Process Optimization of Warm Laser Shock Peening without Coating for Automotive Spring Steel

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

Process Optimization of Warm Laser Shock Peening without Coating for Automotive Spring Steel1 S. Prabhakaran1, S. Kalainathan1,a 1 – Centre for Crystal Growth, VIT University, Vellore, India a – spkaran.kmd@gmail.com, s.kalainathan@gmail.com DOI 10.2412/mmse.91.76.916 provided by Seo4U.link

Keywords: warm laser shock peening (WLSP) without coating, residual stress, hardness, dynamic strain aging, dynamic precipitations.

ABSTRACT. The current study proposes and optimizes the process parameters for warm laser shock peening without ablation coating. Warm laser shock peening brings up the advantage of dynamic strain aging and dynamic precipitation hardening of metallic materials. The low energy Nd: YAG laser at the fundamental wavelength of 1064 nm utilized for the operation and the borosilicate (BK7) glass was used as a confinement medium. The experiment performed with different laser pulse densities and the results revealed that the higher pulse densities lead to surface melting due direct ablation taking place on the pre warmed specimen surface. Also, the process temperature optimization was carried out and the result indicates that there was a hardness drop during the laser peening at 300 oC, which is due to an excess amount of precipitation leads to lose the strength of the metal. The microstructural analysis was performed using the field emission scanning electron microscope (FE-SEM).

Introduction. The post-processing material designing technology in automotive and aerospace industries is playing a vital role. Most of the fatigue cracks are initiating at the surface and it propagates throughout the material leading to fatigue fracture. The surface modification technologies like cold rolling, ball milling, surface attrition treatment, shot peening and laser shock peening (LSP) are used to modify the surface mechanical properties of metallic materials. Among these, the shot peening is a mechanical cold working process that can induce compressive residual stress through a number of successive shots using spherical iron balls, water jet and oil jet. Here, the induced compressive residual stress (RS) effectively retards the fatigue crack initiation and propagation [1], [2], [3], [4]. The laser based materials processing techniques are emerging from a decade because of its all round performance like reliability and consistency in the industries. LSP emerges as a novel cold working surface modification technique through inducing deep and high compressive residual stress [1], [5], [6]. The basic phenomena behind this LSP process are the laser matter interaction producing high pressure ionized gas plasma on the metal surface induces strong compressive shock waves into the material and this compressive stress production is purely a cold working process [5] ,[6]. Normally water or glass is utilized as the transparent confinement medium and black paint or PVC tape used as surface protective ablation medium for LSP process. Ganesh et.al [2] ,[3] introduced the investigation of LSP on spring steel for automotive applications and LSP has effectively repaired the fatigue life of partly fatigued SAE 9260 spring steel using poly vinyl chloride (PVC) tape as an ablative medium. The LSP producing grain refinement induced plastic deformation is liable to the fatigue life enhancement of metallic materials [1-3]. The ambient condition LSP treatment induced internal RS relaxation affects the mechanical properties of metal materials under exposure thermal conditions [1], [4]. The thermal engineering based warm laser shock peening (WLSP) got advantages such as dynamic strain aging (DSA), and dynamic precipitations (DP) hardening of low-alloy steel 1

© 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/

MMSE Journal. Open Access www.mmse.xyz


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