Mario Rosso

The influence of carbon on nitrogen substitution in iron ε-phases

Mössbauer spectroscopy of nitrided and carbonitrided iron powders and plates with total interstitial atom contents of between 24 and 33 at% has allowed the variation with composition of the hyperfine magnetic fields of metal atoms with 2 or 3 close interstitial atoms to be determined. Fe atoms with three non-metal nearest neighbours and paramagnetic behaviour have also been detected. Only when the content of non-metal atoms goes beyond 30 at%, do Fe atoms in ε-carbonitrides display higher magnetic fields than in the corresponding ε-nitrides. The lower number of electrons contributed to the iron 3d-orbitals by C atoms than by N atoms is considered the cause of the above phenomenon. The variation of other hyperfine parameters with composition is also discussed.

Studies of graded cemented carbides components

Graded structures obtained by coupling different WC grades, also containing cubic carbides type (Ta, Nb)C with Co as binder, were produced by vacuum sintering. Different batches of cylindrical samples were compacted by coupling two different grades and were sintered at 1450°C under vacuum. The hardness properties as well as the shrinkage due to sintering process are function of the employed grades and show gradual change between the single values of each of the coupled grades. The residual porosity was always lower than type A02, the observed microstructure shows very good link between the coupled grades, with the presence of a diffusion zone and without any influence of gravity on structure. The results show that it is possible to produce multilayer graded tools with a tough core, constituted by cheap carbide grades with high binder contents, covered by a very hard surface layer, achieving high economical and technical competitiveness for the production of tools for applications in high abrasive as well as in shock loaded environments.

Optimization of heat treatment cycles for automotive parts produced by rheocasting process

This work aims at studying the possibility of optimising the heat treatment cycles of parts produced using the New Rheocasting process in order to reduce the total cost of the operation, attaining good mechanical properties for high performance parts. The mechanical properties and the microstructure features of the considered A 356 alloy and the relative produced parts have been analysed and studied on samples machined both from the as cast and from the heat treated compomnents. The obtained results showed the possibility of successfully modifying the T6 heat treatment cycle with economical benefits, maintaining at the same time comparable high level properties and performances, together with good dimensional precision.

Corrosion resistance and properties of pump pistons coated with hard materials

Abstract In several industrial branches the thermal spray coatings are used to prevent the corrosion of a certain substrate. From the point of view of corrosion resistance, the coating microstructure as well as the chemical, physical and mechanical properties play an important role in the working behaviour of the coated parts. The coating porosity is mainly important because the corrosion media could penetrate towards the substrate through the interconnected pores or other defects located inside the coating. This paper is focussed on the study of wear and corrosion behaviour of Alumina thermal spray coatings for pump piston application. The behaviour of coatings with different percentages of Aluminium Oxide and Titanium Oxide is compared. The whole selected coating materials show high corrosion resistance both in saline and acid environment. The substrate pretreatment, the Ni–Cr bond coat and the spraying procedure ensure to the sprayed layers quite a good adhesion level. The salt spray test and a static immersion test in acid medium have been carried out. Microhardness evaluation, light microscopy and electron microscopy have been used to complete the characterisation programme.

Study of Ti-Enriched CoCrMo Alloy for Dental Application

Metallic materials can be considered to be in a peculiar position for production of medical devices, because of their attractive properties. In this paper, a new enriched Co-based composition is proposed for dental application, starting from a conventional CoCrMo alloy. Macrostructural and microstructural investigations, mechanical and corrosion resistance evaluation, and metal ions release are carried out. The best composition, among the studied alloys, is identified for dental purpose.

Stainless Steels Sintered Form the Mixture of Prealloyed Stainless Steel and Alloying Element Powders

The aim of the presented paper is to describe the sintered duplex stainless steels manufactured in sinter-hardening process and their structural and mechanical properties. Duplex stainless steels were obtained through powder metallurgy starting from austenitic 316L or ferritic 410L prealloyed base powders by controlled addition of alloying elements powder. Prepared mixes were compacted at 700MPa and sintered in a vacuum furnace with argon backfilling at temperature of 1240°C for 1h. After sintering different cooling cycles were applied: rapid cooling (6°C/s) using nitrogen under pressure and slow cooling (0.1°C/s) with furnace in argon atmosphere. Produced sintered duplex stainless steels were studied by scanning and optical microscopy and EDS chemical analysis of microstructure components as well as X-ray analysis. Mechanical properties were studied through tensile and three-point bending tests and Charpy impact test. It was demonstrated that austenitic-ferritic microstructures with regular arrangement of both phases and absence of precipitates can be obtained with properly designed powder mix composition as well as sintering cycle with rapid cooling rate. Produced sintered duplex steels show good mechanical properties which depend on austenite/ferrite ratio in the microstructure and elements partitioning (Cr/Ni) between phases. The optimal mechanical properties were obtained for compositions based on ferritic 410L powder where the balanced distribution of α and γ is present and the tensile strength can reach value about 500MPa with 16% of elongation and impact energy about 120J. The precipitations of hard intermetallic σ-FeCr phase take place when sintering with slow cooling cycle what cause substantial decrease of plastic properties, including reduce of elongation to 7% and in particular decrease of impact energy to 68 J.

