Effect of Multiple Laser Shock Peening on the Mechanical Properties of ETP Copper

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Effect of Multiple Laser Shock Peening on the Mechanical Properties of ETP Copper1 Ayush Bhattacharya1, Siddharth Madan1, Chirag Dashora1, S. Prabhakaran2, V.K. Manupati1,a, S. Kalainathan2, K.P.K. Chakravarthi3 1 – School of Mechanical Engineering, VIT University, Vellore, Tamil Nadu, India 2 – Centre for Crystal Growth, VIT University, Vellore, Tamil Nadu, India 3 – Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India a – manupativijay@gmail.com

DOI 10.2412/mmse.77.9.503 provided by Seo4U.link

Keywords: laser shock peening, ETP copper, mechanical properties, ultimate tensile strength, elongation.

ABSTRACT. The study conducted proposes the effect of multiple laser shock peening (LSP) on ETP copper alloy plates, with a low energy laser to investigate the changes in the mechanical properties of the material. In this paper, a study of the microstructure, micro hardness and surface morphology has been conducted on the parent sample, and the laser peened sample. The aim of this current research work is to investigate the changes in the mechanical properties of ETP Copper after LSP and to study the changes in the microstructure, micro hardness, tensile strength, elongation and surface morphology of the laser peened sample, and comparing it with the parent sample.

Introduction. Electrolytic Tough Pitch (ETP) copper has long been the standard type of commercial wrought copper used in the production of sheet, plate, bar, strip, and wire. It is widely used in the automotive and aerospace sectors, due to its engineering material, owing to its excellent corrosion resistance, good ductility as well as high thermal and electrical conductivity. The motivation of this research work drawn from the fact that this material has its potential to be used for the fabrication of heat sinks in International Thermonuclear Experimental Reactor (ITER) applications. Although ETP copper is not 100% oxygen free, it is considered OFC (oxygen free copper) because it has a minimum conductivity rating of 100% IACS (International Annealed Copper Standard) while the minimum rating to consider OFC is 99.9% pure.

Laser shock Peening (LSP) is a surface enhancement process used to enhance the properties of a material surface. The basic mechanism involved in this process is to create compressive residual stresses to a certain depth in the material which helps delay the premature failures. These deep, high magnitude compressive residual stresses modify the surface hardness and microstructure, therefore, improving different properties of the material such as strength and wear, corrosion and fatigue resistances. The principle involved in laser peening involves a laser pulse with a duration of several nano-seconds is focused on a material surface. The material surface evaporates instantly by ablative interaction. The evaporating material is limited by water, and the resulting high-density metallic vapor is ionized to form a plasma by inverse bremsstrahlung. The absorbed laser energy, in the plasma, generates a heat-sustained shock wave and impinges on the material with an intensity of several gigapascals, far higher than the yield strength of most materials. The shock wave propagates through the material losing energy thus creating a permanent strain. Once after propagation of the shock wave, the strained region is elastically constrained to form compressive residual stresses on the surface.

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© 2017 The Authors. Published by Magnolithe GmbH. This is an open access article under the CC BY-NC-ND license http://creativecommons.org/licenses/by-nc-nd/4.0/

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

However, conventionally, LSP works with protective coatings applied to the metal surface, such as black paint or Al foils or vinyl tape. These coatings protect the surface from the thermal effects causing surface damage. The laser peening without protective coating (LPwC) has been proposed [3] for the advantages: (a) It is applicable for direct treatment of nuclear plant components during maintenance with low laser pulse energies (< 1 Joule) that can be sent through optical ďŹ bers; (b) Surface chemistry of the treated material is not altered.

The study [2] effects of single shot Laser Shock Peening on ETP Copper in both room temperature (RT-LSP) as well as cryogenic conditions (CLSP). They concluded that nanotwins were observed in copper after CLSP and not RT-LSP. In addition to this, they found that more energy was stored in the material as defects (dislocations), unique microstructure changes and higher material strength when LSP was done in cryogenic conditions.

In [5] stated that LSP improved the micro-hardness, surface roughness whereas [4] and [6] had stated that the compressive forces produced by the LSP process resulted in a decrease of the Fatigue Crack Growth Rate as well as Fatigue Crack Initiation Life, thereby highly increasing the fatigue life of Ti alloys.

The aim of this current research work is to investigate the changes in the mechanical properties of ETP Copper after LSP, and to study the changes in the microstructure, micro hardness, tensile strength and surface morphology of the laser peened sample, and comparing it with the parent sample.

Experiments and Characteristics Specimen Preparation. Specimens of dimensions 15 X 15 X 5 mm3 (length X breadth X thickness) were prepared by electric discharge machine (EDM) wire cutting. Mechanical Polishing was carried out with SiC abrasive sheets with grit sizes of 800, 1000, 1200, 1500, 2000. The specimen was then mirror polished in a disc polisher, with alumina powder and then rinsed in acetone. The polished sample was then etched for 15 secs, with an etching solution with the following concentrations: FeCl3 (2.5 gm) + HCl (25 mL) (38% cons) + water (50 mL). An optical microscope (ZEISS, Germany) was used to study the microstructures, before and after the LPwC Operation. Micro-hardness as a function of depth was measured using a Vickers Hardness Tester (Mitutoyo, Japan) with an indent load of 50gf and an indent time of 15 secs. Surface roughness was measured with a stylus profilometer (Marsurf, Germany) operated with roughness filter cut-off of 0.8 mm over a range of 5.00 mm. The tensile test was carried out using the universal material testing servo-hydraulic machine (INSTRON 8801).

LPwC Process

Fig. 1. Schematic representation of LPwC processing setup.

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Table 1. Experimental parameters for conducting laser peening process.

