2 Experimental
2.1 Materials and welding parameters
The base material was 6061-T6 Al alloy sheet with
a thickness of 2 mm and the filler wire adopted was
ER5356 Al−Mg alloy with a diameter of 1.2 mm. The
dimensions of the web and the flange were 200 mm ×
50 mm and 200 mm × 80 mm, respectively. Welding
operation was fully done by an automatic MIG welder at
the self-made fixture. The welding power source used
was DP400 developed by OTC to provide DP-MIG.
To maintain the stability of the welding process, the
mode of one droplet transfer per pulse was obtained by
adjusting the high-frequency pulse parameters, as shown
in Fig. 2 [7]. The thermal frequency and the welding
travel speed were set as 4 Hz and 60 cm/min,
respectively. The welding process throughout this work
was shielded by pure argon at air flow rate of 20 L/min.
The effect of average current (80, 90 and 100 A) was
determined by comparing the fusion condition and the
formation of welding joints. The current differences
between thermal pulse and thermal base at 20, 40 and
60 A were determined under the optimum average
current, respectively. After welding, the properties of the
T-joint formed at three current differences were observed.
The transverse sections taken from the joint beads were
ground, polished and etched for the macrophotograph
and microstructure analysis.