Micro-end milling of NiTi biomedical alloys, burr formation and phase transformation

This paper focuses on burr formation in micro-end milling of two Nickel-Titanium shape memory alloys (SMA), an austenitic and a martensitic NiTi. Phase transformation during machining was also examined. The experimental design approach was used to study the effect of cutting parameters on burr formation. The studied parameters were cutting speed, feed per tooth, depth and width of cut, 20 machining strategy and initial material phase of the NiTi alloy. Different types of burrs were formed during micro-end milling of NiTi alloys; it was observed that top burrs are the most important. The height of top burrs can reach values close to those of the depth of cut. Burrs were observed and characterized using a Scanning Electron Microscope (SEM), confocal and optical microscopes. The affected layer under the machined surface, and phase transformation 25 were investigated by using SEM. The results of the analysis of variance showed a significant formation of burrs, deeply influenced by the feed per tooth and width of cut. An increase in the feed per tooth and a decrease of width of cut tend to decrease the height and width of the top burr. In a thin layer under the machined surface, phase transformation was observed for the martensitic NiTi

Alliage de titane métastable, ressort d’horlogerie à base d’un tel alliage et son procédé de fabrication

L’invention se rapporte à un alliage de titane béta métastable comprenant en % massique entre 24 et 45% de niobium. L’invention se rapporte également à un ressort d’horlogerie réalisé à base d’un tel alliage et à un procédé de fabrication d’un tel ressort

Interaction of bone-dental implant with new ultra low modulus alloy using a numerical approach

International audienceAlthough mechanical stress is known as being a significant factor in bone remodeling, most implants are still made using materials that have a higher elastic stiffness than that of bones. Load transfer between the implant and the surrounding bones is much detrimental, and osteoporosis is often a consequence of such mechanical mismatch. The concept of mechanical biocompatibility has now been considered for more than a decade. However, it is limited by the choice of materials, mainly Ti-based alloys whose elastic properties are still too far from cortical bone. We have suggested using a bulk material in relation with the development of a new beta titanium-based alloy. Titanium is a much suitable biocompatible metal, and beta-titanium alloys such as metastable TiNb exhibit a very low apparent elastic modulus related to the presence of an orthorhombic martensite. The purpose of the present work has been to investigate the interaction that occurs between the dental implants and the cortical bone. 3D finite element models have been adopted to analyze the behaviour of the bone-implant system depending on the elastic properties of the implant, different types of implant geometry, friction force, and loading condition. The geometry of the bone has been adopted from a mandibular incisor and the surrounding bone. Occlusal static forces have been applied to the implants, and their effects on the bone-metal implant interface region have been assessed and compared with a cortical bone/ bone implant configuration. This work has shown that the low modulus implant induces a stress distribution closer to the actual physiological phenomenon, together with a better stress jump along the bone implant interface, regardless of the implant design

Mechanical stability of custom-made implants: Numerical study of anatomical device and low elastic Young's modulus alloy

The advent of new manufacturing technologies such as additive manufacturing deeply impacts the approach for the design of medical devices. It is now possible to design custom-made implants based on medical imaging, with complex anatomic shape, and to manufacture them. In this study, two geometrical configurations of implant devices are studied, standard and anatomical. The comparison highlights the drawbacks of the standard configuration, which requires a specific forming by plastic strain in order to be adapted to the patient’s morphology and induces stress field in bones without mechanical load in the implant. The influence of low elastic modulus of the materials on stress distribution is investigated. Two biocompatible alloys having the ability to be used with SLM additive manufacturing are considered, commercial Ti-6Al-4V and Ti-26Nb. It is shown that beyond the geometrical aspect, mechanical compatibility between implants and bones can be significantly improved with the modulus of Ti-26Nb implants compared with the Ti-6Al-4V

Fatigue performance evaluation of a Nickel-free titanium-based alloy for biomedical application - Effect of thermomechanical treatments

In the present work, structural fatigue experiments were performed on a Ti-26Nb alloy subjected to different thermomechanical treatments: a severe cold rolling, a solution treatment and two aging treatments at low-temperature conducted after cold rolling in order to optimize the kinetics of precipitation. The aim is to investigate the effect of microstructural refinement obtained by these processes on fatigue performances. Preliminary tensile tests were performed on each state and analyzed in terms of the microstructure documented by using X-Ray diffraction and TEM analysis. These tests clearly promote the short-time-aged cold-rolled state with a fine α and ω phases precipitation. An interesting balance between mechanical properties such as high strength and low Young's modulus has been obtained. Cyclic bending tests were carried out in air at 0.5%, 1%, 2% and 3% imposed strain amplitudes. At low straining amplitude, where the fatigue performances are at their best, the cold-rolled state does not break at 3×10E6 cycles and the long-time aged precipitation hardened state seems to be a good competitor compared to the cold-rolled state. All failure characteristics are documented by Scanning Electron Microscopy (SEM) micrographs and analyzed in term of microstructure

Martensite Transformation and Superelasticity at High Temperature of (TiHfZr)74(NbTa)26 High-Entropy Shape Memory Alloy

