Fracture toughness of friction stir welded Invar 36 alloy at low temperature

The fracture toughness of friction stir welded Invar36 alloy, obtained with travelling speed of 2 mm/s and rotational speed from 200 r/min to 1000 r/min, were estimated at low temperature (77 K) by small punch test, and were compared with that at room temperature (298 K) as well. At 298 K, all welds have higher small punch energy than the base material. For the welds acquired with the same welding parameters, the small punch energy tested at 77 K is always higher than the ones at 298 K. The small punch energy of welds trends to increase with decreasing of the rotational speed at both 77 K and 298 K. With rotational speeds equal or less than 400 r/min, the small punch energy of welds is significantly higher than that of the base material at 77 K. The results indicate that the friction stir welds keep excellent ductility at low temperature. With the increase of travelling speed, higher heat input produces a coarser microstructure and leads to the reduction of fracture toughness in the weld.