Ausonio Tuissi

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.

Fatigue of pseudoelastic NiTi within the stress-induced transformation regime: a modified Coffin–Manson approach

Strain controlled fatigue tests of a pseudoelastic nickel?titanium (NiTi) shape memory alloy (SMA) have been carried out in this investigation. In particular, flat dog-bone shaped specimens, obtained from commercial NiTi sheets, have been analyzed, under pull?pull loading conditions, in two subsequent steps: (i)?material stabilization and (ii)?fatigue life estimation. The first step was carried out to obtain a stable pseudoelastic response of the SMA, i.e. with no residual deformations upon unloading, and it can be regarded as a preliminary processing condition of the alloy. Results on functional fatigue, i.e. in terms of stabilized pseudoelastic response, and on structural fatigue, in terms of cycles to failure, are reported and discussed. Furthermore, a modified Coffin?Manson approach for fatigue life estimation of SMAs is proposed, which takes into account the strain mechanisms involved during repeated stress-induced martensitic transformations.

Processing of Shape Memory CuZnAl Open-cell Foam by Molten Metal Infiltration

Foams and other highly porous materials with a cellular structure are known to have many interesting combinations of physical and mechanical properties. In addition, foaming of shape memory alloys (SMA) greatly improves the set of application possibilities. In this work an open-cell metal foam of a CuZnAl SMA is presented. This foam was produced through a recently proposed process, which consists mainly in molten metal infiltration of a bed of silica-gel particles and the subsequent use of hydrofluoric acid (HF) as solvent. The results showed that very regular open-cell foams could be obtained, having an almost spherical cell morphology and relative densities of approximately 0.3. Microstructural and compositional analyses on foamed specimens showed uniform microstructure of ligaments and the absence of SiO2 interaction with the metal. In this way the possibility of foaming CuZnAl system through the proposed low-cost process was clearly demonstrated.

Application of SMA Composites in the Collectors of the Railway Pantograph for the Italian High-Speed Train

In high-speed train operation the flexural modes of the collector play a significant role in the vibrations for the frequencies over 40-50 Hz. In a previous work it has been established that a possible way to increase the modal damping of these flexural modes, without deep modification of the collector structure, is to increase the specific damping of the lateral horns, usually made of glass fiber polymer. Ni-Ti alloy yarns can be used as “smart fibers” embedded in this conventional material in order to make new horns with increased damping capacity, with a configuration of laminated composite material. The first step of the work herein presented consists in setting, through a proper thermal treatment, martensitic structure within the pantograph working temperature range, in order to obtain damping capabilities at low amplitude strain in the range 10−4-10−3. Afterwards a series of dynamic tests aimed at identifying the damping capacity of the NiTi wires has been undertaken. A finite element (FE) model of the SMA composites horn has been finally validated, comparing the results of dynamic numerical analysis with the results of measurements.

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.

Consolidated Al/Al2O3 Nanocomposites by Equal Channel Angular Pressing and Hot Extrusion

This work is aimed at producing Al-matrix nanocomposites through the powder metallurgy (PM) process. An initial study on pure aluminum powder was performed to analyze the performance of different PM techniques. In particular, Al powder was refined via high-energy ball milling and consolidated by hot extrusion and equal channel angular pressing. The latter revealed to be a very efficient procedure for powder compaction (high density and hardness of the products) and it was adopted for the production of Al-based composites reinforced with 2% and 5 wt% of Al2O3 nanoparticles. The composites exhibited higher hardness than the pure aluminum sample.

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.

Functional Characterization of Shape Memory CuZnAl Open-Cell Foams by Molten Metal Infiltration

In the recent years, the research for novel materials with tailored mechanical properties, as well as functional properties, has encouraged the study of porous and cellular materials. Our previous work proposed and reported about the possibility to manufacture open-cell metal foams of CuZnAl shape memory alloy by liquid infiltration in a leachable bed of silica-gel particles. This innovative methodology is based on cheap commercial consumables and a simple technology, focusing on intermediate-density low-cost foams with interesting cost/benefits ratio. Microstructural analyses on foamed specimens showed uniform microstructure of ligaments and a very regular and well reproducible open-cell morphology. Moreover, calorimetric analysis detected a thermo-elastic martensitic transformation in the foamed material. In this study, a CuZnAl shape memory alloy was considered and tested to clarify possible effects of the foaming process on the functional properties of the material. Morphological, calorimetric, and thermo-mechanical analyses were carried out. The results show that it is possible to produce metal foams of CuZnAl shape memory alloy with different functional properties and able to recover mono-axial compressive strains up to 3%.

Microstructure Evolution and Aging Kinetics of Al-Mg-Si and Al-Mg-Si-Sc Alloys Processed by ECAP

A study was carried out on a ECAP processed Sc-containing Al-Mg-Si alloy and on a reference 6082 alloy to investigated grain structure evolution during severe plastic deformation and post-ECAP aging behaviour. The results showed that the mechanism of ultrafine structure development was substantially unchanged with respect to a reference Sc-free alloy. Also the aging sequence and precipitation kinetics of the two alloys revealed to be comparable. The ECAP processed samples of the 6082 reference alloy showed a clear recrystallization peak at temperatures in the range 315-360°C, depending on the amount of strain experienced, whereas the Sc-containing alloy retained its ultrafine structure up to temperatures well exceeding 450°C, under the conditions reproduced in a DSC temperature scan.