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ORIGINAL ARTICLE

On the influence of laser beam welding parameters for autogenous AA2198 welded joints Theano N. Examilioti 1,2,3 & Nikolai Kashaev 1 & Josephin Enz 1 & Benjamin Klusemann 1,2 & Nikolaos D. Alexopoulos 3 Received: 28 April 2020 / Accepted: 9 August 2020 # Springer-Verlag London Ltd., part of Springer Nature 2020

Abstract The effects of different autogenous laser beam welding process parameters on the fusion zone (FZ) geometry, microstructure, and tensile mechanical properties were investigated for 5-mm-thick AA2198 alloy sheets. Porosity formation and hot cracking are observed for low laser powers and welding velocities, while the porosity level is essentially reduced with increasing laser power. The characteristic cross-sectional geometry of the welded joints changes with increasing laser power, taking shapes from narrow V shape to rectangular I shape, and the results are discussed based on the “closed” and “open” keyhole formation during laser beam welding. A methodology is exploited in terms of quantifying the geometrical dimensions of the cross-section of the FZ in order to promote the welded joints with a narrow width as well as with a rectangular shape. The optimal process parameters, leading to FZ close to the desirable rectangular I shape and with a low number of defects, are identified. Microstructural analyses reveal a pronounced transition zone in between the FZ and the heat-affected zone, which is subdivided into two narrow zones, the partially melted zone (PMZ) and the equiaxed zone. The narrow width of the FZ and PMZ, as well as the rectangular shape of the FZ, enables the autogenous welded joint to reach good tensile deformation properties. Keywords Aluminum-lithium alloy . Laser beam welding . AA2198 . Microhardness . Tensile strength

1 Introduction Third-generation aluminum-copper-lithium (Al-Cu-Li) alloys are highly promising as they provide improved mechanical properties and damage tolerance behavior when compared with other aluminum-copper alloys [1]. Al-Cu-Li alloys were developed for lightweight and high-performance design and have already been used as structural materials for aerospace applications [2]. Innovative Al-Cu-Li alloys such as AA2198 and AA2196 provide low density, high specific strength, as well as better corrosion and fatigue crack growth resistance, compared with conventional Al-Cu alloys such as AA2024 [3–5]. The addition of lithium (Li) enables the formation of

* Nikolaos D. Alexopoulos [email protected] 1

Institute of Materials Research, Materials Mechanics, Helmholtz-Zentrum Geesthacht, 21502 Geesthacht, Germany

2

Institute of Product and Process Innovation, Leuphana University of Lüneburg, 21335 Lüneburg, Germany

3

Department of Financial Engineering, School of Engineering, University of the Aegean, 821 32 Chios, Greece

several strengthening precipitates depending on the Li content and heat treatment conditions, besides the S-type particles, also precipitates such as δ΄ (Al 3 Li), δ΄ (AlLi) and T 1 (Al 2CuLi) [6]. It is mentioned by Heinz et al. [7