Mechanical properties and fatigue crack growth rates in friction stir welded nugget of 2198-T8 Al-Li alloy joints

A 2.0mm thick Al-Li alloy 2198-T8 plate is welded by friction stir welding in this work. Four dog-bone type samples are designed to use and measure stress-strain curves and mechanical properties. M(T) samples with transverse weld in the center are designed. Fatigue tests under different R ratio loads are performed on the welded samples with crack growing in the welded nugget and parent material samples. Fatigue crack growth rates in these welded nuggets are obtained and compared. It is shown that there is no peculiar yielding process in the stress-strain curve of the welded nugget compared with the parent material. Yield strength and tensile strength have a "U" shape through the weld zone and a lower value in the weld zone while the elongation is in reverse. Unlike the parent material, fatigue crack growth rates in the welded nugget are not sensitive to R ratios. Based on K_res, finite element models are built and the crack closure method is used to calculate the effect of residual stress. Redistributions of residual stress with crack growth are taken into account. Fracture surfaces are investigated to summarize the fracture feature of the welded nugget under fatigue load.