M. Gelfi

Glancing-incidence X-ray diffraction for depth profiling of polycrystalline layers

On the basis of glancing-incidence X-ray diffraction (GIXRD) spectra collected at different incidence angles, it is possible to obtain structural information at different depths. In the case of an ideal crystalline material, the integrated intensity of each crystalline-phase reflection is correlated to the irradiated volume of the phase. In this work, it is shown that quantitative information on the thickness of thin polycrystalline layers can be obtained by means of GIXRD. Experiments have been performed on thin films of gold with different thicknesses, sputtered on glass slides. The film thickness has been carefully evaluated by X-ray reflectivity (XRR) experiments. XRR and GIXRD data are compared, and the consistency of the thickness values of the crystalline gold layer is shown.

Influence of Material Microstructures in Micromilling of Ti6Al4V Alloy

In the most recent decades the introduction of unconventional machining processes allowed the development of micromachining techniques. In this work, the influence of material microstructures on the micromilling process was investigated. Ti6Al4V alloy was selected as workpiece material since it is a very common material for micro applications and because its duplex microstructure can be easily changed by proper thermal treatments. Four different microstructures (namely bimodal, fully equiaxed, fully lamellar and mill annealed) were obtained through recrystallization annealing treatments carried out at different times and temperatures. The mechanical properties of the samples were assessed by microhardness measurements. Nano-indentations were also performed on single grains to understand how the different hardness of phases and structures present in the Ti6Al4V alloy can affect the micromilling process. Microchannels using two flute flat end mills with a diameter equal to 200 µm were realized on the treated samples. Two different feed-per-tooth values were used during the tests. Cutting force, channel shape and burr dimension were investigated. Morphological and energy dispersive spectroscopy (EDS) analyses were performed on tools by means of a scanning electron microscope (SEM): in this way the phenomena mainly influencing the tool status were also identified. Lower cutting forces and reduced tool wear were observed when working fully lamellar microstructures compared to the other ones.

Micromilling of Lamellar Ti6Al4V: Cutting Force Analysis

The aim of this article is to study the influence of a Ti6Al4V microstructure on cutting forces during the micromilling process. Samples were annealed above the β-transus at three different temperatures—1020, 1050, 1080°C—and then cooled in a furnace, air, and water, in order to produce different Widmastätten microstructures. Micromilling tests were carried out on heat-treated samples, and the cutting forces were measured by means of a load cell. The results were correlated to the sample microstructures, which were thoroughly investigated by means of an optical microscope, X-ray diffraction, and microhardness measurements. The highest cutting forces were observed for soft and ductile furnace-cooled samples, suggesting that the most important factor affecting workability is the material ductility, while hardness is a less relevant parameter.

Effect of microblasting on cathodic arc evaporation CrN coatings

Abstract Cathodic arc evaporation CrN double layer coatings were deposited onto tool steels in two steps by industrial apparatus. Microblasting was performed on CrN first layer in order to remove metallic droplets and surface defects and to promote the adhesion of the second layer. Surface morphology effects were assessed by a scanning electron microscope coupled with Image analyser software. Nanoindentation and scratch tests were performed to evaluate the changes of coating adhesion and mechanical properties. Structure, microstructure and residual stress were evaluated by X-ray diffraction. In particular, by means of Cu and Cr wavelengths, the residual stresse of both steel substrates and coatings was calculated. The results show that microblasting has no effect on coating hardness and slightly affects the adhesion of the coating to the substrate. On the other hand, microblasting increases coating resistance to cohesive failure and the substrate compressive stress at the coating interface.

Effect of Different Corrosion Levels on the Mechanical Behavior and Failure of Threaded Elements

This research concerns the study of corrosion effects on different threaded elements to analyse bolted joints working in a saline environment. In particular, this article examines the mechanical behavior of a M8 junction realized by different steels and subjected to different degrees of corrosion. Four materials were investigated: a low-alloyed structural steel in the original condition and the same after zinc-plating; a quenched and tempered steel; and an austenitic stainless steel. For each steel were tested a certain number of threaded rods exposed to a saline water solution, simulating the atmospheric exposure to an aggressive environment. The samples were characterized by different intervals of exposure. During these periods, the threaded elements (threaded rods) were kept in tension applying a tightening torque proportional to the yield strength of the tested steel, to evaluate their susceptibility to stress corrosion cracking. Before and after the different corrosion steps, some metallographic analyses, static tensile and fatigue tests, were carried out to determine, for each sample, the degree of corrosion and its influence on the reduction of the junction resistance. In order to evaluate the stress-intensifying factor and its variation with the imposed degree of corrosion was carried out a FEM analysis.

