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The inverse analysis approach is recently spreading as a valuable methodology to calibrate constitutive models describing the superplastic behaviour of light alloys. In this work, the superplastic behaviour of a titanium grade for biomedical applications was characterized by means of a Genetic Algorithm based inverse analysis. Free inflation tests at 850 °C were carried out on the Ti-6Al-4V Extra Low Interstitial alloy at two different levels of constant pressure; in addition, a jump pressure test in which the gas pressure was changed during the same test between the same levels was performed. The dome height curve was acquired during each test and subsequently adopted as target data: a simple 2D Finite Element model, in which the material behaviour was implemented using the Backofen power law, was then created to calibrate the material constitutive equation by means of an inverse analysis approach. A set of constants, able to minimize the error between numerical and experimental data, was determined for each investigated load condition. Subsequent numerical simulations, run for validation purposes, demonstrated that the sets determined using as target the data from the free inflation tests under a constant level of pressure were capable of effectively describing the material behaviour only for load conditions close to the ones from which they had been determined. On the contrary, the set of material constants obtained using the same inverse approach but adopting as target data the acquisitions from the jump pressure test, revealed to describe the material behaviour over a wider span of operative conditions. [ABSTRACT FROM AUTHOR]

In the present work, both the surface chemical contamination and the mechanical alteration of Ti-6Al-4V (Ti64) and Ti-6Al-4V extra low interstitial (Ti64ELI) titanium alloys subjected to superplastic forming (SPF) for the manufacturing of highly customized biomedical prostheses have been investigated. As case study, a cranial implant was considered. The design of the manufacturing process was assisted by a numerical model calibrated on free inflation experimental tests. Glow discharge optical emission spectrometry (GDOES) analyses, nanoindentation tests, and metallographic analyses allowed to relate the mechanical alteration to the oxygen enrichment due to the environmental exposition during processing. While similar diffusion kinetics were found, different oxidation rates were measured in the two investigated alloys. The hardness variation was strictly related to the oxygen content. In order to verify the material biocompatibility, cytotoxicity tests were conducted on the most oxidized part. Results highlighted that the oxygen enrichment due to the manufacturing process did not significantly affect the cells viability. [ABSTRACT FROM AUTHOR]

In this work, the gas forming of AZ31 magnesium alloy 0.75-mm-thick sheets at elevated strain rates (fast gas forming) is investigated through an experimental-numerical approach. First, free inflation tests were carried out to find the conditions, in terms of temperature and forming pressure, able to give the best compromise between the alloy formability and the forming time. The analysis was successively moved to a closed die forming application with a stepped geometry case study in order to analyse the real forming process. Both an axisymmetric model of the free inflation test and a 3D model of the closed die forming process were built to correlate the results from free inflation tests (in terms of optimal strain rate values) to the closed die forming test: Numerical simulations were run to find the pressure value to be applied in gas forming tests. Experimental gas forming trials were finally conducted in order to support the approach and to analyse post-forming characteristics of the formed parts. Results showed that very small fillet radii can be reached on a commercial Mg alloy sheet setting very short forming times (few seconds). The choice of the forming temperature and of the corresponding optimal strain rate strongly affects the grain growth and the cavitation phenomena. Even if the alloy is prone to a strong static and dynamic grain growth at elevated temperatures, a small mean grain size value can be reached in the formed component due to the short forming times. [ABSTRACT FROM AUTHOR]

In this work, the mechanical and technological behaviour of AZ31 magnesium alloy laser welded joints is investigated. The forming behaviour of the joints is analysed by both tensile and biaxial stretch tests. Each test is monitored using a digital image correlation system in order to acquire the complete strain field during the whole test. Both in tensile and biaxial stretching tests, the strain maps reveal that the weld bead makes the strain path experienced by the welded specimen more critical than the one experienced by the base material, and this can be related to morphological defects of the weld bead. [ABSTRACT FROM AUTHOR]

