Author ORCID Identifier

https://orcid.org/0000-0003-0070-0958

Year of Publication

2021

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department/School/Program

Mechanical Engineering

First Advisor

Dr. Haluk E. Karaca

Abstract

Laser Powder Bed Fusion (L-PBF) is one of the most promising Additive Manufacturing (AM) methods to fabricate near net-shape metallic materials for a wide range of applications such as patient-specific medical devices, functionally graded materials, and complex structures. NiTi shape memory alloys (SMAs) are of great interest due to a combination of unique features, such as superelasticity, shape memory effect, high ductility, work output, corrosion resistance, and biocompatibility that could be employed in many applications in automotive, aerospace, and biomedical industries. Due to the difficulties with traditional machining and forming of NiTi components, the ability to fabricate complex parts, tailor properties that can show superelasticity, and fabricating highly textured alloys via AM is a paradigm shift for its shape memory alloy applications. Therefore, this study aims to establish process parameters, building strategy, microstructure, and property relationship of NiTi alloys. It will present that process parameters, sample orientation, loading type, and post heat treatments can be used effectively to tailor the transformation temperatures, grain shapes and sizes, texture, strength, transformation strain, and superelasticity of L-PBF-AM NiTi.

The AM process parameters (PPs) govern the solidification characteristics, thermal gradient directions, competitive grain growth mechanisms, partial re-melting of the previous laser tracks and layers, which highly affect the composition, grain shape and size, texture, and thus mechanical properties of fabricated materials. Therefore, a comprehensive and systematic study is conducted to gain an in-depth understanding of the relationship between the AM processing parameters, microstructure, and shape memory properties of NiTi alloys fabricated by using the Ni-rich Ni50.8Ti49.2 (at. %) powder. It was found that the decrease of laser power from 250 to 100 W, scanning speed from 1250 to 125 mm/s, and hatch spacing from 80 to 40 µm alter the texture from the [001] to [111] orientation along the building direction. Moreover, it was revealed that transformation temperatures (TTs), microstructure, and the correlated thermo-mechanical response could be significantly changed with the process parameters. By the careful selection of PPs, as-fabricated NiTi samples can show superelasticity with 6% recovery and a recovery ratio of more than 87%.

The shape memory behavior of NiTi alloys is highly texture/orientation dependent. Thus, it was hypothesized that the properties of NiTi alloys can be tailored by not only altering the process parameters but also adjusting the relative position of the sample orientation to the building direction, BD. Fabrication of the samples along different directions (loading directions, LD) relative to the BD impacts the re-melting and reheating processes of the layers, altering the texture of the fabricated samples along the testing direction. Moreover, since the impact of defects (e.g., cracks, pores) and crystallographic orientation formed during L-PBF-AM on the strength and shape memory behavior of NiTi could significantly be different in respect to the loading mode (compression or tension), both dog bone tensile and rectangular compressive samples were fabricated along the selected directions. It was revealed that when the LD was altered from 0 or 90 degrees (respect to the build plate) to 45 degree, the texture along the LD was altered from [100] to [110] orientation and significantly lowered the transformation stress. These texture variations created anisotropic compression-tension behaviors with deformation patterns consistent with single crystals. The [001]-textured parts showed higher strength and lower transformation strain (2.87% @ 200 MPa in tension for 0 degree), while the [110] samples showed higher transformation strain at lower stresses (5.31% @ 150 MPa in tension for 45 degree).

Heat treatment is a very efficient method to control the TTs and improve the strength of Ni-rich NiTi alloys. The chemical composition, volume fraction, and coherency of the precipitates highly impact the TTs, matrix strength, hardness, and martensitic morphology and shape memory behavior of Ni-rich NiTi alloys. Therefore, post-heat treatment effects on the transformation characteristics (TTs, thermal hysteresis, and recoverable strain) and the microstructure of the L-PBF-AM Ni-rich NiTi SMAs have been investigated through transmission electron microscopy, thermal cycling under stress, and isothermal stress-strain experiments. L-PBF-AM NiTi samples were fabricated with laser power of 250 W, scanning speed of 1250 mm/s, and hatch spacing of 80 µm to show improved shape memory response as they presented strong [001] texture. It was revealed that post heat treatment at 500 °C for 1.5 h increased the TTs, decrease the hysteresis, and significantly improve the strength of the L-PBF-AM samples due to the formation of coherence precipitates. Perfect superelastic behavior with superelastic strain of 7% and superelastic window of 100 ºC was observed for aged L-PBF-AM samples.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2021.212

Available for download on Wednesday, December 01, 2021

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