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Wavelength dependent deformation in a laser peened Ti-2.5Cu alloy
Author’s Accepted Manuscript Wavelength dependent deformation in a laser peened Ti-2.5Cu alloy A. Umapathi, S. Swaroop
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To appear in: Materials Science & Engineering A Received date: 24 October 2016 Revised date: 15 December 2016 Accepted date: 17 December 2016 Cite this article as: A. Umapathi and S. Swaroop, Wavelength dependent deformation in a laser peened Ti-2.5Cu alloy, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2016.12.073 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Wavelength dependent deformation in a laser peened Ti-2.5Cu alloy A. Umapathi, S. Swaroop* Surface Modification Laboratory, School of Advanced Sciences, VIT University, Vellore 632014, India
[email protected] (A. Umapathi) [email protected] (S. Swaroop)
* Corresponding author: S. Swaroop. Tel: +91 416 220 2489; Fax: +91 416 220 3092.
Abstract Laser peening without coating (LPwC) was performed on a Ti-2.5Cu alloy at wavelengths of 1064 and 532 nm and at a constant power density of approximately 7 GW cm-2 with overlap rates of 53, 63 and 73%. Surface softening due to thermal interaction of laser beam with material was observed till a depth of 500 m (at 532 nm) and 200 m (at 1064 nm), based on hardness data. This was corroborated (rather weakly) by residual stress analysis. In addition, softening due to mechanical effects (adiabatic heating) was observed in the bulk. Although there was an increase in mechanical softening with increase in overlap rates at 532 nm, it was observed, upon comparison with peened samples at 1064 nm, that the mechanical softening is a function of wavelength of radiation used for peening. It was observed that the onset of softening was earlier if the wavelength was shorter. Further, evidence of hardening in the form of twinning was found for the 1064 nm case while it was absent for the 532 nm case, for 73% overlap. The workhardened depth was more than 1000 m, not observed in earlier studies based on residual stress analysis. The direct consequence of softening effect was found in the fatigue results. The fatigue life extended by a factor of 1.4 and 2.3 for the
samples peened at 532 nm and 1064 nm respectively, consistent with the observed wavelength dependent onset of softening.
Keywords: Laser peening without coating; Ti-2.5Cu alloy, Residual stress, Hardness, Adiabatic heating, Fatigue.
1. Introduction
Titanium alloys are typically used in aero engines where they undergo fatigue cycles. Under such circumstances, their fatigue lives are determined by the nature of surface defects. Therefore, improvements in the surface conditions where residual compressive stresses are introduced at the surface, resulting in delayed propagation of cracks are common. Examples of techniques that lead to such improvement are shot peening, laser peeing, deep-rolling, ultrasonic impact peening and ball-burnishing. The present study employs the technique of laser peening [1]. A modification of this technique, called Laser Peening without Coating (LPwC), which does not employ a conventional coating of the sample surface is popular and more efficient in the recent times [2-4]. This technique has been used by us with aluminium alloys and steels [5-9]. The technique of LPwC was also recently reviewed by us [10]. Typically, LPwC is used in conjunction with the frequency double radiation of an Nd:YAG laser (532 nm), as the fundamental frequency is absorbed by water. The alloy Ti-2.5Cu can be easily formed and welded and has enhanced mechanical properties at high temperatures (up to 350 °C). It finds applications in bypass ducts of gas-turbine
engines. The earlier work on this alloy [11, 12] focused on comparing the residual stress distribution in LPwC and ball burnishing using the frequency doubled radiation. However, the mechanism involved in the deformation processes was not analysed. The present study is done to understand the process of LPwC from two specific points of view. That is, to understand the influence of wavelength of radiation on (a) the hardening and softening mechanisms involved in the process and (b) the fatigue lives. In order to realize these objectives, the alloy Ti-2.5Cu was double aged through specific heat treatments and subjected to LPwC with fundamental and frequency doubled radiations and investigated for the resulting deformation mechanisms in the process and the fatigue behaviour. 2. Experimental 2.1 Material
The alloy Ti-2.5Cu was commercially purchased from Tianjin Haixing Steel Import and Export Co. Ltd., China. The material was in the form of cold rolled plates (5 mm thick). The composition of the alloy in wt% is 2-3% Cu, 0.08% C, 0.01% H, 0.2% Fe, 0.05% N, 0.2% O and Ti (balance). Samples of size 20 mm × 20 mm × 5 mm were obtained by EDM wire cutting. Solution heat treatment was done at 805 C for 1 h. Double aging was done at 400 C and then at 475 C. At both the temperatures the duration of aging was 8 h and the cooling was done in air. The initial microstructure of sample is shown in Fig. 1 (recorded with Carl Zeiss optical microscope). The precipitates of Ti2Cu are marked in the figure.
2.2 Laser peening without coating (LPwC)
For LPwC, a pulsed Nd:YAG laser was used operating at a wavelength of 1064 nm (fundamental) and 532 nm (second harmonics). For the fundamental radiation, a thin layer (< 1mm) of running water was used as confining layer, simultaneously removing the ablated material and for the second harmonics, peening was done by submerging the sample inside water, with the sample surface below the water surface by 10 mm. The peening scheme was same as the one used in our earlier studies [7, 8]. The peening scheme is also depicted in the top left side of Fig. 2 and the macrograph of a peened area is shown on top right side of the same figure. The same scheme was used for the fatigue samples, whose dimensions and peened area in dark are shown in the bottom portion of Fig. 2. For the square samples (peened area of 20 mm × 20 mm), peening was done on one side while for the fatigue samples peening was done on both sides. The square samples were used for measurements of hardness, residual stress, roughness and XRD. With a constant energy (350 mJ) and spot diameter (0.8 mm), a constant power density was obtained (6.97 GW cm-2). The variable parameter in the experiment was the overlap percentage of the pulses. Three different overlap rates were used, namely, 53, 63 and 73% (constant in peening direction and perpendicular direction). The varying overlap rates were obtained by varying the speed of the servo motor of a movable X-Y translation stage on which the sample was mounted. 2.3 Microhardness Microhardness of the samples was measured with MATUZAWA-MMT-X Vicker’s hardness tester. The load used was 500 g and the time of holding was 10 s. The hardness was measured
along the cross section of the sample, at various depths. At each depth, an average of three measurements was taken. 2.4 Residual stress Residual stress was measured using an X'pert Pro system (PANalytical, Netherlands). The results were analysed with the standard sin2 technique. The radiation used was Cu-Kα with primary aperture dimension of 2 mm × 2 mm. The peak used for the measurement was {213} of phase. The sample was tilted from -40 to +40, involving a total of 33 steps. The corresponding increment in the sin2 value was 0.0250. The average residual stress was calculated from this by the slope of the d versus sin2 plot with the error of 5 MPa. The diffraction elastic constants were obtained using the elastic modulus of 110 GPa and Poisson ratio of 0.33. Successive layers were removed by electropolishing in order to obtain the residual stress as a function of depth. Corrections to measured depths were performed using standard techniques elaborated by [13] and [14]. 2.5 Fatigue tests Fatigue tests were carried out on specimens (unpeened and peened) prepared according to ASTM standard [E 466-07] specified for high cycle fatigue testing. Tests were done with Instron 8801, in the constant load configuration with the R ratio of -1. The load level used for testing was 12.5 kN. The frequency used for cyclic loading was 10 Hz. All experiments were performed in air. 3. Results and discussion 3.1 Microhardness The variation of microhardness as a function of depth is shown for all percentages of overlap in Fig. 3 (a), peened at the wavelength of 532 nm, along with the unpeened sample as the
