Effect of Heat Treatment on Microstructure and Mechanical Behavior of Additively Manufactured Nitinol Shape Memory Alloys

Master thesis (2021)

Authors

W. Zhu Mechanical Engineering

Contributors

V. Popovich Team Vera Popovich - Mechanical, Maritime and Materials Engineering (mentor)

Jia-Ning Zhu Team Vera Popovich - Mechanical, Maritime and Materials Engineering (mentor)

M.J.M. Hermans Team Marcel Hermans - Mechanical, Maritime and Materials Engineering (graduation committee member)

Faculty

Mechanical Engineering, Mechanical Engineering

Mechanical properties Phase transformations Heat treatment Additive manufacturing Shape memory Alloy Precipitates Nitinol L-PBF Laser powder bed fusion

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Published Date

06-10-2021

Language

English

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Faculty

Mechanical Engineering

Abstract

Nitinol shape memory alloys (SMAs) have a unique combination of shape memory capability, making it an attractive material for various engineering and biomedical applications. Additive manufacturing (AM) by laser powder bed fusion (L-PBF) allows to produce Nitinol net shape parts, which broadens its applications. Due to the high heating and cooling rate during L-PBF process, there always exists supersaturated solute elements and metastable structures in L-PBF Nitinol parts. Microstructure and precipitate characteristics have detrimental effects on phase transformation and shape memory behavior of Nitinol alloys. Proper heat treatment is an important method to mitigate these detrimental effects and improve the properties of Nitinol. In this project, the effect of heat treatment on shape memory behavior of equiatomic Nitinol fabricated by L-PBF is studied. The heat treatment process is optimized, which is annealing at 950 °C for 5.5h and subsequent aging at 350 °C for 18 hours. By applying the optimized heat treatment process, the cyclic stability of the Ti50Ni50 SMA are improved by 50% for recoverable strain and 70 MPa for applied stress. Additionally, the relationship among the microstructure and precipitates, functional and mechanical properties, and heat treatment parameters are investigated.

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