Grain refinement of NiTi alloys during ultrasound-assisted wire-arc directed energy deposition

Achieving grain refinement in the wire-arc directed energy deposition (DED) process of NiTi alloy is highly challenging. We have successfully obtained NiTi alloy with a fine microstructure and observed an improvement in superelastic recoverability by introducing an ultrasound-assisted field during the deposition process. The results show that the average grain size of the NiTi alloy samples decreased from 147.3 to 72.2 mu m. Additionally, the recoverable strain of the samples increased significantly from 3.7% to 4.4%. This is attributed to the ultrasonic cavitation and acoustic streaming phenomena during the ultrasound-assisted process, which promotes the columnar-to-equiaxed transition (CET) during the solidification process. Fundamentally, the key to the enhancement of recoverable strain after ultrasound-assisted is that the formation of equiaxial grains provides more strain compatibility to the grain boundaries, which reduces the formation of dislocations during deformation.