reference. The hardness is observed to increase from a lower value and then reach a constant level at a depth of approximately 500 m in all cases of overlap. This is indicative of a softening effect near the surface caused by the thermal interaction of laser beam with the material. Therefore, there is more of a thermal mechanism restricted only at the surface which disappears due to rapid dissipation of heat as a result of short duration of laser pulses (in nanoseconds). Deeper inside the material, mechanical effects (as increase in hardness) due to the shock waves produced and propagated at high strain rates are observed. Even though the mechanical effects are present at the surface (originating from plasma pressure), since the thermal effects are also present at the surface (and predominant at the surface), the overall hardness profile is a thermo-mechanical effect. Although not uncommon, such effects have been observed, for example by Salimianrizi et al. [15] in a 6061 aluminium alloy. A similar overall trend of surface thermal softening was observed for the samples peened at the wavelength of 1064 nm (Fig. 3(b)), although within the depth of 200 m from the surface, as the fundamental radiation is more absorbed in water than the second harmonics. To ascertain thermal softening, the average roughness of the peened samples are compared with the unpeened sample and shown in Fig. 4. Clearly the peened samples show increased roughness (from 0.06 m to 0.20 m for the 532 nm case and up to 2.723 m for the 1064 nm case), indicating surface damage due to thermal interaction between laser beam and the material surface, thereby decreasing the dislocation density and hardness. These results indicate a predominant amount of energy is dissipated as heat at the surface in the case of 1064 nm (thereby increasing the surface roughness) while it is delivered more into the material in the 532 nm case. Any surface inhomogeneity resulting from laser beam overlap rate should be of the order of the beam diameter. Since the beam diameter is 0.8 mm in the present study and the roughness
is few m, the inhomogeneity cannot be because of overlap rates. Further, the plastic strains are microscopically accommodated in the compression of lattice planes (as reflected in residual stress measurements) and there was no appreciable macroscopic strains observed after peening. Moreover, as the plasma pressure is same irrespective of the wavelength, if no thermal effects are present as a function of wavelength, then same levels of strains (reflected as surface roughness) should have been observed at both wavelengths. However, clearly it was not the case. Therefore, the inhomogeneity is due to the non-uniform ablation of the material, which in turn is a function of laser-material interaction. This explains why roughness scales with overlap rates, as with increase in overlap rates laser-material interaction time increases, in turn increasing ablation. Thermal effects accompanied by rapid cooling can also cause formation of soft martensite, as reported by Carpeno et al.[16] in nanoblades of Ti-6Al-4V peened with UV laser. The XRD patterns of the samples at all peened samples are shown in Fig. 5 with unpeened sample as the reference. The phase identified as P in Fig. 5 is the precipitate Ti2Cu (JCPDS No. 150717). However, this phase leads to precipitation hardening of the material, unlike the soft martensite, which is absent in the present study. It should be noted that the precipitation hardening is due to initial heat treatment and not because of the LPwC process, as aging was done for 16 hours (Section 2.1) before LPwC while LPwC lasted for few minutes. Therefore, in this time frame, there cannot be further significant growth of precipitates. That is to say, hardness profiles are not affected by precipitates, as no new precipitates are formed during LPwC. It can be seen that the maximum increase in microhardness for the lowest overlap rate is about 20% (above a depth of 500 m for 532 nm case and 200 m for 1064 nm case) from the reference level, that is, from 215 to 260 HV (Fig. 3 (a) and (b)). This may in itself not be
very significant. On the other hand, a flat hardness profile (within the error bar) observed in all overlap percentages beyond a particular depth could indicate a large depth of workhardening, something of the order of few mm as evident from Fig. 3. Maawad et al. [11] also investigated LPwC of Ti-2.5Cu at a power density of 5 GW cm-2 using a beam diameter of 0.8 mm. They also reported insignificant increase in hardness (only about 10%) with a far lesser workhardened depth of 500 m. The difference can be attributed to the larger power density in the present study (about 7 GW cm-2). Flat hardness profiles are normally observed when harder beta phase is present, as in Ti-6Al4V [17]. Although addition of copper to titanium could be expected to increase the ductility, comparison with the trend of Ti-6Al-4V [17] suggests that the precipitates of Ti2Cu (Fig. 1) would have contributed to the precipitation hardening of the material. In still softer materials such as aluminium, a clear decreasing trend has been reported, for example by Sathyajith et al. [18] in a 6061 alloy, although the increase in the surface hardness is not better than the saturation hardness in the present study, with lesser workhardened depth. In an earlier investigation on 17-4 PH steel [7], we have observed 15% increase in surface hardeness with a workhardened depth of about 600 m. The preceding discussion indicates that irrespective of the material, a lower increase in surface hardness is advantageous without a large increase in dislocation density, as hardness and dislocation density are correlated. The mentioned advantage is over the conventional process of shot peening [19], especially considering the fact that the introduced dislocations are unstable at higher temperatures [12]. Most interesting feature depicted in Fig. 3 (a) is the decrease in the saturation hardness with the increase in overlap rates for the 532 nm case. Although an increase in the overlap rates would mean an increased strain in the high strain rate process of LPwC, a corresponding
decrease in hardness is indicative of a softening mechanism. This softening is however different from the surface softening indicated above in the sense that the cause is mechanical rather than thermal. In contrast, in Fig. 3(b), it can be observed there is a progressive hardening in the 1064 nm, with increase in overlap rates. This indicates, the softening is a function of wavelength. As it was pointed out earlier that less heat is dissipated at the surface in the 532 nm case, it appears that more energy is delivered to the bulk of the material, resulting in increased strain and strain rates, with the eventual result of earlier softening. The mechanism will be taken up in detail in Section 3.3, comparing with the residual stress results. 3.2 Residual stress The residual stress depth profile is shown in Fig. 6 (a) for the overlap rates of 53, 63 and 73% (at 532 nm) along with the profile for the unpeened material. The unpeened material shows a relatively flat profile at low compressive stress level (-180 MPa on the average) and the maxima observed at a depth of 50 m for the unpeened sample is not significant from the baseline value, in comparison with the peened samples. Similar to the results of hardness, here also thermal softening is observed up to a depth of 50 m. After this trend, there is a continuous increase in the compressive stress till 1000 m. The apparent inconsistency in the depth to which thermal softening extends can be explained based on the difference in the nature of hardness and residual stress measurements. The former is macroscopic and nonuniform due the possibility of precipitates being measured while the latter is based on the lattice strain and more sensitive. Besides, the underlying mechanism of hardness and residual stress are not correlated to the dislocation density and lattice spacing in the same way. That is to say, measured hardness is generally due to the effects of dislocation density and residual stress increments. Therefore it is not surprising to observe this contrast. However, what is emphasized here is the agreement between the thermal softening at and near surface in two