Innovation in materials and joining processes continues to play a key role in the evolution of the transport sector, in general, and in the aviation in particular. Important factors of innovation are the improvement in material and processes performances, coupled to the associated cost reduction. Titanium alloys, characterized by high mechanical properties and corrosion resistance, are particularly interesting for the aeronautical sector. Their joining, from metal sheets with equal or different thicknesses, is mainly carried out by using laser or arc welding techniques. With the laser process, a high amount of energy is applied in a very limited area for a veiy short time, resulting in a reduced heat affected zone. With the arc welding, a large amount of heat is applied in a larger area for a longer time, obtaining a more extended heat affected zone. The combination of the process with high energy density (laser) with the high energy contribute (MIG) has been shown to enhance the beneficial effects of both processes (key-hole, ability to bridge sheet gap) while limiting their drawbacks (high thermal gradient, low mechanical resistance). In this paper, the hybrid Laser-MIG welding of Ti-6Al-4V was investigated. The welded joints were obtained from with 3-mm thick sheets using the double fillet welded T-joint. The main factors of evaluation were the morphology of the weld bead and energetic considerations. [ABSTRACT FROM AUTHOR]

Abstract: The research activity reported in the present paper aims to evaluate the Forming Limit Curves (FLC) of the Mg alloy AZ31 in warm conditions (200°C) while keeping the equivalent strain rate constant. Specific tools to carry out such a formability test were designed and created: a flat punch (in line with Marciniak’s test) embedding a heating system was adopted in order to heat the central part of the specimen both rapidly and uniformly, where ruptures were forced due to the presence of a driving sheet between the specimen and the punch. A Digital Image Correlation system was also embedded in the formability equipment in order to acquire major and minor strains continuously and evaluate the moment and location of failures. Finite Element simulations were run in order to define punch speed profiles (which differ according to the geometry of the specimen) that were able to keep a constant equivalent strain rate in the region where ruptures were forced. Experimental tests implementing the punch speed profiles were carried out in order to obtain temperature, load and strain data. FLCs at two different strain rate levels (0.02s−1 and 0.002s−1) both confirmed and allowed us to quantify the noticeable strain rate effect of such an alloy on the FLC at a temperature of 200°C. The proposed approach for FLC evaluation is effective for materials whose properties are strongly influenced by the strain rate. Such FLC data can be usefully implemented in numerical simulations of sheet metal forming processes: while tensile tests can be used to determine variations in mechanical behaviour according to the strain rate, the FLCs evaluated in this work allow us to determine the occurrence of strain path-dependent critical conditions according to the strain rate. [Copyright &y& Elsevier]

Abstract: The present work aims at studying the tangential bending process (wiping) and the combined effect of a bending and stretching stress on thin (0.7mm) magnesium alloy (AZ31) sheets when working in warm conditions. The test equipment was designed in order to heat the sheet only in the bending region and to stretch the sheet after the wiping process; it was used for investigating the parameters affecting the stretch-bending state of stress in the sheet, quite common in the stamping process. A preliminary screening analysis over a large set of process parameter was performed using a full factorial design. As a result, only the effect of the temperature, the speed and the bend radius were further investigated according to a central composite design and using the springback factor and the maximum stretching load as response variables. The present work can give useful guidelines in the warm forming of AZ31 Mg alloy sheets in terms of key process parameters, fillet radii of the equipment, material stretch-bending strength (according to temperature, bending radius and speed) and precision of the final part (springback amount). [Copyright &y& Elsevier]

Abstract: In this study, a test procedure is proposed to investigate the formability of the AZ31 Mg alloy in warm conditions (below 200°C) while keeping the equivalent strain rate constant. Both the variables (temperature and strain rate) play a key role on the plastic behaviour of Mg alloy sheets. A numerical–experimental approach was adopted with the aim of designing equipment and test procedures, which can perform formability tests in warm conditions. Since finite element simulations need reliable and accurate data to model the forming process, the authors propose a methodology to evaluate the forming limit curve (FLC) according to both the temperature level and the (equivalent) strain rate value. Using the designed equipment and proposed approach, it is possible to: (i) force material failure in the central part of the specimen; (ii) define a constant temperature in the failure region; (iii) determine a constant equivalent strain rate in the failure region. Experimental tests in the present paper were carried out using a digital image correlation system, which is able to acquire strain maps during the entire forming process; experimental data allowed for both a validation of numerical results and the acquisition of important information about the effects of strain rate and temperature on the FLC of the AZ31 Mg alloy. [Copyright &y& Elsevier]

