SHAPE MEMORY BEHAVIOR OF NiTiHf ALLOYS FABRICATED BY LASER POWDER BED FUSION

Author

Guher Pelin Toker, University of KentuckyFollow

Author ORCID Identifier

https://orcid.org/0000-0001-6361-9079

Date Available

12-18-2024

Year of Publication

2022

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department/School/Program

Mechanical Engineering

First Advisor

Haluk Karaca

Second Advisor

Julius Schoop

Abstract

NiTiHf-based alloys have emerged as potential materials for applications requiring high transformation temperatures (> 100 °C) with high strength and work output. Currently, it is challenging to produce complex shapes of these materials with tailored shape memory properties or reduce the material waste from machining operations given the cost of these materials. Additive manufacturing via Laser Powder Bed Fusion (L-PBF) represents an attractive way to fill this gap by producing unique geometries from powder-based feedstock. In this method, process parameters such as laser power, scanning speed, hatch spacing and energy density play significant roles in controlling the mechanical behavior and microstructure of the final parts.

The L-PBF process parameters such as laser power, scanning speed, hatch spacing and energy density are the main factors to determine the composition, microstructure shape memory behavior of fabricated NiTiHf alloys. Thus, systematic study is conducted on slightly Ni-lean Ni_49.8Ti_30.2Hf₂₀ and slightly Ni-rich Ni_50.4Ti_29.6 Hf₂₀ (at%) alloys to investigate the effects of processing parameters. It is revealed that transformation temperatures and hardness were only slightly changed as a function of process parameters in Ni-leanNi_49.8Ti_30.2Hf₂₀ alloys due to the selection of initial composition.

In Ni-rich Ni_50.4Ti_29.6Hf₂₀ alloys, it was revealed that process parameters can be used to tailor the transformation temperatures from 150 ^oC to 360 ^oC. The transformation strain is also strongly affected by the process parameters. Crack formation was observed mostly in the samples fabricated with high laser power and high energy density while porosity was observed in low laser power and low energy density samples. It was shown that shape memory effect and superelasticity with recovery strain of about 2% in compression can be obtained in L-PBF fabricated NiTiHf alloys without heat treatment. Nano-precipitate formation is one of the common ways to improve the shape memory behavior of NiTi based shape memory alloys. L-PBF NiTiHf samples were characterized in three conditions: (i) as-fabricated, (ii) solution annealed and (iii) aged (550 ^oC for 3h), where the transformation temperatures (TTs), shape memory strains and superelasticity were quantified. After solution annealing, TTs and hysteresis were decreased while aging resulted in higher TTs and strength due to the formation of Ni-rich precipitates. Aged L-PBF NiTiHf samples recorded a recoverable strain of 1.03% under stress levels above 500 MPa in compression. It was found that the low and mid power settings resulted in Ni-rich compositions, and aging improved the shape memory effect and superelasticity compared to the as-fabricated samples. On the other hand, the higher laser power yielded Ti-rich composition due to the Ni-evaporation during the process, and no major effect from the heat treatments were observed.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2022.438

Recommended Citation

Toker, Guher Pelin, "SHAPE MEMORY BEHAVIOR OF NiTiHf ALLOYS FABRICATED BY LASER POWDER BED FUSION" (2022). Theses and Dissertations--Mechanical Engineering. 207.
https://uknowledge.uky.edu/me_etds/207