Papers by Author: Manuele Dabalà

Abstract: Aluminum alloys are one of the main materials employed in aerospace and automotive applications. One of the problems that affect these alloys in certain application is the poor corrosion resistance that can, however, be enhanced with proper surface treatments, such as anodizing. Among these treatments, Plasma Electrolytic Oxidation (PEO) is one of the most promising and the production of PEO coatings on traditional aluminum alloy was extensively studied in literature. Recently, the production of a lot of components is going into the direction of using innovative manufacturing systems and customized components, with improved mechanical and physical properties, can be manufactured by additive manufacturing (AM) techniques. Among the AM methods, laser-based AM has an immense potential for producing fully dense metallic structures, using a variety of available metal powders and has attracted more and more attention. The resulting AM samples are characterized by different microstructures in comparison with the conventionally manufactured ones and this can cause differences also in the production of PEO coatings and other surface treatments. Objective of the present work is to produce PEO coatings on AM samples in order to increase the corrosion and wear performances of the samples. PEO coating were produced on the samples testing different parameters and the coatings were characterized, in terms of microstructure and composition, with SEM analysis. The corrosion resistance of the samples was also evaluated with electrochemical tests. The results were compared with the ones obtained on traditionally manufactured samples. PEO coatings were successfully produced on AM samples obtaining samples with good coatings thickness and improved corrosion performances compared to the untreated ones.

Abstract: Waste of electrical and electronic equipment (WEEE) is the fastest growing advanced type of solid waste streams in the urban environment worldwide and contains interesting amounts of precious metals. Hydrometallurgical technique is fast emerging as preferred process for the recovery of a variety of metals due to its lower energy consume and lower smelter emissions than conventional pyrometallurgical processes. In this work, a hydrometallurgical process for the recovery of gold and silver from electronic scraps was studied. In place of cyanide, thiosulfate was chosen as complexing agent for gold. Thiosulfate leaching can be considered a non-toxic process and the gold dissolution rates can be faster than conventional cyanidation. The electronic scraps, obtained from “end of life” mobile phones, were crushed and pre-treated with nitric acid before the leaching. Different parameters were studied: concentration of thiosulfate, temperature and reaction time. Moreover, the use of ultrasound to assist the hydrometallurgical gold extraction was investigated, as its application in ores leaching shows a greater metals release in shorter time and the advantage of working at lower concentration of reagents and at lower temperature. In this work, the use of ultrasound allowed a higher recovery of the precious metals than conventional leaching in all the conditions studied (different concentrations of reagents, temperature and reaction time). Moreover, the studied process allowed also the recovery of the other metals present in the waste (Cu, Sn and Ag).

Abstract: Anodizing is one of the most promising surface treatments for lightweight metals as titanium because it can increase wear and corrosion-resistance, as well as provide aesthetic appearance and electrical insulation.Three different types of anodizing can be performed on titanium alloys: type I for elevated temperature forming, type II as anti-galling application and type III for coloured surfaces. The type II anodization, called also grey anodizing, is often requested in aerospace or biomedical applications. It is characterized by the use of alkaline electrolyte and it is standardized according to the SAE AMS 2488D norm. However, in literature it is difficult to find information about the process parameters of grey anodizing.In this work, different parameters of the grey anodizing process on a grade 5 titanium alloy were investigated and optimized, in order to obtain an anodized layer with the desired properties, in terms of corrosion resistance, thickness of the coating and wear properties. In particular, the effect of current and voltage applied, treatment time, temperature and electrolyte composition on the characteristics of the anodized layer was studied.The thickness, the composition, the morphology and the adhesion of the protective layers were characterized by SEM/EDS analysis. The chemical and phase composition were analyzed by XRF and XRD techniques. The corrosion resistance of the samples was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy tests. The best results were obtained using as electrolyte a solution containing sodium hydroxide, titanium dioxide, sodium silicate and activated charcoal, with the formation of an anodized layer mainly constituted by titanium oxides and silicates. Intermediate treatment times and ambient temperature were the best conditions to produce the anodized layer. The sample with the best performances showed a homogeneous protective layer about 5 μm thick and was characterized by a lower corrosion current density, higher corrosion potential and polarization resistance, compared with the other anodized samples.

Abstract: The continuous improvement of the applications of duplex stainless steels (DSS) is justified mainly by their excellent corrosion resistance in very aggressive environments and by their mechanical strength, often twice than that of conventional austenitic grades. Therefore more extensive applications have been suggested, like constructions, structural components, automotive applications, etc. However for such not advanced but quantitatively important applications the base cost of the alloy is one of the more constraining features. A way to obtain leaner alloys is to reduce the Ni content and to compensate with manganese and nitrogen additions. In the present paper the structural and mechanical properties of two low Ni duplex grades are analyzed in order to investigate the structural stability of the austenite against its possible transformation to martensite and the secondary phase’s precipitation. The detailed characterization has been performed with SEM-EDS on as received and heat treated (600-850 °C) alloys. A limited precipitation of chromium carbides and nitrides at the grain boundaries has been detected in both grades. The martensite formation has been revealed only in the 2101 grade. With regard to the impact toughness the behaviour of 2101 grade is quite similar to that of the common DSS, while in the 2304 the drastic drop of toughness is avoided. The corrosion behaviour of both grades in aggressive chloride environments is quite similar to that of austenitic AISI 304 grade.