Investigating the Effect of PWHT on microstructural features and fatigue crack growth behavior of friction stir welded AA2024-T6 aluminum alloy joints

Friction stir welding (FSW) was employed to join 10 mm thick AA2024-T6 aluminium alloy plates to overcome the fusion welding related problems such as grain coarsening in fusion zone, heat affected zone (HAZ) softening, lower joint efficiency and premature joint failure. The center cracked test (CCT) specimens were used to evaluate the fatigue crack growth rate of the welded joints. The microstructural features of joint were characterized using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The fractured surfaces of CCT specimens were analyzed using scanning electron microscopy. Results showed that the FSW joints exhibit lower fatigue strength than the parent metal due to the dissolution of second phase strengthening precipitates. The post weld heat treatment (PWHT) specifically solution treatment (ST), artificial ageing (AA), solution treatment and ageing (STA) were employed to improve the fatigue properties of FSW joints. It was observed that the STA treated FSW joint exhibit higher fatigue life than the ST and AA treated joints. It is attributed to the precipitation of second phase strengthening precipitates in welded joint after STA.