Study on tensile and fatigue failure in the low-hardness zone of AA2519-T62 FSW joint

Abstract

The aim of this research was to investigate the basic performance and failure of AA2519-T62 friction stir welded joint in tensile test and in low cycle fatigue regime. It has been reported that at the retreating side, the layer of overgrowth grains undergoes deformation in the TMAZ and forms a characteristic large-grain band partly surrounding the SZ. The reported UTS is very high and it equals 405 MPa, what corresponds to 86.5% joint efficiency value. The failure occurred in the LHZ at the retreating side with the fracture mechanism characterized by simultaneously cracking in several parallel planes. The LCF behavior of the tested joint indicates three stages of fatigue life: a relatively long period of cyclic hardening (up to 500–1000 cycles), the longest period of cyclic stabilization, followed by cyclic softening until failure. The fatigue crack initiation takes place in the near-surface layer of overgrown grains and then propagates through the low-hardness zone.

Effect of Shot Peening on the Low-Cycle Fatigue Behavior of an AA2519-T62 Friction-Stir-Welded Butt Joint

“…In the performed investigation, the welded joints subjected to the shot peening are characterized by a higher value of stress amplitude and a lower value of plastic strain amplitude. These results overlap with other studies on the low-cycle fatigue of shotpeened samples [22,24]. The cycle fatigue curves are presented in Figure 6.…”

“…Beneficial deformation of large near-surface grains was observed (Figure 4b). This layer of coarse grains occurs in extruded heat-treated aluminum alloys, and in the previous investigation [22] was identified as a factor promoting the initiation of fatigue cracks in the low hardness zone. Simultaneously, the impact of steel balls led to the unfavorable introduction of small cracks in the ultrafine-grained part of the welded joint at a depth of about 30 µm (Figure 4c).…”

“…The first stage of the research involved macro-and microstructural analysis of treated welded joints. This study focused solely on the impact of the peening process on the structure of the welded joints, rather than on the structure of the joints in the as-welded state, as has been described in previous works [2,22]. The macrostructural and microstructural images of the welded joint after shot peening are presented below in Figure 4.…”

“…In many cases, despite providing data on, for example, microstructure and residual stress, papers rarely address the fatigue properties. In the previous research on the FSW of Sc-modified AA2519-T62 extrusion, the authors investigated the impact of welding parameters on its LCF behavior [2] and the role of the low-hardness zone (LHZ) in LCF failure [22]. As the continuation of this research, the authors aim to investigate the effect of post-weld processing in the form of shot peening on LCF behavior of the considered FSW joints.…”