Search

Friction stir processing (FSP), a severe plastic deformation process, was applied on commercially pure Ti to obtain an improved microstructure. The process yielded a refined microstructure and higher mechanical properties at room temperature (RT). Yet the microstructure was found to contain bright bands demonstrating high hardness values of about 500 HV. High-resolution scanning electron microscopy (HRSEM) as well as electron backscattering diffraction (EBSD) analysis indicated that these bands were composed of extra-fine equiaxed α-Ti grains with an average radius of 1–2 microns. In addition, a retained β phase was detected at the boundaries of these α-Ti grains, together with a small quantity of separate β grains. The results of a fractography study conducted on broken tensile specimens showed that the material that underwent FSP was free of defects and that the fracture started at these bands. It is proposed that these bright bands are due to excessive deformation occurring during the processing stage, leading to an accelerated dynamic recrystallization (DRX) process. In turn, these heavy deformation regions act as a strengthening constituent, making the material superior to the parent material as far as its mechanical RT properties are concerned. Consequently, this means that the FSP of CP-Ti has the potential to serve as an industrial means of improving the mechanical properties of the material. [ABSTRACT FROM AUTHOR]

Metal additive manufacturing (AM) technologies can be classified according to the physical process involving the raw material as fusion-based and solid-state processes. The latter includes sintering-based technologies, which are aligned with conventional fabrication techniques, such as metal injection molding (MIM), and take advantage of the freeform fabrication of the initial green part. In the present work, 17-4PH stainless steel samples were fabricated by material extrusion, or rather bound metal deposition (BMD), a solid-state AM technology. The powder-based raw material was characterized together with samples fabricated using different angular infill strategies. By coupling different characterization technologies, it was possible to identify and classify major properties and defects of the raw material and the fabricated samples. In addition, microstructural modifications were found to be linked with the mesostructural defects typical of the BMD solid-state additive manufacturing technology applied to metals. [ABSTRACT FROM AUTHOR]

The austenitic stainless steels utilized in the production of osteosynthesis devices are susceptible to crevice corrosion. Several studies have compared the corrosive behavior of austenitic and duplex stainless steels (DSS), both of which are recognized as viable biomaterials for tissue engineering applications. All of the in vitro and in vivo studies on animals and clinical results reported to date indicate that austeno-ferritic duplex stainless steel can be recommended as a suitable alternative to ASTM F138 steel, since it is resistant to crevice corrosion in the human body and presents superior mechanical properties. The use of DSS for biomedical applications is still under discussion, mainly due to the lack of knowledge of its behavior in terms of device heating or induced movement when exposed to magnetic fields, a potentially harmful effect for the human body. As a breakthrough production technology, additive manufacturing (AM) has demonstrated significant benefits for the fabrication of metal devices with patient-specific geometry. Laser powder bed fusion has particularly been used to manufacture DSS-based components. A fine control of the processing conditions allows for an understanding of DSS microstructural evolution, which is essential for selecting processing parameters and estimating performance, including mechanical properties and corrosion resistance. Furthermore, scientific investigation is necessary for determining the relationships among material, process, and magnetic properties, in order to establish the underlying principles and critical responses. The purpose of this review is to highlight the key performances of DSS for biomedical applications and to point out the relevant role of advanced processing technologies such as additive manufacturing. [ABSTRACT FROM AUTHOR]

Commercially pure titanium (CP-Ti) plates were friction-stir welded (FSWed) using a welding tool with a tungsten carbide (WC) pin. The bead-on-plate technique was applied to reduce the effects of welding defects, such as incomplete penetration. An X-ray inspection and fractography showed that the FSWed material was free of defects and of WC particles, which may have originated from the welding tool. The appearance of refined equiaxed grains in the thermo-mechanically affected zone (TMAZ) may have been related to dynamic recrystallization (DRX) occurring during the FSW due to the high temperature and intensive plastic deformation involved in the process. Grain refinement, mechanical twinning, and increased dislocation density were detected within the TMAZ, and these microstructural changes were considered to be responsible for the improved mechanical properties of the TMAZ. The TEM study reported in the current paper revealed the presence of nano-sized grains in the FSWed material due to dynamic recrystallization (DRX) occurring during the processing stage. The microstructure obtained during FSW of Ti has been reported in a several publications, yet many discrepancies can be found in these publications. Among these discrepancies are the size and the shape of the grains at the various zones, as well as the presence or non-presence of various zones at the vicinity of the weld. The current study contradicts the argument for correlations between the conditions prevailing at different points across the TMAZ and microstructural changes, which were previously proposed by several researchers. [ABSTRACT FROM AUTHOR]

