PSI - Issue 18

Benjamin Möller et al. / Procedia Structural Integrity 18 (2019) 556–569 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Keywords: steel-aluminium joint, lap joint, compact adapter, laser beam welding, explosion welding, fatigue assessment, constant amplitude loading, variable amplitude loading, corrosive environment, corrosion fatigue, ship building, ship design

1. Introduction Lightweight design materials, such as aluminium, can be directly used in the form of total material substitutions, but also through hybrid material combinations. For the latter case and the combination of aluminium with conventional materials, e.g. steel, joining is necessary. In the past, welding processes were developed for joining thin sheets of steel and aluminium. On the one hand, laser beam welding (LBW) was tested and reliably implemented in order to join this hybrid material combination, as shown in Kaierle et al. (2014). On the other hand, cold metal transfer with low heat input was developed for use in automotive manufacturing by Fronius International GmbH (2018) as well as Fronius International GmbH and Magna Steyr (2019). An overview of the various mechanical and thermal technologies for joining steel to aluminium is given by Lahdo et al. (2016). As in the automotive industry, the use of lightweight construction principles is of great interest in the maritime industry. The introduction of the MEPC resolutions and Guidelines related to MARPOL Annex VI (2011), which contain the Energy Efficiency Design Index (EEDI), requires increased environmental awareness in the development of ships. According to the state of the art, hybrid material combinations of steel and aluminium are currently being joined by explosive welding in yacht design, in order to reduce fuel consumption and increase transport weight. The joining process of explosive welding leads to a flat composite of steel and aluminium sheets. For the realisation of the process and to ensure the formability of the composite suitable for shipbuilding, a sheet of unalloyed aluminium, i.e. technically pure aluminium, is required as an intermediate layer between the steel and high-strength aluminium alloy. The design of an explosive-welded adapter is explained in more detail by Buijs (2004). The field of application of explosive welded details in shipbuilding design is shown in the FSW-Ship project (2013). A considerable disadvantage of this type of connection is the limited freedom of design. Therefore, alternative methods for joining steel and aluminium are sought. The aim of the research project in this respect was to provide a replacement for the adapters, which are currently surface-connected and elaborately manufactured by means of explosive welding. The aim was to develop and optimise a laser beam welding process, as well as to evaluate the quasi-static and cyclic strength of the joint. However, in the thermal joining of hybrid materials, due to different melting points and coefficients of thermal expansion in combination with the formation of intermetallic phases, the consequent embrittlement must be taken into account, caused by the limited solubility of the elements, Klock and Schroer (1977) and Kreimeyer et al. (2004). The presence of the intermetallic phases, particularly those rich in aluminium, Fe 2 Al 5 and FeAl 3 , in the weld seam leads to a decrease in the strength of the joint due to high hardness values of about 1000 HV and low ductility, as shown by Kallage (2013). Nomenclature d f focus diameter D F fiber diameter E energy per unit length F a force amplitude f focusing length f c collimator focal length l c crack length N f cycles to failure P L laser power p d penetration depth R m tensile strength R p0.2 yield strength t sheet thickness v w welding speed