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IN recent years, the economic and political pressure on the automotive and aircraft industry to reduce fuel consumption and CO2 emissions has constantly increased. Because of this, the efforts for lightweight design became more important. A possibility for reducing weight of a structure is the use of lightweight and high-strength materials. Aluminum alloys are increasingly employed for diverse applications and also as a substitute for steel in many sheet applications. This is mainly due to their high strength-to-density ratio and machinability as well as their easy availability. Although heat-treatable Al-Zn alloys are attributed to the aluminum alloys with the highest strength, there is no widespread use of these alloys.[1] The reason for this disregard of the industry lies in the fact that Al-Zn alloys exhibit distinct weldability problems such as the formation of porosity and hot cracking,

JOSEPHIN ENZ, PhD Student in the Department of Joining and Assessment, is with Helmholtz-Zentrum Geesthacht, Institute of Materials Research, Materials Mechanics, Max-Planck-Strabe 1, 21052 Geesthacht, Germany, and also Scientist with TuTech Innovation GmbH, Harburger Schlobstrabe 6-12, 21079 Hamburg, Germany. Contact email: [email protected] STEFAN RIEKEHR and VOLKER VENTZKE, Scientists in the Department of Joining and Assessment, NORBERT HUBER, Head of the Division of Materials Mechanics, and NIKOLAI KASHAEV, Head of the Department of Joining and Assessment, are with Helmholtz-Zentrum Geesthacht, Institute of Materials Research, Materials Mechanics. Manuscript submitted June 16, 2015. Article published online April 1, 2016 2830—VOLUME 47A, JUNE 2016

which predominantly arise during fusion welding.[2–9] This is also the reason why only a few studies have been conducted on fusion welding these alloys. Up to now, defect-free welds were only achieved by solid-state welding processes such as friction stir welding.[5,9] However, among the fusion welding processes, laser beam welding (LBW) belongs to the most efficient welding processes, due to high welding speeds, the high energy density of the laser beam, and the beneficial depth-to-width ratio of the resulting weld seams. Moreover, LBW possesses a higher flexibility—concerning the geometry and dimensions of the structure to be welded—in comparison to solid-state welding processes, due to the possibility of contactless welding and small beam dimension. By overcoming the weldability problems during LBW of Al-Zn alloys, these alloys may gain recognition as promising structural materials for lightweight structures. Recent work by Zhang et al. offers the first promising results in partially improving the weld quality by using a fiber laser for welding.[10] However, further progress in this direction requires obtaining a deeper understanding of the existing weldability problems and, based on this, the development of welding processes that significantly reduce or avoid these problems. The present study concerns the theoretical considerations of laser weldability of Al-Zn alloys. Significant