# Influence of Multiple Laser Shock Peening without Coating on Ti-6Al-4V Alloy for Aircraft Applications

Sandeep Varin, Yash Jain, S Prabhakaran, S Kalainathan

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Sandeep Varin, Yash Jain, S Prabhakaran, S Kalainathan. Influence of Multiple Laser Shock Peening without Coating on Ti-6Al-4V Alloy for Aircraft Applications. Mechanics, Materials Science & Engineering MMSE Journal. Open Access, 2017, 9, 10.2412/mmse.69.60.586. [https://hal.science/hal-01504762]

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Submitted on 10 Apr 2017

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# Influence of Multiple Laser Shock Peening without Coating on Ti-6Al-4V Alloy for Aircraft Applications

Sandeep Varin¹, Yash Jain¹, 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

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DOI 10.2412/mmse.69.60.586 provided by [Seo4U.link](https://link.springer.com/10.2412/mmse.69.60.586)

**Keywords:** laser shock peening without coating (LSPwC), residual stress, hardness, roughness.

**ABSTRACT.** The current research proposes multiple laser shock peening without coating (LSPwC) process for aircraft titanium Ti-6Al-4V alloy using low energy Nd: YAG laser. The low energy of 350 mJ in at the fundamental wavelength 1064 nm is utilized for the laser surface modification process. The compressive residual stress measurements are accomplished according to X-ray diffraction sin²Ψ method. The surface morphology and surface roughness profiles were characterized using atomic force microscope and roughness profilometer respectively. The surface microstructure is examined by scanning electron and optical microscopes. Further, the mechanical properties of laser shock depth hardened layers were systematically analyzed through Vickers microhardness tester.

**Introduction.** Laser shock peening is a nonconventional surface modification technique. Laser peening uses impulsive pulse of high energy density that laser ablation induces shock waves which triggers compressive residual stress due to plastic deformation of material than conventional shot peening thus enhances the properties such as increase in hardness, roughness, prevent crack growth, increase in fatigue life, increase in corrosion resistance. Laser shock peening can be done by two ways low energy or high energy. This paper is completely followed low energy with focused beam by using a converging lens. Material was confined with aqueous (water) medium. Plasma pressure generation takes place as the beam strikes the surface hence, plasma has the tendency to not escape the water. Water acts as confinement layer [1 - 4].

Laser Shock Peening without Coating (LSPwC) generally implements laser (Nd: YAG) for generation of impulsive beam by the use of converging lens which has been projected on the surface of Ti-6Al-4V. The surface with no ablative layer was confined by water medium. As the beam strikes the surface high temperature plasma generates which was confined by the aqueous medium and the portion of energy propagates as a shock wave in to the metal. Hence, once the plasma pressure exceeds the dynamic yield strength of the metal plastic deformation starts induces compressive residual stress [3, 4]. The generated compressive residual stress within the surface till certain depth concluded grain refinement and dislocations. Each spot produces a patch on the surface and the shape matches the output laser source. Due to repeated cycles-high strain/ stress rates produces that lead to significant microstructural changes [2, 5].

Titanium alloys have many applications in aircraft as it possesses high strength, low density, good corrosion resistance properties, and less weight. The coating of titanium powder metallurgy gives superior corrosion resistance paired with high strength and low density. Rapid/multiple laser shocks over surface of titanium promises greater depth of residual stress. Titanium has 2 phases alpha (hcp) and beta (bcp) which makes it heat treatable and weld-able. Presence of Alpha/Beta transformation in titanium assures a wide variety of microstructure and properties changes through surface
modification technique-Laser shock peening. The surface modification technique credits the manufacturer with the range of price and weight characteristics of components for both airframes and engines [1 - 6].

**Experiments and Characterizations.**

**Specimen preparation.** Cold working was used during the laser shock peening technique on Ti-6Al-4V rectangular piece of area (15x15x3) mm has been cut by the wire (Electrical Discharge Machine) EDM machine into 4 pieces. Microstructure has been observed by polishing the surface with grit surface range (600-2000) grains. Heat treatment took place in muffler furnace for 4-hours at temp 890° C. Leading by water quenching for 5-10 mins.

**Laser Shock Peening without Coating (LSPwC) process.** Laser shock peening took place at ambient conditions. Generally, Nd: YAG laser terminates laser selection process as it possesses characteristics best suitable for the operation during experiment with wavelength 1064nm. The energy consumption by the laser was 350mJ with a gap of 10ns for each pulse. The cycle rate was constraint at 10 Hz but frequency increment can boost up the process by avoiding effects. Multiple and rapid shots over the surface has created a black colour patch (circumspect as laser peened region). The surface of material' was covered by a very thin layer of water (1-2 mm) which acts as confinement layer and has tendency to confine plasma. Overlapping rate of laser shots on the surface of titanium alloy Ti-6Al-4V was 75%.

