Fatigue crack growth rates in friction stir welding joints of Al-Li 2060-T8X Alloy

Friction stir welding (FSW) has been emerged as one of the most reliable, and cost-effective method for joining of the aerospace-grade aluminium alloys. This study was carried out to examine the fatigue crack growth behavior in the friction-stir welding joints of Al-Li base alloyAA2060-T8X.The mechanical properties, fatigue crack growth rate and the microstructural features of the weld zone resulted from the friction-stir processing were compared with that of the base metal. Fatigue tests were performed according to the BS7448 standard at a constant amplitude stress ratio R=0.1 and plot of crack growth rate versus stress intensity range was constructed. Fractography was also performed to examine the failure mode, and crack propagation mechanisms. The results showed that tensile strength, yield strength, and elongation of the base metal averaged at 533 MPa, 380MPa and 15.5%, respectively. The mechanical properties of the FSW joints were found 24-30% lower with respect to the base metal. The tensile strength, yield strength and elongations of the FSW joints were ranged 350-378 MPa, 335-351 MPa, and 6.5-8.5%, respectively. Micro hardness of the FSW joints was in the range of 124 to 143 HV with an average value of 134.8 HV, as compared to the base metal that averaged at 173 HV. The da/dN-ΔK curves of the base metal and the FSW joints showed a similar fatigue crack growth propagation rate in the near-threshold region. However, fatigue cracks in the FSW joints propagated at a much faster rate during stage II (Paris region). This anomaly was attributable to the freshly developed micro structure inside the stir zone.