PSI - Issue 18

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

558

3

weld width focal position

w w  z

2. Specimen design for fatigue testing 2.1. Materials and laser beam welding (LBW) processes used for steel-aluminium combinations

The development and optimisation of an adequate one-sided welding process was carried out on dissimilar lap joints made of the aluminium alloy EN AW-6082 T651 (sheet thickness t = 8 mm) and S355 steel ( t = 5 mm) with the mechanical properties defined in Table 1. Therefore, a laser beam welding process for joining steel to aluminium for maritime applications was set up by the Laser Zentrum Hannover e.V. using the high-power laser welding system Trumpf TruDisk 16.002 with a maximum laser power of 16 kW, as shown by Lahdo et al. (2016) and Möller et al. (2017). A theoretical spot diameter of 0.30 mm resulted from a focal length of 300 mm and a line of collimation with a length of 200 mm, when considering a fiber diameter of 200 µm. The influence of the fusion zone with respect to the formation of intermetallic phases was determined by investigations on the weld width and penetration depth as well as existing cracks in weld trails with a constant laser power of 6 kW and a variation in welding speed. Welding trials were carried out using spot diameters of 0.6 mm and 1.2 mm, corresponding to a focal position of  z = -5.0 mm and  z = -12.5 mm.

Table 1: Mechanical properties of applied materials Material

Yield strength [MPa]

Tensile strength [MPa]

Elongation [%]

S355 ( t = 5 mm)

431

527

28

EN AW-6082 T651 ( t = 8 mm)

276  284

326  334

17  18

Simultaneously, laser beam welding using a high-power diode laser (Laserline LDF 10000-60) with a maximum power of 10.4 kW was investigated, as shown by Lahdo et al. (2018). A constant optical fiber with a diameter of 600 μm was used, in order to create a variation in the spot diameters depending on different focal and collimator focal lengths. However, on the basis of the TruDisk 16.002 system, a spectrometer (wave length: 396.148 nm) and specifically designed laser head (Scansonic MI GmbH) were used to include a control of the penetration depth with the help of an adjustment of the laser power. For a fibre diameter of 600 µm, a focal length of 250 mm and collimator focal length of 120 mm, a spot diameter of 1.25 mm results. In both of the process variants (controlled and un-controlled), an oscillation of the laser beam with a frequency of 100 Hz was applied in order to increase the weld seam width.

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Table 2: Process parameters series for fatigue testing of laser beam and explosion welded steel-aluminium joints Test series Specimen type Number of weld seams Fiber diameter Energy per unit length Laser beam oscillation Remark

Weld width Penetration depth

(TS)

D F [µm]

E [kJ/m]

w w [mm]

p d [mm]

1A 1B 1C 1D 1E 3B 3C 6C 6E Ex

lap lap lap lap lap lap lap

1 1 1 1 1 3 3 6 6

200 200 200 600 600 200 200 200 600

300 360 300 409 409 360 300 300 409

no no no

1.4 1.6 1.6 2.5 2.1 2.0 1.7 1.6 2.7

2.2 3.2 1.4 1.1 0.9 2.5 1.5 2.2 1.6

0.6 0.5 1.1 2.2 2.3 0.8 1.2 0.7 1.7

yes, 100 Hz yes, 100 Hz

controlled

no no no

adapter V1 adapter V2

yes, 100 Hz

controlled

explosion w. -

-

-

no

-

-

-

-