In this work, a (TiHfZr)(NbTa) 26 (%at) high-entropy quinary alloy has been developed especially for high-temperature superelastic applications and studied over a large range of temperatures. The mechanical properties of this new material were compared with those of other superelastic alloys. The different ingots have been made in a cold crucible from pure metals. Several thermomechanical treatments have been performed on the microstructure of four alloys among them (TiHfZr)(NbTa) 26 alloy. The microstructure of each alloy has been characterized by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and x-ray diffraction technique (XRD) and the mechanical behavior was investigated through three-point bending tests between - 40 and 200C, in quasi-static monotonic and low cycle loading conditions. The effects of the thermomechanical treatments on the static and cyclic thermomechanical mechanical responses have been analyzed in combination with the microstructure investigations of the four studied alloys. It has been shown that the (TiHfZr)(NbTa) 26 alloy presents a martensitic transformation and a superelastic effect over the studied range of temperatures, in the cold-worked state or after solution treatment. Finally, the obtained experimental results have been compared with those of other superelastic alloys demonstrating the features of the developed high-entropy high-temperature superelastic alloy

Stress Concentration and Mechanical Strength of Cubic Lattice Architectures

The continuous design of cubic lattice architecture materials provides a wide range of mechanical properties. It makes possible to control the stress magnitude and the local maxima in the structure. This study reveals some architectures specifically designed to reach a good compromise between mass reduction and mechanical strength. Decreased local stress concentration prevents the early occurrence of localized plasticity or damage, and promotes the fatigue resistance. The high performance of cubic architectures is reported extensively, and structures with the best damage resistance are identified. The fatigue resistance and S–N curves (stress magnitude versus lifetime curves) can be estimated successfully, based on the investigation of the stress concentration. The output data are represented in two-dimensional (2D) color maps to help mechanical engineers in selecting the suitable architecture with the desired stress concentration factor, and eventually with the correct fatigue lifetime

Journal of the Mechanical Behavior of Biomedical Materials

Porous structures, including those with lattice geometries, have been shown to mimic the mechanical properties of the human bone. Apart from the widely known strut-based lattices, the Triply Periodic Minimal Surfaces (TPMS) concept has been introduced recently to create surface-based lattices and to tailor their mechanical behaviors. In this study, the numerical investigation of the effective elastic properties, the anisotropic behavior, and the local stress distributions of a broad range of topologies provide us with a complete numerical tool to assist bone implant design. The comparison database of the lattices includes TPMS-based lattices, both sheet, and skeletal, as well as strut-based lattices. The lattices are subjected to periodic boundary conditions and also, a homogenization method is deployed to simulate the response of the lattice unit cells determining their apparent equivalent stiffness. A correlation among the lattice topologies, their effective mechanical properties, and the local Von Mises stress concentrations in them is observed. The stress distribution of various topologies with the same elastic modulus is examined to combine all the investigations. Finally, a large variety of numerical results are presented to allow the comparison of the lattice structures and the selection of the optimal configuration that mimics the elastic properties of the bone

In situ elaboration of a binary Ti–26Nb alloy by selective laser melting of elemental titanium and niobium mixed powders

Ti–Nb alloys are excellent candidates for biomedical applications such as implantology and joint replacement because of their very low elastic modulus, their excellent biocompatibility and their high strength. A low elastic modulus, close to that of the cortical bone minimizes the stress shielding effect that appears subsequent to the insertion of an implant. The objective of this study is to investigate the microstructural and mechanical properties of a Ti–Nb alloy elaborated by selective laser melting on powder bed of a mixture of Ti and Nb elemental powders (26 at.%). The influence of operating parameters on porosity of manufactured samples and on efficacy of dissolving Nb particles in Ti was studied. The results obtained by optical microscopy, SEM analysis and X-ray microtomography show that the laser energy has a significant effect on the compactness and homogeneity of the manufactured parts. Homogeneous and compact samples were obtained for high energy levels. Microstructure of these samples has been further characterized. Their mechanical properties were assessed by ultrasonic measures and the Young's modulus found is close to that of classically elaborated Ti–26Nb ingot

Elastically Graded Titanium Alloy Produced by Mechanical Surface Deformation

The objective of this study was to develop a thermo-mechanical strategy to create a radial elasticity gradient in a β metastable Ti-Nb-Zr alloy, and to characterize it in terms of microstructural and mechanical properties. A first investigation was conducted on thin samples of Ti-20Nb-6Zr (at.%) submitted to various thermo-mechanical treatments. Microstructure-properties relationships and elastic variability of this alloy were determined performing uniaxial tensile tests, X-ray diffraction and scanning and transmission electron microscopies. Based on these preliminary results, mechanical deformation was identified as a potential way to lower the elastic modulus of the alloy. In order to create elastically graded pieces, shot-peening was therefore carried out on thicker samples to engender surface deformation. In this second part of the work, local mechanical properties were evaluated by instrumented micro-indentation. Experimental observations demonstrated that shot-peening enabled to locally induce martensitic transformation on surface, and a decrease in indentation elastic modulus from 85 to 65 GPa over 400 μm was highlighted. Surface deformation proved to be an efficient way of creating an elasticity gradient in β metastable titanium alloys. This combination of material and process could be suitable to produce dental implants with mechanically enhanced biocompatibility