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Laser powder bed fusion (L-PBF) is one of the most promising additive manufacturing technologies for metals. In this work, the discrete element method (DEM) was used to reproduce a powder bed of particles distributed in a random way to be as close as possible to reality. Single and multiple scan tracks simulations were performed for Ti6Al4V alloy using a commercial CFD software, FLOW-3D AM®. The output from the numerical simulations was elaborated to obtain shape and size of melt pools, morphology of scan track surfaces and porosity content. In particular, a specific model was used in order to predict air entrainment in the melt pool and, therefore, to estimate gas porosity content, as an innovative approach to predict such defects. Results from simulations were compared with experimental data from Ti6Al4V samples produced by L-PBF in terms of melt pools size and morphology, as well as density. The good agreement between calculated and experimental results indicates that simulation of L-PBF can represent a powerful tool not only for the optimization of process parameters, but also for the prediction of porosity level.

The present study is focused on the wear behavior of an innovative AlMgScZr alloy manufactured using the laser powder bed fusion process. This material, commercially known as Scalmalloy®, exhibits high strength and ductility, together with low anisotropy, which instead represents a strong limitation for other alloys produced by additive manufacturing processes. The typical heat treatment for this alloy is an annealing treatment at 325 °C, which leads to a further increase in mechanical performances as a result of precipitation of intermetallic particles containing Sc and Zr. These precipitates usually exhibit good thermal stability, as shown by studies of alloys with similar chemical composition but manufactured via conventional processes. This ensures good performance of the alloy also at high temperatures, in the range 100-250 °C. Among the mechanical properties of this innovative alloy, wear resistance has rarely been studied so far, especially as a function of temperature. However, its evaluation can provide valid ideas for new applications, not only in the aerospace industry, but also in the automotive field. For this reason, the wear behavior of AlMgScZr alloy at high temperatures has been studied in this work. Pin-on-disk tests were performed at different temperatures on samples produced by additive manufacturing followed by annealing at 325 °C for 4 h. The coefficient of friction was monitored during the tests and the wear rate was calculated from the measurement of the track profile. Furthermore, the damaging mechanisms were accurately studied by observations of the morphology of the sample surface after progressive testing. These analyses, performed by using a scanning electron microscope, allowed the identification of the evolution of the wear mechanism with temperature. These results can be used for the determination of optimum ranges for application of this alloy for components that can experience high temperature exposure during their service.

UNS N06625 is a nickel-based superalloy used for oil and gas applications and commonly produced according to NACE MR0175 in the annealed/solution annealed condition. The annealing/solution annealing treatment makes the material corrosion resistant in the most challenging environments, in the presence of sulfides and chlorides at high pressure and temperature. However, thanks to its chemical composition, UNS N06625 can also be considered as an age-hardenable material whose mechanical strength can be improved by promoting the metastable second phase γ′′ precipitation into the γ matrix. However, the corrosion behavior of the aged alloy has never been investigated in NACE environments. This paper aims to understand the suitability of the age-hardened condition of UNS N06625 for oil and gas applications through the evaluation of the material corrosion performance in NACE level VII environments by using NACE TM0177 tests. Three heats of UNS N06625 have been produced and forged in different bar diameters: 152 mm (6 in), 203.2 mm (8 in), and 254 mm (10 in). Afterward, the bars have been annealed and age-hardened according to optimized time-temperature parameters and finally tested to assess their mechanical properties and resistance to stress corrosion cracking, sulfide stress cracking, galvanic-induced hydrogen stress cracking, and hydrogen embrittlement.

The application of an innovative high-pressure T6 treatment (HPT6) to additively manufactured AlSi10Mg alloy is reported in this paper. The aim of this treatment is to obtain the effective densification of the material together with the hardening effects typical of T6 heat treatment in one step. For comparison, a two-step treatment consisting of hot isostatic pressing followed by conventional T6 treatment was applied. Standard annealing and T6 treatment alone were also considered. The microstructural and mechanical properties of alloys treated under all the studied conditions were analyzed and their density was measured to quantify their densification. Although the application of high pressure hindered the diffusion mechanisms, and thus could limit the hardening effect of heat treatment, HPT6 treatment was found to ensure suitable mechanical properties and high densification. Furthermore, it required less time; therefore, it can be considered as a time-efficient process for high-performance applications.