Parameter

Magnitude

Laser Wavelength

1064 nm

Pulse Repetition Rate

10 Hz

Pulse Duration

10 ns

Laser Energy

300 mJ

Number of Shots

5

Spot Diameter

0.8 mm

Pulse Power Density

6 GW/cm^2

Shock Wave Pressure

8.41 GPa

Pulse Density

1600 Pulses/cm^2

A Nd:YAG laser (Litron, UK) with operating fundamental wavelength of 1064 nm was used for the LPwC process. The experiment was carried out in ambient conditions (25 degrees C), one can find the detailed process as shown in Fig. 1.

The energy utilized was 300 mJ with a pulse duration of 10 ns. Multiple shocks were given with a repetition rate of 10 Hz. In general LPwC process, the target metal surface is mirror polished, and a confinement medium like water or a transparent glass is used. No protecting coating was pre-owned for the material being used. Hence direct laser-matter interaction was made to take place on the decarburized surface that could behave as an ablation medium as well as an opaque medium (colour of decarburized surface was black). The decarburized layer was partially removed by grinding it until getting a thin film (100 â&#x20AC;&#x201C; 150 micro m) [7], [8]. This was done to avoid effects of impedance mismatch for the full ablation and to form a full plasma for LPwC operation. A smooth and uniform surface was prepared for the multiple shock LPwC operation. Water was used as the confinement layer, with a thickness of 1-2 mm. For maintaining uniformity in the application of water, a water jet setup was used. The LSP parameters, which have been shown in Table 1, tuned to get a match of shock impedance between the water and the decarburized copper surface to attain peak pressure. A 2D XY translation motorized stage (SVP Lasers, India) was employed to perform LPwC experiments in transverse and longitudinal directions. The overlapping rate (75%) was fixed at 90% in the longitudinal direction and at 40% in the transverse direction.

For tensile testing, transverse specimens were prepared according to ASTM: E8/E8M Standard test methods and tested for a nominal strain rate of 0.5 mm/min under ambient conditions. Double sided LSP was performed for this study, and these are shown in Fig. 2 (b), (c), and (d).

Results and Discussion Fig. 3 shows the microstructures of the sample that is without laser shock processing and with laser shock peening (Fig. 3 (a-b) as observed by optical microscopy images. The grains were almost equated having micron-sized precipitates distributed throughout the matrix apart from the presence of annealing twins in Fig. 3 (a), and almost circular inclusions in Fig. 3 (b). A very few large grains are present in both the samples, with no signs of abnormal grain growth observed.

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a)

`

b)

c)

d) Fig. 2. Laser peened specimen (a), (b) tensile test specimen dimensions, (c) unpeened tensile test specimen, (d) LPWC tensile test specimen.

a)

b)

Fig. 3. (a) Without laser peened microstructure at 20X, (b) laser shock peened microstructure at 20X. The micro-hardness as a function of depth was measured performing the Vickers Hardness Test. The average hardness of the untreated specimen was 76.5 HV. Average Hardness of LPwC specimen was found to be increased at 104 HV. The results indicated an approximately 36% increase in hardness after LPwC process. The hardness at 50 micro meters was observed to be highest at 141.2 HV and then gradually started decreasing with increasing depth Fig. 4 (a), (b) till it showed similar hardness values as the untreated sample at 700 micro meter depth. This effect can be explained due to the

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reduction in intensity of the shock wave, during its propagation into the material and the strain hardening effect of the LPwC process.

The tensile results showed an increase of 3% in UTS values accompanied with decrease of 6% in elongation values. This can be attributed due to higher material strength and increased brittleness character in the material due to the coherent precipitates, which are dispersed at the sub-grain boundaries and inside the grains.

Table 2. Tensile test results.

Tensile Sample

UTS (GPa)

Elongation (mm)

Unpeened

0.236

41.340

LPwC

0.243

38.825

Fig. 4. (a) Variation of hardness as a function of depth, (b) tensile stress-strain curves for unpeened and LSP samples.

Summary. In this study, the effect of LPwC was studied on ETP copper specimens, with a laser wavelength o 1064nm, laser energy of 300mJ and a power density of 6 GW cm⁻². Some of the observations after the analysis were, microstructure was observed both before and after LPwC on ETP Copper specimens. Annealing twins were found to occur in the LPwC microstructure, due to lowering of stacking fault energy (SFE). Higher values of hardness, surface roughness and UTS were recorded with decrease in ductility of the material, after LPwC Operation. The increase in hardness and UTS is caused by coherent precipitates or the second phase particles, which are dispersed at the sub-grain boundaries and inside the grains. The precipitates obstruct the movement of dislocations giving rise to higher UTS and strength in the material.

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[3] N. Mukai, N. Aoki, M. Obata, A. Ito, Y. Sano and C. Konagai: Proc. 3rd JSME/ASME Int. Conf. on Nuclear Engineering (ICONE-3), Kyoto, 1995 p.III-1489.

[4] Sokol D.W., Clauer A.H., Dulaney J.L., Lahrman D. W. Applications of laser peening to titanium alloys. In Photonic Applications Systems Technologies Conference (p. PTuB4). Optical Society of America. 2005.

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[5] Qiao H., Zhao J., Gao Y. Experimental investigation of laser peening on TiAl alloy microstructure and properties. Chinese Journal of Aeronautics, 28(2), 2015, 609-616.

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[7] S. Prabhakaran, S. Kalainathan. Compound technology of manufacturing and multiple laser peening on microstructure and fatigue life of dual-phase spring steel. Materials Science & Engineering A, 2016, 634–645 [8] S. Prabhakaran, S. Kalainathan. Warm laser shock peening without coating induced phase transformations and pinning effect on fatigue life of low-alloy steel. Materials and Design 107, 2016, 98–107.

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