independent measurements. This conclusion is further strengthened by noting that a lower power density may not produce such a softening effect due to the reduced thermal interaction between the laser beam and the material. That this is the case is confirmed by the absence of thermal softening in earlier studies by Maawad et al. [11, 12] where a lower power density of 5 GW cm-2 was used. Similar to the hardness results, a reversal in the softening trend is observed in the 1064 nm case. That is, as shown in Fig. 6 (b), there is no progressive softening with increase in overlap rates, indicating that the softening mechanism is wavelength dependent. Further, the surface stress levels are large at 1064 nm compared to 532 nm. This indicates again that the heat dissipation at the surface is relatively more at 1064 nm, leading to the relaxation of stresses at the surface. This further strengthens the conclusion that the thermal softening is the mechanism responsible for residual stress maxima in peened samples. The measurements were terminated at 1000 m because most of the beneficial effects due to the compressive stresses are well within this range. For example, in enhancing the fatigue lives of materials, compressive stresses play a major role only at the surface by introducing a closure force on the cracks, thereby arresting them from further propagating [17, 12, 20-25]. Similar arguments, though of chemical nature, can be extended to reduction of corrosion rates in materials [6, 26-28]. Nevertheless it would be instructive to check the lack of stress reversal even till 1000 m, although it demonstrates a favourable workhardening at least till a depth of 1000 m, which is far deeper than the earlier studies [11, 12], due to the larger power density used in the present study. To check this, residual stress measurements were carried out from the other side of another specimen with 73% overlap, at 532 nm. This resulted in datum points (Fig. 7) from a depth of 4000 to 5000 m, as the thickness of the sample was 5 mm. The stress did reverse the sign and crossed the zero stress level and
entered into tensile stress range, with 496 MPa at the surface. Such large tensile stresses can be anticipated to compensate for the large maximum compressive residual stresses in the interior. Even though this can be anticipated to be favourable for crack propagation at the other end, it should be noted that the stress levels will fall rapidly in thicker samples, as shown by Ding [29]. Further, it can be circumvented by peening the other side, as in some earlier studies [12, 30] although it may not be a viable solution in practice. Similar trend was also observed at 1064 nm. The results of Maawad et al. [11,12] are compared with the present study in Fig. 8. Three striking contrasts can be observed upon comparison.
1. There is no surface thermal softening as in the present case. 2. The maximum compressive stress is lower than the present case (-650 MPa [11, 12] and 746 MPa (present study)). 3. The workhardened depth is much lower compared to the present study (300 m [11, 12] and minimum of 1000 m (present study)).
The reason for the first contrast was already pointed out while the reason for the second contrast could be due to the peening on both sides of the sample by Maawad et al. [12], where the possible large compressive residual stress on the first peened surface would have got reduced due to the subsequent peening of the other side, to balance the stresses. However, the observed contrast is too large to account for by this balancing mechanism and hence it can be concluded that the lower power density used by Maawad et al. [12] overrides the balancing effect. Lower power density can again be attributed to explain the third contrast.
An important feature, as observed in the case of hardness data is also evident from the results of residual stress. With the increase in overlap rates, there is a mechanical softening throughout the measured depth, which is not the same as the surface softening. Further, this depends on the wavelength of the radiation used. This necessitates the discussion of possible softening mechanisms in the following section. 3.3 Deformation mechanisms
Softening in the bulk in titanium alloys could be caused by adiabatic heating. Whereas phase transformation discussed earlier involves thermal cause with mechanical effect at the surface, adiabatic heating involves mechanical cause with thermal effects both at the surface as well as in the bulk, as the cause of mechanical effect is the plasma pressure at the surface of the sample. However, as the temperature due to laser-material interaction is very high at the surface, it dominates the mechanical effect at the surface. It dissipates quickly due to very short and localized interactions at the surface, leading to surface softening. Hence, in the bulk, adiabatic heating dominates. Overall, the entire hardness or residual stress profile is a result of thermal and mechanical effects. In a high strain rate irreversible plastic deformation process such as LPwC, the lattice friction encountered by the dislocations is released as heat, thereby increasing the temperature in the bulk, in turn decreasing the dislocation density, accounting for the softening in the bulk. As expected, this should increase with increase in strain and strain rates (see Appendix). Obviously, increasing overlapping rates would imply increasing strains and hence increasing softening. Adiabatic heating of this sort was reported earlier by Shen et al. [31] and Meyers et al. [32]. However, the lack of adiabatic shear bands show the increase in the temperature is not high enough (without flow localization) to cause them. This indeed was observed in the
earlier studies [31, 32] where adiabatic heating was reported without flow localization at earlier stages of deformation. It is also proposed here that copper in the matrix, with its higher thermal conductivity would assist in propagating heat efficiently throughout the bulk. The foregoing discussion raises the most important question as to why there is such an early onset of softening at 532 nm. Considering carefully the very early onset of softening in the investigation of Carpeno et al. [16] with UV radiation and the early onset of softening in the 532 nm compared to 1064 nm in the present study, there seems to be a dependence of wavelength on the onset of softening process. Lower the wavelength, earlier the onset of softening. This above conclusion might be true considering the fact that LPwC is done with green light in water submerged samples, as the absorption of green light (532 nm) in water is not significant, compared to infrared radiation (1064 nm). Therefore, the heat energy dissipated at the surface at 1064 nm is larger compared to the case at 532 nm. This leads to the decrease in the levels of compressive stresses in the infrared regime (Fig. 6 (b)). The opposite is the case at 532 nm (Fig. 6 (a)). Consequently, higher transfer of energy (the undissipated part) can be expected to occur into the bulk of the material at 532 nm, leading to larger activation of dislocations and hence larger dissipation of heat due to adiabatic heating, eventually leading to early softening compared to the case where absorption in water is more (at 1064 nm). This clearly demonstrates the fact that the onset of softening is a function of wavelength. Further, with the increase in overlap rates, the laser beam-material interaction time increases, increasing the transfer of energy to the bulk and the resulting softening behaviour, provided the adiabatic heating is significant. The argument holds good even for increase in power density, following the results of Maawad et al. [11, 12].
