Quasi-equal strength friction stir welding of Invar alloys towards better service performances

Invar alloys, renowned for their low coefficient of thermal expansion (CTE), are critical for high-precision applications, yet conventional welding methods face challenges of heat-induced softening and service performance degradation. Here, friction stir welding (FSW) was employed to join 2-mm-thick Invar alloys by designing a split-type tungsten-based composite tool. The welding tool enhances thermo-mechanical stirring efficiency, enabling sound joints at a welding speed of 70 mm/min speed and rotational velocities of 200–500 rpm within a large welding process window. The joint exhibited fine equiaxed grains with an average grain size of 5.4 μm, representing 28.9 % of the base material (BM), contributing to equal-strength joints with significant work hardening in stirred zone. The joints exhibited better dimensional stability than the BM with an average CTE of 1.64 × 10⁻⁶ °C⁻¹ (89.3 % of the BM). The joints also showed superior corrosion resistance, mainly due to microstructural homogenization to trigger the formation of more stable and protective passive films during electrochemical corrosion.