PSI - Issue 13

Ann-Christin Hesse et al. / Procedia Structural Integrity 13 (2018) 2053–2058 Ann-Christin Hesse et al./ Structural Integrity Procedia 00 (2018) 000 – 000

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Furthermore, the base material grade was altered; a mild S355J2+N as well as a quenched S960Q were used. While the S355J2+N can commonly be found in structural applications, the S960Q is used in applications that demand high yield and ultimate strength, such as mobile cranes. Additionally, different sheet thicknesses ranging from 3 mm to 10 mm were used during the tests. The strength values of the different base materials can be found in Table 2. To examine the influence of axial misalignment on the fatigue strength of the joints, some samples were manufactured with an axial misalignment of 15% of the sheet thickness. This value correlates with the highest permitted value for the axial misalignment in quality level C according to ISO 13919-1 (1996). The geometry of the samples is shown in Figure 1. Before welding, the sheets were sandblasted in the fusion zone and the joint edges were milled, cleaned and degreased. Welding was performed without pre-heating. Laser welding was carried out by Fraunhofer IWS in Dresden, Germany. The welding parameters are listen in Table 3 and Table 4. Exemplary macrographs of samples made from S355J2+N in 6 mm thickness are shown in Figure 2 and Figure 3.

Table 3: Welding parameters of the electron beam welded specimen Material Sheet thickness Voltage

Beam current

Welding speed

Defocusing

Working distance

S355J2+N and S960Q

6 mm

120 kV 120 kV

28 mA 38 mA

18 mm/s 15 mm/s

-25 mA -15 mA

900 mm 896 mm

10 mm

Table 4: Welding parameters of the disk laser welded specimen Material Sheet thickness Laser power

Welding speed

Focus position

Beam diameter

Remarks

3 mm 6 mm

4 kW 8 kW 8 kW 8 kW 6 kW

5 m/min 3 m/min 2 m/min 3 m/min 3 m/min

-1 mm -2 mm -5 mm -2 mm 0 mm

375 µm 375 µm 375 µm 375 µm 375 µm

- - - -

S355J2+N

10 mm

S960Q

6 mm

10 mm

Welded from both sides

Figure 1: Geometry of the samples used for fatigue testing, additionally marked are the laser scanning paths which were used to determine the weld geometry

Figure 2: Cross-section of a disk laser welded specimen, made from S355J2+N in 6 mm thickness

Figure 3: Cross-section of electron beam welded specimen, made from S355J2+N in 6 mm thickness

3. Weld geometry characterization

After welding, the sample geometry was measured using a laser scanner, which is based on the measurement principle of laser triangulation. Three tracks on the top and the bottom side of the samples, rectangular to the welding direction, were measure, as marked in Figure 1. The data were analyzed in terms of axial misalignment, angular misalignment, excessive weld metal, incomplete filled grooves, excessive penetration and root concavity. As an example, the data that were obtained for the axial misalignment for all S-N curves are displayed in Figure 4. If no axial misalignment was intended, the measured values are within the quality level (QL) B, which corresponds to the highest quality level according to ISO 13919- 1. If an axial misalignment of 0.15∙t was targeted, the mean values for all samples of the S-N curves lie close to the target value. However, the target value lies exactly on the limit value of QL C, so that some of the samples fall into quality level D, others remain in quality level C.