The peened microstructures are compared for both the radiations in Fig. 9. It is evident that twinning is observed for the 73% overlap case (Fig. 9 (f)) for the infrared radiation while it is absent for green. Twinning is very common in titanium alloys at high strain rates (as in LPwC) or in high temperatures deformation processes [33-38]. It was observed in such independent measurements by various researchers. For example, Carpeno et al. [16] observed compression twins in Ti-6Al-4V. Chichili et al. [39] and Meyers et al. [32] reported increased twin density in -titanium. Therefore, it is reasonable to conclude that the hardening process is not overtaken by softening as long as the wavelength of the radiation was at the infrared level. On the other hand, with the decrease in wavelength, softening sets in much earlier, strengthening the earlier drawn conclusion that the onset of softening is a wavelength dependent phenomena. Inspection of Fig. 9 also shows that the volume fraction of precipitates is not different from the unpeened sample in Fig. 1. Hence, the effect of precipitates can be ruled out in the peening process. Further, the surface roughness is highest in the infrared range (Fig. 9 (f)), as discussed earlier. 3.4 Fatigue behaviour
The number of cycles to failure for the unpeened sample and the samples peened at wavelengths of 532 nm and 1064 nm at the power density of nearly 7 GW cm-2 and the overlap rate of 73% is shown in Fig. 10, for the fatigue tests done at a load level of 12.5 kN with the R ratio of -1 at room temperature. The fatigue life has extended by a factor of 1.4 and 2.3 for the samples peened at 532 nm and 1064 nm respectively. Clearly, there is an improvement in the fatigue life due to peening. However, the improvement depends on the wavelength of the radiation. It was pointed out by Sano et al. [40] that in the technique of
LPwC, the sample is submerged inside water and correspondingly the radiation of 532 nm which is not absorbed by water is used. It was discussed earlier that this leads to earlier onset of softening. Together with this, the improvement in fatigue life is not very appreciable compared to the peening radiation of 1064 nm. The fractographs of the unpeened samples and samples peened at 532 nm and 1064 nm are shown in Fig. 11 respectively. The sites where cracks are detected are shown at larger magnification for unpeened samples and samples peened at 532 nm and 1064 nm in Fig. 11. The peened surfaces are also marked with red coloured arrows in the figure. In the case of unpeened samples, the cracks originate at the surface and they have grown larger in size. For the peened samples, the cracks originate at the sub-surface. For the sample peened at 532 nm, the cracks were observed at a depth of ~1.2 mm while for the sample peened at 1064 nm they were observed at ~2.1 mm. Sub-surface cracks are less detrimental compared to surface cracks, as they were also observed in earlier studies [12, 20] and it is reflected in the corresponding fatigue lives. Evidently, samples peened at 1064 nm have deeper sub-surface cracks of shorter grown lengths. This is consistent with the previously drawn conclusions based on earlier onset of softening in the case of 532 nm radiation. Hence it is clear that 1064 nm radiation is preferable from the point of view of improving fatigue life, even though it may be more absorbed in water in the LPwC process. 4. Conclusions
Laser peening without coating (LPwC) was performed on a Ti-2.5Cu alloy at wavelengths of 1064 and 532 nm, at a constant power density of 6.97 GW cm-2 and at overlap rates of 53, 63 and 73%. The following conclusions were drawn.
1. From the hardness measurements, surface softening due to thermal interaction of laser
beam with material was observed till a depth of 500 m (at 532 nm) and 200 m (at 1064 nm). 2. Thermal softening was also supported by residual stress analysis. 3. Softening due to mechanical effects (adiabatic heating) was observed in the bulk at 532
nm. 4. On comparison with peened samples at 1064 nm, mechanical softening was found to be a
function of wavelength of radiation used for peening, that is, onset of softening was earlier if the wavelength was shorter. 5. Twinning was found for the 1064 nm while it was absent for the 532 nm (for 73%
overlap). 6. The workhardened depth was more than 1000 m, not observed in earlier studies. 7. Fatigue life extended by a factor of 1.4 and 2.3 for the samples peened at 532 nm and
1064 nm respectively (73% overlap), consistent with the observed wavelength dependent onset of softening.
Acknowledgement
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. Appendix If the work done to move dislocations at a stress of to cause an incremental strain of d, then from simple heat balance we have, (1) Here, is the fraction of work converted to heat (typically 63% [41]), is the density of the material, Cp is the specific heat at constant pressure (as in the peening process) and dT is the increase in the temperature in the adiabatic process. The softening in the material can be expressed as,
(2)
We may write the incremental stress as, ( ) Here is the decrease in the flow stress with temperature. Combining the above equations, we have,
(3)
(4)
In terms of the rates, we can write, ̇ ̇
(5)
From equations (1) and (5) it is clear that both increase in temperature and softening increase with increase in strain and strain rate.
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Figure captions Figure 1: Optical micrograph of Ti-2.5 Cu alloy solution heat treated (805 C, 1 h), air cooled and double aged (400 C, 8 h and 475 C, 8 h), air cooled.
Figure 2: (Top left) Peening scheme for square and fatigue samples; (Top right) macrograph of a peened sample and (Bottom) Peened fatigue sample, with the dark shaded area indicating peened area. Peening was on single side for square sample and on double sides for fatigue samples.
Figure 3: Variation of microhardness along the cross section of the sample (a) peened at 532 nm and (b) peened at 1064 nm.
Figure 4: Surface roughness before and after peening with different overlap rates.
Figure 5: XRD patterns of the peened samples for different overlap rates, along with that of unpeened sample.
Figure 6: Comparison of the residual stress data along the cross section of the samples peened at (a) 532 nm and (b) 1064 nm radiations.
Figure 7: Variation of the residual stress along the entire cross section of the sample for the samples peened at an overlap percentage of 73%.
Figure 8: Comparison of the residual stress data from the present study with earlier study [11].
Figure 9: Cross-sectional optical microstructures of samples peened at (a) and (b) 53% overlap, (c) and (d) 63% overlap and (e) and (f) 73% overlap. All micrographs on left are peened at 532 nm and on right are peened at 1064 nm.
Figure 10: Comparison of number of cycles to failure for samples peened at 532 and 1064 nm radiation. The load level used for testing was 12.5 kN with R=-1 and at room temperature..
Figure 11: Fractographs of (a) unpeened, (b) higher magnification image of crack area highlighted in (a), (c) sample peened at 532 nm, (d) higher magnification image of crack area highlighted in (c), (e) sample peened at 1064 nm and (f) higher magnification image crack area highlighted in (e). The overlap for peened samples was 73%.