In this paper, wear properties of samples manufactured using thixocasting were compared with those of components obtained using low-pressure die-casting and additive manufacturing in order to assess the relationship between material performance and production technologies, both conventional and innovative. The investigated items were made with AlSi7Mg alloy. First, microstructural analysis and hardness measurements were carried out. Subsequently, pin-on-disk wear tests were performed. Wear behavior of the samples was studied considering both coefficient of friction and wear rate, while the damage mechanism was analyzed by observation of the worn paths using scanning electron microscope, correlating the behavior to the specific microstructure. In addition, the effect of selected heat-treated conditions, relevant for real applications, on wear properties was also evaluated.

A series of commercial and non-conventional heat treatments was performed to modify the initial as-cast microstructure of two high-chromium (14 wt% and 18 wt% Cr) white cast irons and a low chromium nickel-bearing grade (Ni-Hard 2) for slurry pumping applications. The effect of each heat treatment on the microstructure of the alloys was carefully investigated by optical and scanning electron microscope and X-ray diffraction analysis. Samples were tested for impact erosion resistance in flowing slurry by using a modified-Coriolis apparatus and their weight losses were related to microstructure and hardness. The analysis showed that the erosion resistance of these alloys is related to the carbides volume fraction and hardness, in combination with the matrix microstructure. In case of high-chromium white irons, the destabilization heat treatment (950°C-2h) produced a hard martensitic matrix with secondary carbides that gave superior erosion resistance compared to all other conditions. Excellent performances were obtained also for Ni-Hard 2 in as-cast condition, indicating that this alloy could be an economic alternative to high chromium irons in applications where good erosion resistance is required.

In this paper we discuss the effects of deslagging practices and the size of electric arc furnace (EAF) slags on structural, microstructural, composition and leaching tests. The samples were collected from seven steelmakers located in Brescia (Lombardy Region, Italy). Nine granularity fractions of four samples were tested to evaluate the influence of the granularity on the leaching tests. The results showed that, in general, the release of the elements arises when the size of the particle decreases, except in one sample, in which vanadium and zinc displayed the opposite trend. X-ray diffraction results suggest that behavior may be ascribed to the effect of the grinding, which causes a different percentage of the phases in the various fractions. In conclusion, the possible effects of the size should also be carefully considered when defining new leaching test requirements for EAF slags.

Electric arc furnace (EAF) slags were investigated by micro-Raman spectroscopy. A slag sample characterised by well-developed crystalline phases was obtained. The EDXS elemental composition made it possible to recognize the grains corresponding to the brownmillerite, larnite and magnesioferrite phases, as identified by XRD in the same powdered sample. The grains were collected and analysed by &micro, Raman spectra, and the results showed good reproducibility in each grain and good agreement with spectra of the phases reported in the literature. A Raman database devoted to crystalline phases of EAF slag identification was created to be used by portable Raman instruments, allowing the phase characterisation of the slag directly during steel production.

In the present work, the effect of Cr and Mn addition on corrosion resistance was investigated on AlSi3Mg alloy. Potentiondynamic corrosion tests in a 3.5 wt. % NaCl solution were performed on samples in different heat-treated conditions, and corrosion current density and potential were determined by Tafel method. Brinnel hardness measurements were also carried out in order to couple corrosion resistance with mechanical properties. It was interestingly found that Cr presence enhanced mechanical properties and corrosion resistance in comparison with the base alloy.

Heavy-section ductile iron castings solidify under low cooling rates, giving the risk to form defects like non-metallic inclusions, microporosities and degenerated shapes of graphite that have a negative effect on fatigue resistance, favoring the crack nucleation. The aim of this work is to precisely identify the defects that most affect the fatigue behavior in heavy-section castings, in order to guide the foundry to take the proper countermeasures. For this purpose, fatigue rotating bending tests were carried out on specimens machined from small-scale samples opportunely cast to reproduce long solidification times. The fracture surface of broken samples were investigated by means of Scanning Electron Microscopy in order to identify crack initiation points and fracture mechanisms. Shrinkage porosities and spiky graphite were found to play the most important effect on crack nucleation, lowering the fatigue resistance of the castings, while chunky graphite just behaved as a preferential path for crack propagation.

The goal of this paper is to investigate the failure of a mandrel pin mounted on a rolling contact test bench. For this purpose, different methods of analysis were conducted, including metallography and stress analysis. The stress values in the failed pin were determined by Finite Elements. The high notch effect and the incorrect mounting of the specimens were identified as the failure causes. A pin design change is proposed to reduce the maximum stress even in the case of incorrect specimen mounting.

