The titanium alloy Ti-6Al-4V was subjected to laser peening (without coating) at power densities of 3, 6 and 9 GW cm−2. The cross sectional profiles of microhardness and residual stress indicated softening in the bulk (away from the surface) at power density of 9 GW cm−2. This behavior was analyzed using a combination of Zerilli-Armstrong phenomenological model in conjunction with the concept of adiabatic heating. The implications of this analysis was supported by structural changes in terms of α → β transition (analyzed through synchrotron radiation) and microstructural changes in terms of primary α (equiaxed) to secondary α (lamellar) transformation, which is observed for the first time here. Based on this, optimum levels of peening to improve fatigue lives are suggested through relevant experiments. © 2019 Elsevier Ltd

Sathya Swaroop N-R

Department of Physics

School of Advanced Sciences

Vellore Campus

Umapathi A

Vellore Institute of Technology (VIT) is a private university located in&nbsp;Tamil Nadu, India. Founded in 1984, as Vellore Engineering College, the institution offers 20 undergraduate, 34 postgraduate, four integrated and four research programs. It has campuses in Vellore, Amravati, Bhopal and Chennai.

VIT is one of the top ranked private universities in India according to NIRF, THE and QS Rankings.&nbsp;Govt. of India has recognized&nbsp;VIT, Vellore as an&nbsp;Institution of Eminence. This has allowed VIT to take independent quality initiatives and move up in world ranking.

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VIT University

Optics & Laser Technology

Laser peening without coating (LPwC) was done on the titanium TC6 alloy at a wavelength of 532 nm using an Nd:YAG laser. The laser power densities of 3, 6 and 9 GW cm−2 were used to peen the samples. Samples were also peened multiple times (1, 3 and 5 passes) at 6 GW cm−2. Microhardness showed an overall 23% increase from the baseline value. Further, softening of α phase in the bulk was observed above 6 GW cm−2 in the samples peened once and above 1 pass in multiply peened samples. A similar trend was observed from the residual stress analysis of the samples. The maximum compressive residual stress was −1780 MPa at a depth of 50 μm at 9 GW cm−2. The observed softening of α phase was proposed due to adiabatic heating. Microstructural changes due to adiabatic heating resulting in increased β volume fractions were observed and confirmed by synchrotron radiation measurements. © 2017 Elsevier Ltd

Deformation of single and multiple laser peened TC6 titanium alloy

Laser peening without protective coating (LPwC) is an advanced mechanical surface modification method being used for a wide range of metallic materials for improving their fatigue and corrosion properties. The literature review of LPwC for the last two decades is systematically presented. The mechanism and experimental parameters of LPwC are described comprehensively. The major factors that influence the fatigue or corrosion properties, for example, compressive residual stress (CRS) and surface roughness, are analyzed with great care using the available data from the literature. A list of suggestions for future work in LPwC is given at the end. © 2017 Taylor &amp; Francis.

Materials and Manufacturing Processes

Laser peening without coating—an advanced surface treatment: A review

The titanium alloy TC6 was laser peened without coating (LPwC) with a pulsed Nd:YAG laser (1064 nm) at power densities of 3, 6 and 9 GW cm− 2. Microhardness data revealed that hardness does not linearly scale with power density. Residual stress analysis indicated that the profile has two distinct regions, one near surface (&lt; 100 μm) showing thermal effects due to laser beam-material interaction and a far-surface region (&gt; 100 μm) with decreasing compressive residual stresses with increasing power density. Transmission electron microscopy (TEM) showed high dislocation density with dislocation tangles and dislocation walls without dislocation cell or nanocrystals in the near surface region. Synchrotron radiation showed the increase in volume fraction of β phase with increase in power density in the far-surface region. Fatigue performance of the samples (number of cycles to failure) subjected to LPwC at a power density of 6 GW cm− 2 was found to have increased by a factor of 2–4, in the load range of 12.5–15 kN. Further, it was observed that in addition to the normally observed sub-surface crack nucleation of LPwC samples, additional formation of β phase in the far-surface region improves fatigue performance. © 2017 Elsevier Inc.

Materials Characterization

Phase gradient in a laser peened TC6 titanium alloy analyzed using synchrotron radiation

Influence of deformation induced martensitic (α′) phase transformation and compressive residual stresses on electrochemical properties of austenitic (γ) 321 steel laser peened without protective coating in acidic solution was investigated. Peening induced compressive residual stresses lowered the corrosion rate 19 times compared to untreated condition, subdued the deleterious role of α′ phase. Presence of dual phases (γ and α′) ensued multiple peaks in the active region of the polarization curve and pitting at α′ phase sites. Roughness after peening (≤1 μm) had no effect on corrosion properties. A fivefold increase in charge transfer resistance observed after peening. © 2017 Elsevier B.V.

Materials Chemistry and Physics

Effect of laser peening on electrochemical properties of titanium stabilized 321 steel

Laser peening without coating (LPwC) is an advanced surface modification treatment applied nowadays for many of the industrial alloys, as it tremendously enhance the fatigue and corrosion properties. We report the influence of LPwC on residual stresses, hardness, microstructure, surface roughness and topography of a nickel based superalloy, Inconel 600. LPwC parameter optimization for Inconel 600 that results in larger and deeper compressive residual stresses with relatively smaller surface roughness was carried out. It was noted with optimized LPwC condition that maximum compressive residual stresses was nearly −672 MPa and depth of work-hardening was about 1400 μm, with slight increase in the hardness with a case depth of about 400 μm. The 3D topography indicated roughening at LPwC treated surface and represented the column or steps like feature whilst the average roughness was found to be smaller (≤1 μm). Grain refinement was not observed. The corrosion resistance of LPwC specimen showed tremendous increase with a 106fold decrease in corrosion rate from that of untreated condition. Consequently, larger and deeper pits were observed in the case of untreated specimen while LPwC treated specimen showed no pitting. The chemical composition and chemical state analyzed with XPS indicated a better passivation of the surface film on LPwC specimen. © 2016 Elsevier B.V.

Journal	Data powered by TypesetOptics & Laser Technology
Publisher	Data powered by TypesetElsevier BV
ISSN	0030-3992
Open Access	0