Abstract: The present work aims at studying the combined effect of tangential bending and stretching stress (quite common in the stamping process, e.g. in the pinch or die shoulder regions) on thin magnesium (Mg) sheets when working at elevated temperature. An experimental/numerical approach was adopted using specific equipment able to heat the sheet only in the bending region and to stretch the sheet after the wiping process. Stretch-bending tests were aimed at understanding the process parameters really affecting the stretch-bending process at elevated temperature using the design of experiment (DOE) technique. In particular the temperature, the punch speed, the bending radius and the rolling direction were considered in the screening analysis. Beside the maximum stretching load, also the springback phenomenon was measured, in order to quantify the amount of plastic strain in the bending section when changing the process parameters. The finite element (FE) analysis was focused on the process parameters which resulted to play a key role: the bending radius and the temperature. Plastic strain distributions in the bending area and the critical condition occurrence in the stretching step were evaluated using 2D fully coupled thermo-mechanical models. [Copyright &y& Elsevier]

Abstract: The present work has been focused on the warm deep drawing process by superimposing a thermal gradient between the blank centre in contact with the punch and the blank flange (which is going to be drawn). Using such a technique the limit drawing ratio of Mg alloys can be dramatically improved. As a case of study, the forming of cylindrical AZ31 cups was investigated. In particular, the attention was focused on the forming temperature (in both the flange region and in the blank centre). Finite element simulations were run in order to investigate the stress and strain distribution in the material determined by the temperature gradient and to validate experimental results. Tests highlighted that a LDR equal to 3.375 is possible when setting the temperature of the blank holder at 230°C and the punch speed at 6mm/min. [Copyright &y& Elsevier]

Coastal sediments from the northern Adriatic Sea were analyzed for 137Cs and 134Cs from the Chernobyl accident to better understand the transport and sedimentation processes in a marine area strongly influenced by the delivery of suspended matter from the Po River. In particular, the suitability of 137Cs as a tool for studying sediment accumulation rates was evaluated. Radionuclide distributions are determined by the influence of the river and by the grain size composition that is a function of the hydrodynamic processes. With respect to pre-Chernobyl values, 137Cs surficial activities were increased from 3 to 13 times, and inventories up to 3 times. Four types of 137Cs activity-depth profiles were found, being representative of different combinations of sediment accumulation and radiotracer displacement within the surficial layers. At some sites sediment accumulation rates of 1.6–4.8 cm y−1 caused the formation of a sub-surficial peak. At the other locations, the Chernobyl peak of 137Cs was still confined at the water–sediment interface with some downward penetration due to mixing and diffusion. In general, molecular diffusion is the prevailing mechanism but the incidence of mixing is very significant at three sites (58–71%). [Copyright &y& Elsevier]

Abstract: The effect of laser cutting parameters on the mechanical behavior of laser butt welded joints whose edges were obtained by laser cutting was investigated. The paper aims to demonstrate that new high power solid-state fiber lasers not only represent a valid and reliable alternative to the most established CO2 and Nd:YAG laser sources, but also allow to obtain cuts having edges well suited for subsequent direct laser welding. First Ti6Al4V 1mm thick sheets having edges machined by milling were laser welded. Once the optimal welding condition was determined, the mechanical characterization of sheets cut by fiber laser and then laser welded was performed. Comparative strain analysis performed by a digital image correlation technique highlighted the effect of the gap between the sheets resulting from the different cut edge quality. Experimental results showed that the correct selection of laser cutting parameters allows to obtain butt joints characterised by mechanical properties comparable with the ones obtained by milling. Cutting edge quality in the optimal range of gap values allows to obtain the best mechanical performances of the joint. [Copyright &y& Elsevier]

