3. Results and discussion
The hardness distribution of Al-Al joint is shown in Fig. 1(a). In the regions of welding seam, as well as upper and lower sheets, the hardness value varies from 23 to 28 HV. However, the hardness distribution of the Al-steel joint varies much as displayed in Fig. 1(b). The welding seam and lower steel sheet exhibit higher hardness between 120 and 150HV, while the upper Al sheet’s hardness is only about 30 HV, resulting in inhomogeneous hardness distribution.
The static overlap-shear strengths (σb) of Al-Al and Al-steel joints are 4691 N and 3846 N. To quantitatively compare the fatigue behaviors of Al-Al and Al-steel joints, the stress amplitudes (Δσ) were set as 0.2, 0.3 and 0.4 of the joint shear strength. The testing results are illustrated in Figs. 1(c&d), where both the Al-Al and Al-steel RFSSW joints exhibit outstanding fatigue properties, compared with other reported results [12 ,13 ]. It is noted that the Al-steel joint with Δσ/σb=0.2 does not suffer obvious damages after 105 cycles.