**Characterization method.** Residual stress measurements were done on laser peened and un-peened by Sin² Ψ method using X'pert Pro system (PANalytical, Netherlands) at an operating voltage of 45 kV and current of 40 mA using Cuₖα-radiation with PRS X-ray detector. Roughness was measured from surface profilometer (MarTalk) by electronic stylus method. Micro-Hardness test was done by Vickers hardness tester using constant indentation load condition, Cross sectional mounting for micro-hardness was done as given by ASTM: E384 standard. Scanning Electron Microscope (ZEISS EVO 18, Germany) was used for measuring surface characteristics of both un-peened and laser peened region of Ti-6Al-4V. Atomic force microscope was used for measuring surface integrity of both un-peened and LSPwC treated specimen by Nanosurf EasyScan 2 (Switzerland) with static force operating mode controlled by software version 3.1.0.22, firmware version 3.1.3.13, controller S/N 023-06-154 with cantilever type-ContAl-G and head type-EZ2-AFM.

**Results and Discussions.**

**Residual stress analysis.** Compressive residual stress during LSPwC is due to thermal-mechanical behaviour at surface and subsurface grains. Multiple laser shocks on surface generates high shock wave results in severe plastic deformation which induces greater compressive residual stress. This analysis indicates that excessive shocks over surface would not escalates residual stress due to plastic deformation saturation of material [2-6, 7-9]. Increase in compressive residual stress offers resistance to crack growth initiation. Compressive residual stress generated in laser treated specimen is much higher comparatively un-peened specimen of Ti-6Al-4V, so was measured at surface is -176 MPa (Maximum). Un-peened specimen has tensile stress of 204.7 MPa within the surface.

**Scanning electron microscopic analysis.** Surface characteristics were investigated by scanning electron microscope. Both the Ti-6Al-4V LSPwC and un-peened specimen were observed at scale bar of 10 µm. Analysing un-peened specimen shows micro-pores. Analysing laser peened specimen shows grain refinement on the surface which induces refined grain boundary, grain boundary dislocations features. Microstructure variation was easily identified in laser treated specimen [1, 6, 7-11].
<figure><img src="image_3.png" /><figcaption>Fig. 1. (a) and (b) SEM surface morphology image of un-peened and LSPwC Ti-6Al-4V.</figcaption></figure>

**Surface roughness analysis.** Using Stylus method profilometer, Surface roughness has been observed. The Ra (arithmetic average of the all values of the profiles) has increased from 1.97 to 3.02 µm compared with un-peened surface. The irradiation of laser on surface increases surface roughness enhancing the mechanical property such as wear or friction for significant applications [1-3, 6, 10, 11].

<figure><img src="image_4.png" /><figcaption>Fig. 2. Roughness mapping of (a) un-peened and (b) LSPwC Ti-6Al-4V.</figcaption></figure>

**Atomic force microscopic analysis.** Atomic Force Microscope was used to observe surface integrity of Ti-6Al-4V in both LSPwC and un-peened specimen. Area roughness was observed and enhanced
due to various refined grain characteristics in LSPwC specimen. Twinning deformation was observed
in LSPwC sample. Depth of laser treatment over surface can easily be identified.

<figure><img src="image_5.png" /><figcaption>Fig. 3. AFM images of (a) un-peened and (b) LSPwC Ti-6Al-4V.</figcaption></figure>

**Micro-hardness investigation.** Using Vickers micro-hardness test, Hardness of LSPwC and un-peened Ti-6Al-4V have measured at constant indentation load of 50 gf at every 50 µm depth. The average hardness value of treated Ti-6Al-4V compared with un-peened has been increased measured till 750 µm. Increase in dislocation density increases the hardness. Laser shock pressure impact decreases on depth leads to low plastic deformation within the surface hence, low dislocation [1,3,6,10-13].
<figure><img src="image_6.png" /><figcaption>Fig. 4. Vickers micro-hardness profile of Ti-6Al-4V.</figcaption></figure>

**Summary.** LSPwC increased the compressive residual stress within the surface and sub-surface due to plastic deformation which enhances surface characteristics of material. Hence, it was observed in LSPwC surface the compressive residual stress increased to -176MPa (max.). This process has increased the hardness of the surface due to increase in grain density which induces grain boundary features within the surface when measured till 750 µm. The roughness for laser peened surface was observed and it has increased from 1.972 to 3.0231 µm. Microstructures changes were observed in SEM images. Variations in microstructure have been scrutinized. Grain refinement, dislocation density, twinning deformation was seen. These variations induce changes in mechanical properties within the laser peened region.

References

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Cite the paper

Sandeep Varin, Yash Jain, S. Prabhakaran, S. Kalainathan (2017). *Influence of Multiple Laser Shock Peening without Coating on Ti-6Al-4V Alloy for Aircraft Applications*. Mechanics, Materials Science & Engineering, Vol 9. Doi 10.2412/mmse.69.60.586