Paola Bassani

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.

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.

Effects of Isothermal Ageing and Continuous Cooling after Solubilization in a Duplex Stainless Steel

The kinetics of precipitation of secondary phases in a duplex stainless steel (SAF 2205) after isothermal and continuous cooling treatment were investigated. The evolution of the phases chemical composition in relation with time and cooling rate is presented.

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.

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%.

Characterization of a Cold-Rolled 2101 Lean Duplex Stainless Steel

Duplex stainless steels (DSS) may be defined as a category of steels with a two-phase ferritic-austenitic microstructure, which combines good mechanical and corrosion properties. However, these steels can undergo significant microstructural modification as a consequence of either thermo-mechanical treatments (ferrite decomposition, which causes σ- and χ-phase formation and nitride precipitation) or plastic deformation at room temperature [austenite transformation into strain-induced martensite (SIM)]. These secondary phases noticeably affect the properties of DSS, and therefore are of huge industrial interest. In the present work, SIM formation was investigated in a 2101 lean DSS. The material was subjected to cold rolling at various degrees of deformation (from 10 to 80% thickness reduction) and the microstructure developed after plastic deformation was investigated by electron backscattered diffraction, X-ray diffraction measurements, and hardness and magnetic tests. It was observed that SIM formed as a consequence of deformations higher than ~20% and residual austenite was still observed at 80% of thickness reduction. Furthermore, a direct relationship was found between microstructure and magnetic properties.

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.

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.

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.