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Abstract
We report on laser shock peening (LSP) of tungsten, a material used as a divertor in Tokamak machine for magnetic confinement fusion reactions such as the ITER facility (France) and JET facility (UK). Peak compressive stresses of -370 MPa and depths of up to 1.75 mm were recorded when 0.25 cm2 area of tungsten (99.95% pure) was irradiated by a 1030 nm Yb:YAG laser operating at 10 J, 10 ns. Furthermore, we demonstrate enhancement of compressive stresses in one direction, by application of circular polarised light in hard material like tungsten. However, no enhancement of compressive stresses with circular polarisation was observed in soft material like aluminium.
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Corrections
29 November 2022: A typographical correction was made to the author affiliations.
1. Introduction
Laser shock peening (LSP) of materials has become an indispensable tool to the industry for improving the fatigue life of metals and thus the overall operational life of a system. This advanced surface modification technique has benefited critical components for the nuclear industry [1], the automobile industry [2] as well as the aviation industry [3] and have found numerous novel applications [4]. Laser shock peening utilise high energy, short pulses (typically ns pulses) of light generated by laser, which when focused on to a workpiece, initiates the formation of plasma, which expands and is confined by a confinement layer (typically water). The resultant of this expansion and subsequent confinement of the plasma is a plasma- induced pressure impulse. The pressure impulse travelling through the workpiece introduces compressive residual stresses and plastic deformation which in turn improves the stress corrosion cracking (SCC) resistance as well as the fatigue performance of the workpiece. Advantages of LSP includes introduction of compressive stresses to depths typically exceeding 1.5 mm and an unaltered surface roughness after peening (which is not the case for shot peening, where small metal or ceramic balls are bombarded on the workpiece).
Pure tungsten and its alloys have numerus applications ranging from armaments (military), aviation, sports (NASCAR and Formula One) to nuclear fusion. These applications exploit the high strength, density and melting point of tungsten as a material. Magnetic confinement fusion (MCF) machine such as ITER facility, France and JET facility, UK utilise Tokamak design to confine a hot plasma in the shape of a torus for sufficient duration of time to initiate and sustain controlled thermonuclear fusion reaction. This controlled thermonuclear fusion reaction produces heat which can be converted to electricity and generate power. The temperatures within a Tokamak inner vessel reaches millions of degrees, thus at the bottom of the vessel a divertor is used to extract heat and ash produced by the reaction, minimise plasma contamination and protect surrounding walls from thermal and neutronic loads. Tungsten, withthe highest melting point of all metals is used as the suitable material for divertor to withstand extreme temperatures. At present, no data is available for peening of tungsten owing to it being a difficult material to work with i.e. brittle at room temperature and susceptible to rapid oxidation at elevated temperature. Moreover, it will have a significant impact on the fusion reactor’s lifetime and operational cost if the fatigue life of tungsten can be improved. In this paper, we report introduction of compressive stresses in tungsten using an cryogenic gas cooled Yb:YAG diode pumped solid state laser capable of producing energies in excess of 10 J at 10 ns at its fundamental wavelength of 1030 nm. Furthermore, an enhancement of compressive stresses was observed for circularly polarised light compared to linearly polarised light for tungsten LSP. This dependency was not observed for soft metals like aluminum, thus introducing another laser parameter, “Polarisation”, (along with traditionally used parameters like energy, pulse-width, intensity and wavelength) to be considered for LSP of hard metals like tungsten.
2. Experimental setup
Laser shock peening experiments were carried out using the DiPOLE laser system at the Central Laser Facility (CLF). The DiPOLE laser is a scalable diode pumped, cryogenic gas cooled, multi-slab ceramic Yb:YAG laser system, capable of generating temporally shaped pulses (2 ns to 10 ns in duration) at 1030 nm wavelength in excess of 10 J at 10 Hz. Details of the DiPOLE laser can be found in the following Refs. [5,6]. Figure 1 shows the schematic diagram of the amplification stages for the DiPOLE laser. An ytterbium doped fiber oscillator and amplifier seeds a regenerative amplifier (PA1) to generate ∼1 mJ of IR output. This is further amplified in a room temperature Yb:YAG based multi-pass booster amplifier (PA2) to 80 mJ. Finally a multi-slab Yb:YAG ceramic is cryogenically gas cooled to amplify the PA2 output to 10 J, 10 Hz in a square 18mm by 18mm footprint in six passes. For all the experiments reported in this paper, the pulse shape was kept as a square flat-top pulse (shown in Fig. 2), and repetition rate was limited to 1 Hz.
