# Effect of Multiple Laser Shock Peening without Coating on Al-2024-O Alloy for Automotive Applications

Yash Jain, Sandeep Varin, S Prabhakaran, S Kalainathan

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Yash Jain, Sandeep Varin, S Prabhakaran, S Kalainathan. Effect of Multiple Laser Shock Peening without Coating on Al-2024-O Alloy for Automotive Applications. Mechanics, Materials Science & Engineering MMSE Journal. Open Access, 2017, 9, 10.2412/mmse.65.57.424. [https://hal.science/hal-01504761](https://hal.science/hal-01504761)

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# Effect of Multiple Laser Shock Peening without Coating on Al-2024-O Alloy for Automotive Applications

Yash Jain¹, Sandeep Varin¹, S. Prabhakaran², S. Kalainathan²

1. School of Mechanical Engineering, VIT University, Vellore – 632 014, India

2. Centre for Crystal Growth, VIT University, Vellore - 632 014, India

<figure><img src="image_2.png" /></figure>

DOI 10.2412/mmse.65.57.424 provided by [Seo4U.link](https://link.springer.com/10.2412/mmse.65.57.424)

**Keywords:** laser shock peening without coating (LSPwC), hardness, X-ray diffraction, roughness, residual stress, surface morphology.

**ABSTRACT.** The present work discusses the influence of surface compressive residual stress induced by multiple laser shock peening without coating process using low energy Nd: YAG laser with the fundamental wavelength. Laser shock peening without absorbent coating (LSPwC) was employed to Al-2024 in order to improve its surface microstructure and mechanical properties. The compressive residual stress measurements are accomplished according to X-ray diffraction $sin^2\Psi$ method. The surface morphology and roughness analysis were carried out using atomic force microscope and roughness profilometer respectively. Also, the microstructure analysis was performed using an optical microscope and scanning electron microscope. The Vickers micro-hardness revealed the improvement of surface and sub-surface hardness after multiple laser shock peening without coating process using low energy laser.

**Introduction.** Laser shock peening is a recently developed advanced surface modification technology which has been very effective in improving strength, wear, hardness, roughness and corrosion resistance of metallic materials. It is cold deformation process where pulses hit the surface through high power intensity and shock waves are generated. Laser shock peening (LSP) utilizes high energy laser pulses to impact the surface of the metallic material covered with a transparent confinement layer (usually water).

When a laser pulse of sufficient intensity is imparted on the material, the absorbent layer is vaporized. The vapor absorbs the remaining laser energy and is heated and ionized into a high pressurized plasma. The transparent confinement layer amplifies the plasma pressure which propagates into the metallic material as a shock wave. In turn, shock wave will lead to plastic deformation which will lead to compressive stresses being induced in the surface layers of the metallic material, provided the plasma pressure is of sufficient magnitude to exceed the elastic limit of the metal. Due to the shock waves thus generated, the dislocation density considerably increases and therefore the resistance offered for external load also increases. The increasing compressive stress value will significantly increase the mechanical properties of the surface layer such as hardness, strength, wear and roughness compared to the conventional shot peening. Laser shock peening (LSP) can induce far deeper compressive residual stresses, better surface finish and uniform distribution of intensity across the surface. LSP also improves the crack growth resistance post foreign object damage (FOD) with delayed crack initiation i.e. LSP decreases the crack growth rate to a substantial amount.

Aluminum alloys are widely used in automotive and aerospace industries because of their light weight and high strength to weight ratio. Considerable research has been carried out to examine the effects of LSP on the mechanical properties of aluminium alloys. Most of the research showed that the mechanical properties improved significantly for aluminium alloys due to the compressive residual
stresses after surface modification technique. The objective of this work is to examine the effects of LSPwC, change in microstructure, micro-hardness of Al-2024 alloy.

