Improving fatigue property of linear friction welded cruciform joints of low carbon steel

Improving the fatigue performance of welded structures is a critical engineering challenge. While previous studies have shown that modifying the weld toe geometry can enhance the fatigue properties of linear friction welded (LFWed) butt joints, industrial applications often require cruciform joints. In this study, we aimed to improve the fatigue properties by fabricating a cruciform joint using linear friction welding (LFW) and changing the welding conditions. LFWed cruciform joints were fabricated using short-side oscillation, in which the long side of the rib vibrated perpendicular to the oscillation direction, as established in our previous study. The results showed that all LFWed cruciform joints with flash exhibited superior fatigue performance, with fatigue cycles exceeding FAT63, the design curve for cruciform joints defined by the International Institute of Welding. Increasing the upset was found to enhance fatigue life more effectively than increasing post-oscillation pressure. When both the post-oscillation pressure and upset were increased, fracture occurred at the weld toe, regardless of fatigue stress level. Under a nominal stress range of 161 MPa, the longest fatigue life was observed, with the joint remaining unbroken even after 1 × 10⁷ cycles. The transition from welding interface fracture to weld toe fracture, which significantly improved fatigue life, was influenced by an increased welding interface area due to a larger upset and a corresponding reduction in local stress at the weld toe of the welding interface. These findings indicate that increasing the upset to expand the welding interface area is an effective approach to improve the fatigue properties of LFWed cruciform joints. To fabricate defect-free cruciform joints using LFW, short-side oscillation with a larger upset is recommended.