Fig. 2. (a) Temporal profile of the output (black (3)) and subsystems i.e. PA1 (red (1)) and PA2 (green (2)).(b) Spatial profile at the output of DiPOLE amplifier of 18 mm by 18 mm, which was reduced to 5 mm by 5 mm for peening experiments.
The spatial profile of the laser beam was preserved by image-relaying and was transported to the peening work station through vacuum spatial filter (VSF) and set of 45° mirrors coated for high reflection (HR) at 1030 nm. Further, the beam was focused onto the workpiece using a 400 mm focal length lens to produce a 5 mm by 5 mm spot. The workpiece was angled at approximately 10° with respect to the input beam to reduce back reflections to the cryogenic gas cooled laser amplifier. A thin layer of water (∼4mm) was flowed over the surface of the workpiece to act as a confinement layer. To protect the focusing lens from water and debris ejecting from the workpiece during LSP process, an AR-coated fused silica window was placed in front. The whole area was enclosed by a 3mm thick aluminum sheet to stop any scattering or stray reflections produced by the LSP process. An automatic XY-stage with speed control held the workpiece and was syncronised to control the overlapping of subsequent laser shots. During the experiments described in this manuscript, two different types of overlapping was implemented; (a) Single shot: one shot at one site (L1) with no overlap to the next shot (b) multi shot: Four shots at one site with 100% overlap (L4). Figure 2 shows the temporal and spatial profile of the DiPOLE output used for the peening experiments.
Pure tungsten (99.95%, W00032) samples were supplied by Goodfellow Cambridge Ltd. with dimentions of 50 mm by 50 mm and thickness of 10 mm. They were manufactured by cold isostatic pressing of tungsten powder into sheet ingots, which were subjected to sintering (exhausting gas and impurities) and cold rolling. Finally the samples were cut and polished to a surface roughness of Ra ∼ 10µm. Aluminium (AW7075-T651) samples were supplied by Thames Stockholders Ltd. of same size and roughness. Samples were used as supplied and no further polishing was done before experiments. Figure 3 shows the picture of tungsten target after laser shock peening (LSP) at 9.5 J, 10 ns.
Fig. 3. Laser shock peening locations (Bottom and Top) at 9.5 J, 10 ns, with water for 1 (L1) and 4 (L4) shots for pure Tungsten.
3. Measurement and analysis of residual stresses
Residual stresses introduced by the laser shock peening (LSP) can be measured using various techniques like incremental central-hole drilling (ICHD), Neutron Diffraction, contour, slitting, X-ray diffraction, ultrasound and acoustic measurements. In this study, two different methods of measurements were employed, (a) ICHD and (b) Acoustic signal measurements.
The Incremental Central Hold Drilling (ICHD) technique [7] is an invasive, mechanical strain relief method which is performed by incremental machining of a shallow hole in a component to relief the residual stresses and measuring it simultaneously. The principle is that the removal of the stressed material results in the surrounding material readjusting its stress state to re-attain residual stress equilibrium. The measured surface strains allow for the back calculation of the previously existing residual stresses. The formulae and calculations derived for the back calculation process are developed from a combination of experimental and finite element analyses [8]. All ICHD measurements presented in this study, were performed in StressCraft Ltd. in accordance to ASTM837 standard [9]. Further, for this study the installation details, drilling cutters, strain gauge type and hole diameters all comply with ASTM E837-13 standard. Tungsten carbide inverted cone cutters were used for the orbital hole drilling (circular milling) at a speed of 15,000 rpm with a pre-set orbit eccentricity to give the required hole diameter. Figure 4 shows the installed gauge (EA-06-125RE-120) on a tungsten sample and the setup for incremental drilling.
Fig. 4. Incremental Central Hold Drilling (ICHD) on tungsten samples (A) Gauge attached to the tungsten sample (B) Hole drilling for residual stress measurements.
Acoustic signals generated during Laser Shock Peening (LSP) can be analysed to provide a qualitative evaluation of residual stress of the material [10]. The parameters produced by analysis can be benchmarked using measurements of residual stress by other techniques like ICHD. Figure 5 shows the layout and typical acoustic signals generated during LSP. Details of the analysis and signal processing is given in ref. [10].
Fig. 5. (a) Setup used for acoustic signal measurement and (b) Temporal evolution of the acoustic signal, (c) expanded view of the temporal signal showing convolution of multiple signals.
