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 critical processing parameters are: (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.
After welding, the cross-section of Al-Al and Al-steel joints were electron spark cut and polished for optical microscope observation. The hardness test was measured along the welding seam, upper and lower sheets (1 mm distance from the welding seam). The loading is 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 is 30 Hz by Instron 8801. The stress amplitude (Δσ=(σmaxmin)/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.