Fatigue strength improvement of linear friction welded butt joints of low carbon steel by pressurizing after oscillation

Although linear friction welding (LFW) has received significant attention as an effective solid-state welding method, the fatigue properties of LFW joints have not been fully investigated. In this study, the fatigue strength of the steel joints fabricated using LFW was investigated and improved by increasing the pressure after oscillation. Low-carbon steel SM490A was successfully joined using LFW without unbonded areas or apparent softening in the heat-affected zone (HAZ). The microhardness, microstructure, weld toe shape, and fatigue life were assessed to investigate their relationships with the pressure conditions after oscillation. The results indicated that increasing the pressure after oscillation improved the area fraction of martensite (M) and the hardness of the weld interface center. The weld toe radius (rho) and weld toe angle (alpha) also increased regardless of the upset control condition. The increasing of the pressure after oscillation from 50 MPa to 300 MPa increased rho from 0.4 mm to 2.1 mm and the alpha increased from 92.7 degrees to 103.4 degrees at the weld center, accompanied by a reduced stress concentration factor (Kt) from 3.0 to 1.7. Under the fatigue test conditions of the stress range of 380 MPa, the fatigue life of the LFW as-welded joints was increased by approximately 31 % when the pressure after oscillation was 250 MPa compared to that of 50 MPa and by 87 % when it was 300 MPa compared to that of 250 MPa. Despite the possibility of fracture initiation at the weld toe at the edge of the LFW joints, the fatigue strength demonstrated a substantially longer fatigue life than the S -N design curve FAT90 recommended by the International Institute of Welding.