Carlo Alberto Biffi

Laser shape setting of thin NiTi wires

An unconventional method for shape setting of NiTi shape memory alloy wires is proposed. A laser beam was used to induce straight shape, superelasticity and shape memory effects to thin NiTi wires. Laser treatment revealed to be a suitable and reliable alternative to the conventional final thermal treatment of NiTi wires, which is normally carried out in annealing furnaces. Thermo-mechanical and calorimetric tests showed that the laser heat treatment confers optimal functional properties, analogous to those of commercially available NiTi wires.

Fibre Laser Cutting and Chemical Etching of AZ31 for Manufacturing Biodegradable Stents

The use of magnesium-alloy stents shows promise as a less intrusive solution for the treatment of cardiovascular pathologies as a result of the high biocompatibility of the material and its intrinsic dissolution in body fluids. However, in addition to requiring innovative solutions in material choice and design, these stents also require a greater understanding of the manufacturing process to achieve the desired quality with improved productivity. The present study demonstrates the manufacturing steps for the realisation of biodegradable stents in AZ31 magnesium alloy. These steps include laser microcutting with a Q-switched fibre laser for the generation of the stent mesh and subsequent chemical etching for the cleaning of kerf and surface finish. Specifically, for the laser microcutting step, inert and reactive gas cutting conditions were compared. The effect of chemical etching on the reduction in material thickness, as well as on spatter removal, was also evaluated. Prototype stents were produced, and the material composition and surface quality were characterised. The potentialities of combining nanosecond laser microcutting and chemical etching are shown and discussed.

Al/Al2O3 Nanocomposite Produced by ECAP

Metal matrix nanocomposites have been produced by powder metallurgy route. Al and nanoAl2O3 powders were grinded through high energy ball milling. Then, the composite powders were sintered by Equal Channel Angular Pressing (ECAP). 12 ECAP passes were carried out in order to improve the dispersion of the hard particles. SEM analysis was performed to investigate the distribution of the ceramic nanoparticles within the matrix. Hardness tests were executed to evaluate the mechanical behavior of the nanocomposites. Finally, mechanical strength values obtained by numerical models were compared with those estimated from hardness measurements. High energy ball milling followed by ECAP process revealed to be a suitable route for the production of metal matrix composites reinforced with well dispersed nanoparticles.

Effect of laser microcutting on thermo-mechanical properties of NiTiCu shape memory alloy

The machining of shape memory alloys (SMAs), such as NiTi based alloys, is a very interesting and relevant topic for several industrial applications in the biomedical, sensor and actuator fields. Laser technology is one of the most suitable methods for the manufacturing of products in the aforementioned fields, mainly when small and precise features have to be included. Due to the thermal nature of this process, study of its effect on the functional properties of these materials is needed. Except for binary NiTi, few results on the laser machining of NiTi based alloys are available in the literature. In this work, thin sheets of Ni40Ti50Cu10 (at.%) were processed by a fibre laser and the effect of process speed on the material properties was analysed. Scanning electronic microscopy was adopted for observation of the laser cut edges’ morphology. Chemical composition of the processed material was evaluated by energy dispersion spectroscopy and nanohardness measurements were used to estimate the heat affected zone. SMA functional properties were studied on both base and laser machined material. These characteristics are affected by laser machining for the presence of melted material; this effect can be minimised by increasing the laser process speed.

CuZr Based Shape Memory Alloys: Effect of Cr and Co on the Martensitic Transformation

In the present work an investigation on CuZr based shape memory alloys was proposed. In particular, this study has been addressed the effect of the addition of Cr and Co on the martensitic transformation behaviour. The characterization was performed using DSC in terms of evolution of characteristic temperatures. The analysis of the proposed alloys was completed with the evaluation of the microhardness and the microstructure, observed by means of a scanning electron. Moreover, X-rays diffraction analysis was also carried out to check the crystal structures in the different alloys. It was shown how the addition of Co can improve thermal stability and the thermal hysteresis of the martensitic transformation by the first thermal cycles, even if the characteristic temperatures were significantly decreased.

