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•The interaction of an electric current with the electrical properties of tungsten carbide green bodies triggers the conditions for flash sintering, which results in ultrafast ...densification up to 95% in less than 10 s.•A thermal runaway of Joule heating is activated by the negative temperature dependence of resistivity of the material in the green state.•Flash sintering of tungsten carbide is characterized by the absence of an incubation stage and by a self-extinguishing power surge.•The microstructure achieved after the flash can be tailored and optimized by prolonging the sintering into a resistive heating regime, thus increasing the final density further, as well.
This work explores the possibilities for the ultrafast sintering of binderless tungsten carbide by electric/pressure assisted sintering. A limited voltage (3–4 V) in AC condition was applied to WC powder compact in combination with uniaxial pressure. The thermal insulating ceramic die allows the ultrafast heating (104 °C/min) of the powder compact which undergoes a rapid transition of its electrical properties, from negative to positive dependence of resistivity on temperature, i.e. from NTC to PTC behaviour. Such effect is fundamental for inducing a thermal runaway phenomenon associated with ultrarapid temperature increase and massive electric power dissipation, thus inducing very rapid sintering. The relationship between electrical properties of tungsten carbide and the possibility to achieve “flash sintering” conditions to complete densification in a couple of seconds was investigated. At the optimal conditions of 3.5 V and 4 MPa pressure, pure WC sinters up to 95% in less than 10 s. Longer sintering time after the flash improves only slightly the density, despite a significant energetic consumption. It is also shown that if larger pressure is applied, the flash event duration and final density decrease.
The intrinsic heat treatment (IHT) during laser directed energy deposition (L-DED) of a maraging steel powder was investigated. The thermal history of each layer during the building process was ...simulated to evaluate the effect of the interlayer pause on the maximum and minimum temperatures experienced by the material. The minimum temperature decreases with the interlayer pause, enhancing the austenite to martensite transformation needed to trigger the intermetallic precipitation during the deposition of the subsequent layers. Specimens were then produced with different interlayer pauses, and the hardness profile, compressive strength, and the microstructure along the building direction were investigated and compared to the ageing curves. The results agree with the simulation and demonstrate that the intrinsic heat treatment may allow avoiding post-building heat treatment.
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Purpose - The aim of the paper is the study of the change in the mechanical properties (and in particular in ductility), with the microstructure, of a biomedical Ti-6Al-4V alloy produced by different ...variants of selective laser melting (SLM).Design methodology approach - Ti-6Al-4V alloy produced by different variants of SLM has been mechanically characterized through tensile testing. Its microstructure has been investigated by optical observation after etching and by X-ray diffraction analysis.Findings - SLM applied to Ti-6Al-4V alloy produces a material with a martensitic microstructure. Some microcracks, due the effect of incomplete homologous wetting and residual stresses produced by the large solidification undercooling of the melt pool, are observable in the matrix. Owing to the microstructure, the tensile strength of the additive manufactured parts is higher than the strength of hot worked parts, whereas the ductility is lower. A pre-heating of the powder bed is effective in assisting remelting and reducing residual stresses, but ductility does not increase significantly, since the microstructure remains martensitic. A post-building heat treatment causes the transformation of the metastable martensite in a biphasic a-b matrix, with a morphology that depends on the heat treatment. This results in an increase in ductility and a reduction in strength values.Originality value - The study evidenced how it is possible to obtain a fully dense material and make the martensite transform in Ti-6Al-4V alloy through the variation of the SLM process. The stabilization of the microstructure also results in an improvement of the ductility.
Purpose - The purpose of this paper is the microstructural and mechanical characterization of a biomedical Ti-6Al-4V alloy produced by electron beam melting, and the study of the stability of the ...as-built microstructure upon heat treatment.Design methodology approach - Ti-6Al-4V alloy produced by electron beam melting has been mechanically characterized through tensile and fatigue testing. Its microstructure has been investigated by optical observation after etching and by X-ray diffractometry analysis. The stability of the microstructure of the as-built material has been deepened carrying out suitable heat treatments, after an analysis by dilatometry test.Findings - The microstructure of a Ti-6Al-4V alloy produced by electron beam melting has a very fine and acicular morphology, because of the intrinsically high-solidification rate of the process. This microstructure is very stable, and the traditional thermal treatments cannot modify it; the microstructure changes significantly only when an amount of strain is introduced in the material. However, the mechanical properties of the alloy produced by electron beam melting are good.Originality value - The paper provides evidence of the microstructural stability of the material produced by electron beam melting. Even if the microstructure of the as-built material is not recommended by the specific ISO standard, the related mechanical properties are fully satisfactory. This is a significant indication from the point of view of the production of Ti-6Al-4V orthopaedic and dental prostheses by electron beam melting.
The aim of this paper was to investigate the compression properties of several trabecular structures produced by additive laser manufacturing of a Ti6Al4V, having different densities and unit cells. ...Filling space structures were investigated, with different unit cells characterized by both bending-dominated and stretching-dominated behaviour. The stiffness and yield strength were correlated to relative density according to the Gibson and Ashby model. For a constant porosity, the stiffness and the yield strength varied between two extremes represented by the cubic structure (stretching-dominated deformation) and the cross structure (bending-dominated deformation). The properties of the deformed structures did not differ substantially from those of the regular structures. Only in the cubic structure did distortion enhance the contribution of bending to deformation and both stiffness and strength decreased. Cross structures displayed the highest strength at constant stiffness than the others, since they are characterized by the most favourable orientation of the struts.