Abstract: The aluminum/steel structural transition joints are widely used in shipbuilding industry due to the advantages of joining these two materials with important weight savings while exploiting their best properties. The research objective is the evaluation of process feasibility to laser weld structural transition joints to overcome manufacturing limitations of traditional TIG/MIG welding. This study presents results achieved during as-simulated laser welding by imposing severe thermal cycles to specimen obtained from structural transition joints with time periods longer than those normally recorded during laser welding. Metallurgical and mechanical characterization of heat treated specimen is performed to evaluate the influence of the thermal cycles on final joint properties. [Copyright &y& Elsevier]

Abstract: Laser beam welding experiments have been carried out on 3-mm thick aluminium–magnesium alloy 5083 specimens in butt-joint configuration. The mechanical properties of the joints have been evaluated by performing tensile tests, hardness profiles across the weld sections, porosity measurements and EDX analyses. A design of experiment technique has been used to study the effects of the welding speed () and the incident laser power (P) on the aforementioned response variables that are considered to be representative for the weld quality. By comparing the welds obtained by operating at constant linear energy input released onto the material (P/ ratio), the best results have been found for higher laser powers and welding speeds. A clear correlation was found between the incidence of porosity, the tensile strength and the hardness of the fused zone. Welding reliability was enhanced for selected sets of process parameters capable of producing butt-joints showing mechanical properties very competitive if compared with the performances obtained, on similar aluminium alloys, using alternative joining technologies like friction stir welding or gas tungsten arc welding. [Copyright &y& Elsevier]

The single-pulse laser hardening of a hypereutectoid steel coated by a graphite layer was investigated using a numerical/experimental approach. Experimental tests were conducted on coated samples using a fibre laser source and without any gas shielding aiming to explore the effect of laser power, pulse energy and defocusing distance on the dimensions of the hardened region. The process operating window of the discrete spot laser hardening using the graphite layer was determined through a finite element model and compared with previous results obtained on uncoated samples. For the same laser power and interaction times, an enlargement of the hardened region was found when using the graphite coating, especially when operating at the lowest laser energy level. The process operating window remains similar in shape to the one of the uncoated steel but moves towards larger hardened diameters and much larger defocusing distances. Once the maximum temperature has been fixed, a linear relationship between the hardened diameter and the defocusing distance exists. No obvious surface oxidation occurs since the graphite coating acts as a protective layer. [ABSTRACT FROM AUTHOR]

In this paper, the feasibility of using a fiber laser to perform a dissimilar metal joining was explored. AZ31B magnesium and 316 stainless steel were autogenously joined in butt configuration. The weldability between different materials is often compromised by a large difference in thermal properties and poor metallurgical compatibility. Thus, the beam was focused onto the top surface of the magnesium plate, at a certain distance from the interfaces (offset), and without using any interlayer or groove preparation. Such a method was called laser offset welding (LOW). Results proved a very good capability. The ultimate tensile strength exceeded the value of 100 MPa, since a resistant and thin layer of hard intermetallic compounds is formed within the fusion zone. The rupture was observed within the magnesium side, far from the centerline. The metallurgy of fusion zone indicated the effectiveness of phases coalescence, without mixing at liquid states. LOW was demonstrated to be a promising technique to join dissimilar metal welds, being capable to produce an effective bonding with good tensile strength. [ABSTRACT FROM AUTHOR]

In this work the single-pulse laser irradiation of a hypereutectoid steel was investigated using a fiber laser source, in a range of process parameters enabling surface hardening and remelting. Effects of laser power, pulse energy and defocusing distance were investigated using a numerical/experimental approach. Laser surface treatments were conducted on uncoated samples without any gas shielding, changing both the laser power and the pulse energy, and exploring a wide range of defocusing distances. Numerical simulations were conducted using a finite element model calibrated by means of an optimization procedure based on a specific calculation algorithm and using a subset of experimental data producing surface melting. Using both simulations and experiments, the process operating windows of the discrete spot laser treatment were determined: it was found that, when varying the laser power between 250 W and 750 W, melt-free hardened zones are produced with a maximum extension between 0.7 mm and 1.0 mm; on the contrary, in case of more tightly beam focusing conditions, surface melting occurred with a size of the re-melted areas ranging between 1.0 mm and 1.4 mm. Results further showed that a small change (generally 2–3 mm) of the defocusing distance suddenly brings the material from melting to a non-hardening condition. [ABSTRACT FROM AUTHOR]

