A parametric investigation on friction stir welding of Al-Li 2099

Aluminum-lithium alloys have lower density compared to other traditional aluminum alloys and as such, are being used in lightweighting applications. However, while the microstructure and mechanical behavior of the wrought form of aluminum-lithium alloy 2099 are well characterized, the effect of process variability in friction stir welding (FSW) of this alloy is not well understood. As such, a parametric investigation was performed to examine the effects of tool rotational rates and transverse speeds during FSW on the corresponding microstructure and tensile strength of aluminum-lithium alloy 2099. In general, a strong correlation between tool translation speed and tensile strength and ductility was found. Specifically, the elongation to failure and ultimate tensile strength increased as the transverse speeds increased due to a direct decrease in the heat input. However, elongation to failure and ultimate tensile strength were less dependent on the rotational rate since adequate material mixing was achieved and strain rate sensitivity is negligible at these strain rates. Postmortem analysis revealed a greater presence of micro void and coalescence on the fracture surface for the more ductile specimens as opposed to a higher occurrence cleavage fracture due to the coarsening of intermetallics.