## EXPERIMENT AND PROCEDURE

**Experimental material.** The chemical composition of Al-2024 alloy used in this work is given in the table below. The hot rolled sheet was cut into work pieces of dimensions 150x150x100mm by CNC EDM machine. The samples are polished using different grades of Si paper and disc polishing for uniform scratch less surface. Heat treatment of the sample was performed in a muffler furnace at a temperature 883°C for 4 hours. After heat treatment, water quenching of the sample was done for about 5-10 minutes.

<table><caption>Table 1. Chemical composition.</caption><thead><tr><th>Al</th><th>Cu</th><th>Mg</th><th>Mn</th><th>Cr</th><th>Fe</th><th>Si</th><th>Ti</th><th>Zn</th></tr></thead><tbody><tr><td>90.7-94.7</td><td>3.8-4.9</td><td>1.2-1.8</td><td>0.3-0.9</td><td>(Max)0.1</td><td>(Max)0.5</td><td>(Max)0.5</td><td>(Max)0.15</td><td>(Max)0.25</td></tr></tbody></table>

**Laser Shock Peening without Coating procedure.** The LSP was conducted using generally Nd: YAG laser and the shock waves were generated. When the power density of the laser pulse is sufficiently high, then the shock waves are generated on the metal target. These shock waves propagate through the metal target and compressive residual stresses are being induced on the metal surface due to plastic deformation. During LSP, shock waves were induced using a Q switched repetition rate laser of 10Hz with a fundamental laser beam wavelength of 1064nm, pulse energy and pulse duration were 350mJ and 10ns respectively. In this process, the metal target surface is mirror polished and water is used as a confinement layer. LSPwC process is being carried on the decarburized surface which acts as an ablative surface. A smooth and uniform surface is created for multiple LSPwC. The thickness of the confinement layer (water) is kept 1-2mm by employing water jet setup.

**Characterization method.** The residual stress measurements are performed using X-ray diffraction according to the $\sin^2\Psi$ method using X'pert Pro system (PANalytical, Netherlands) at an operating voltage of 45 kV and current of 40 mA using Cu$_{K\alpha}$-radiation with PRS X-ray detector. Scanning Electron Microscope (ZEISS EVO 18, Germany) is used to study surface morphology. The surface roughness profile (Martalk) is determined using surface profilometer (Stylus Method). The micro-hardness distribution of the sample was determined using a Vickers micro-hardness tester. A cross section of the specimen was prepared given by (ASTM: E384 standard) for hardness testing and hardness distribution was acquired under a constant load of 1.96N(200g). Atomic force microscope investigates surface attributes for both laser treated and untreated Al-2024 specimen by Nanosurf EasyScan2 (Switzerland) with constant static force operating type controlled by software version 3.1.0.22, firmware version 3.1.3.13, controller S/N 023-06-154 with head type EZ2-AFM and cantilever type ContAl-G.

## Results and Discussion

**Residual stress analysis.** After LSPwC, high compressive residual stress is induced. Severe plastic deformation takes place due to shock waves generated which tends to increase the compressive residual stress. It was observed Al-2024 LSPwC treated specimen at surface stress has been increased to -126 MPa(maximum)
<table><caption>Table 2. Compressive residual stress analysis.</caption><thead><tr><td>Specimen</td><td>Surface residual stress(MPa)</td></tr></thead><tbody><tr><td>1. LSPwC</td><td>-126±16</td></tr><tr><td>2. Un-peened</td><td>71±3</td></tr></tbody></table>

**Surface characteristics analysis.** The surface morphology of the specimen was measured by scanning electron microscope. Both were investigated at 20µm scale bar, wide varieties of changes was seen. Variation in patterns were observed b/w un-peened and LSPwC. Grain size deformation took place after LSPwC which induces grain factors such as dislocation density and twinning boundary. Pores/ pits were observed in un-peened specimen. LSPwC treated specimen has showing decrement in number of pits/ pores due to compressive residual stress within surface and subsurface. Solidification patches was observed in laser treated specimen.