A numerical/experimental procedure is proposed for calculating the residual stress state during the cooling phase of the casting process of a superduplex stainless steel (ASTM A890 Gr. 5A). The experimental activity consisted of casting, tensile and creep tests. Casting tests were used to set (by acquiring temperature data at different points) and validate (by measuring displacements after releasing residual stresses by cutting) the Finite Element model. Tensile and creep tests were used to determine the material properties from room temperature to 1200 °C. A fully coupled thermo-mechanical analysis was conducted by neglecting the presence of the sand mould; instead, attention was focused on the material by modelling the creep behaviour using the Bailey–Norton formulation. Measurements of the of the displacements due to the stress release after EDM wire cuts revealed to be in good agreement with the numerical model and confirmed the key role played by viscosity during the cooling phase. Neglecting the viscous strain led to an overestimation (more than twofold) of the stress level in the cast part after the cooling phase in the sand mould. The good matching between experimental and numerical data indicated that a numerical model that incorporates the creep behaviour is able to accurately capture the investigated phenomenon, despite the simplification in the modelling of the casting process (sand mould replaced with virtual convection) which does not substantially affect its accuracy. The robustness of the methodology, which is characterized by small computational cost and good quality of results, was further proved simulating and comparing numerical and experimental results concerning a second casting geometry. [ABSTRACT FROM AUTHOR]

Abstract: Warm forming of magnesium alloys has attracted much attention due to the very poor formability of Mg alloys at room temperature. In the present paper, the warm deep drawing of magnesium alloy AZ31 (3wt.% Al, 1wt.% Zn) sheets was studied by both the experimental approach and the finite element analysis. The results indicated that the formability of the AZ31 sheets could be improved significantly at elevated temperatures. Sound cups could be formed at 150°C with the highest punch speed of 6mm/min, while when the forming temperature was increased up to 250°C, sound cups could be drawn with the highest punch speed of 120mm/min. Finite element analyses were performed to investigate the effects of the process parameters on the drawability of rectangular cups and to predict the formation of the process defects. The reasonable agreement between the numerical simulation results and experimental data validated the accuracy of the finite element analysis. [Copyright &y& Elsevier]

Abstract: To obtain a quantitative estimate of the sediment accumulating on the muddy belt extending along the western Adriatic continental shelf, a budget was calculated based on the mass accumulation rates measured from more than one hundred activity–depth profiles of 210Pb. 137Cs depth-distributions were used, when possible, to support 210Pb results. 210Pb profiles show quasi-monotonic declines in the foreset region of the central Adriatic clinoform. In the prodelta of the rivers Po and Isonzo, 210Pb profiles are characterized by intervals of low and uniform activity, which are thought to be produced by flood deposition. Since the north Adriatic is shallow and occasionally affected by storms from Bora wind, sediments are not in steady-state and their accumulation is the net result of recurring deposition, resuspension and winnowing events. In the western Adriatic, mass accumulation rates range between 0.04 and 6.6 g cm−2 yr−1. Peak values were recorded in the Po and Isonzo prodeltas and along a narrow belt between Pescara and the Gargano promontory. Furthermore, the latter depocentre is located immediately seaward of the offlap break (40–50 m water depth) and parallels that of the most recent sigmoid of the HST. The integration of mass accumulation rates throughout the mud wedge of the north and central Adriatic gave a value of ∼42.0 Tg yr−1 of sediment accumulating along the Italian coast. The balance between sediment accumulation and riverine sediment supply (∼46.6 Tg yr−1) suggests an export of 4.6 Tg yr−1 to the south Adriatic basin and/or the Mediterranean Sea. The along-shore sediment transport is very effective and, in particular, 16.2 Tg yr−1 of sedimentary material (∼35% of the riverine supply) is transferred from north to central Adriatic, through the Ancona section. [Copyright &y& Elsevier]