Biodegradable magnesium coronary stents: material, design and fabrication

Biodegradable cardiovascular stents in magnesium (Mg) alloys constitute a promising option for a less intrusive treatment, due to their high compatibility with the body tissue and intrinsic dissolution in body fluids. The design and fabrication aspects of this medical device require an integrated approach considering different aspects such as mechanical properties, corrosion behaviour and biocompatibility. This work gathers and summarises a multidisciplinary work carried out by three different research teams for the design and fabrication of Mg stents. In particular, the paper discusses the design of the novel stent mesh, the deformability study of the Mg alloys for tubular raw material and laser microcutting for the realisation of the stent mesh. Although, the results are not fully validated as the device has not been fully tested, they show the feasibility of the used approaches, as the first prototype stents in Mg alloy were produced successfully.

Fiber laser micromachining of magnesium alloy tubes for biocompatible and biodegradable cardiovascular stents

Magnesium alloys constitute an attractive solution for cardiovascular stent applications due to their intrinsic properties of biocompatibility and relatively low corrosion resistance in human-body fluids, which results in as a less intrusive treatment. Laser micromachining is the conventional process used to cut the stent mesh, which plays the key role for the accurate reproduction of the mesh design and the surface quality of the produced stent that are important factors in ensuring the mechanical and corrosion resistance properties of such a kind of devices. Traditionally continuous or pulsed laser systems working in microsecond pulse regime are employed for stent manufacturing. Pulsed fiber lasers on the other hand, are a relatively new solution which could balance productivity and quality aspects with shorter ns pulse durations and pulse energies in the order of mJ. This work reports the study of laser micromachining and of AZ31 magnesium alloy for the manufacturing of cardiovascular stents with a novel mesh design. A pulsed active fiber laser system operating in nanosecond pulse regime was employed for the micromachining. Laser parameters were studied for tubular cutting on a common stent material, AISI 316L tubes with 2 mm in diameter and 0.2 mm in thickness and on AZ31 tubes with 2.5 mm in diameter and 0.2 in thickness. In both cases process parameters conditions were examined for reactive and inert gas cutting solutions and the final stent quality is compared.

FLEXURAL VIBRATION SUPPRESSION OF GLASS FIBER/CuZnAl SMA COMPOSITE

This work proposes the functional characterization of a composite material, suitable for passive suppression of flexural vibration of beams and shells. Two patterned thin sheets of CuZnAl Shape Memory Alloy (SMA) are embedded into a layered beam of glass fiber. The composite combines the density and stiffness of the glass fiber with high damping properties of SMA in martensitic state. Properly shaped patterning of the SMA sheets, for improving adhesion between the SMA and glass fiber, is performed by means of laser technology. The effect of the laser micromachining on transformation temperatures and internal friction properties of the SMA elements are analyzed. Finally, measurements of the structural damping of the layered glass fiber/SMA composite are reported and the flexural vibration suppression, due to the embedded CuZnAl sheets, is shown.

High performance shape memory effect in nitinol wire for actuators with increased operating temperature range

In this research, the high performance shape memory effect (HP-SME) is experimented on a shape memory NiTi wire, with austenite finish temperature higher than room temperature. The HP-SME consists in the thermal cycling of stress induced martensite and it allows achieving mechanical work higher than that produced by conventional shape memory actuators based on the heating/cooling of detwinned martensite. The Nitinol wire was able to recover about 5.5% of deformation under a stress of 600 MPa and to withstand about 5000 cycles before failure. HP-SME path increased the operating temperature of the shape memory actuator wire. Functioning temperatures higher than 100°C was reached.

CuZnAl Shape Memory Alloys Foams

Foams and other highly porous metallic materials with cellular structures are known to have many interesting combinations of physical and mechanical properties. That makes these systems very attractive for both structural and functional applications. Cellular metals can be produced by several methods including liquid infiltration of leachable space holders. In this contribution, results on metal foams of Cu based shape memory alloys (SMAs) processed by molten metal infiltration of SiO2 particles are presented. By using this route, highly homogeneous CuZnAl SMA foams with a spherical open-cell morphologies have been manufactured and tested. Morphological, thermo-mechanical and cycling results are reported.