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

High strength Fe–36 wt%Ni alloy sheets with 3 mm thickness were successfully friction stir welded and defect-free welds were acquired at travelling speed of 2 mm/s and rotational speed between 600 and 1000 rpm. Essentially, the friction stir welding process did not change the excellent thermal expansion properties of Fe–36 wt%Ni alloy. Friction stir welding resulted in a uniform coarser-grained microstructure in the stir zone with comparable fractions of low angle boundaries and Σ3 twin boundaries to the base material. However, approximate 10% reductions of hardness and tensile strength were found in the joint. The relationship between grain size and mechanical properties of friction stir welds was discussed. The base material has higher mechanical properties, not only because of the finer average grain size, but also attribute to its inhomogeneous grain structure.