Papers by Author: Roberto Montanari

Abstract: Dynamic modulus vs. temperature was measured in different alloys (stainless steels, Al alloys, Ti alloys, Ni-base superalloys) prepared by additive manufacturing and an anomalous trend was observed in some of them. Dynamic modulus, measured in successive mechanical spectroscopy test runs with heating-cooling cycles, exhibits an anomalous trend in the first test run that is no longer present in the successive runs. The phenomenon consists in the inversion of the decreasing trend of modulus occurring during heating and gives rise to its permanent increase at the end of the complete heating-cooling cycle. The temperature range where the modulus anomaly takes place and the permanent increase observed after cooling depend on the specific alloy. Scanning electron microscopy (SEM) observations and density measurements revealed that the irreversible process causing the anomalous behavior is the closure of pores of nanometric size leading to material densification. This result has been discussed by considering lattice diffusion.

Abstract: Mechanical spectroscopy (MS) is a dynamic technique for the characterization of material properties providing information that can not be obtained otherwise, and is important for a variety of engineering fields. To illustrate the potentiality of MS, this work provides some examples regarding different metallic systems: (i) thin Al foils for MEMS, (ii) complex structures of point defects in Cr martensitic steels for structural applications in future nuclear fusion reactors, (iii) depinning of dislocations from point defects and precipitates.

Abstract: This work briefly discusses the applications of Layered Double Hydroxides (LDHs) to medicine and presents a study regarding the growth of LDHs on the biodegradable AZ31 alloy foreseen to manufacture a rib-fixator. Mg is one of the most investigated metallic materials for biomedical applications owing to its high biocompatibility and osteointegration, as well as a value of the elastic modulus close to that of human bone. Since Mg is essential for metabolism, when it degrades forming Mg²⁺ ions, it promotes healing and growth of bone tissue. Experiments have been carried out to grow LDHs on the alloy surface in view to retard corrosion in human body and intercalate drugs to be released in-situ, with anti-inflammatory, analgesic, and antimicrobial action.

Abstract: Austenitic stainless steels produced by Laser Powder Bed Fusion (L-PBF) are interesting materials because of their excellent corrosion resistance. Due to their relatively low hardness, the tribological response of these materials is poor, which limits their use in applications where control of wear degradation is important. Nevertheless, low-temperature plasma-assisted carburisation is an interesting process for improving the wear resistance of austenitic stainless steels, as has been observed for wrought materials. In fact, the increase in hardness is guaranteed by a surface layer of expanded austenite (S-phase) with a thin top layer of amorphous carbon. In this work, AISI 316 L, produced by the L-PBF technique, was carburised using 5 different plasma gas mixtures (by varying the CH4/H2 ratio) at 475°C for 7 hours. The samples obtained were then subjected to a detailed microstructural characterisation in order to obtain information on surface modification. The morphological features of the surface were examined by SEM observations in top view and in cross-section. The tribological performance was evaluated by pin-on-flat tests (alumina sphere as counter-material) with 2 different applied loads and a stroke length of 5 mm. Friction coefficient, wear rate (stylus profilometer) and wear mechanisms (SEM) were also evaluated. Preliminary results show an increase in wear resistance of all plasma treated materials compared to the untreated material. The improved tribological performance was discussed in relation to the abrupt increase in surface hardness.

Abstract: EUROFER97 martensitic steel is recognized in EU as the reference material for the test blanket module in ITER reactor and for structural sections subject to high radiation doses in DEMO reactor. An extended experimental campaign has been carried out with the scope of improving strength without loss of ductility. The main idea behind the present study is to reach the goal through grain refinement achieved by cold rolling and heat treatments for inducing recrystallization of the work hardened structure. A combination of five cold rolling reduction ratios (CR) (20%, 40%, 50%, 60%, 80%) and eight heat treatments in the temperature range 400-750°C (steps of 50 °C) with soaking time of 1 hour has been examined to describe the evolution of microstructure and mechanical properties. The strength of deformed samples decreases as the heat treatment temperature increases and the change is more pronounced in the samples cold-rolled with higher CR ratios. The results showed that cold rolling with CR of 80% followed by a treatment at 650 °C produces a fully recrystallized structure with sub-micrometric grains which guarantees improved yield stress and hardness than standard EUROFER97 steel, with a comparable total elongation. In conclusion, this work demonstrated the feasibility to strengthen EUROFER97 without compromising its ductility.