Effects of T5 and T6 Heat Treatments Applied to Rheocast A356 Parts for Automotive Applications

The correlation between the evaluation of the mechanical and of the fatigue behaviour of the rheocast, T5 and T6 heat treated SSM A356 aluminium alloy with respect to the microstructures of the component has been investigated. The study has been carried out on a suspension arm injected in a rheocasting 800 tons plant in Stampal S.p.A. The new rheocasting is a process that allows obtaining the alloys in a semisolid state directly from the liquid state, by controlled cooling of the molten alloys. The resulting microstructures are very fine, free from defects and homogeneous: these characteristics improve the mechanical properties of the alloys and specially the response to cyclic stress, an important issue for a suspension component. After a preliminary tensile test analysis, axial high frequency fatigue tests have been carried out at room temperature on specimen cut out from the suspension arm to determine the Wöhler curve and the number of cycles to failure. The results of this work allow a comparison of the effects of heat treatment process, T5 or T6, on Semi-Solid components for industrial applications in the automotive field. On the basis of these analysis the correlation between microstructure and mechanical performances can be established.

Self-Hardening Alloys for Automotive Application

In this paper a self-hardening Al-based alloy (AlZn10Si8Mg) is proposed for automotive applications as an alternative to the A357 (AlSi7Mg0.6) T6 heat treated alloy. The properties of the AlZn10Si8Mg alloy have been monitored and compared to those of the A357 alloy which is already employed in the targeted automotive industry. The samples have been submitted to microstructural analysis and mechanical characterization, while the presence of defects on the fractured surface has been identified by fracture surface analysis. A relationship between defects and mechanical performances has been identified. The corrosion resistance of the alloys has also been investigated according to the Standard BS 11846. On the basis of the results obtained till now, the AlZn10Si8Mg alloy is a good candidate for the proposed automotive application.

New frontiers for thixoforming

The continuous increase in demand for light alloys in different applications involves manufacturing high integrity and superior performance components using cost-effective and safety processes. In this direction, the research communities together with manufacturing industries are focusing their attention to develop new products using innovative procedures, i.e., Semi-Solid Metal (SSM) processing routes. The central issue is the technological transfer to industry for affordable mass production. The purpose of this paper is to present an overview of alternative methods of production of enhanced performance light alloys components for critical areas. The origin, development and current evolution of the SSM processing are illustrated with characteristics of real-life components for automotive industry. Different techniques are presented to provide a general idea prevalently on the commercially available SSM processing techniques. A comparison with traditional methods, in particular High- and Low-Pressure Die Casting technologies, in terms of process parameters and properties is illustrated and discussed.

Effect of Different Vacuum Heat Treatments on the Microstructure of a Low Alloyed Sintered Steel

The main aim of the present contribution is to show how different heat treatment conditions influence the microstructure of a Fe - [1.5Cr - 0.2Mo] - 0.6C powder system. In vacuum furnaces, the cooling rate is generally determined by the pressure of the gas (basically N2) introduced into the chamber. Different gas pressures have been applied, from 0 to 6 bars. The average cooling rates were calculated in the range of 1180 °C to 400 °C and were varying from 0.1 to 6 °C/s, according to the gas pressure. Considering the cooling rates, increasing the nitrogen pressure resulted in an increased amount of bainite/martensite microstructure. The microstructure constituents ranged from 97% pearlite + 3% ferrite in the system treated at 0 bar to 82 % martensite + 18 % bainite (with small amount of tempered martensite) in the system cooled applying N2 at 6 bars. Mechanical properties have been evaluated in terms of toughness, TRS and hardness for all processing conditions; the analysis of the properties allowed to plot graphs correlating the different properties as function of the characteristic microstructures.