1,721,064 research outputs found
A unified constitutive approach for creep response of AISI 316 steel produced by conventional technologies or additive manufacturing techniques
No full textThe creep response of AISI 316 and AISI 316 L was analysed to provide a coherent picture of the material behaviour, valid for both conventional wrought and additively manufactured steels. Literature evidences were considered. The analysis demonstrated that the presence of a fine distribution of particles, precipitated during creep exposure in both AISI 316 and 316 L(N), should be taken into account. A recent model, expressly developed for particle-strengthened alloys, was successfully used to describe the minimum creep rate dependence on applied stress for AISI 316 and 316 L(N). The same set of constitutive equations, in combination with the composite model, was then used for describing creep data obtained by testing the AISI 316 L produced by additive manufacturing. The main difference with wrought materials was that the microstructure of the additively manufactured alloy was composed by cells (soft zones) and thick walls heavily decorated with ultrafine oxide particles (hard zones)
Study of mechanical, microstructural and thermal stability properties of friction stir processed aluminum 2024-T3 alloy
No full textFriction Stir Processing (FSP) was applied on AA2024-T3 plates. The hardness and tensile properties of the stir zone were tested, and a microstructure study was conducted. During the FSP the coarse elongated grains of the parent material changed into fine equiaxed ones, refinement of the coarse precipitates detected at the parent material occurred at the same time. The material subjected to FSP was found to be thermally unstable, as revealed by aging experiments at 300◦C and precipitate chemical analysis. TEM study showed that the rolled parent material contained low-angle sub-grains with dislocation network boundaries. During FSP these were replaced by fine equiaxed grains with clear boundaries and no dislocation tangles. This led to the conclusion that the material underwent Dynamic Recovery (DRV) during the rolling process, as opposed to Dynamic Recrystallization (DRX) during FSP. Thus, FSP appears to have the technological potential for improving the microstructure and mechanical properties of AA2024
Oxidation Effects on Short-Term Creep Response in Air of Commercially Pure Titanium (CP-2 Ti)
Full text linkThe creep response in the air of commercially pure titanium was investigated at 550, 600, and 650 °C to assess the effect of oxidation on the mechanical response. Experiments demonstrated that prolonged exposures at high temperatures produced a marked reduction in the minimum creep rate under a given applied stress. Microhardness measurements showed that a hardened zone formed in proximity to the surface due to oxygen penetration into the metal. A simplified composite model was then used to describe the creep response. In this model, the sample consisted of two zones, the hard case that was enriched in oxygen and the soft pure-titanium core, both creeping with similar strain rates. Calculations led to an estimation of the dependence of the minimum creep rate on stress and temperature for the hard high-oxygen zone. The simplified composite model presented here provided a good description of the experimental creep data for pure titanium, tested in its air, and a reliable picture of the effect of oxidation on complex Ti alloys
A study of the metallurgical and mechanical properties of friction-stir-processed cu
Full text linkFriction stir processing (FSP), a severe plastic deformation process, was applied on pure Cu to obtain a stir zone with a very fine grain size. Yet, when FSP is used, the stir zone is as wide as the diameter of the shoulder at the upper surface of the weld and markedly narrower near its opposite surface. This property, as well as the differences between the advancing side and the retreating side, makes it impossible to obtain a uniform cross-section as far as the microstructure and mechanical properties are concerned. For these reasons, a new approach is proposed in which the material was processed on both sides, thus yielding a wider, rectangular and more homogenous stir zone from which all the specimens were machined out. Processing the material from both sides eliminated any microstructural difference between the upper and the lower side, at least within the gauge length’s cross-section of the creep specimens. Although grain refinement was detected, the mechanical properties of the friction-stir-processed (FSP’ed) material are inferior relative to those of the parent material. The TEM study reported in the current paper revealed the existence of nanosized grains in the FSP’ed material due to dynamic recrystallization (DRX) occurring during the processing stage. Because both X-ray inspection and fractography showed that the FSP’ed material was free of defects, the material may not comply with the Hall–Petch relation due to lower dislocation density caused by XRD occurring during FSP. The inverse Hall–Petch effect may also be considered as an assistive mechanism in mechanical property deterioration
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No full textBasic creep modelling of aluminium
