2. Experimental procedure
The studied materials include Aluminum (Al 1060) and steel (DIN 1623)
sheets, cleaned and grinded, with thickness of 2.0 mm and area of 50
x
150 mm.
The
welding was conducted using RFSSW machine provided by Aerospace
Engineering Equipment (Suzhou) Co., Ltd. The sequence of the welded
joint from up to down is Al and steel, as well as the overlap area is 50
x 50 mm. The critical processing parameters and processes were: (1)
rotation speed: 2000 RPM; (2) penetration depths 2.40 and 1.85 mm for
Al-Al and Al-steel joints; (3) penetration speed: 1 mm/s. It shall be
noted that the refill material comes from the upper sheet. The joint
locates in the center position of the overlap area, and the joint
diameter is 9 mm.
After welded, the cross-section of Al-Al and Al-steel joints were
electron spark cut and
polished
for hardness test, where hardness was measured along the welding seam,
upper and lower sheets (1 mm distance from the welding seam) using
Vickers indenter. The load was 0.245 N and a dwell time of 10 s. The
Al-Al and Al-steel specimens were tensile tested by
Zwick/Roell
Z250 with a strain rate of 1mm/min. The tension-tension fatigue tests
were conducted using sinusoidal wave in load control mode with R=0, and
the testing frequency was 30 Hz by Instron 8801. The stress amplitude
(Δσ=(σmax-σmin)/2) was set as 0.2, 0.3
and 0.4 of the joint shear strengths (σb) to
quantitative analysis and comparison. After the fatigue test, all the
specimens were stretched to fracture for observing fatigue damages.