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S0921-5093(16)31574-X http://dx.doi.org/10.1016/j.msea.2016.12.073 MSA34504
To appear in: Materials Science & Engineering A Received date: 24 October 2016 Revised date: 15 December 2016 Accepted date: 17 December 2016 Cite this article as: A. Umapathi and S. Swaroop, Wavelength dependent deformation in a laser peened Ti-2.5Cu alloy, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2016.12.073 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Wavelength dependent deformation in a laser peened Ti-2.5Cu alloy A. Umapathi, S. Swaroop* Surface Modification Laboratory, School of Advanced Sciences, VIT University, Vellore 632014, India
[email protected] (A. Umapathi) [email protected] (S. Swaroop)
* Corresponding author: S. Swaroop. Tel: +91 416 220 2489; Fax: +91 416 220 3092.
Abstract Laser peening without coating (LPwC) was performed on a Ti-2.5Cu alloy at wavelengths of 1064 and 532 nm and at a constant power density of approximately 7 GW cm-2 with overlap rates of 53, 63 and 73%. Surface softening due to thermal interaction of laser beam with material was observed till a depth of 500 m (at 532 nm) and 200 m (at 1064 nm), based on hardness data. This was corroborated (rather weakly) by residual stress analysis. In addition, softening due to mechanical effects (adiabatic heating) was observed in the bulk. Although there was an increase in mechanical softening with increase in overlap rates at 532 nm, it was observed, upon comparison with peened samples at 1064 nm, that the mechanical softening is a function of wavelength of radiation used for peening. It was observed that the onset of softening was earlier if the wavelength was shorter. Further, evidence of hardening in the form of twinning was found for the 1064 nm case while it was absent for the 532 nm case, for 73% overlap. The workhardened depth was more than 1000 m, not observed in earlier studies based on residual stress analysis. The direct consequence of softening effect was found in the fatigue results. The fatigue life extended by a factor of 1.4 and 2.3 for the
samples peened at 532 nm and 1064 nm respectively, consistent with the observed wavelength dependent onset of softening.
Keywords: Laser peening without coating; Ti-2.5Cu alloy, Residual stress, Hardness, Adiabatic heating, Fatigue.
1. Introduction
Titanium alloys are typically used in aero engines where they undergo fatigue cycles. Under such circumstances, their fatigue lives are determined by the nature of surface defects. Therefore, improvements in the surface conditions where residual compressive stresses are introduced at the surface, resulting in delayed propagation of cracks are common. Examples of techniques that lead to such improvement are shot peening, laser peeing, deep-rolling, ultrasonic impact peening and ball-burnishing. The present study employs the technique of laser peening [1]. A modification of this technique, called Laser Peening without Coating (LPwC), which does not employ a conventional coating of the sample surface is popular and more efficient in the recent times [2-4]. This technique has been used by us with aluminium alloys and steels [5-9]. The technique of LPwC was also recently reviewed by us [10]. Typically, LPwC is used in conjunction with the frequency double radiation of an Nd:YAG laser (532 nm), as the fundamental frequency is absorbed by water. The alloy Ti-2.5Cu can be easily formed and welded and has enhanced mechanical properties at high temperatures (up to 350 °C). It finds applications in bypass ducts of gas-turbine
engines. The earlier work on this alloy [11, 12] focused on comparing the residual stress distribution in LPwC and ball burnishing using the frequency doubled radiation. However, the mechanism involved in the deformation processes was not analysed. The present study is done to understand the process of LPwC from two specific points of view. That is, to understand the influence of wavelength of radiation on (a) the hardening and softening mechanisms involved in the process and (b) the fatigue lives. In order to realize these objectives, the alloy Ti-2.5Cu was double aged through specific heat treatments and subjected to LPwC with fundamental and frequency doubled radiations and investigated for the resulting deformation mechanisms in the process and the fatigue behaviour. 2. Experimental 2.1 Material
The alloy Ti-2.5Cu was commercially purchased from Tianjin Haixing Steel Import and Export Co. Ltd., China. The material was in the form of cold rolled plates (5 mm thick). The composition of the alloy in wt% is 2-3% Cu, 0.08% C, 0.01% H, 0.2% Fe, 0.05% N, 0.2% O and Ti (balance). Samples of size 20 mm × 20 mm × 5 mm were obtained by EDM wire cutting. Solution heat treatment was done at 805 C for 1 h. Double aging was done at 400 C and then at 475 C. At both the temperatures the duration of aging was 8 h and the cooling was done in air. The initial microstructure of sample is shown in Fig. 1 (recorded with Carl Zeiss optical microscope). The precipitates of Ti2Cu are marked in the figure.
2.2 Laser peening without coating (LPwC)
For LPwC, a pulsed Nd:YAG laser was used operating at a wavelength of 1064 nm (fundamental) and 532 nm (second harmonics). For the fundamental radiation, a thin layer (< 1mm) of running water was used as confining layer, simultaneously removing the ablated material and for the second harmonics, peening was done by submerging the sample inside water, with the sample surface below the water surface by 10 mm. The peening scheme was same as the one used in our earlier studies [7, 8]. The peening scheme is also depicted in the top left side of Fig. 2 and the macrograph of a peened area is shown on top right side of the same figure. The same scheme was used for the fatigue samples, whose dimensions and peened area in dark are shown in the bottom portion of Fig. 2. For the square samples (peened area of 20 mm × 20 mm), peening was done on one side while for the fatigue samples peening was done on both sides. The square samples were used for measurements of hardness, residual stress, roughness and XRD. With a constant energy (350 mJ) and spot diameter (0.8 mm), a constant power density was obtained (6.97 GW cm-2). The variable parameter in the experiment was the overlap percentage of the pulses. Three different overlap rates were used, namely, 53, 63 and 73% (constant in peening direction and perpendicular direction). The varying overlap rates were obtained by varying the speed of the servo motor of a movable X-Y translation stage on which the sample was mounted. 2.3 Microhardness Microhardness of the samples was measured with MATUZAWA-MMT-X Vicker’s hardness tester. The load used was 500 g and the time of holding was 10 s. The hardness was measured
along the cross section of the sample, at various depths. At each depth, an average of three measurements was taken. 2.4 Residual stress Residual stress was measured using an X'pert Pro system (PANalytical, Netherlands). The results were analysed with the standard sin2 technique. The radiation used was Cu-Kα with primary aperture dimension of 2 mm × 2 mm. The peak used for the measurement was {213} of phase. The sample was tilted from -40 to +40, involving a total of 33 steps. The corresponding increment in the sin2 value was 0.0250. The average residual stress was calculated from this by the slope of the d versus sin2 plot with the error of 5 MPa. The diffraction elastic constants were obtained using the elastic modulus of 110 GPa and Poisson ratio of 0.33. Successive layers were removed by electropolishing in order to obtain the residual stress as a function of depth. Corrections to measured depths were performed using standard techniques elaborated by [13] and [14]. 2.5 Fatigue tests Fatigue tests were carried out on specimens (unpeened and peened) prepared according to ASTM standard [E 466-07] specified for high cycle fatigue testing. Tests were done with Instron 8801, in the constant load configuration with the R ratio of -1. The load level used for testing was 12.5 kN. The frequency used for cyclic loading was 10 Hz. All experiments were performed in air. 3. Results and discussion 3.1 Microhardness The variation of microhardness as a function of depth is shown for all percentages of overlap in Fig. 3 (a), peened at the wavelength of 532 nm, along with the unpeened sample as the