Friction stir processing (FSP) makes it possible to obtain a stir zone with very fine grain size with the aid of severe plastic deformation. Yet using FSP, it is impossible to obtain a uniform cross section as far as the microstructure and mechanical properties are concerned. To reduce the effect of this limitation, in the current study, the material was processed on both sides, thus yielding a wider, rectangular and more homogenous stir zone. In a recent publication, the authors focused on the mechanical properties, thermal stability and transmission electron microscopy (TEM) study of friction-stir-processed AA2024-T3, comparing this alloy to the parent material. While the previous study mentioned focused on the parent and on the as friction-stir-processed material, the current study focuses on post-creep specimens and hence completes the previous one with microstructural processes occurring during creep. The current paper completes the above precipitate analysis using electron-dispersive x-ray spectroscopy (EDS) mapping of the various precipitates reported after exposure to creep temperatures. The TEM study reported in the current paper revealed the formation of dislocation structures during creep, in parallel to the dynamic recrystallization (DRX) reported by the authors in the past. In addition, fractography study indicated premature cracking as the prevailing failure mechanism as well as in the case of friction stir welded creep specimens. [ABSTRACT FROM AUTHOR]

Abstract Friction stir extrusion is an innovative process designed to recycle metal chips from various machining operations. In this research, the feasibility of solid-state recycling of pure aluminum AA1090 machining chips using FSE process is investigated. In the early stage, a FE simulation was conducted in order to optimize the die design (spiral scroll of the plunge, hole size and bearing distance) and the process parameters in terms of plunge rotational speed and extrusion rate. The AA1090 aluminum chips were produced by turning off an as-received bar without lubrication. The chips were compacted on a MTS machine up to 150KN of load. The resulting chip-billets had a diameter of 40mm and 30mm high. The chip-based billet was FS Extruded at 1000rpm rotational speed and 0.8mm/s of plunge displacement. The extruded samples were analyzed by optical microscope in order to see the material flow and to characterize the microstructure. Finally, micro-hardness Vickers profiles were carried out, in both longitudinal and transversal direction, in order to investigate the homogeneity of the mechanical properties of the extrudate. [ABSTRACT FROM AUTHOR]

The poor weldability of the AA2024 aluminum alloy limits its use in industrial applications. Because friction stir welding (FSW) is a non-fusion welding process, it seems to be a promising solution for welding this alloy. In the current study, FSW was applied to butt weld AA2024-T3 aluminum alloy plates. Creep tests were conducted at 250 and at 315 °C on both the parent material and the friction stir welded specimens. The microstructures of the welded and non-welded AA2024-T3 specimens before and after the creep tests were studied and compared. A comprehensive transmission electron microscopy study together with a high-resolution scanning electron microscopy study and energy-dispersive x-ray spectroscopy analysis was conducted to investigate the microstructure stability. The parent material seems to contain two kinds of Cu-rich precipitates—coarse precipitates of a few microns each and uniformly dispersed fine nanosized precipitates. Unlike the parent material, the crept specimens were found to contain the two kinds of precipitates mentioned above together with platelet-like precipitates. In addition, extensive decoration of the grain boundaries with precipitates was clearly observed in the crept specimens. Controlled aging experiments for up to 280 h at the relevant temperatures were conducted on both the parent material and the welded specimens in order to isolate the contribution of exposure to high temperatures to the microstructure changes. TEM study showed the development of dislocation networks into a cellular dislocation structure in the case of the parent metal. Changes in the dislocation structure as a function of the creep strain and the FSW process were recorded. A detailed creep data analysis was conducted, taking into account the instability of the microstructure. [ABSTRACT FROM AUTHOR]