Abstract: Ni base superalloys are commonly employed in the industrial fields of aerospace, automotive and energy production due to their excellent mechanical properties and corrosion resistance at high temperature. Superficial defects and cracks may occur during both manufacturing process of components and their service life. High energy density welding techniques, electron beam (EBW) and laser beam (LBW) welding, can be used to create efficient repairs. Joints, obtained by EBW and LBW of IN792 directionally solidified (DS) superalloy, have been investigated to determine the presence of defects, and evaluate the mechanical properties related to specific microstructural features. The results showed that a pre-heating temperature (PHT) higher than 200 °C is always necessary to prevent the formation of hot cracks in the molten zone (MZ) and heat affected zone (HAZ). The process parameters have been optimized to get a good quality of the seams (lack of macro-defects, a good penetration depth and width). Some preliminary test of post-welding heat treatments (PWHTs) have been investigated to homogenize as far as possible the microstructure and the mechanical properties across the seams. The results obtained by the two techniques, EBW and LBW, have been compared and discussed.

Abstract: Additive manufacturing of Al alloys can represent an interesting solution for high-performance components in various industrial fields, as for instance the automotive and aerospace industry. Often, for these applications, the alloys are required to withstand exposure to high temperatures. Therefore, the investigation of the evolution of material properties with increasing temperature is of utmost importance in order to assess their suitability for this kind of applications. In the present study, tensile properties at high temperature were investigated for an AlSi10Mg alloy. Samples were manufactured by laser-based powder bed fusion in horizontal and vertical direction in order to examine the influence of building direction on material behavior. The samples were tested in as-built condition and after exposure to high temperature. Tensile tests were performed up to 150 °C and the effect of holding time at the test temperature was evaluated. Furthermore, the alloy was characterized by mechanical spectroscopy in order to evaluate the behavior of dynamic modulus with temperature and, thus, to provide a comprehensive characterization of the material behavior. It was found that the peculiar microstructure of the alloy produced by additive manufacturing is responsible for good high-temperature strength of the material up to 150 °C. The material also exhibits a good thermal stability even after holding at test temperature for 10 h.

Abstract: The problems for fixing rib fractures are usually challenged with different rib fixators whose design strongly depends of the material selected for construction. Main issues refer to rib surgery implantation and tissue acceptance for a duration longer than the osteosynthesis. In this paper we discuss how a material selection can strongly suggest different design solutions both in shape of a rib fixator and even constraining or directing the surgical application with an invasive or noninvasive implantation. In particular, in discussing the general issues and specific experiences of the authors the paper illustrates examples of two different solutions under patent request that are dictated by the material whose selection give the design solutions with innovative aspects.

Abstract: The work investigates refractory metals (bulk W, W produced via plasma spraying, W-1% La₂O₃ and Mo) of interest as plasma facing materials in future nuclear fusion reactors. They have been irradiated by a single Nd:YAG laser pulse to simulate the effects of transient thermal loads of high energy occurring in a tokamak under operative conditions and then examined by SEM observations. In all the materials the laser pulse induces a crater in the central area of laser spot surrounded by a ridge due to movement of molten metal while in a more external area a network of cracks is observed. Diameter and depth of the crater, ablated volume and morphological features of the surrounding area exhibit differences depending on the specific metal, its physical and microstructural characteristics which affect vaporization, melting and heat propagation from the irradiated spot.

Abstract: In Europe EUROFER 97 has been recognised as reference steel for the nuclear costructions under high radiation density for first wall of a fast breeder reactors as well as in other high stressed primary structures such as the divertors, blanklet and vessels. Following to this a EUROFER 97 detailed knowledge of the microstructure evolution after thermo-mechanical processing is required, because the material mechanical properties are interesting also for innovative solar plants, i.e. NEXTOWER project. A detailed knowledge of process optimisation is mandatory because EUROFER 97 steel mechanical properties and microstruture are heavily influenced and improved (and easily affeted) by thermomechanical treatments. In this paper the effect of thermo-mechanical parameters on the grain refinement of EUROFER 97 has been investigated by cold rolling and heat treatment on pilot scale.