reference. The hardness is observed to increase from a lower value and then reach a constant level at a depth of approximately 500 m in all cases of overlap. This is indicative of a softening effect near the surface caused by the thermal interaction of laser beam with the material. Therefore, there is more of a thermal mechanism restricted only at the surface which disappears due to rapid dissipation of heat as a result of short duration of laser pulses (in nanoseconds). Deeper inside the material, mechanical effects (as increase in hardness) due to the shock waves produced and propagated at high strain rates are observed. Even though the mechanical effects are present at the surface (originating from plasma pressure), since the thermal effects are also present at the surface (and predominant at the surface), the overall hardness profile is a thermo-mechanical effect. Although not uncommon, such effects have been observed, for example by Salimianrizi et al. [15] in a 6061 aluminium alloy. A similar overall trend of surface thermal softening was observed for the samples peened at the wavelength of 1064 nm (Fig. 3(b)), although within the depth of 200 m from the surface, as the fundamental radiation is more absorbed in water than the second harmonics. To ascertain thermal softening, the average roughness of the peened samples are compared with the unpeened sample and shown in Fig. 4. Clearly the peened samples show increased roughness (from 0.06 m to 0.20 m for the 532 nm case and up to 2.723 m for the 1064 nm case), indicating surface damage due to thermal interaction between laser beam and the material surface, thereby decreasing the dislocation density and hardness. These results indicate a predominant amount of energy is dissipated as heat at the surface in the case of 1064 nm (thereby increasing the surface roughness) while it is delivered more into the material in the 532 nm case. Any surface inhomogeneity resulting from laser beam overlap rate should be of the order of the beam diameter. Since the beam diameter is 0.8 mm in the present study and the roughness
is few m, the inhomogeneity cannot be because of overlap rates. Further, the plastic strains are microscopically accommodated in the compression of lattice planes (as reflected in residual stress measurements) and there was no appreciable macroscopic strains observed after peening. Moreover, as the plasma pressure is same irrespective of the wavelength, if no thermal effects are present as a function of wavelength, then same levels of strains (reflected as surface roughness) should have been observed at both wavelengths. However, clearly it was not the case. Therefore, the inhomogeneity is due to the non-uniform ablation of the material, which in turn is a function of laser-material interaction. This explains why roughness scales with overlap rates, as with increase in overlap rates laser-material interaction time increases, in turn increasing ablation. Thermal effects accompanied by rapid cooling can also cause formation of soft martensite, as reported by Carpeno et al.[16] in nanoblades of Ti-6Al-4V peened with UV laser. The XRD patterns of the samples at all peened samples are shown in Fig. 5 with unpeened sample as the reference. The phase identified as P in Fig. 5 is the precipitate Ti2Cu (JCPDS No. 150717). However, this phase leads to precipitation hardening of the material, unlike the soft martensite, which is absent in the present study. It should be noted that the precipitation hardening is due to initial heat treatment and not because of the LPwC process, as aging was done for 16 hours (Section 2.1) before LPwC while LPwC lasted for few minutes. Therefore, in this time frame, there cannot be further significant growth of precipitates. That is to say, hardness profiles are not affected by precipitates, as no new precipitates are formed during LPwC. It can be seen that the maximum increase in microhardness for the lowest overlap rate is about 20% (above a depth of 500 m for 532 nm case and 200 m for 1064 nm case) from the reference level, that is, from 215 to 260 HV (Fig. 3 (a) and (b)). This may in itself not be
very significant. On the other hand, a flat hardness profile (within the error bar) observed in all overlap percentages beyond a particular depth could indicate a large depth of workhardening, something of the order of few mm as evident from Fig. 3. Maawad et al. [11] also investigated LPwC of Ti-2.5Cu at a power density of 5 GW cm-2 using a beam diameter of 0.8 mm. They also reported insignificant increase in hardness (only about 10%) with a far lesser workhardened depth of 500 m. The difference can be attributed to the larger power density in the present study (about 7 GW cm-2). Flat hardness profiles are normally observed when harder beta phase is present, as in Ti-6Al4V [17]. Although addition of copper to titanium could be expected to increase the ductility, comparison with the trend of Ti-6Al-4V [17] suggests that the precipitates of Ti2Cu (Fig. 1) would have contributed to the precipitation hardening of the material. In still softer materials such as aluminium, a clear decreasing trend has been reported, for example by Sathyajith et al. [18] in a 6061 alloy, although the increase in the surface hardness is not better than the saturation hardness in the present study, with lesser workhardened depth. In an earlier investigation on 17-4 PH steel [7], we have observed 15% increase in surface hardeness with a workhardened depth of about 600 m. The preceding discussion indicates that irrespective of the material, a lower increase in surface hardness is advantageous without a large increase in dislocation density, as hardness and dislocation density are correlated. The mentioned advantage is over the conventional process of shot peening [19], especially considering the fact that the introduced dislocations are unstable at higher temperatures [12]. Most interesting feature depicted in Fig. 3 (a) is the decrease in the saturation hardness with the increase in overlap rates for the 532 nm case. Although an increase in the overlap rates would mean an increased strain in the high strain rate process of LPwC, a corresponding
decrease in hardness is indicative of a softening mechanism. This softening is however different from the surface softening indicated above in the sense that the cause is mechanical rather than thermal. In contrast, in Fig. 3(b), it can be observed there is a progressive hardening in the 1064 nm, with increase in overlap rates. This indicates, the softening is a function of wavelength. As it was pointed out earlier that less heat is dissipated at the surface in the 532 nm case, it appears that more energy is delivered to the bulk of the material, resulting in increased strain and strain rates, with the eventual result of earlier softening. The mechanism will be taken up in detail in Section 3.3, comparing with the residual stress results. 3.2 Residual stress The residual stress depth profile is shown in Fig. 6 (a) for the overlap rates of 53, 63 and 73% (at 532 nm) along with the profile for the unpeened material. The unpeened material shows a relatively flat profile at low compressive stress level (-180 MPa on the average) and the maxima observed at a depth of 50 m for the unpeened sample is not significant from the baseline value, in comparison with the peened samples. Similar to the results of hardness, here also thermal softening is observed up to a depth of 50 m. After this trend, there is a continuous increase in the compressive stress till 1000 m. The apparent inconsistency in the depth to which thermal softening extends can be explained based on the difference in the nature of hardness and residual stress measurements. The former is macroscopic and nonuniform due the possibility of precipitates being measured while the latter is based on the lattice strain and more sensitive. Besides, the underlying mechanism of hardness and residual stress are not correlated to the dislocation density and lattice spacing in the same way. That is to say, measured hardness is generally due to the effects of dislocation density and residual stress increments. Therefore it is not surprising to observe this contrast. However, what is emphasized here is the agreement between the thermal softening at and near surface in two
