Papers by Author: Sergio Baragetti

Abstract: Ti-6Al-4V titanium alloy is a high strength-to-mass ratio material common in a lot of engineering fields. Its surface oxide can guarantee the protection of the substrate from various corrosive media. Unluckily, this film can be scratched in presence of mechanical and chemical loads and for this reason the corrosion resistance can decrease. The Structural Mechanics Laboratory (SM-Lab) is carrying out a characterization of the alloy in different environments under quasi-static loading. In this paper, a summary of the outcomes of the investigation and the description of the fracture surface of a specimen with EDM notches quasi-statically tested in methanol is provided

Abstract: Light alloys are a very interesting challenge in order to have light components with high mechanical features. One of these is the 7075 aluminum alloy, which is commonly employed in aeronautic, automotive and maritime fields.On the other hand, the application of a PVD (Physical Vapor Deposition) coating can improve the hardness of the surface and the tribological properties of the component.The effectiveness of these coatings on the fatigue behavior of the sublayer material is not already clear. For this reason, bending tests on uncoated and coated specimens in air were performed in order to evaluate the S-N diagrams

Abstract: Ti-6Al-4V alloy in solutionized and aged condition was subjected to axial fatigue testing in air and corrosive environments respectively. Severity of the methanol damage as evidenced through fractographic studies, corroborates loss in fatigue strength of samples tested in methanol environment in contrast to samples tested in air. Samples subjected to fatigue loading in NaCl environment revealed extensive secondary cracks along alpha grain boundaries.

Abstract: 7075-T6 aluminium alloy is under investigation in the last decades because of its sensitivity to corrosion and its poor surface behaviour. The alloy is widely used in many mechanical, aeronautical and structural engineering appliances because of its high strength-to-weight-ratio. Notwithstanding its strength and light weight advantages, its applications in corrosive environments, with fatigue applied loads, is still under research. The mechanical and chemical driving forces have still to be accurately evaluated and the role of each one identified. Results of tests on rotating bending fatigue (R = -1) 7075-T6 uncoated and PVD DLC coated specimens are reported in this paper under various stress levels and number of cycles to failure. Fracture surfaces were observed via SEM characterization in order to assess the role of the chemical aggressive environment, the residual stress field and the applied load.

Abstract: Diamond-like coating (DLC) of 2.5 micron thick coated on the aluminum alloy samples, were subjected to rotating bending fatigue test in methanol and air environments respectively. DLC coating is X-ray amorphous in nature as revealed by grazing incidence X-ray diffraction studies. Residual stresses generated due in the coatings were measured using Raman peak shift analysis of G band of carbon. Fractographic analysis of uncoated fatigue tested samples in methanol revealed severe damage in contrast to coated samples in methanol. Diamond polished and DLC coated samples revealed better coating adherence and fatigue performance than 1200 grit polished and fatigue tested samples.

Abstract: Despite its high tensile strength and its brilliant fatigue behaviour, 7075-T6 aluminium alloy usage is dramatically reduced, due to its sensitivity to corrosion and its poor surface characteristics. In order to be adopted for advanced applications in the aeronautic, automotive and maritime fields, as well as in further innovative applications, 7075-T6 resistance must be improved. Controversial results are found in literature on coated 7075-T6 fatigue strength, due to the complex mechanical interaction between the substrate and the coating, and the high temperatures involved in the coating process. In the present work, testing on rotating bending fatigue (R = -1) 7075-T6 uncoated and PVD DLC coated specimens has been conducted. Different stress levels were considered, to assess the impact of the applied stress on the fatigue life of the substrate-coating system. SEM characterization of the fracture surfaces has been performed, to investigate the influence of the coating on the fracture mechanism

Abstract: High strength-to-mass ratio light alloys, such as 7075-T6 aluminium alloys and Ti-6Al-4V titanium alloys, are commonly adopted for high performance structural components in the aeronautic, automotive and maritime sectors. For this reason, it is crucial to investigate the effects of the external environment on their mechanical properties, to avoid dramatic component failure. In the present work, experimental tests were performed on Ti-6Al-4V and 7075-T6 light alloys. Ti-6Al-4V notched flat dogbone specimens, with K_t = 1.18, were tested for quasi-static and SCC effects in a methanol-water aggressive environment at different concentrations. Rotating bending R = -1 fatigue tests were performed on 7075-T6 in air and methanol environment, to evaluate the effects of an aggressive environment on the fatigue strength at 200’000 cycles. The influence of DLC and WC/C PVD coatings on fatigue limit at 200’000 cycles has been evaluated in air and aggressive environment, to assess their mechanical and protective effects on the 7075-T6 substrate

Abstract: 7075-T6 aluminium alloy is commonly adopted in high performance structures and components. Its fatigue behaviour is however dramatically worsened by exposure to aggressive environments. The deposition of PVD coatings, which are commonly adopted to increase the surface properties of structural elements in terms of hardness, contact fatigue and wear resistance, could be beneficial also for the fatigue behaviour of a 7075-T6 substrate in an aggressive environment. In the present work, Diamond Like Carbon (DLC) PVD coated 7075-T6 specimens immersed in methanol have been analysed, by means of step-loading rotating bending fatigue tests (R = -1) at 2·10⁵ cycles. Coated specimens were tested in laboratory air for comparison, and uncoated polished samples were studied in both the environments to obtain reference values. SEM micrographs of the fracture surfaces were taken to investigate the effects of the corrosive environment on the failure mechanism.

Abstract: 7075-T6 is one of the most performing aluminium alloys, considering its mechanical properties and good fatigue behaviour. In this work the influence of WC/C and DLC PVD coatings on the fatigue behaviour was investigated by rotating bending tests at 2·10⁵ cycles. The fatigue behaviour of polished and untreated specimens was considered as reference. In order to decouple the effect of the deposition temperature from the coating one, some uncoated specimens were submitted to the thermal cycles of the WC/C and DLC processes and then tested under fatigue loading. Fatigue life was determined using a step-loading technique. SEM micrographs of the fracture surfaces were taken in order to characterize the fatigue mechanisms of coated and uncoated specimens.

Abstract: In the present experimental work, a WC/C coated 7075-T6 aluminum alloy was considered from the corrosion point of view. The coating was deposited by PVD technique with a final thickness of about 2.5μm. In order to study the influence of the coating on the corrosion behavior of the aluminum alloy, the samples surfaces were partially coated and the interface among the metal and the coating was analyzed after the corrosion tests described into the ASTM G110 standard. Such experimental plan was decided in order to simulate the possible in-service local removal of the thin and hard coating. This kind of damage, due for example to a foreign object impact, can occur because of the great hardness difference between the coating and the substrate. The experimental tests were carried out on samples with different surface finishing, ranging from about 0.02μm Ra (mirror-polished surface) to about 0.8μm Ra (320 grit paper). The aim of such choice was to investigate the effect of a surface roughness different from the optimal one (mirror polished) on the coating deposition. Moreover a different corrosion resistance is expected.