Simulation and Experimental Forging of Cast NiTiHf Shape Memory Alloy Using Shaped Dies

High temperature NiTi-20Hf (at%) shape memory alloy has generated significant interest in the past decade due to its combination of transformation temperatures, actuation properties, and strength. The alloy has been successfully scaled up to heats exceeding 200 kg and diameters over 300 mm using a variety of melting methods while maintaining compositional control. Historically, this alloy has been processed via extrusion. However, for larger heats, extrusion becomes increasingly costly and ingot sizes are limited by extrusion press capacity (tonnage and liner dimensions). Commercial adoption of NiTi-20Hf alloy would benefit from the ability to process ingots to smaller diameters with conventional methods used for processing commercial NiTi.

Currently, most binary NiTi cast ingots are converted to bar via cogging (radial pressing) using open die forging. However, preliminary cogging forging trials on Hf containing ternary NiTi-20Hf cast ingot using flat dies resulted in catastrophic cracking along the diameter of the material. Several causes for the cracking were postulated, including die chill, oxidation, and tensile forces. Gleeble testing was performed on cylinders of cast NiTi-20Hf at temperatures from 700-1100ºC and strain rates from 0.001sec^-1 to 10sec^-1 to determine appropriate temperature and strain rate regimes for hot forging. This data was integrated into DEFORM forging software to model cogging of the alloy using a variety of shaped dies to mitigate tensile forces and determine appropriate forging parameters. Using output from the model, shaped dies were manufactured, and trial forging was performed. The methodology and results of the modeling and experimental forging will be discussed.