The influence of chunky graphite on mechanical properties, especially on fatigue resistance, of an EN-GJS-400 ferritic ductile iron, was investigated by tensile and rotating-bending tests. The samples for the experiments were machined from two heavy section castings. The first one was cast with the addition of a Ce-containing inoculant to promote chunky formation and the second one by using a standard Ce-free inoculant. The presence of chunky graphite in the samples decreased tensile strength and elongation at break, while no significant effect on the hardness and yield strength was measured. Also fatigue strength was negatively affected, with a decrease of fatigue limit of about 12%. This value was calculated also taking into account the different amount of microporosity in the two sets of samples and its effect on the fatigue strength by using the Murakami approach.Fracture analysis showed that chunky graphite has a limited influence on fatigue crack nucleation if compared to micro-shrinkages, while it behaves as a preferential path for crack propagation. A simple way to consider the presence of chunky graphite in the fatigue design of heavy section ductile irons is proposed on the basis of the present results and taking into consideration the published data too. Keywords: Ductile cast iron, Chunky graphite, Fatigue limit, Microporosity, Analysis of statistics of extremes, Fracture analysis

Ultrasound treatment is one of the known method to obtain the globular microstructure for semisolid processing of aluminum alloys. Notwithstanding the numerous advantages, the main industrial limit of this method is the low service life of the horn due to the combined hot corrosion and erosion phenomena. Various approaches are reported in literature to overcome this problem, but a real satisfactory option does not seem to be available yet. In this paper, the behavior of a H11 tool steel sonotrode coated with a thin layer of TiAlCN is investigated and compared with the performance of an uncoated steel one. The lifetime of the horn was evaluated in terms of effectiveness in semisolid feedstock material preparation. For this purpose, the microstructure of semisolid cast samples obtained after different working times was analyses by optical and scanning electron microscopy. Moreover, melt contaminations due to the probe dissolution were also evaluated. The applied coating exhibited a significant resistance to separate hot corrosion in liquid Al alloy and cavitation erosion phenomena in water. However, the combined effect of the two damaging mechanisms during ultrasound treatment of the molten alloy leaded to coating removal and to a horn consumption comparable to that of bare steel.

Fatigue resistance of Direct Metal Laser Sintered (DMLS) AlSi10Mg alloy after sand-blasting was investigated in the present study. A preliminary characterization of the samples was carried out in order to identify material microstructure, surface roughness and superficial residual stresses. It was found that the applied post-processing treatment was responsible for a strong decrease in average surface roughness as compared to the as-built condition and induced compressive residual stresses on the samples surface. Axial fatigue tests were performed in both finite and infinite (i.e. 2x106 cycles) life regimes and the obtained results were compared with literature values for the same alloy after various post-treatments, including heat treatment, machining, polishing and shot peening. A general positive effect of the applied sand-blasting on fatigue resistance was observed. This, coupled with the improvements of surface finishing, encourages the use of sand-blasting as a simple and effective post-treatment. Finally, observations of the fractured surfaces allowed also the identification of porosities located on the surface as the main crack initiation sites. Once the crack has started, it moves along a large flat area, as typical of fatigue propagation, with a small final overload region.

A solution, quenching and ageing heat treatment is often performed on additive manufactured AlSi10Mg parts to dissolve the anisotropy due to the layer-by-layer building. This study investigates the influence of temperature and duration of solution and ageing treatment on microstructure, hardness and density of AlSi10Mg alloy produced by direct metal laser sintering. A parallel investigation is carried out on AlSi10Mg samples produced by gravity casting to analyse the different response to the same heat treatment conditions. The highest hardness, combined with an acceptable increase of porosity, is reached after selected heat treatment parameters. It was also found that, compared to the as-produced condition, this treatment leads to a decrease of ultimate tensile strength, without affecting the yield strength of additive manufactured samples, and reduces the difference in properties along the two building directions. The high properties of the as-produced samples are related to the finer microstructure and, as proved by the differential scanning calorimetric measurements, to the self-quenching phenomenon.

In the present paper, fatigue tests were performed on sand-blasted AlSi10Mg samples produced by Direct Metal Laser Sintering (DMLS). The effect of sand-blasting on surface properties was evaluated by roughness and residual stresses measurements, together with morphological analysis, in comparison with as-built condition. An evident improvement of surface finishing was observed after sand-blasting, which also leaded to the presence of compressive residual stress on the external surface of samples, as revealed by XRD2 measurements. Furthermore, defects analysis allowed the identification of a uniform distribution of porosities in the cross section in terms of number of defects, while larger porosities seem more abundant close to the surface. It was found that the tested material exhibits good fatigue resistance, supporting the positive role of sand-blasting as a simple post-processing treatment. Superficial defects are the preferential crack initiation sites, as demonstrated by SEM analysis of fracture surfaces.

