Microstructure and properties of friction stir welded high strength Fe-36Â wt%Ni alloy

► Defect-free friction stir welds of high strength Fe-36 wt%Ni alloy were acquired. ► The welds maintain the excellent thermal expansion coefficient. ► The welds have uniform and slightly coarser austenite microstructure in stir zone. ► About 10% reduction of hardness and strength were found in the joints. ► The relationship between mechanical properties and grain size of FS welds was discussed with Hall-Petch relation.