Tailoring Texture, Microstructure, and Shape Memory Behavior of NiTi Alloys Fabricated by L-PBF-AM

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

Influence of SLM on Compressive Response of NiTi Scaffolds

Porous Nickel-Titanium shape memory alloys (NiTi-SMAs) have attracted much attention in biomedical applications due to their high range of pure elastic deformability (i.e., superelasticity) as well as their bone-level modulus of elasticity (E≈12-20 GPa). In recent years, Selective Laser Melting (SLM) has been used to produce complex NiTi components. The focus of this study is to investigate the superelasticity and compressive properties of SLM NiTi-SMAs. To this aim, several NiTi components with different level of porosities (32- 58%) were fabricated from Ni50.8Ti (at. %) powder via SLM PXM by Phenix/3D Systems, using optimum processing parameter (Laser power-P=250 W, scanning speed-v=1250mm/s, hatch spacing-h=120μm, layer thickness-t=30μm). To tailor the superelasticity behavior at body temperature, the samples were solution annealed and aged for 15 min at 350°C. Then, transformation temperatures (TTs), superelastic response, and cyclic behavior of NiTi samples were studied. As the porosity was increased, the irrecoverable strain was observed to be higher in the samples. At the first superelastic cycle, 3.5%, 3.5%, and 2.7% strain recovery were observed for the porosity levels of 32%, 45%, and 58%, respectively. However, after 10 cycles, the superelastic response of the samples was stabilized and full strain recovery was observed. Finally, the modulus of elasticity of dense SLM NiTi was decreased from 47 GPa to 9 GPa in the first cycle by adding 58% porosity

The effects of powder feedstock and process parameters on the material characteristics of Ti6Al4V thin strut features fabricated by laser powder bed fusion additive manufacturing

In this work, three different types of Ti6Al4V powder feedstock of different particle size ranges (fine, medium, and coarse) were utilized to fabricate thin strut lightweight features using laser powder bed fusion additive manufacturing (L-PBF-AM) using different process parameter settings. Thin strut features of varying dimensions from 0.1mm to 0.5mm were fabricated. The resulting sample sets allow for the analysis of the compound powder feedstock-process- geometry-material (PPG-M) characteristics for lightweight features fabricated by L-PBF-AM, which have not been previously explored. Various material characteristics were experimentally determined and analyzed, including success rate, geometry quality, porosity, pore size, grain size, and mechanical properties of the lightweight thin strut samples. The results clearly demonstrated the significance of the compound PPG-M relationships for lightweight structures, which calls for further studies to “re-establish” the knowledge base for L-PBF-AM materials at small dimension scales.Mechanical Engineerin

Energy damping in shape memory alloys: A review

In recent years shape memory alloys (SMAs) have gained significant attention as potential damping device materials. This article presents an extensive review of the damping characteristics of SMAs, as well as experimental methods used to characterize their damping properties. The shape memory response and associated damping quality are discussed for three popular families of SMAs; Fe-based, Cu-based, and NiTi-based alloys including their behaviors and limitations. This review article also summarizes the most important parameters that impact the damping behavior of SMAs which are necessary to be investigated by researchers and manufacturers to address the current design challenges