J. Zhu
info
Please Note
<p>This page displays the records of the person named above and is not linked to a unique person identifier. This record may need to be merged to a profile.</p>
3 records found
1
NiTi Shape Memory Alloys (SMAs) are highly valued for their unique Superelastic (SE) properties, Shape Memory Effect (SME) and recoverable deformation. However their application is often limited by functional fatigue and the accumulation of residual strain in cyclic loading, compromising their cyclic stability.
The performance of NiTi SMAs relies heavily on the microstructure and composition, both of which are strongly affected by the manufacturing process.
Additive Manufacturing (AM) and specifically Laser Powder Bed Fusion (LPBF), offers an unprecedented ability to tune the microstructure, and thus the properties and performance of NiTi SMAs, by adjusting the process parameters.
It also offers the possibility to fabricate NiTi components with complex geometries and implement techniques such as Functional Grading (FG), where two different microstructures co-exist in the same sample to give rise to novel functional behaviours.
This thesis aims to investigate how processing route and microstructure govern the compressive superelastic response and cyclic degradation of NiTi, with emphasis on LPBF manufacturing.
Homogeneous samples fabricated with LPBF (using various sets of process parameters), casting and rolling, were mechanically characterized. Furthermore, another FG sample was fabricated with LPBF, where the core had strong texture preferable for superelasticity, and non-textured outer sections.
All samples were subjected to cyclic compression tests at austenitic state (80 °C) to ensure superelasticity, with in-situ Digital Image Correlation (DIC) setup to map strain evolution. LPBF samples produced measurable differences in texture/defects that resulted in distinct stress-strain loops and strain localization. A4 with strong texture showed signs of early slip activation, while A6 with weaker texture and porosity promoted stress localization and faster cyclic degradation.
All homogeneous samples accumulated over -2% residual strain. The rolled sample exhibited high stresses and eventually buckling, with the deformation being accommodated by mechanical twinning. Lastly, the cast sample exhibited early functional fatigue in the first 3 cycles, and localized deformation in the top section as shown in optical micrographs.
In contrast the FG sample exhibited superior cyclic stability accumulating only 0.89% of residual strain after 10 compression cycles. The DIC stain maps showed that most of the strain was carried out by the core, while the outer sections remained comparatively elastic and shared load. Optical micrographs also revealed that irreversible damage concentrated at the interface between the textured core and non-textured outer sections. FG sample was also tested to 6 compression cycles at 6 different temperatures (60-110 °C), where at lower temperatures it behaves more homogeneously while at higher temperatures the core carries most of the strain.
The results of this work demonstrate that LPBF process parameters can be used to tailor compressive superelasticity and cyclic stability and that functional grading of crystallographic texture is a highly effective strategy to mitigate cyclic degradation of NiTi, enabling the design of durable and high performance NiTi components.
...
NiTi Shape Memory Alloys (SMAs) are highly valued for their unique Superelastic (SE) properties, Shape Memory Effect (SME) and recoverable deformation. However their application is often limited by functional fatigue and the accumulation of residual strain in cyclic loading, compromising their cyclic stability.
The performance of NiTi SMAs relies heavily on the microstructure and composition, both of which are strongly affected by the manufacturing process.
Additive Manufacturing (AM) and specifically Laser Powder Bed Fusion (LPBF), offers an unprecedented ability to tune the microstructure, and thus the properties and performance of NiTi SMAs, by adjusting the process parameters.
It also offers the possibility to fabricate NiTi components with complex geometries and implement techniques such as Functional Grading (FG), where two different microstructures co-exist in the same sample to give rise to novel functional behaviours.
This thesis aims to investigate how processing route and microstructure govern the compressive superelastic response and cyclic degradation of NiTi, with emphasis on LPBF manufacturing.
Homogeneous samples fabricated with LPBF (using various sets of process parameters), casting and rolling, were mechanically characterized. Furthermore, another FG sample was fabricated with LPBF, where the core had strong texture preferable for superelasticity, and non-textured outer sections.
All samples were subjected to cyclic compression tests at austenitic state (80 °C) to ensure superelasticity, with in-situ Digital Image Correlation (DIC) setup to map strain evolution. LPBF samples produced measurable differences in texture/defects that resulted in distinct stress-strain loops and strain localization. A4 with strong texture showed signs of early slip activation, while A6 with weaker texture and porosity promoted stress localization and faster cyclic degradation.
All homogeneous samples accumulated over -2% residual strain. The rolled sample exhibited high stresses and eventually buckling, with the deformation being accommodated by mechanical twinning. Lastly, the cast sample exhibited early functional fatigue in the first 3 cycles, and localized deformation in the top section as shown in optical micrographs.
In contrast the FG sample exhibited superior cyclic stability accumulating only 0.89% of residual strain after 10 compression cycles. The DIC stain maps showed that most of the strain was carried out by the core, while the outer sections remained comparatively elastic and shared load. Optical micrographs also revealed that irreversible damage concentrated at the interface between the textured core and non-textured outer sections. FG sample was also tested to 6 compression cycles at 6 different temperatures (60-110 °C), where at lower temperatures it behaves more homogeneously while at higher temperatures the core carries most of the strain.
The results of this work demonstrate that LPBF process parameters can be used to tailor compressive superelasticity and cyclic stability and that functional grading of crystallographic texture is a highly effective strategy to mitigate cyclic degradation of NiTi, enabling the design of durable and high performance NiTi components.
