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

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/

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

contributing an extensive improvement in fatigue life cycle [1], [7]. LSPwC method of producing high compressive RS works effectively with low energy lasers, also it is economical for commercial applications [1], [4], [5].

Experiments and methods. A high content of Si & Mn medium carbon low alloy steel SAE 9254 hardened (900 0C) and tempered (500 0C) used for the laser surface modification process. The LSPwC performed at room temperature (25 0C) and pre-warmed (250 ± 15 0 C ) specimens with a low energy Nd: YAG laser (300 mJ) of pulse duration 10 ns by the fundamental frequency of 1064 nm without any confinement medium for both the processes. An experiment performed with the optimized parameters such as laser spot diameter of 0.8 mm. The laser power density used for the current experimental process is ~ 6 GW cm-2. The borosilicate glass (BK7) is used as the confinement layer for the experiments. In order to avoid fast cooling of pre-heated specimen the electrical dryers are used for continues heating of targeting specimen and its surroundings during WLSP experiment.

Subsequently, the WLSP treated specimen slowly cooled from the processing temperature to avoid RS relaxation by fast cooling [1,7,8]. The mirror polished surface would not act as an opaque medium but since it is polished transparent surface. An aluminium foil is not an opaque layer and there is an experimental coating difficulty because of the high-temperature processing. In the case of high energy laser, an increased thickness of the protective surface needs to be maintained [1].

Results and discussion Laser pulse density optimization based on the residual stress analysis Table 1. Residual stress results for the different pulse densities of LSPwC. Specimen with pulse density

Surface residual stress (MPa)

Residual stress at 50 μm depth (MPa)

Unpeened

124

196

LSPwC (800 pulses/cm )

-302

-305

LSPwC (1600 pulses/cm2)

-330

-397

LSPwC (2500 pulses/cm2)

-349

-489

LSPwC (3900 pulses/cm2)

-294

-463

2

The residual stress was measured using to X-ray diffraction sin2Ψ method. The X-ray irradiations at the diffractive angle (81.920) are measured by X’pert Pro system (PANalytical, Netherlands) using CuKα-radiation. The electrolyte polishing successive layer removal technique adopted for sub-surface analysis of residual stress. The surface and sub-surface (at 50 μm) residual stress values were considered for the optimization of laser pulse density. The laser pulse density of 800 pulses/cm2 was induced the compressive residual stress of - 302 MPa and - 305 MPa on the surface and sub-surface (at 50 μm) respectively. Likewise, the laser pulse density of 1600 pulses/cm2 induced -330 MPa and -397 MPa compressive stress on the surface and sub-surface (at 50 μm) respectively. The laser pulse density of 2500 pulses/cm2 induced -349 MPa and -489 MPa compressive stress on the surface and sub-surface (at 50 μm) respectively and which is the maximum compressive residual stress. Because the higher pulse density at 3900 pulses/cm2 induced only -294 MPa and -463 MPa compressive stresses on the surface and sub-surface (at 50 μm) respectively due thermal effect producing surface damage [1], [4].

Process temperature optimization: Vickers microhardness evaluation

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

Fig. 1. Vickers microhardness profile of different temperature war laser shock peening.

The transverse cross-sectional specimens are used to measure Vickers microhardness with a constant load of 200 g. The excess amount of precipitation will lead to lose the strength of any material.

Likewise, the process temperature optimization is an important task to control the precipitation level with the study metal SAE 9254 spring steel. The process temperature optimization were carried out from 100 0C to 300 0C at an interval of 50 0C. The WLSP without coating at 100 0C to 250 0C is showing the improved hardness drastically. In the case of 300 0C WLSP, the hardness was decreased due to excess amount precipitaions formed during this process lead lose the strength of the material.

The average microhardness of unpeened specimen is around 343 HV. The WLSP at 250 0C improved to 427 HV from 343 HV and it shows around 25% of improvement in hardness. Also, the graph indicates that the hardening effect is more in the sub-surface than the surface for all the temperature range of process due to direct laser ablation treatment[1,4,5,7,8-10].

Microstructure analysis

(a)

(b)

Fig. 2. SEM of unpeened and FE-SEM of warm laser shock peening without coating specimen surface morphologies.

The SEM microstructure indicates unpeened specimen microstructure as shown in Fig. 2a. The FESEM shows the precipitations and refined grains produced by WLSP process as shown in Fig. 2b.

The precipitations in the nano range are clearly seen in Fig. 2a. These carbide precipitations are MMSE Journal. Open Access www.mmse.xyz

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

blocking or filling the grain boundaries and increases the dislocation density of the metallic materials.

In such a case excess amount of precipitation will affect the material strength. Due to dynamic strain aging and dynamic precipitations the material will get hardened and the mechanical properties such that fatigue life can be improved[1], [4], [9], [10].

Summary. The low energy Nd: YAG laser with the fundamental wavelength was utilized for the conventional warm laser shock peening process successfully. The higher laser pulse densities producing thermal effect affect the induction of compressive residual stress and its magnitude. The laser pulse density of 2500 pulses/cm2 is optimized for the SAE 9254 spring steelfor automotive applicatons.The process temperature for the warm laser shock peening without coating is optimized and the higher temperture (above 250 0C) is lead lost the hardness of the metal. So, this indicates that the excess amount of precipitation may affect the machanical properties of the metallic materials.

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Optics & Laser Technology, 81, 2016, pp.137-144, DOI 10.1016/j.optlastec.2016.02.007 [6] Ramkumar, K. Devendranath, et al. Influence of laser peening on the tensile strength and impact toughness of dissimilar welds of Inconel 625 and UNS S32205. Materials Science and Engineering: A 676, 2016, 88-99, DOI 10.1016/j.msea.2016.08.104 [7] Ye, Chang, et al. Fatigue performance improvement in AISI 4140 steel by dynamic strain aging and dynamic precipitation during warm laser shock peening. Acta materialia 59 (3), 2011, pp. 10141025, DOI 10.1016/j.actamat.2010.10.032 [8] Liao, Yiliang, Chang Ye, Gary J. Cheng. A review: Warm laser shock peening and related laser processing technique. Optics & Laser Technology 78, 2016, pp.15-24, DOI 10.1016/j.optlastec.2015.09.014 [9] Podgornik, B., Leskovšek, V., Godec, M., b. Sencic. Microstructure refinement and its effect on properties of spring steel. Mater Sci Eng A 599, 2011, pp. 81–86.

[10] Scuracchio, B.G., de Lima, N.B., Schon, C.G,. Role of residual stresses induced by double peening on fatigue durability of automotive leaf springs. Mater Des 47, 2013, pp. 672–676, DOI 10.1016/j.matdes.2012.12.066

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