Laser welding of highly reflective metal with absorbance-enhanced surface structure fabricated using picosecond laser
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S.I. : CURRENT STATE-OF-THE-ART IN LASER ABLATION
Laser welding of highly reflective metal with absorbance‑enhanced surface structure fabricated using picosecond laser Tetsuo Sakai1 · Masatoshi Hirono1 Received: 16 October 2019 / Accepted: 26 May 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract We present a method to stably weld a copper at lower laser power by blackening the surface. Since the reflectivity of copper is approximately 97% in the infrared range, a precise and stable processing of copper is difficult even at high power using infrared laser. To decrease the reflectivity, we fabricated light-absorbing nano-structure on copper surface using picosecond laser pulses. The structure consists of aggregations of nano-particles, and is capable of absorbing as much as 80% of incident light. We demonstrated that a copper with this structure can be welded using 70% lower irradiation power than a copper without it. Additionally, we theoretically investigated the light-absorbing nano-structure using 3D finite-difference time-domain (FDTD) method. Keywords Laser ablation · Copper · Nano-structure · Absorbance-enhanced surface structure · Picosecond laser · Femtosecond laser · Ultrashort pulse laser
1 Introduction Laser welding has become popular in manufacturing process for mass production since the appearance of high-power CW infrared lasers [1–3]. In recent years, high-speed laser welding using galvanometer optical scanner is possible even for material with reflectivity as much as aluminum [4]. Since this welding method enables precise processing [4], it has shown to be effective in welding fine aluminum structure like a lithium ion battery [5]. However, in case of copper, which not only has high reflectivity but also high melting point and high thermal conductivity, a precise and stable welding process is difficult. To stabilize a welded state, various studies have been done such as laser power modulation [6] and harmonic beam combination [7]. Recently, CW Nd:YAG SHG laser and blue laser diode have been developed for a stable processing of copper due to its high absorbance at shorter wavelength [8, 9]. The process using a short wavelength is simple and manageable, but their laser power is still insufficient to weld at high speed. * Tetsuo Sakai [email protected] 1
Corporate Manufacturing Engineering Center, Toshiba Corporation, 33, Shin‑Isogo‑Cho, Isogo‑ku, Yokohama 235‑0017, Japan
We focused on a method to increase absorbance without using short wavelength laser. The method is a blackening of material surface by ultra-short pulse laser processing [10–15]. Such surface blackening had been studied to increase opto-electron conversion efficiency of silicon [16]. Vorobyev and Guo investigated the fabrication method using femtosecond laser pulse and controlled a color of various kinds of metal surfaces [11–13]. Recently, a potential to increase fabrication efficiency of the blackened surface using femtosecond fiber laser and picosecond laser at high repetition rate was rep
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