<figure><img src="image_3.png" /><figcaption>Fig. 1. SEM surface morphology image of un-peened and peened Al 2024 specimen</figcaption></figure>

**Surface roughness analysis.** Using profilometer by stylus method. The surface roughness was scrutinized for both LSPwC and unpeened Al-2024 specimens. The investigation states some amount of roughness has been escalated in LSPwC specimen compared to un-peened specimen. The increase in surface roughness is due multiple laser beam interactions which resulted in generation of shock waves which increases the dislocation density. The surface roughness (Ra) increased from 0.29 µm before treatment to 7.22 µm after LSP treatment.

<figure><img src="image_4.png" /><figcaption>Fig. 2. Roughness profile of un-peened Al-2024.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 3. Roughness profile of LSPwC Al-2024.</figcaption></figure>

**Atomic force microscopic analysis.** Atomic force microscope (AFM) 3D topography of un-peened and LSPwC specimen is presented in fig. 4 respectively. It is evaluated that larger depths and areas are observed in LSPwC due to lower overlapping rate. Area roughness was detected in LSPwC treated region.

<figure><img src="image_6.png" /><figcaption>Fig. 4. AFM 3D topography of un-peened Al-2024.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 5. AFM 3D topography of LSPwC Al-2024.</figcaption></figure>
**Micro-hardness analysis.** The micro-hardness of the specimen(Al-2024) was measured using Vickers micro-hardness tester. The hardness of the specimen was obtained under a constant load of 1.96N(200g). It was observed that the micro-hardness of the metallic material increased from before treatment to after LSP treatment. The strength of the material Al-2024 greatly increased as a result of LSPwC. After LSP, plastic deformation of the specimen occurred resulting in an increase in the number of dislocation density and pinning of dislocation takes place. Thus, the micro-hardness of the material increased as result of high dislocation density and grain refinement.

**Summary.** LSPwC can induce greater compressive residual stress thereby improvising the resistance offered for crack growth initiation.

The surface roughness after laser treatment possesses visible changes within the surface and sub-surface due to value increased to 7.22 µm after LSPwC.

The micro-hardness has greatly increased in LSPwC specimen due to grain refinement and refined grain characteristics formation due to multiple shocks over surface and sub-surface.

Grain refinement took place due to high pressure shocks within the surface and sub-surface which induces grain attributes thus ultimately enhances mechanical characteristics.

**References**

[1] Sathyajith, S., S. Kalainathan, and S. Swaroop. "Laser peening without coating on aluminum alloy Al-6061-T6 using low energy Nd: YAG laser." Optics & Laser Technology 45 (2013): 389-394.

[2] Kalainathan, S., and S. Prabhakaran. "Recent development and future perspectives of low energy laser shock peening." Optics & Laser Technology 81 (2016): 137-144.

[3] Prabhakaran, S., and S. Kalainathan. "Compound technology of manufacturing and multiple laser peening on microstructure and fatigue life of dual-phase spring steel." Materials Science and Engineering: A 674 (2016): 634-645.

[4] Fu Zhao, William Z. Bernstein a "Environmental assessment of laser assisted manufacturing: case studies on laser shock peening and laser assisted turning" Journal of Cleaner Production 18 (2010) 1311e1319.

[5] L. Zhang, J.Z. Lu, "Effects of different shocked paths on fatigue property of 7050-T7451 aluminium alloy during two-sided laser shock processing" Materials and Design 32 (2011) 480-486.

[6] S. Baragetti, G. D Urso, "Aluminium 6060-T6 friction stir welded butt joints: fatigue resistance with different tools and feed rates" Journal of Mechanical Science and Technology 28 (3) (2014) 867~877.

[7] Omar Hatamleh, Jed Lyons, "Laser and shot peening effects on fatigue crack growth in friction stir welded 7075-T7351 aluminium alloy joints" International Journal of Fatigue 29 (2007) 421-434.

Cite the paper

Yash Jain, Sandeep Varin, S. Prabhakaran, S. Kalainathan (2017). [Effect of Multiple Laser Shock Peening without Coating on Al-2024-O Alloy for Automotive Applications. Mechanics, Materials Science & Engineering, Vol 9. Doi 10.2412/mmse.65.57.424](https://www.mmsexyz.com/mmse/65.57.424)