3.1 Acoustic data analysis and results for LSP of tungsten
Acoustic signals from LSP of tungsten were analysed for water as confinement medium. As mentioned in a previous publication [10], this technique is for qualitative measurement and only show indication or a trend. ICHD measurement generate quantitative data which can then be used as benchmarks for acoustic data. Details regarding fitting functions (lognormal distribution) and parameters (Xc,σ, ξ and δ) can be found in reference [10]. Figure 6(a) shows the change of parameters when peening laser pulse energy is changed from 6 J to 9 J, also shown is the change of parameters for one shot (L1) and four shots with 100% overlap (L4) for linear and circular polarisation. As seen from the Fig. 6, both Xc and σ reduce when the pulse energy is increase from 6 J to 9 J (for both linear and circular polarisation). Moreover, there is a further reduction of Xc and σ when number of overlapping laser shorts are increased from L1 to L4. This reduction of both Xc and σ indicate an increase of compressive stresses for tungsten with the increase of pulse energy as well as overlapping (L4). Exact values of compressive stresses will be discussed in section 3.2 where ICHD results are shown.
Fig. 6. (a) Acoustics signal analysis for tungsten LSP (water as confinement medium) at DiPOLE lab with different pulse energy and overlapping (L1 and L4) for linear and circular polarisation. (b) Acoustics signal analysis for tungsten LSP for 9 J pulse energy, 10 ns pulse width and linear and circular polarisation.
Figure 6(b) shows the acoustic signal analysis for linear and circular polarised light at 9 J pulse energy and 10 ns pulse width. As seen for the linear polarised light (red line) the parameter δ increases slightly when number of shots increase from L1 to L4. However, for the circularly polarised light (blue line) the δ-parameter shows a clear reduction in value from L1 to L4. This indicates that for linear polarised light the change in stress in negligible at the surface. However, in case of circularly polarised light the same energy and pulse width should show significant improvement of compressive stresses when number of pulses are increased from 1 to 4 (L1 to L4).
3.2 ICHD results for LSP of tungsten
In the first set of experiments, a total of 9 samples were sent to StressCraft for ICHD measurements, Table 1 list the samples and its laser parameters. Each specimen surface was first lightly abraded (400 grade SiC paper with light hand pressure). Target site surfaces were then degreased with acetone and one EA-06-125RE-120 target gauge was installed at each site; gauges were orientated with element 1 aligned with the longitudinal direction (with respect to the LSP zone) and element 3 with the transverse direction; Fig. 4(A) shows the gauge orientation. Gauges were drilled using a miniature, PC-controlled, orbital driller; depth increments were set at 4 × 64 µm + 4 × 128 µm + 8 × 256 µm, giving a completed hole-depth of 2816 µm for residual stress data to depth 2048 µm.
Table 1. List of samples and laser parameters measured by ICHD technique in experimental set 1.
3.2.1. Residual stresses of an un-peened tungsten sample
The residual stresses of an un-peened tungsten sample is used as the reference to the level of stresses developed during laser shock peening process. Figure 7 shows the transverse (σ1) and longitudinal (σ3) residual stresses for the BID-W00032-1 sample. Owing to the manufacturing process the samples had an initial peak compressive stresses of −75MPa at 500µm depth for both σ1 and σ3 direction.
3.2.2. ICHD results for LSP of tungsten with water as confinement medium
In all of LSP experiments detailed in the manuscript, water was used as a confinement medium. In the first set of experiments, laser parameters like pulse energy (9 J and 6 J), pulse duration (10 ns and 2 ns), polarisation (linear and circular) and number of layers/ pulse overlap (100%) (L1 and L4) were varied during the experiments. Figure 8(a) and (b) shows the effect of change of pulse energy and polarisation for single shot (L1) at 10ns pulse width for longitudinal and transverse directions. As seen, 6 J pulse energy introduces tensile stresses which max at the surface of the sample. An increase of compressive stresses of up to −100MPa is noted for 9 J pulse energy and linear polarisation (0°). However, a significant increase of compressive stresses to −225 MPa is seen at the depth of 250 µm for 9 J circularly polarised light. To further understand the LSP process for tungsten, same energy and polarisation states were repeated with four 100% overlapping shots (L4). Figure 8(c) and (d) shows the residual stresses of the samples for four overlapping shots (L4). A maximum compressive stresses of −280 MPa was observed at a depth of 160 µm for transverse direction and circular polarisation. The overall depth of compressive stresses also increases to 1.75 mm for 9 J, circularly polarised light for L4. Note that the peak compressive stresses as well as the depth of plastic deformation for linear polarised light is same for both σ1 and σ3 directions. However, for circularly polarised light the peak compressive stress as well as depth of plastic deformation is significantly more for σ1 direction compared to σ3 direction. This experimental result shows that utilisation of circularly polarised light enhances the compressive stresses compared to linearly polarised light for same pulse energy and pulse width for both single (L1) as well as multiple shots (L4).
