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, respectively. The higher shear
strength of Al-Al joint shall be resulted from the higher penetration
depth and more solid mixture of similar materials. 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 reliable fatigue
properties, compared with other reported results14 ,15 .
It is noted that the Al-steel joint with Δσ/σb=0.2 does
not suffer obvious damages after 105 cycles.