The main aim of the current study is to evaluate the compressive quasi‐static and fatigue properties of titanium alloy (Ti6Al4V) cellular materials, with different topologies, manufactured via laser ...powder bed fusion (LPBF) process. The topologies herein considered are lattice‐based regular and irregular configurations of cubic, star, and cross‐shaped unit cell along with trabecular‐based topology. The results have indicated that the effective stiffness of all configurations are in the range of 0.3–20 GPa, which is desirable for implant applications. The morphological irregularities in the structures induce bending‐dominated behavior affecting more the topologies with vertical struts. The S–N curves normalized with respect to the yield stress indicate that the behavior of star regular structures is between purely stretching‐dominated cubic and purely bending‐dominated cross‐based structures. Trabecular structures have shown desirable quasi‐static and fatigue properties despite the random distribution of struts.
Near-full density and crack-free AISI H13 hot-work tool steel was fabricated using laser-directed energy deposition (L-DED). Two different heat-treatment scenarios, i.e., direct tempering (ABT) from ...the as-built (AB) condition and systematization and quenching prior to tempering (QT), were investigated, and their effect on the microstructure, hardness, fracture toughness (Kapp), and tempering resistance of the L-DED H13 is reported. For this purpose, the optimal austenitization schedule was identified, and tempering curves were produced. At a similar hardness level (500 HV1), QT parts showed higher Kapp (89 MPa√m) than ABT (70 MPa√m) levels. However, the fracture toughness values obtained for both parts were comparable to those of wrought H13. The slightly larger Kapp in the QT counterpart was discussed considering the microstructural homogenization and recrystallization taking place during high-temperature austenitization. The tempering resistance of the ABT material at 600 °C was slightly improved compared with that of the QT material, but for longer holding times (up to 40 h) and higher temperatures (650 °C), ABT showed superior resistance to thermal softening due to a finer martensite substructure (i.e., block size), a finer secondary carbide size, and a larger volume fraction of secondary V(C,N) carbides.
The expressive power of interval temporal logics (ITLs) makes them one of the
most natural choices in a number of application domains, ranging from the
specification and verification of complex ...reactive systems to automated
planning. However, for a long time, because of their high computational
complexity, they were considered not suitable for practical purposes. The
recent discovery of several computationally well-behaved ITLs has finally
changed the scenario.
In this paper, we investigate the finite satisfiability and model checking
problems for the ITL D, that has a single modality for the sub-interval
relation, under the homogeneity assumption (that constrains a proposition
letter to hold over an interval if and only if it holds over all its points).
We first prove that the satisfiability problem for D, over finite linear
orders, is PSPACE-complete, and then we show that the same holds for its model
checking problem, over finite Kripke structures. In such a way, we enrich the
set of tractable interval temporal logics with a new meaningful representative.
In powder metallurgy (PM), the compaction step is fundamental to determining the final properties of the sintered components. The deformation and defectiveness introduced in the powder material ...during uniaxial die compaction can be correlated to the activation and enhancement of the dislocation pipe diffusion, a lattice diffusion mechanism during the sintering process. Its coefficient depends on the dislocation density. The powder particles are mostly deformed along the direction of the compaction (longitudinal direction) rather than along the compaction plane; consequently, the contact areas perpendicular to the direction of the compaction present a higher density of dislocations and lattice defects. This high density intensifies the shrinkage along the direction of compaction. To demonstrate the influence of uniaxial cold compaction on the material’s stress state the powder particles and their contacts were modeled using spheres made of pure copper. These spheres are compacted in a die at different pressures to better analyze the system’s response at the grade of deformation and the consequent influence on the material’s behavior during the sintering. In the different zones of the sphere, the micro-hardness was measured and correlated to the concentration of dislocations using the model for indentation size effect (ISE). After the compaction, the spheres were more deformed along the longitudinal than the transversal direction. The results obtained using hardness indentation show differences in the dislocation density between the undeformed and deformed spheres and, in the case of the compacted sphere, between the contact area along the longitudinal and the transversal direction.
Cobalt is the most used metal binder in hard metals since its extraordinary wetting, adhesion and mechanical properties. Nevertheless, it has been recognized genotoxic and cancerogenic with higher ...toxicity in combination with WC. To substitute Co with an alternative binder, the interaction between the binder and WC must be taken into account. In this work, IN625 is considered as a binder alternative due to its desirable combination of high-temperature strength and corrosion/oxidation resistance. A characterization of the interaction between WC and IN625 was carried out by means of Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDXS) and X-Ray Diffraction (XRD). Depending on the sintering temperatures, different phases were evidenced at the WC–IN625 superalloy interface. From 1250 °C to 1300 °C, where solid-state sintering takes place, (Cr,Mo)23C6, W2C and (Cr,W) solid solutions were detected. At a sintering temperature of 1350 °C, IN625 melts and the formation of additional phases, such as an intermetallic Ni4W phase and (Mo,W) and (Mo,Nb) solid solutions, were observed. The precipitation of NbC and (Mo,Cr)23C6 carbides in IN625 was also detected.
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