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 Widmastatten 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.

30NiCrMoV12 quenched and tempered samples were coated with 4 μm of CrN produced by cathodic arc deposition technique. Micro-blasting treatment was performed on the coated samples, after the deposition, to remove defects like droplets from the surface. The fatigue limit was assessed and compared to that of the bare steel. A basic characterization of the coating/substrate was performed (hardness, roughness and adhesion). The residual stress distribution next to the surface of the samples was measured and its effect on the fatigue limit was evaluated. The micro-blasting treatment does not adversely affect the hardness and the adhesion of the coating nor the fatigue resistance of the coated samples.

Residual stress has a significant effect on the performance of thin films, in terms of adhesion, hardness, wear and fatigue resistance. Thus, when assessing innovative coatings or new deposition technologies, it is important to perform residual stress evaluation by means of a suitable and reliable technique. X-ray diffraction (XRD) is one of the commonly used techniques, because it is non-destructive, surface sensitive and phase selective. However, it is subject to certain limitations: X-ray diffraction allows stress evaluation (i.e. its indirect deduction from the measured diffraction profile) only in case of crystalline materials, and the results may be subject to aberrations in the presence of texture or stress gradients often occurring in thin films. Recently, a new class of methods for residual stress evaluation has been proposed, based on incremental focused ion beam (fib) milling, combined with high-resolution in situ scanning electron microscopy (SEM) imaging and full field strain analysis by digital image correlation (DIC). The aim of the present paper is to explore in some detail the significance of the stress values obtained for the same coating by X-ray diffraction and focused ion beam milling, and to demonstrate that the analysis of residual stress depth gradients is possible by using FIB-DIC techniques. Finally, a comparative assessment of the weaknesses and strengths of the two techniques will be carried out. For this purpose, a chromium nitride highly textured thin film sample was chosen. The residual stress state evaluation by the two methods was carried out for the coating deposited by the cathodic arc evaporation (CAE-PVD) technique. Although many significant differences were identified between the X-ray diffraction and focused ion beam milling methods, careful consideration of the gauge volumes and weighting demonstrated that satisfactory agreement was obtained. The analysis highlighted the importance of the issues related to (a) probe-to-sample interaction volume (gauge volume), (b) film texture, and (c) the elastic anisotropy. All these factors must be taken into account in order to enable valid comparisons to be drawn. (C) 2014 Elsevier B.V. All rights reserved.

In heavy section cast iron components, anomalous microstructures can be formed especially at the core, because of low cooling rate, with a consequent decrease in mechanical properties. The matrix microstructure, the presence of surface defects, such as inclusions and porosities, and the size and morphology of graphite nodules are critical issues to ensure the adequate performance of the component in fatigue applications. Proper design of heavy section ductile iron castings requires a database of information about microstructures and mechanical properties that nowadays is missing or is very poor. In this paper, we presented the mechanical characterization of ductile irons with solidification times up to 20 h, carried out to identify the metallurgical and the process parameters that most affect the improvement in fatigue strength. An innovative production line was designed, aimed at reproducing on a small-scale sample the characteristics of heavy walled components. A standard ferritic ductile cast iron compared to a ferritic cast iron with high silicon content and a fully pearlitic ductile cast iron were studied by tensile, Brinell hardness and rotating bending fatigue tests. Optical microscope analysis and scanning electron microscope observations of the most significant fracture surfaces were also performed. It was concluded that the ferritic cast iron is more suitable for the production of large thickness castings undergoing fatigue loads, because the decrease in ductility and its effect on other properties is less pronounced.

Damaged DLC coatings usually require remanufacturing of the entire coated components starting from an industrial chemical de-coating step. Alternatively, a complete or local coating repair can be considered. To pursue this approach, however, a local coating removal is needed as first operation. In this context, controlled decoating based on laser sources can be a suitable and clean alternative to achieve a pre-fixed decoating depth with high accuracy. In the present study, we investigated a laser-based decoating process executed on multilayered DLC films for advanced tribological applications (deposited via a hybrid PVD/PE-CVD technique). The results were acquired via multifocal optical digital microscopy (MF-ODM), which allowed high-resolution 3D surface reconstruction as well as digital profilometry of the lasered and unlasered surface. The study identifies the most critical process parameters which influence the effective decoating depth and the post-decoating surface roughness. In particular, the role of pulse overlap (decomposed along orthogonal directions), laser fluence, number of lasing passes and assist gas is discussed in text. A first experimental campaign was designed to identify the best conditions to obtain full decoating of the DLC + DLC:Cr layers. It was observed that decreasing the marking speed to 200 mm/s was necessary to obtain a sufficient pulse overlap and a nearly planar ablation profile. By operating with microsecond pulses and 1 J/cm2 (fairly above the ablation threshold), less than 10 passes were needed to obtain full decoating of the lasered area with an etching rate of 1.1 μm/loop. Further experiments were then executed in order to minimise the roughness of the rest surface with the best value found at around 0.2 μm. Limited oxidation but higher R a values were observed in Ar atmosphere.