The microstructural features of INCOLOY alloy 617 in the solution annealed condition and after long-term creep tests at 700 and 800 °C were characterized and correlated with hardness and creep strength. Major precipitates included (Cr,Mo,Fe)23C6 carbides and the δ-Ni3Mo phase. M6C and MC carbides were also detected within the austenitic grains. However, minor precipitates particularly γ′-Ni3(Al,Ti) was found to play an important role. At different exposure temperatures, the microstructural features of the Ni–22Cr–12Co–9Mo alloy changed compared with the as-received condition. The presence of discontinuously precipitated (Cr,Mo,Fe)23C6 carbides and their coarsening until the formation of an intergranular film morphology could be responsible both for a reduction in rupture strength and for enhanced intergranular embrittlement. The fraction and morphology of the γ′-phase, precipitated during exposure to high temperature, also changed after 700 or 800 °C exposure. At the latter test temperature, a lower volume fraction of coarsened and more cubic γ′ precipitates were observed. These microstructural modifications, together with the presence of the δ-phase, detected only in specimens exposed to 700 °C, were clearly responsible for the substantially good creep response observed at 700 °C, compared with that found at 800 °C. [ABSTRACT FROM AUTHOR]

Friction 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. [ABSTRACT FROM AUTHOR]

Friction stir welding (FSW) is the most widely used solid-state joining technique for light-weight plate and sheet products. This new joining technique is considered an energy-saving, environment friendly, and relatively versatile technology. FSW has been found to be a reliable joining technique in high-demand technology fields, such as high-strength aerospace aluminum and titanium alloys, and for other metallic alloys that are hard to weld by conventional fusion welding. Several studies accounted for the microstructural modifications induced by solid-state FSW, based on the resulting mechanical properties obtained at the FSW joints, such as tensile, bending, torsion, ductility and fatigue responses. In the last few years with the need and emerging urgency to widen the FSW application fields, broadening the possible alloy systems, and to optimize the resulting mechanical properties, this joining technique was further developed. In this respect, the present contribution focuses on two modified-FSW techniques and approaches applied to aluminum alloys plates. In a first case, an age-hardening AA6082 sheets were double side friction stir welded (DS-FSW). In a second case a non-age-hardening AA5754 sheet was FSW by an innovative approach in which welding pin was forced to slightly deviate away from the joining centreline (defined by authors as RT). In both the cases different pin heights were used, the sheets were subjected to heat treatments (peak hardening T6 for the AA6082, and annealing for the AA5754) and compared to the non-heat treated FSW conditions. Microstructural modifications were characterized by optical microscopy (OM). The mechanical properties were characterized both locally, by nanoindentation techniques, and globally, by tensile (yield, YT; ultimate, UT; and elongation, El) or forming limit curve (FLC) tests. Both the new approaches were directly compared to the conventional FSW techniques in terms of resulting microstructures and mechanical responses. [ABSTRACT FROM AUTHOR]

Abstract: The Ni–22Cr–12Co–9Mo alloy (UNS alloy N06617, also known as alloy 617), of relatively common use at high temperature, is also one of the candidate materials for the most ambitious EU and US projects for future generations of power plants. Its microstructure in the solution-annealed condition and after long-term creep tests at 700 and 800°C was characterized by means of light optical, scanning electron and transmission electron microscopy. Differential scanning calorimetry (DSC) analyses were also performed to check the usefulness of this technique to detect microstructural modification in specimens exposed to high temperature. Microstructural features were correlated to the hardness evolution in a chronological range of several thousand hours. The major microstructural features were identified to be the presence of inter- and/or intragranular particles ((Cr,Mo,Fe)23C6 and other types of carbides, δ-Ni3Mo and γ′-Ni3(Al,Ti)) that clearly played an important role in the mechanical behaviour of the alloy. The microstructural features of the investigated alloy changed at different exposure temperatures (and times) with respect to the as-received condition. The observed microstructural instability of the alloy suggested that creep data obtained at the highest temperature should be very carefully considered in extrapolating the creep strength of the alloy at about 700°C, since they should lead to a substantial underestimation of the creep life. [Copyright &y& Elsevier]