbed in water, more shock wave energy transfer to the bulk of the material can be expected to occur, leading to increased dislocation density and hence increasing the chances of adiabatic heating. Moreover, severe plastic deformation increases with increase in laser power density, causing increase in dislocation density, as reflected in the peak shifts of deformed material compared unpeened material. These higher angle peak shifts are well correlated with residual stresses as discussed in Section 3.2. This is an indication of increasing CRS in the material [41]. In contrast to the present study, in the same material phase transformation was not observed by Nie et al. [21], because of three reasons. First is the variation in laser power density, as they used 5 GW cm2 compared present study very less (6–9 GW cm2). Second, they used sacrificial layer coating as aluminium foil, unlike the present study as this would decrease the shock wave pressure. Third, they peened the samples with a laser wavelength of 1064 nm, while in the present study water penetrable 532 nm radiation was used, as laser wavelength influences the deformation in titanium alloy and significant variation in results were noticed as a function of wavelength [8]. It should be noted that all the three enumerated reasons (high power density, no coating on the surface and lesser wavelength of radiation) increase the chances of adiabatic heating leading to harder b phase formation. Further, increasing the number of passes (in multiple peening) also increases the chance of adiabatic heating, due to increased dislocation activity. Interestingly, LPwC induced transformation of softer to harder phase is not uncommon even in athermal cases. For example, LPwC induced harder martensite phase transformation (athermal) in an originally austenitic AISI321 steel was also observed by Karthik et al. [12]. Further, phase transformation was also supported by microstructural observations as detailed in next section. The detection of harder b phase may appear contradictory to the claim of softening in the bulk discussed in Sections 3.1 and 3.2 on hardness and residual stress. However, it is important to note the softening is with respect to a phase alone and the contribution to residual stress from b phase is difficult to evaluate due to insufficient intensity of b peaks at higher angles.
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presented in the previous section about the formation of b phase resulting from adiabatic heating. The microstructure of multiple peened samples at 6 GW cm2 are presented in Fig. 10. Similar trends in increase of b phase in the bulk can be observed from the figure, indicating the increased chances of adiabatic heating with increase in number of passes. The grain sizes were measured from the micrographs and are presented in Table 2. From the table it can be observed that there is no any significant variation in the grain sizes. 4. Conclusions Laser peening without coating was performed on a TC6 alloy with laser power density of 3, 6 and 9 GW cm2 and multiple times at 6 GW cm2 with laser pulses of wavelength of 532 nm, pulse duration of 10 ns and overlapping percentage of 70%. Following conclusions could be drawn out of the study. 1. Microhardness data showed that the hardness variations were within 23% of the baseline hardness (for unpeened sample). Saturation in hardness and softening (in the bulk) with single peening (beyond 6 GW cm2) and in multiple peening (from 3 times onwards at 6 GW cm2) was observed. 2. Residual stress analysis showed largest CRS around 1780 MPa at a depth of 50 lm at 9 GW cm2. Softening of a phase in the bulk of the samples was confirmed in residual stress data also and it was proposed due to adiabatic heating. 3. Surface roughness in all peened samples was found to have increase within 12–14% compared to the peened samples. The increase in surface roughness was found to be insignificantly affected by power density and/or multiple peening. 4. Synchrotron radiation diffraction studies revealed that a to b phase transformation was noticed on peened specimens (due to adiabatic heating) and it increased with increase in power density. This was confirmed with microstructures after LPwC.
Acknowledgment We thank DST-SERB, India (Grant No. SB/S3/ME/36/2013) for the financial support, VIT University for the infrastructure and constant support throughout the project and National Facility of OIM and Texture at IIT-Bombay for the residual stress measurements. The Centre for Advanced Material Processing and Testing at VIT University (funded by DST-FIST) is acknowledged for assistance in mechanical testing. RRCAT, Indore, India is acknowledged for the Synchrotron facility (AD-XRD, BL-11). References
3.5. Microstructure Fig. 9(a) represents unpeened specimen and (b), (c), and (d) represents the samples peened with laser power density of 3, 6, and 9 GW cm2 respectively. All the micrographs are taken in the cross section. The unpeened microstructure shows predominantly aphase (grey area) and very less b phase (dark area). After LPwC, the b phase volume fraction increases with increasing laser power density. The sample peened with a power density of 3 GW cm2 indicates small increase in dark region (b phase) and corresponding b phase volume fraction. In the same way, 6 and 9 GW cm2 peened samples exhibit substantial increase of the dark area, as confirmed by SR-XRD patterns shown in Figs. 7 and 8. There is a good correlation of phase information between microstructure and SR-XRD patterns. This is also consistent with the arguments
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