# Influence of Surface State in Micro-Welding of Copper by Nd:YAG Laser
## 1 Introduction  
The advances in laser technology have increased its competitiveness in various industrial fields, and this has enhanced its increased applications to various processes such as cutting [1], welding, and, more lately, for mechanical characterization [2]. In this work, laser was used for welding of copper. Copper finds great applications in automotive and renewable energy industries, since it is an important material for conducting electric power and for building up electrical systems. Miniaturization of products has necessitated micro-joining of copper in these fields. However, it is challenging to raise the temperature of copper locally owing to its high thermal conductivity and joining using conventional processes result in welding defects [3]. In addition, copper shows low light absorption rate around 1000 nm wavelength, which results in process instability [4]. The absorption rate of copper increases rapidly below 600 nm wavelength. [PERSON] et al. reported the possibility of process stabilization at a wavelength of 532 nm [5]. [PERSON] et al. also developed a 532 nm wavelength green Nd:YAG laser and they investigated its performance in copper welding. They reported a stable process with high efficiency, which led to a narrow weld bead with deep penetration. Absorption rate of copper increased stably with increase in pulse duration, while pulse shaping helped to control porosity [6]. When near infrared light of 1064 nm fundamental wavelength Nd:YAG laser is used to micro-weld copper, the absorption rate increases with increasing power density. Green Nd:YAG laser of 532 nm wavelength shows higher and constant absorption rate regardless of change in power density. However, the power of the 532 nm laser system decreases because of frequency doubling of the Nd:YAGlaser. Moreover, the 532 nm laser system is more complex and expensive compared to fundamental wavelength laser systems [7]. Therefore, shorter wavelength has a possibility of high efficient process for copper, but use of 1064 nm is desired in industrial applications because of its affordable cost and high reliability. However, since light absorption rate is very low at 1064 nm wavelength, stabilization of the process at this wavelength is challenging. Creation of surface ripples during keyhole generation affects surface absorptivity [8]. [PERSON] et al. investigated keyhole behavior in laser welding and they showed that light absorption is increased by multi-reflections effect in the keyhole [9]. [PERSON] et al. also investigated the process behavior in keyhole welding and they reported that most of the laser power was absorbed within the keyhole [10]. [PERSON] et al. carried out laser welding of copper thin sheets using green and infrared pulsed laser beam sources and they noted that low absorptivity of copper increased sensitivity to variations in surface roughness and oxidation [11]. These characteristics lead to a highly sensitive and unstable process in laser welding of copper. [PERSON] et al. investigated laser welding under four different surface conditions namely; as received, sandblasted, black-painted and nano-composite material added on pure copper surface. The highest welding efficiency was obtained by applying the nano-composite material on the surface of pure copper coupons. The sandblast surface treatment and black-painting could also decrease the surface reflectivity from the laser beam and thus enhanced welding efficiency [12]. [PERSON] et al. have also noted that with low laser absorption, changing surface conditions leads to a highly sensitive process. In addition, infrared light absorption increases remarkably at the transition region between solid phase and liquid phase [13]. Since the absorption rate by fundamental wavelength of Nd:YAG laser increases because of multi-reflections effect on the keyhole wall, it is expected that the absorption rate might increase by irradiating the laser on the wall of a concave hole similar to a keyhole.