1 Introduction
FSW is an efficient way to join light alloys. Among these light alloys, aluminum alloy is the most common one. Almost all the aluminum alloys can be joined using FSW [1-4]. Poor fatigue performance is one of the symbols of aluminum friction stir welding (FSW) joint. Yang et al [5] performed the fatigue test of double-side fraction stir welded AA6082 joint. The results showed that the failure occurred in the low hardness heat affected zone (HAZ) and SZ which resulted in a shorter fatigue life when compared with the as-received base metal (BM). He et al [6]indicated the earlier fatigue failure located on the thermo-mechanically affected zone (TMAZ) and SZ in AA6061 joint. Deng et al[7] found the poor fatigue performance of AA7075 FSW joint which fractured in the HAZ, TMAZ, and SZ. Liu et al[8,9] found the shorter fatigue life of AlZnMgCu FSW joint. Besel et al [10-12] systematically investigated the fracture behavior of Al-Mg-Sc alloy. The results showed that the low hardness SZ contributed to the earlier fatigue failure and shorter fatigue life. According to the above investigations, the fatigue failure tended to occurred in the low hardness zone, including HAZ, TMAZ, and SZ in FSW joint. Among them, SZ was a key zone in FSW joint. It usually exhibited a lower hardness and strength[2, 13-16]. Therefore, the fatigue performance of SZ should be paid attention.
Higher strength contributed to better fatigue performance. Leng et al[17] investigated the aging treatment on the hardness, strength, and fatigue life of AA7075. The fatigue life of the AA7075 was increased as the increase of hardness and strength during aging. Esmaeili et al [18] investigated the fatigue behavior of AA7075 deformed by ECAP. The results showed that the strength and fatigue life remarkably enhanced after ECAP process. Xu et al [19] investigated the corrosion fatigue behaviors of gradient structured AA7B50. The results showed that the surface hardening from ultrasonic surface rolling greatly improved the corrosion fatigue performance. Therefore, improving hardness or strength was an important way to modify the fatigue performance of aluminum alloy.
Plastic deformation was an effective way to improve hardness and strength of FSW joint and SZ. Xin et al [20]observed that the strength of AZ31 FSW joint was improved after the post-rolling process. Wang et al [21] also revealed the improved strength of SZ in ZK61 FSW joint after rolling process. Recently, we have reported the improving effect of rolling process on the strength of AA6061 DS-FSW plate[22]. The mechanism for strength improvement was systematically revealed.
This investigation was aimed to reveal the effect of rolling process on the fatigue performance of SZ of AA6061 DS-FSW plate. The fatigue life of original BM, as-welded SZ, and rolled SZs was characterized. The improving effect of rolling process on the fatigue performance of SZ was found. The fracture mechanism of as-welded SZ and rolled SZs of AA6061 DS-FSW joint was discussed. Then, the effect of lamellar structure on the fatigue crack initiation in SZs was revealed.