Microstructure, mechanical and fracture properties of friction stir welded 2195 Al-Li alloy joints

The 8 mm-thick 2195 Al-Li alloy joints were achieved by Friction Stir Welding (FSW). The microstructural evolution, temperature-dependent mechanical properties, and fracture properties were studied. The T₁, δ′/β′ and θ′ precipitates were observed in the Base Metal (BM) and the Heat Affected Zone (HAZ). Most of the precipitates, except for re-precipitated δ′/β′ phases, were dissolved in the Nugget Zone (NZ). The tensile specimens that failed at cryogenic temperatures (–196 °C) had the maximum Ultimate Tensile Strength (UTS), and the fracture surface showed the inter-granular fracture characteristics. Compared to those at room temperature (25 °C), the decreasing tensile properties at high temperature (180 °C) were related to microstructure and strain hardening effects. The NZ presented the optimal fracture toughness, and the Crack Tip Opening Displacement (CTOD) was mutually dominated by microhardness and grain size. Analysis on Fatigue Crack Growth (FCG) rates indicates that the Thermal-Mechanically Affected Zone (TMAZ) exhibited the most superior fatigue resistance performance at stress intensity range below 17 MPa·m^1/2 due to compressive residual stress and the crack closure effect. The fatigue fracture surfaces reveal that the crack propagation zone was characterized by the striations and secondary cracks. Also, inter-granular fracture behavior was responsible for the fastest FCG rates in the NZ.