independent measurements. This conclusion is further strengthened by noting that a lower power density may not produce such a softening effect due to the reduced thermal interaction between the laser beam and the material. That this is the case is confirmed by the absence of thermal softening in earlier studies by Maawad et al. [11, 12] where a lower power density of 5 GW cm-2 was used. Similar to the hardness results, a reversal in the softening trend is observed in the 1064 nm case. That is, as shown in Fig. 6 (b), there is no progressive softening with increase in overlap rates, indicating that the softening mechanism is wavelength dependent. Further, the surface stress levels are large at 1064 nm compared to 532 nm. This indicates again that the heat dissipation at the surface is relatively more at 1064 nm, leading to the relaxation of stresses at the surface. This further strengthens the conclusion that the thermal softening is the mechanism responsible for residual stress maxima in peened samples. The measurements were terminated at 1000 m because most of the beneficial effects due to the compressive stresses are well within this range. For example, in enhancing the fatigue lives of materials, compressive stresses play a major role only at the surface by introducing a closure force on the cracks, thereby arresting them from further propagating [17, 12, 20-25]. Similar arguments, though of chemical nature, can be extended to reduction of corrosion rates in materials [6, 26-28]. Nevertheless it would be instructive to check the lack of stress reversal even till 1000 m, although it demonstrates a favourable workhardening at least till a depth of 1000 m, which is far deeper than the earlier studies [11, 12], due to the larger power density used in the present study. To check this, residual stress measurements were carried out from the other side of another specimen with 73% overlap, at 532 nm. This resulted in datum points (Fig. 7) from a depth of 4000 to 5000 m, as the thickness of the sample was 5 mm. The stress did reverse the sign and crossed the zero stress level and
entered into tensile stress range, with 496 MPa at the surface. Such large tensile stresses can be anticipated to compensate for the large maximum compressive residual stresses in the interior. Even though this can be anticipated to be favourable for crack propagation at the other end, it should be noted that the stress levels will fall rapidly in thicker samples, as shown by Ding [29]. Further, it can be circumvented by peening the other side, as in some earlier studies [12, 30] although it may not be a viable solution in practice. Similar trend was also observed at 1064 nm. The results of Maawad et al. [11,12] are compared with the present study in Fig. 8. Three striking contrasts can be observed upon comparison.
1. There is no surface thermal softening as in the present case. 2. The maximum compressive stress is lower than the present case (-650 MPa [11, 12] and 746 MPa (present study)). 3. The workhardened depth is much lower compared to the present study (300 m [11, 12] and minimum of 1000 m (present study)).
The reason for the first contrast was already pointed out while the reason for the second contrast could be due to the peening on both sides of the sample by Maawad et al. [12], where the possible large compressive residual stress on the first peened surface would have got reduced due to the subsequent peening of the other side, to balance the stresses. However, the observed contrast is too large to account for by this balancing mechanism and hence it can be concluded that the lower power density used by Maawad et al. [12] overrides the balancing effect. Lower power density can again be attributed to explain the third contrast.
An important feature, as observed in the case of hardness data is also evident from the results of residual stress. With the increase in overlap rates, there is a mechanical softening throughout the measured depth, which is not the same as the surface softening. Further, this depends on the wavelength of the radiation used. This necessitates the discussion of possible softening mechanisms in the following section. 3.3 Deformation mechanisms
Softening in the bulk in titanium alloys could be caused by adiabatic heating. Whereas phase transformation discussed earlier involves thermal cause with mechanical effect at the surface, adiabatic heating involves mechanical cause with thermal effects both at the surface as well as in the bulk, as the cause of mechanical effect is the plasma pressure at the surface of the sample. However, as the temperature due to laser-material interaction is very high at the surface, it dominates the mechanical effect at the surface. It dissipates quickly due to very short and localized interactions at the surface, leading to surface softening. Hence, in the bulk, adiabatic heating dominates. Overall, the entire hardness or residual stress profile is a result of thermal and mechanical effects. In a high strain rate irreversible plastic deformation process such as LPwC, the lattice friction encountered by the dislocations is released as heat, thereby increasing the temperature in the bulk, in turn decreasing the dislocation density, accounting for the softening in the bulk. As expected, this should increase with increase in strain and strain rates (see Appendix). Obviously, increasing overlapping rates would imply increasing strains and hence increasing softening. Adiabatic heating of this sort was reported earlier by Shen et al. [31] and Meyers et al. [32]. However, the lack of adiabatic shear bands show the increase in the temperature is not high enough (without flow localization) to cause them. This indeed was observed in the
earlier studies [31, 32] where adiabatic heating was reported without flow localization at earlier stages of deformation. It is also proposed here that copper in the matrix, with its higher thermal conductivity would assist in propagating heat efficiently throughout the bulk. The foregoing discussion raises the most important question as to why there is such an early onset of softening at 532 nm. Considering carefully the very early onset of softening in the investigation of Carpeno et al. [16] with UV radiation and the early onset of softening in the 532 nm compared to 1064 nm in the present study, there seems to be a dependence of wavelength on the onset of softening process. Lower the wavelength, earlier the onset of softening. This above conclusion might be true considering the fact that LPwC is done with green light in water submerged samples, as the absorption of green light (532 nm) in water is not significant, compared to infrared radiation (1064 nm). Therefore, the heat energy dissipated at the surface at 1064 nm is larger compared to the case at 532 nm. This leads to the decrease in the levels of compressive stresses in the infrared regime (Fig. 6 (b)). The opposite is the case at 532 nm (Fig. 6 (a)). Consequently, higher transfer of energy (the undissipated part) can be expected to occur into the bulk of the material at 532 nm, leading to larger activation of dislocations and hence larger dissipation of heat due to adiabatic heating, eventually leading to early softening compared to the case where absorption in water is more (at 1064 nm). This clearly demonstrates the fact that the onset of softening is a function of wavelength. Further, with the increase in overlap rates, the laser beam-material interaction time increases, increasing the transfer of energy to the bulk and the resulting softening behaviour, provided the adiabatic heating is significant. The argument holds good even for increase in power density, following the results of Maawad et al. [11, 12].