Full text linkIn recent years a basic creep model that does not involve adjustable parameters has been developed. The main feature of this model is that it is fully predictable and the assumptions at its basis can be easily verified once the output is compared with experimental data. This model, initially developed for pure copper, has been here applied to pure aluminium. A critical issue has been identified with the controlling mechanisms during power law breakdown. The increase in the creep rate at high stresses and low temperatures can be quantitatively explained from the raised climb rate due to the deformation-induced increase in concentration of vacancies. The model can also account for the fairly wide range of stresses where aluminium follows power-law creep with a creep exponent of 4-5. At slightly lower stresses, the creep exponent increases somewhat due to the presence of an internal stress. Since no adjustable parameters have been required, the model represents a notable enhancement over the conventional approach, which is based on the use of the power-law equation and requires fitting of experimental data to determine the values of the material parameters.</p
Creep response of Ti–6Al–4V alloy produced by additive manufacturing: Effect of annealing at 1050 °C
Full text linkThe present study mainly aims at investigating the creep response of a Ti-6Al-4V alloy produced by additive manufacturing and annealed above the 8-transus, and at rationalizing the differences observed when comparing its behaviour to that of the same alloy annealed at lower temperatures. Herein, the creep response of this alloy produced by additive manufacturing and subsequently annealed at 1050 degrees C is investigated at temperatures ranging from 500 to 650 degrees C. The heat treatment produces the typical Widmansta center dot tten microstructure with thin 8 -lamellae interposed between coarse a-lamellae. The minimum creep rate dependence on the applied stress and temperature is compared with literature data of tests of alloys with Widmansta center dot tten microstructures produced by traditional technologies. A modified form of an equation, successfully used to describe the creep responses of the Ti-6Al-4V alloy with different initial microstructures, was here proposed. The suggested modification introduces a threshold stress, which is related to the presence of finely spaced a-8 interfaces and a2-Ti3Al particles. This threshold stress is considered to be negligible when the distance between the a-8 interfaces is long and the a2-Ti3Al particles are absent or excessively spaced. In contrast, in the materials with Widmansta center dot tten microstruc- tures, even minor differences in the heat treatment conditions and/or the processing history cause considerable variations in the distance between the a-8 interfaces. This coupled with the occasional precipitation of a2-Ti3Al particles results in different threshold stress values
Analysis of the microstructural evolution during hot extrusion of AA6060 by means of FEM simulation
No full textIn this work an experimental methodology to evaluate the prediction of recrystallized structures in aluminum extrusion was presented and validated. In the first part of the work an experimental procedure to investigate the evolution of recrystallization in aluminum alloys is presented and discussed. Several cups, obtained by means of inverse extrusion, were produced at different temperatures and process speeds. The specimens were analyzed in order to examine the grain size distribution. The coefficients for dynamic recrystallization models were obtained by regression analysis after thermo-mechanical FEM simulations of the experiments realized with the code Deform 3D. A complete set of coefficients was regressed for the available microstructure evolution models inside the code environment. The specimens were then heated in a furnace and cooled in order to reproduce static recrystallization of the material. The grain distribution was examined and the coefficients for the equation for static recrystallization prediction were regressed, too. In the second part of the work the extrusion of a round-shaped profile is described and the grain size distribution on the profile and on the billet rest is analyzed. The obtained models were applied to the real extrusion of a round profile and a comparison between experimental measurements and simulation results was performed. The simulated results were in very good agreement with experimental data, except in zones where peripheral coarse grain and grain growth appeared. Here, a further investigation effort and specific modeling equations are required
Creep behaviour of a AlSiMg alloy produced by additive manufacturing
No full textThe present study aims at investigating the effect of the peculiar microstructure of additive manufactured samples on the creep behavior of a AlSiMg alloy. Constant load creep experiments were carried out between 150 and 205°C on an AlSiMg alloy produced by Powder Bed Fusion Additive Manufacturing (AM). The samples were mostly strained up to rupture, although in some cases the test were interrupted at the early onset of the tertiary region. Analyzing the time-to rupture, in the different load and temperature conditions, as a function of the applied stress, it can be clearly seen that the alloy produced by AM is substantially comparable, in terms of time to rupture, with an alloy of similar composition, tested in the die-cast state. The high values of the stress exponent suggest that the creep behavior is strongly affected by the presence of secondary-phase particles
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