Soheil Saedi

Achieving biocompatible stiffness in NiTi through additive manufacturing

This article seeks to reduce the stiffness of NiTi parts from a nonporous state to that of human bone by introducing porosity. Compact bone stiffness is between 12 and 20 GPa while the currently used bone implant materials are several times stiffer. While very stiff implants and/or fixation hardware can temporarily immobilize healing bone, it also causes stress shielding of the surrounding bone and commonly results in stress concentrations at the implant or immobilization hardware’s fixation site(s). Together these processes can lead to implant or fixation hardware and/or the surrounding bone’s failure. Porous NiTi can be used to reduce the stiffness of metallic implants while also providing necessary stabilization or immobilization of the patient’s reconstructed anatomy. In this work, mechanical behavior of porous NiTi with different levels of porosity is simulated to show the relation between the stiffness and porosity level. Then porous structures are fabricated through additive manufacturing to validate the simulation results. The results indicate that stiffness can be reduced from the bulk value of 69 GPa to as low as 20.5 GPa for 58% porosity. The simulation shows that it is possible to achieve a wide range of desired stiffness by adjusting the level of porosity.

Thermomechanical characterization of Ni-rich NiTi fabricated by selective laser melting

This study presents the shape memory behavior of as-fabricated and solution annealed Ni50.8Ti49.2 alloys fabricated using the selective laser melting (SLM) technique. Results were compared to the initial ingot that was used to fabricate powders. Optical microscopy was employed to reveal the microstructure. The shape memory effect under constant compressive stress and isothermal compressive stress cycling tests were utilized to investigate the shape memory characteristics of the initial ingot and fabricated alloys. It was revealed that the SLM method and post heat treatments can be used to tailor the microstructure and shape memory response. Partial superelasticity was observed after the SLM process. Solutionizing the fabricated samples increased the strength and improved the superelasticity but slightly decreased the recoverable strain.

Mechanical and shape memory properties of porous Ni50.1Ti49.9 alloys manufactured by selective laser melting

Near equiatomic NiTi shape memory alloys were fabricated in dense and designed porous forms by Selective Laser Melting (SLM) and their mechanical and shape memory properties were systematically characterized. Particularly, the effects of pore morphology on their mechanical responses were investigated. Dense and porous NiTi alloys exhibited good shape memory effect with a recoverable strain of about 5% and functional stability after eight cycles of compression. The stiffness and residual plastic strain of porous NiTi were found to depend highly on the pore shape and the level of porosity. Since porous NiTi structures have lower elastic modulus and density than dense NiTi with still good shape memory properties, they are promising materials for lightweight structures, energy absorbers, and biomedical implants.

Structural Vibration Control Using High Strength and Damping Capacity Shape Memory Alloys

Designing structures to withstand dynamic environmental hazards such as earthquakes, strong winds, and hurricanes is of primary concern for civil engineers. In addition, recent advances in architectural forms, structural systems, and high performance materials have enabled the design of very slender and lightweight structures. These flexible structures are susceptible to be exposed to high levels of vibrations under strong winds and earthquakes, which may lead to structural damage and potential failure. Over the past two decades, shape memory alloys (SMAs) have emerged as a smart material that can be used in passive vibration control devices for energy dissipating and re-centering purposes. However, the quantity of equivalent viscous damping provided by superelastic NiTi SMA wires or bars is not sufficient to render the use of SMAs as the sole damping device implemented in a tall structure subjected to severe dynamic loadings. This study explores the performance of recently developed NiTiHfPd alloys that have very high strength, high dissipation/damping capacity, good cyclic stability and large operating temperature for vibration control applications. In particular, superelastic response of single crystal and polycrystalline NiTiHfPd alloys were investigated systematically to reveal the effects of composition alteration and heat treatments. The design of a novel passive vibration control device by utilizing the superelastic effect of NiTiHfPd SMAs under compression was illustrated.

Shape memory response of porous NiTi shape memory alloys fabricated by selective laser melting

AbstractPorous NiTi scaffolds display unique bone-like properties including low stiffness and superelastic behavior which makes them promising for biomedical applications. The present article focuses on the techniques to enhance superelasticity of porous NiTi structures. Selective Laser Melting (SLM) method was employed to fabricate the dense and porous (32–58%) NiTi parts. The fabricated samples were subsequently heat-treated (solution annealing + aging at 350 °C for 15 min) and their thermo-mechanical properties were determined as functions of temperature and stress. Additionally, the mechanical behaviors of the samples were simulated and compared to the experimental results. It is shown that SLM NiTi with up to 58% porosity can display shape memory effect with full recovery under 100 MPa nominal stress. Dense SLM NiTi could show almost perfect superelasticity with strain recovery of 5.65 after 6% deformation at body temperatures. The strain recoveries were 3.5, 3.6, and 2.7% for samples with porosity levels of 32%, 45%, and 58%, respectively. Furthermore, it was shown that Young’s modulus (i.e., stiffness) of NiTi parts can be tuned by adjusting the porosity levels to match the properties of the bones.

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.

Selective laser melting of Ni-rich NiTi: selection of process parameters and the superelastic response

Material and mechanical properties of NiTi shape memory alloys strongly depend on the fabrication process parameters and the resulting microstructure. In selective laser melting, the combination of parameters such as laser power, scanning speed, and hatch spacing determine the microstructural defects, grain size and texture. Therefore, processing parameters can be adjusted to tailor the microstructure and mechanical response of the alloy. In this work, NiTi samples were fabricated using Ni50.8Ti (at.%) powder via SLM PXM by Phenix/3D Systems and the effects of processing parameters were systematically studied. The relationship between the processing parameters and superelastic properties were investigated thoroughly. It will be shown that energy density is not the only parameter that governs the material response. It will be shown that hatch spacing is the dominant factor to tailor the superelastic response. It will be revealed that with the selection of right process parameters, perfect superelasticity with recoverable strains of up to 5.6% can be observed in the as-fabricated condition.

A local hybrid actuator for robotic surgery instruments

A major problem in the design of miniature scale motorised endoscopic instruments is the actuation mechanism at the instrument’s tip, especially when a high level of functionality and multi degrees of freedom (DOF) are concerned. In the present study, a new design for hybrid local-actuation of endoscopic instruments is proposed which includes a micro DC motor and a piezoelectric (PZT) actuator. A compact serial configuration was considered for the actuators. The DC motor provides the long movement course required for opening-closing function of the gripper while the PZT guaranteed the high gripping force needed for holding the needle. The efficacy of the design was evaluated in a simulation study using FEM, and it was shown that the needle holder is capable of applying a sufficiently high gripping force, up to 22 N. The feasibility of the design was further approved in experimental examination of a 4× scaled prototype of the instrument.