PSI - Issue 18

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

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In addition to a lap joint with a single weld seam, the influence of the number of welds was investigated using specimens with multiple weld seams. An optimum of three weld seams has been found. On the basis of multiple weld seam lap joints, an adapter made of S355 and EN AW-6082 T651 has been developed (variant V1) and optimised in its geometry (variant V2). As an alternative to explosive welded adapters, this laser beam welded adapter could be used as a semi-finished product and integrated into the production process of a ship. This adapter can be applied as a link between the hull, made of shipbuilding steel, and the superstructure, made of an aluminium alloy. Welding process parameters of the test series for corresponding specimen types, which have been water cut from welded sheets, are summarised in Table 2. The geometrical ratio between weld width and penetration depth has been calculated from mean values of each and is also documented in Table 2. 2.2. Laser beam welded steel-aluminium lap joints In order to characterise the mechanical properties of the lap joint, specimens of 25 mm width were manufactured from the welded sheets by water cutting. The single weld lap joint has a total length of 200 mm with a leg length of the steel and aluminium sheet of 120 mm, resulting in an overlap of 40 mm, Fig. 1(a). In tensile shear tests, a maximum shear force of 9.7 kN has been recorded for a spot diameter of 1.2 mm using an energy per unit length of E = 360 kJ/m, corresponding to a welding speed of 1.0 m/min, already reported by Lahdo et al. (2016) and Möller et al. (2017). For the same specimen geometry manufactured by high-power diode laser welding with a maximum power of 10.4 kW, a maximum shear force from 8 to 9 kN, depending on the spot diameter, has been published by Lahdo et al. (2018). A potential increase of the shear force has been investigated by two, three and four weld seams in parallel. Three welds seams have been chosen for fatigue investigations, since the maximum shear force from tensile testing is approximately 20 kN for joints with three and four weld seams. The leg length of the steel and aluminium is 160 mm. Together with an overlap of 80 mm, the total specimen length is 240 mm, Fig 1(b). For a spot diameter of 1.2 mm, the highest shear forces have been achieved for E = 360 kJ/m (test series 3B) and E = 300 kJ/m (test series 3C). An increase in the fatigue strength of lap joints welded without control of the penetration depth and without oscillation, i.e. test series 1A to 1C and 3B to 3C, has been found with an increasing geometrical ratio of weld width to penetration depth w w / p d , Möller et al. (2017). For these test series, the highest fatigue strength has been found for single welds of test series 1C with F a ( N f = 2ꞏ10 6 ) = 0.90 kN and test series 3C with F a ( N f = 2ꞏ10 6 ) = 0.85 kN, where w w / p d > 1. With these investigations, the feasibility of using a laser beam welding process for steel-aluminium lap joints was demonstrated at benchmarking scale.

(a)

(b)

Fig. 1. Laser beam welded lap joint with (a) one weld seam; (b) three weld seams.

2.3. Laser beam welded steel-aluminium adapters Variant 1 (V1) of the steel-aluminium adapter was manufactured by a double-sided welding of three weld seams for each side using a spot diameter of 1.2 mm. A specimen with a length of 280 mm and a width of 25 mm, Fig. 2(a), was water cut from the welded adapter in order to determine the maximum tensile force under quasi-static loading