The peened microstructures are compared for both the radiations in Fig. 9. It is evident that twinning is observed for the 73% overlap case (Fig. 9 (f)) for the infrared radiation while it is absent for green. Twinning is very common in titanium alloys at high strain rates (as in LPwC) or in high temperatures deformation processes [33-38]. It was observed in such independent measurements by various researchers. For example, Carpeno et al. [16] observed compression twins in Ti-6Al-4V. Chichili et al. [39] and Meyers et al. [32] reported increased twin density in -titanium. Therefore, it is reasonable to conclude that the hardening process is not overtaken by softening as long as the wavelength of the radiation was at the infrared level. On the other hand, with the decrease in wavelength, softening sets in much earlier, strengthening the earlier drawn conclusion that the onset of softening is a wavelength dependent phenomena. Inspection of Fig. 9 also shows that the volume fraction of precipitates is not different from the unpeened sample in Fig. 1. Hence, the effect of precipitates can be ruled out in the peening process. Further, the surface roughness is highest in the infrared range (Fig. 9 (f)), as discussed earlier. 3.4 Fatigue behaviour
The number of cycles to failure for the unpeened sample and the samples peened at wavelengths of 532 nm and 1064 nm at the power density of nearly 7 GW cm-2 and the overlap rate of 73% is shown in Fig. 10, for the fatigue tests done at a load level of 12.5 kN with the R ratio of -1 at room temperature. The fatigue life has extended by a factor of 1.4 and 2.3 for the samples peened at 532 nm and 1064 nm respectively. Clearly, there is an improvement in the fatigue life due to peening. However, the improvement depends on the wavelength of the radiation. It was pointed out by Sano et al. [40] that in the technique of
LPwC, the sample is submerged inside water and correspondingly the radiation of 532 nm which is not absorbed by water is used. It was discussed earlier that this leads to earlier onset of softening. Together with this, the improvement in fatigue life is not very appreciable compared to the peening radiation of 1064 nm. The fractographs of the unpeened samples and samples peened at 532 nm and 1064 nm are shown in Fig. 11 respectively. The sites where cracks are detected are shown at larger magnification for unpeened samples and samples peened at 532 nm and 1064 nm in Fig. 11. The peened surfaces are also marked with red coloured arrows in the figure. In the case of unpeened samples, the cracks originate at the surface and they have grown larger in size. For the peened samples, the cracks originate at the sub-surface. For the sample peened at 532 nm, the cracks were observed at a depth of ~1.2 mm while for the sample peened at 1064 nm they were observed at ~2.1 mm. Sub-surface cracks are less detrimental compared to surface cracks, as they were also observed in earlier studies [12, 20] and it is reflected in the corresponding fatigue lives. Evidently, samples peened at 1064 nm have deeper sub-surface cracks of shorter grown lengths. This is consistent with the previously drawn conclusions based on earlier onset of softening in the case of 532 nm radiation. Hence it is clear that 1064 nm radiation is preferable from the point of view of improving fatigue life, even though it may be more absorbed in water in the LPwC process. 4. Conclusions
Laser peening without coating (LPwC) was performed on a Ti-2.5Cu alloy at wavelengths of 1064 and 532 nm, at a constant power density of 6.97 GW cm-2 and at overlap rates of 53, 63 and 73%. The following conclusions were drawn.
1. From the hardness measurements, surface softening due to thermal interaction of laser
beam with material was observed till a depth of 500 m (at 532 nm) and 200 m (at 1064 nm). 2. Thermal softening was also supported by residual stress analysis. 3. Softening due to mechanical effects (adiabatic heating) was observed in the bulk at 532
nm. 4. On comparison with peened samples at 1064 nm, mechanical softening was found to be a
function of wavelength of radiation used for peening, that is, onset of softening was earlier if the wavelength was shorter. 5. Twinning was found for the 1064 nm while it was absent for the 532 nm (for 73%
overlap). 6. The workhardened depth was more than 1000 m, not observed in earlier studies. 7. Fatigue life extended by a factor of 1.4 and 2.3 for the samples peened at 532 nm and
1064 nm respectively (73% overlap), consistent with the observed wavelength dependent onset of softening.
Acknowledgement
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. Appendix If the work done to move dislocations at a stress of to cause an incremental strain of d, then from simple heat balance we have, (1) Here, is the fraction of work converted to heat (typically 63% [41]), is the density of the material, Cp is the specific heat at constant pressure (as in the peening process) and dT is the increase in the temperature in the adiabatic process. The softening in the material can be expressed as,
(2)
We may write the incremental stress as, ( ) Here is the decrease in the flow stress with temperature. Combining the above equations, we have,
(3)
(4)
In terms of the rates, we can write, ̇ ̇
(5)
From equations (1) and (5) it is clear that both increase in temperature and softening increase with increase in strain and strain rate.
References
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Figure captions Figure 1: Optical micrograph of Ti-2.5 Cu alloy solution heat treated (805 C, 1 h), air cooled and double aged (400 C, 8 h and 475 C, 8 h), air cooled.
Figure 2: (Top left) Peening scheme for square and fatigue samples; (Top right) macrograph of a peened sample and (Bottom) Peened fatigue sample, with the dark shaded area indicating peened area. Peening was on single side for square sample and on double sides for fatigue samples.
Figure 3: Variation of microhardness along the cross section of the sample (a) peened at 532 nm and (b) peened at 1064 nm.
Figure 4: Surface roughness before and after peening with different overlap rates.
Figure 5: XRD patterns of the peened samples for different overlap rates, along with that of unpeened sample.
Figure 6: Comparison of the residual stress data along the cross section of the samples peened at (a) 532 nm and (b) 1064 nm radiations.
Figure 7: Variation of the residual stress along the entire cross section of the sample for the samples peened at an overlap percentage of 73%.
Figure 8: Comparison of the residual stress data from the present study with earlier study [11].
Figure 9: Cross-sectional optical microstructures of samples peened at (a) and (b) 53% overlap, (c) and (d) 63% overlap and (e) and (f) 73% overlap. All micrographs on left are peened at 532 nm and on right are peened at 1064 nm.
Figure 10: Comparison of number of cycles to failure for samples peened at 532 and 1064 nm radiation. The load level used for testing was 12.5 kN with R=-1 and at room temperature..
Figure 11: Fractographs of (a) unpeened, (b) higher magnification image of crack area highlighted in (a), (c) sample peened at 532 nm, (d) higher magnification image of crack area highlighted in (c), (e) sample peened at 1064 nm and (f) higher magnification image crack area highlighted in (e). The overlap for peened samples was 73%.
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