Fig. 8. Residual stresses developed during laser shock peening of tungsten (experiment set-1) for different pulse energy (9 J and 6 J), different polarisation (Linear and circular) and different shots (L1 and L4) (a) Residual stresses in longitudinal direction (σ1) for one shot (L1) (b) Residual stresses in transverse direction (σ3)for one shot (L1) (c) Residual stresses in longitudinal direction (σ1) for four shots at same location with 100% overlap (L4) (d) Residual stresses in transverse direction (σ3) for four shots at same location with 100% overlap (L4).
Further, for single shot experiments (L1) with circularly polarised light, although the peak compressive stresses (−225 MPa) is similar for both σ1 and σ3 directions, the depth of plastic deformation is higher in σ1 direction. For L4 the peak compressive stresses increases to −280 MPa for σ3 direction, along with the depth of plastic deformation increasing from 1 mm to about 1.75 mm. Figure 9 shows the effect of pulse duration for LSP of tungsten. A pulse duration of 2 ns was selected with an energy of 2 J, corresponding to a peak power density of 3.8 GW/cm2 (similar peak power to 9 J, 10 ns experiment). For single shot (L1), only tensile stresses are introduced in the tungsten sample for application of 2 J, 2 ns pulses. For multiple shots (L4), compressive stresses are introduced only on the surface with a maximum stresses of −43 MPa, the depth of plastic deformation also is only till 250 µm. This experiment emphasise the importance of using high energy pulse as oppose to high peak power density pulses for water as confinement medium.
To confirm the validity of the first set of experimental results (above) and to re-observe the enhancement of compressive stresses in one direction with the application of circularly polarised light, we repeated the experiments. Table 2 shows the parameters used for repeat experiments for LSP of tungsten. Note that LSP was performed for additional polarisation direction of 45° to compare it with circular polarisation.
Table 2. List of samples and laser parameters measured by ICHD technique in experimental set 2.
Figure 10 shows the results of the repeat experiments for LSP on tungsten. Figure 10(a) and (b) show the induced compressive stresses for application of different polarisation at 10 J and 10 ns for single shot (L1). Although, for single shot (L1), the ICHD results do not match the experimental set-1 results, however, for experiments set-2 an enhancement of compressive stresses is seen in the longitudinal direction (σ1) compared to transverse direction for circular polarisation (red curve). For multi shots (L4) shown in Fig. 10(c) and (d), the ICHD results for circular and linear polarisation match well with the experiment set-1, but the directions are flipped (i.e in experimental set-1 transverse direction (σ3) (Fig. 8(d)) match well with longitudinal direction (σ1) (Fig. 10(c)) for experimental set-2). It was found that for experimental set-2, the tungsten sample was 90° rotated when placed into the sample holder for LSP, compared to its position for experimental set-1. This explains the flipping of directions for the compressive stresses introduced due to L4 LSP of tungsten. Note that the peak compressive stress is enhanced in one direction for both the experiments on application of circular polarised light, compared to linearly polarised light. The 90° rotation of the sample (although serendipitous) for experimental set-2, equates to application of linear polarisation light at 90° for LSP of tungsten with the same orientation of the sample in experimental set-1 (although the polarisation direction was 0° (same as exp-set-1) in experimental set-2). Thus linear polarisation states (0°, 45° and 90°) were measured for LSP in tungsten and the induced compressive stresses were found to be less than what was induced by circular polarisation (in one direction).
Fig. 10. Residual stresses developed during laser shock peening of tungsten in experiments set 2 (repeat) for pulse energy (10J), different polarisation (Linear and circular) and different shots (L1 and L4) (a) Residual stresses in longitudinal direction (σ1) for one shot (L1) (b) Residual stresses in transverse direction (σ3) for one shot (L1) (c) Residual stresses in longitudinal direction (σ1) for four shots at same location with 100% overlap (L4) (d) Residual stresses in transverse direction (σ3) for four shots at same location with 100% overlap (L4).
