A New Laser Peening Technique: Femtosecond Laser Peening Without a Sacrificial Overlay Under Atmospheric Conditions

Author:  Tomokazu Sano
Source:  ICSP-13
Doc ID:  2017104
Year of Publication:  2017
Abstract:  
Introduction: Laser peening, or laser shock peening, is a surface modification technology using laser-driven shock compression to improve the properties of metals such as hardness, residual stress, fatigue properties,and corrosion resistance [1-4]. A nanosecond pulsed laser is presently utilized as a laser peening tool in aerospace, automotive, medical, and nuclear industries [4]. The solid material which is irradiated by a nanosecond laser pulse transforms into gas or plasma via a liquid, accompanied by a volume expansion. A shock wave is driven as a recoil force during the expansion on the surface and propagates into the material [5,6]. The plastic deformation of the material via the shock wave contributes to the peening effect [7]. In the case where a laser pulse with a near infrared wavelength (~ 1.05 m) is used, the material’s surface needs to be covered with a protective coating or a sacrificial layer such as a black paint or an aluminum tape to prevent the surface from melting or sustaining damage from the laser pulse [4,8]. After the laser treatment, the remaining coating needs to be removed. Laser peening without coating process was developed using 532 nm wavelength lasers by optimizing process conditions, which has been applied to practical uses in nuclear industries [3]. However, the surface needs to be covered with a transparent medium such as water to suppress the plasma expansion and obtain a high amplitude of the shock wave sufficient to deform the material plastically for both wavelengths. Although a micro laser shock peening process has been developed using the shorter wavelength of 355 nm with tens of nanosecond pulse width to suppress thermal damage, this process also requires both a coating and water [9]. The nanosecond laser process does not produce a sufficient shock wave without covering the surface with a plasma confinement medium. Although the applicability of laser peening will clearly be increased if a plasma confinement medium is not required, such a technique has never been realized for the nanosecond laser process. The intensity of a femtosecond laser pulse, which is equivalent to the energy per unit time and unit area and is proportional to the square of the electric field intensity, is extremely high even at a low energy because the pulse width is extremely short [10]. Therefore, direct irradiation of a solid surface with a femtosecond laser pulse drives an intense shock wave that propagates into the solid [11]. Such a shock wave driven by the femtosecond laser pulse irradiated under atmospheric conditions deforms a material plastically, resulting in quenching metastable high-pressure phases [12,13] or forming a high density of dislocations [14-17] Heat-affected and melted zones formed by a femtosecond laser pulse are much smaller than those produced by a nanosecond laser pulse due to its extremely short pulse width [18,19]. Therefore, peening without a sacrificial overlay under atmospheric conditions is considered to be possible using a femtosecond laser pulse. Femtosecond laser peening of steel under water [20,21] and femtosecond laser peen forming of thin metal sheet in the air [22,23] have been reported. However, femtosecond laser peening without a sacrificial overlay under atmospheric conditions aiming to improve mechanical properties has never been reported.