Tailored absorptivity: Increasing the laser weldability of copper through surface structuring
Introduction
Welding of copper and its alloys possess high relevance for industrial engineering but is up to now a challenging task using laser sources due to inherent process instabilities [1], [2]. The main reason for this behavior results from some of its particular material properties. Probably most essential, the absorptivity of copper for a wavelength of around 1 µm is only about 3% at room temperature [3], while at the melting point it is much increased. Experimentally determined values in literature spread between 8 and 13% [4], [5], which are also quite different compared to a theoretically calculated value of 25% [6]. In any case, the energy coupling dramatically increases during the solid/liquid phase change, which can cause welding defects like voids [7], melt ejections and in consequence a collapsing keyhole and sputter on the surface [8].
Several techniques were proposed to stabilize the laser welding process. For example, the use of laser sources operating in the visible wavelength (λ) range [9] have been suggested due to the up to 10 times higher absorptivity of Cu at room temperature. However, interband effects always exhibit a negative temperature coefficient, wherefore the absorptivity of copper decreases for a wavelength of λ = 515 nm from 42% at room temperature to 25% at the melting point [4] leading to similar defects as mentioned before [10]. Other strategies involve the combination of infrared and visible laser radiation [11], the modulation of the average power [8], scanning techniques [12] as well as preheating of the specimen to reduce the energy requirements for the necessary transition into the liquid phase.
While all these strategies tried to adjust the process to the specification of the highly temperature dependent material properties, an alternative approach is addressed in this article, which consist on adjusting the optical material properties to guarantee a stable process. It has been shown in literature that textured surfaces can significantly change the absorption characteristics for electromagnetic radiation [13]. These improvements rest on multiple reflections [14] as well as wave theory based effects such as the excitation of surface plasmons [15].
In this article, Direct Laser Interference Patterning (DLIP) [16] is used to change the absorption behavior of copper. In particular, the goal is to produce a periodic surface structure that exhibits the same optical absorptivity at room temperature as well as at the melting point and thus obtaining a homogenous temperature evolution during the welding process.
Section snippets
Material and methods
The laser structuring experiments were performed on 1.0 mm thick pure (99.9%) copper (Cu) plates. The samples were mechanically polished providing a surface roughness Sq of 17 nm. Prior to the laser process, the substrates were cleaned using ethanol. The interference experiments were conducted using a compact DLIP system (DLIP-µFAB, Fraunhofer IWS), which mounts a four-beam DLIP module producing dot-like patterns. A detailed description of the utilized laser setup is found elsewhere [16]. To
Results and discussion
The produced surface structure can be seen exemplarily in Fig. 1 using confocal microscopy (a) and SEM (b). The structure has a periodic 2D dot structure with a spatial period Λ of 2.2 µm. The peak-to-valley height depends thereby on the used fluence (see Table 1).
The measured absorptivities of the different probes are plotted in Fig. 2 and listed in Table 1. The reference sample with a polished surface exhibits an absorptivity of 4.1% which is in good agreement with values documented in
Conclusion
Laser welding of copper is still a challenging task due to highly temperature dependent material properties. An improvement of the weldability was achieved by adapting the optical material properties to the welding process. For this purpose, DLIP surface structures were produced in this study. The resulting surface structure depth of the dot like texture varied between 100 and 300 nm. The corresponding absorptivity values almost tripled for the 300 nm deep structure and equal the one for liquid
CRediT authorship contribution statement
Robert Baumann: Writing - original draft, Investigation, Methodology, Formal analysis, Conceptualization, Visualization. Dominik Hipp: Investigation, Conceptualization, Resources, Methodology, Formal analysis, Writing - review & editing. Andrés Fabián Lasagni: Funding acquisition, Writing - review & editing, Supervision. Achim Mahrle: Writing - review & editing, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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