Semisolid lead-antimony alloys for cars batteries

The application of semisolid process was investigated on Pb-Sb alloys that are normally used for metallic parts in car batteries, in order to increase their mechanical properties and corrosion resistance. The semisolid behaviour of a Pb-4%Sb alloy was analysed by means of rheological and calorimetry differential scanning experiments. The alloy was melted, treated by ultrasound to obtain the semisolid state and then poured into permanent moulds. Some of these samples were also heated in semisolid temperature range and injected in thixo-casting apparatus. Both poured and injected samples were characterized by metallographic analysis, mechanical test and electrochemical corrosion experiments. The results show an improvement of mechanical and corrosion resistance

Effect of Globular Microstructure on Cavitation Erosion Resistance of Aluminium Alloys

In this paper the effect of globular microstructure on the cavitation erosion resistance was assessed and compared to that of conventional dendritic one. Three different wrought aluminum alloys in as-cast conditions were investigated. The samples were completely characterized by metallographic analyses and microhardness measurements. Cavitation erosion tests were performed according to ASTM G 32 standard. The volume loss was evaluated during the test by periodical interruptions. It was identified the damaging mechanism in case of both dendritic and semisolid microstructure. It was also found that the globular microstructure increases the cavitation erosion resistance only for one of the studied alloys.

Semisolid Processing of Al-Sn-Cu Alloys for Bearing Applications

The aim of this work was to evaluate the advantages in using semisolid processing of Al-Sn alloys in order to improve microstructural homogeneity and, consequently, wear resistance. Different tests were performed on an Al3Sn4Cu by using ultrasound treatments during solidification to obtain an almost globular microstructure. Metallurgical characterizations and pin on disk wear tests were carried out on semisolid as well as on conventionally cast samples in order to assess the benefit of the treatment.

Aluminum Segregation in ZA27 Rheocast Alloy

Aluminum segregation in zinc alloys is a well-known problem during melting and casting. In molten Zn alloys, in fact, Al tends to float on the surface of the bath because of its lower density than zinc. The same problem also occurs during casting solidification, causing chemical gradients along part thickness and soundness problems. As a consequence, the use of high aluminum content zinc alloys, such as ZA27, can be limited.In this paper the effectiveness of rheocasting method on decreasing Al-segregation phenomenon in ZA27 was investigated. The slurry was obtained by using the ultrasound technique, starting from the fully liquid down to different solid fraction percentages. The metal was then cast into a permanent steel mold. The produced samples were completely characterized by metallographic analysis (optical and scanning electron microscope), to assess the microstructure modification induced by the treatment. Hardness tests were also performed to evaluate mechanical properties. For comparison, the same tests were performed on conventionally cast samples.The analyses show that the use of a semisolid alloy increases the homogeneity of the part in terms of hardness, chemical composition and microstructure; furthermore casting’s shrinkage porosity is significantly reduced.

Corrosion and Wear Behavior of CAE Deposited CrN-PVD Coatings

Corrosion and wear resistance of Cathodic Arc Evaporation (CAE) CrN coatings deposited on a tool steel were investigated considering the effect of the number of layers (mono or double) and the influence of defects. The CrN coatings were characterized for mechanical (scratch, nanoindentation test), corrosion behavior (polarizarion tests) and tribological properties (pin-on-disk tests). Scanning Electron Microscope analyses on samples before and after the tests were also made in order to identify the type of the defects and the role they play in the coating damage. Monolayer coating shows a good wear behavior with a stable and low friction coefficient, but its corrosion performance is poor. On the contrary, the presence of an interface into the double layer improves the corrosion resistance, but it is a weak point for the coating delamination during wear tests.