Shape Memory Alloys (SMAs) are a class of metallic multi-functional materials which possess sensing and actuation capabilities, thanks to their unique ability to couple thermal and mechanical fields. NiTi-based Shape Memory Alloys exhibit properties which guarantee their high performance in adverse environments and, with the added benefit of two functionalities, the Shape Memory Effect and Superelasticity, these materials have become ideal candidates for a variety of applications. Spatial orientation of the NiTi crystals is crucial for achieving the superelastic functional response. According to studies on NiTi single crystals, when the grains adopt a [001] orientation along the direction of compression, slip is inhibited in the austenite phase and recoverable transformation strains similar to the theoretically estimated values of 5.3% are possible, defining the benchmark for superelastic responses. Additive Manufacturing techniques, due to their inherent thermal processing conditions, have created unprecedented opportunities for fabrication of NiTi-based Shape Memory Alloys so that the thermomechanical behaviour of the material can be tailored to any application is intended for. During Laser- Powder Bed Fusion, the scanning strategy influences the thermal processing to which the alloy is subjected, affecting its solidification process as well as the heat fluxes and temperature gradients that arise during manufacturing. Meanwhile, it can play a decisive role in achieving epitaxial solidification of columnar grains extending over multiple deposition layers. In this study, five different scanning strategies were employed during Laser-Powder Bed Fusion (L-PBF) of a Ni-rich NiTi alloy and were evaluated with respect to the grain morphology and crystallographic texture that developed, as well as the superelastic response of the samples produced. It was found that a 67° interlayer rotation of the scanning vector promotes the fusion of neighbouring melt pools, resulting in columnar grains that solidify epitaxially along the build direction (BD) of the sample. Meanwhile, a strong ⟨001⟩ fibre texture emerges along the BD. The superelastic response was stabilised at 4.5% recoverable strain with 74.2% recovery ratio after 16 cycles of axial compression loading. When an island scanning pattern was incorporated into the 67° rotation scanning strategy, the crystallographic texture strengthened and the superelastic response improved, (5.0% stabilised recoverable strain and 84% recovery ratio). Furthermore, an increase in the volumetric energy density, achieved by using a flat top laser beam, produced a nearly single-crystalline microstructure, with the highest intensity of the ⟨001⟩∥BD texture. The superelasticity in this case was stabilised at 5.5% recoverable strain with 91.5% recovery ratio. The effect of the loading direction on the superelastic response was also investigated, as was the nature of the residual strain left in the samples after their superelasticity stabilised. Therefore, this study successfully demonstrated that the scanning strategy can be a vital tool in designing the crystallographic texture and the grain morphology of NiTi parts fabricated by L-PBF, and this way, effectively tailor the superelastic functional behaviour to specific requirements of potential applications.
...
Shape Memory Alloys (SMAs) are a class of metallic multi-functional materials which possess sensing and actuation capabilities, thanks to their unique ability to couple thermal and mechanical fields. NiTi-based Shape Memory Alloys exhibit properties which guarantee their high performance in adverse environments and, with the added benefit of two functionalities, the Shape Memory Effect and Superelasticity, these materials have become ideal candidates for a variety of applications. Spatial orientation of the NiTi crystals is crucial for achieving the superelastic functional response. According to studies on NiTi single crystals, when the grains adopt a [001] orientation along the direction of compression, slip is inhibited in the austenite phase and recoverable transformation strains similar to the theoretically estimated values of 5.3% are possible, defining the benchmark for superelastic responses. Additive Manufacturing techniques, due to their inherent thermal processing conditions, have created unprecedented opportunities for fabrication of NiTi-based Shape Memory Alloys so that the thermomechanical behaviour of the material can be tailored to any application is intended for. During Laser- Powder Bed Fusion, the scanning strategy influences the thermal processing to which the alloy is subjected, affecting its solidification process as well as the heat fluxes and temperature gradients that arise during manufacturing. Meanwhile, it can play a decisive role in achieving epitaxial solidification of columnar grains extending over multiple deposition layers. In this study, five different scanning strategies were employed during Laser-Powder Bed Fusion (L-PBF) of a Ni-rich NiTi alloy and were evaluated with respect to the grain morphology and crystallographic texture that developed, as well as the superelastic response of the samples produced. It was found that a 67° interlayer rotation of the scanning vector promotes the fusion of neighbouring melt pools, resulting in columnar grains that solidify epitaxially along the build direction (BD) of the sample. Meanwhile, a strong ⟨001⟩ fibre texture emerges along the BD. The superelastic response was stabilised at 4.5% recoverable strain with 74.2% recovery ratio after 16 cycles of axial compression loading. When an island scanning pattern was incorporated into the 67° rotation scanning strategy, the crystallographic texture strengthened and the superelastic response improved, (5.0% stabilised recoverable strain and 84% recovery ratio). Furthermore, an increase in the volumetric energy density, achieved by using a flat top laser beam, produced a nearly single-crystalline microstructure, with the highest intensity of the ⟨001⟩∥BD texture. The superelasticity in this case was stabilised at 5.5% recoverable strain with 91.5% recovery ratio. The effect of the loading direction on the superelastic response was also investigated, as was the nature of the residual strain left in the samples after their superelasticity stabilised. Therefore, this study successfully demonstrated that the scanning strategy can be a vital tool in designing the crystallographic texture and the grain morphology of NiTi parts fabricated by L-PBF, and this way, effectively tailor the superelastic functional behaviour to specific requirements of potential applications.
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.
...
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.