3.2.3 ICHD results for LSP of aluminium with water as confinement medium
Aluminum is another widely used material in automobile and aerospace industry, and has many times been reported to be a suitable material for LSP [11]. Aluminum, which has about 7 times lower density than tungsten (19.3 g/cm3) and about 5 times lower melting point compared to tungsten (3410 °C), is a metal at the other side of the spectrum to tungsten and an interesting material for comparison in laser shock peening experiments. Further, the comparision will show if polarisation dependency for enhancement of compressive stresses is a material specific phenomenon. The tungsten samples were replaced by Aluminum samples in the same setup as mentioned above. Figure 11(a) and (b) shows the induced compressive stresses and its depth for LSP with 9J, 10ns pulses from the DiPOLE laser, for both single (L1) and four shots with 100% overlap (L4). As expected the peak compressive stresses as well as the depth increased for L4 experiments in both transverse (σ3) and longitudinal direction (σ1). However, note that for soft metals like aluminum, the magnitude of the induced peak compressive stresses for both transverse (σ3) as well as longitudinal direction (σ1) is the same for both L1 and L4 cases when linear or circular polarised light is used. This observation is in contrast to the compressive stresses induced by circular polarised light in tungsten, confirming that the compressive stresses introduced in LSP process is material specific along with the laser parameters.
Fig. 11. Residual stresses developed during laser shock peening of Aluminum 7075-T651 for application of linear and circular polarisation in (a) Longitudinal direction and (b) Transverse direction.
4. Surface characterisation of tungsten
To investigate the surface morphology of tungsten, before and after laser shock peening, scanning electron microscope (SEM) imaging was performed for peened and un-peened tungsten samples. Figure 12(a-f) shows the SEM images for un-peened, single shot (L1) and multi shots (L4) tungsten surfaces both at 10µm and 2µm resolution. Figure 12(a) and (d) shows the surface of an un-peened pure tungsten, It should be noted that although the surface roughness values (Ra) remains the same before and after LSP, the surface morphology changes significantly after LSP for both L1 and L4. This can be attributed to the instantaneous melting and re-solidification at 10ns pulse duration where thermal effects starts playing an important role in formation of surface modification. Further, for multi-shots (L4) case the melting and re- solidification is more pronounced due to the formation of wave like structures seen in Fig. 12(f). Micro-cracks appear on the sample surface on application of four 100% overlapping shots (L4) as seen in Fig. 12(f), this observation is similar to the cracks seen when tungsten was exposed to plasma accelerators under ITER-relevant conditions [12]. The formation of micro-cracks are attributed to the ductile-to-brittle transition effect during re-solidification process. This observation also suggests the use of single or double pulses is beneficial for introducing compressive stresses within tungsten as the probability of micro-crack formation is minimal. Furthermore, it will be interesting to subject the LSP tungsten samples (L1) to such plasma conditions (as reported in [12]) to compare the resilience to the formation of cracks with the introduction of compressive stresses on the sample surface.
Fig. 12. Scanning electron microscope image of un-peened, single shot (L1) and multi shots (L4) tungsten surfaces both at 10 µm and 2 µm resolution.
Figure 13, shows the Zygo NexView NX2 optical profiler image for a 3mm by 3mm area of an unpeened tungsten sample (Fig. 13(a)), also shown is the surface roughness measurement showing Ra of 11.95µm (Fig. 13(b)). Note that both the SEM (Fig. 12(a & d)) as well as the optical profiler (Fig. 13(a & b)) show a clear regular pattern of linear grooves on the surface of pure tungsten sample, which manifests due to the manufacturing process of pure tungsten (cold isostatic pressing, sintering and cold rolling). The enhancement of compressive stresses in one direction for tungsten LSP on application of circularly polarised light can be attributed to the fact that the surface texture of pure tungsten show regular pattern of linear grooves, which on application of circular polarised light, generates unequal pressures applied by the plasma shock wave in two orthogonal directions.
Fig. 13. Zygo NexView NX2 optical profiler image of 3 mm by 3 mm square of an unpeened tungsten sample. (a) is the intensity image and (b) is the roughness measurement with Ra of 11.95 µm.
5. Conclusion
In summary, laser shock peening to tungsten (an important material from the point of view of magnetic confinement fusion machines) is reported for the first time to the best of our knowledge. Peak compressive stresses of −370 MPa and depths of up to 1.75 mm were recorded when 0.25 cm2 area of tungsten (99.95% pure) was irradiated by a 1030 nm Yb:YAG laser operating at 10 J, 10 ns. Furthermore, we demonstrate and validate (via repeat experiments) enhancement of compressive stresses in one direction, by application of circular polarised light in hard material like tungsten. However, no enhancement in compressive stresses with circular polarisation was observed in soft material like aluminum. Furthermore, based on the observations of scanning electron microscope images, it seems beneficial to use single shot or maximum of two 100% overlapping pulses to introduce compressive stresses in tungsten owing to minimal formation of micro-cracks on the sample surface.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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