PDF《Hybrid Al/steel-joints》

25 % for USE-FSW compared to conventional FSW can be achieved directly. Based on this positive effects in the recent work the potential of the USE-FSW for dissimilar joints of aluminum with steel will be presented. Therefore, FSW- as well as USE-FSW-joints were realized and analyzed by light-microscopic and scanning electron microscopic methods. Additional nondestructive computer laminography examinations were carried out to find possible weld defects in the bonding area. 2. Experimental Procedure For the investigations the commercially available aluminum wrought alloy EN AW-6061 T6 (AlMg1SiCu) and the deep-drawing steel DC04 (1.0338) are used in a sheet geometry of

280 mm length,

100 mm width and

3 mm thickness. The chemical composition of the materials according to the manufacturer specifications can be seen in table 1. Table 1. Chemical composition of the investigated materials. Material Elements (wt%) EN AW-6061 T6 Si Fe Cu Mn Mg Cr Zn Ti Al 0.64 0.51 0.21 0.14 0.89 0.15 0.04 0.05 bal. DC04 C Si Mn P S Al Ti Nb Fe 0.041 0.015 0.3 0.01 0.0077 0.49 0.0007 0.0037 Bal For the friction stir welding process a universal four axis machining center DMU80T from DMG MORI was used extended with a pneumatic clamping for the metal sheets as well as four load cells from Kistler to realize a force controlled process (figure 1). Figure 1. Machining center DMU80T extended for FSW. For the investigations a butt-joint configuration was chosen, whereby the steel plate was placed on the advancing side and the aluminum plate on the retreating side. The welding direction was perpendicular to the rolling direction of EN AW-6061 and parallel to the rolling direction of DC04. For the FSW tool a hot-work steel 1.2344 (X40CrMoV51) with a shoulder diameter of

16 mm, a probe length of 2.8 mm and a metric thread of M4.5 was used. An axial force of

4 kN, a tool rotation speed of

1500 rpm, a travel of

20 mm/min, a tilt angle of 2°, and a lateral offset of the probe surface to the faying surface of the steel of 0.3 mm were chosen for conventional friction stir welding. For USE- FSW the machining center was additionally equipped with a Lab VIEW-controlled ultrasonic roll seam module of Schunk Sonosystems, which moves synchronously and parallel to the tool (figure 2). Figure 2. Schematic USE-FSW configuration. The roll seam module works with a resonant frequency of

20 kHz, a generator maximum of

3000 W, and a constant amplitude of

18 ?m. The transmission of the ultrasound takes place in the steel sheet located on the advancing side 3. Results and Discussion The analysis on the microstructure of the developing joining area was carried out by light microscopic investigations first. The comparison of the two cross-sections clearly show the influence of additional ultrasound power on the joining area (figure 3). Figure 3. Cross section images of Al/steel-joints: (a) FSW and (b) USE-FSW. For conventional friction stir welding larger steel particles can be detected in the nugget area (white circles in figure

3 (a)). Furthermore, an extensive steel hook from the base material DC04 is apparent. In comparison to this for USE-FSW Al/steel-joints the interface between EN AW-6061 and DC04 is nearly perpendicular and the nugget clearly defined (white dashed line figure

3 (b)). It is recognizable that simultaneous transmitted power ultrasound improves the development of the bonding zone during the FSW process. Furthermore, a detailed analysis of the microstructure of the stirred zone for FSW and USE-FSW was carried out by scanning electron microscopy (SEM). Thereby different types of particles could be found in the welding area of FSW-joints. In figure