Friction stir welding (FSW) was one of the most successful joining techniques in the 20th century [1–3]. Due to its many advantages such as high energy efficiency, low cost, environmental friendliness and high welding quality [4, 5], FSW had gained considerable scientific and technological attention in many fields, including aerospace, railway, renewable energy and automobile [6–11]. During the welding, a suitable shoulder plunge depth could provide adequate forging force and heat input [12, 13], improved the material flow in stirring zone, avoided the formation of internal defects, and finally obtained a sound FSW joint [14, 15]. However, due to the existence of plunge depth, the flash defects and burrs were inevitably formed on the weld edge. Flash defects could cause the loss of material, and the thickness of weld was thinner than that of the base material, which was called weld thinning [16, 17]. The weld thinning had many negative effects on the joint. Firstly, the integrity of external dimension of joint was destroyed. The flash defects induced by weld thinning maybe become a potential security risks during service. Secondly, the effective load bearing area was reduced due to weld thinning [16]. Kumar et al. [18] indicated that when the plunge of the rotating shoulder increased to a certain extent, weld thinning markedly increased, which easily caused joint fracture in the weak region of the weld and drastically deteriorated the mechanical properties. Thirdly, the aluminum alloy structures, such as those used in aerospace and railway, usually beard both static load and dynamic load during service. The thinning of joint can greatly reduce its fatigue properties. In addition, the stress concentration caused by thinning can greatly reduce the stress corrosion resistance of the joint. Li et al. [19] has welded 2219 aluminum alloy by non-rotational shoulder assisted friction stir welding (NRSAFSW). Although the weld thinning was eliminated by NRSAFSW, the maximum mechanical property was only 307 MPa, equivalent to 69% of the BM. However, the tensile strength of the conventional FSW (CFSW) joint obtained at the optimal welding parameter with the same base material was 346 MPa [20], which demonstrated the mechanical properties of joints obtained by NRSAFSW were unsatisfactory. Zhang et al. [21] designed a new type of welding tool to carry out non-thinning friction stir welding (NTFSW). A concave geometrical feature was utilized to the end surface of tool shoulder and three equally spaced scrolls were machined on the concave shoulder. In such case, the weld thinning phenomenon was avoided. However, the shoulder plunge depth was only 0 mm, and thus insufficient heat input resulted in the formation of kissing bond at the joint root.
The weld thinning reduced the actual bearing area of the weld and deteriorated the dimensional accuracy of the whole joint. At the same time, the existence of depression also led to stress concentration and reduced the mechanical property and corrosion resistance of the joints. In the conventional friction stir welding (CFSW), the length of the stirring pin was a bit shorter than the thickness of workpiece [22, 23]. Therefore, a kissing bond was formed at the joint root, resulting in the deterioration of mechanical properties. Especially, under the dynamic loading conditions, the kissing bond became a crack source in the joint, and significantly reduced fatigue strength of the joint. In order to completely cancel the king bond, Hu et al [24] designed a slight longer stirring probe than the thickness of workpiece in order to accomplish tilt probe penetrating friction stir welding (PFSW), and the kissing bond at the root was successfully eliminated, but the weld thinning phenomenon was still exist.
All in all, the previous studies have indicated that the weld thinning and kissing bond are difficult to remove from the FSW joints in the same time. To break through the limitations, a non-thinning and penetrating friction stir welding (NTPFSW) is put ward, which is expected to efficiently eliminate the weld thinning phenomenon and to avoid the formation of kissing bond at the joint root in the present study. By using the same process parameters, the microstructure and mechanical properties of NTPFSW and PFSW joints were compared and analyzed, and thus the advantage of NTPFSW was revealed.