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.

The aim of this work is to experimentally evaluate the notch ductility of a TWinning Induced Plasticity (TWIP) steel, a Dual Phase (DP) steel and two Quenching and Partitioning (Q&P) steels for automotive components. The notch ductility was investigated by studying the resistance to fracture initiation in tensile specimens with notches of various root radii. Fracture toughness is discussed in terms of the critical J -integral J c . It was found that J c progressively increases with the notch tip radius ρ for all the studied steels, demonstrating a high sensitivity of ductility upon notch severity. The relationship between J c and ρ allowed to calculate the maximum strain at fracture initiation e max , that is proposed as a deformation limit parameter useful to rank the forming capacity of the steels. It can be considered as the maximum strain that can be sustained ahead of the notch tip up to the onset of fracture advancement. Afterwards, a metallographic analysis by means of light optical microscope and scanning electron microscope was carried on etched specimens. The fracture mechanisms were also investigated by means of scanning electron microscope.

In this paper, a complete three-dimensional numerical model of mold filling and solidification of steel ingots is presented. The risk of powder entrapment and defects formation during filling is analyzed in detail, demonstrating the importance of using a comprehensive geometry, with trumpet and runner, compared to conventional simplified models. By using a case study, it was shown that the simplified model significantly underestimates the defects sources, reducing the utility of simulations in supporting mold and process design. An experimental test was also performed on an instrumented mold and the measurements were compared to the calculation results. The good agreement between calculation and trial allowed validating the simulation.

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.

Zinc alloys are known for their low melting point, high castability, and useful wear resistance in certain applications, and at a relatively low cost. In the recent years, many attempts have been made to substitute more expensive copper alloys for zinc-based materials in wear-critical parts. In this regard, the most investigated zinc alloy is ZA27, but good wear resistance can be also achieved by using an alloy with lower aluminium content, like Zamak2. In this paper, the sliding wear behavior of a novel zinc alloy ZnAl15Cu1 was compared to that of ZA27 and Zamak2 in order to assess its relative suitability for applications where sliding wear resistance is required. Pin-on-disk tests were carried out using both bearing steel and a leaded brass sliding partners under three loads and various sliding distances. Friction coefficient and wear rates were compared. Analyses of the worn surfaces was done to identify the damage mechanisms and to understand the friction coefficient behaviour. The ZnAl15Cu1 alloy exhibited wear resistance that is comparable to or better than that for ZA27 and Zamak2.

Lamellar gray cast iron, with a mainly pearlitic microstructure, is widely used in the automotive industry, mostly in the manufacturing of brake disks. This work analyzes in depth the effects of small variations of titanium content on the microstructure and mechanical properties of cast iron brake disks. For this purpose, eight different heats of EN-GJL-250 cast iron were selected, with a similar chemical composition but with different titanium contents, varying from 0.013 to 0.031%. The drops in mechanical strength and hardness values measured on the high-Ti samples were correlated to microstructural variations quantitatively observed by means of optical and scanning electron microscope. It was found that titanium combines to form titanium nitrides, suppressing the beneficial microstructural effects of nitrogen at solidification. Residual nitrogen, if present in sufficient quantity, promotes the nucleation of primary austenite from the liquid and the formation of a fine microstructure, with small eutectic cells and lower graphite content. Such a microstructure provides brake disks with better mechanical properties. The interpretation of results was further supported by thermal analysis and thermodynamic calculations.

The water-cooled ductwork of an electric arc furnace direct evacuation system, installed in a steelmaking plant, experienced premature failure. The damaged pipe was investigated by means of optical microscope, scanning electron microscope equipped with energy dispersive X-ray spectroscopy micro-analysis and differential scanning calorimetry, in order to find out the failure causes and to suggest preventive solutions. The study shows that failure was mainly due to sulphur dewpoint corrosion formed in the hot service and continued also during furnace shutdown. Some modifications to the process were recommended to cope this problem.

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