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Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 2016"


Journal ArticleDOI
TL;DR: In this paper, the effects of fabrication orientation, surface polishing, and hot isostatic pressing upon mechanical behavior of four metallic alloys fabricated with layered, laser-heated methods of additive manufacturing (AM) was compared to that of similar alloys produced with conventional methods (wrought and machined).
Abstract: Mechanical behavior of four metallic alloys fabricated with layered, laser-heated methods of additive manufacturing (AM) was compared to that of similar alloys produced with conventional methods (wrought and machined). AM materials were produced by a leading commercial service provider, as opposed to incorporating material specimens produced by unique or specially-adapted equipment. The elastic moduli were measured in flexure, stress–strain characteristics were measured in tensile deformation, and fatigue strengths were measured in fully reversed bending. The effects of fabrication orientation, surface polishing, and hot isostatic pressing upon mechanical behavior were studied. The fatigue strengths exhibited by SLM AlSi10Mg and DMLS Ti6Al4V in the as-fabricated condition proved to be significantly inferior to that of conventional material. These lower fatigue strengths are a consequence of multiple fatigue cracks initiating at surface defects, internal voids and microcracks, and growing simultaneously during cyclic loading. Measured fatigue strengths of DMLS 316L and 17-4PH approached those of corresponding wrought materials when subjected to principal stresses aligned with the build planes. When cyclic stresses were applied across the build planes of the DMLS stainless steels, fatigue fractures often developed prematurely by separation of material. Post-processing the DMLS Ti6Al4V and SS316L with hot isostatic pressure elevated the fatigue strength significantly. Measurements of surface roughness with an optical profilometer, examinations of the material microstructures, and fractography contribute to an understanding of the mechanical behavior of the additive materials.

720 citations


Journal ArticleDOI
TL;DR: In this paper, the influence of solution and artificial aging heat treatments on the microstructures and mechanical properties of SLM-produced AlSi10Mg alloy parts was investigated.
Abstract: The present paper systematically investigated the influence of solution and artificial aging heat treatments on the microstructures and mechanical properties of SLM-produced AlSi10Mg alloy parts. Due to the high cooling rate of SLM, an ultrafine eutectic microstructure in the as-built samples is characterized by spherical nano-sized network eutectic Si embedded in the Al matrix, which gives rise to significantly better tensile properties and Vickers micro-hardness. The solubility of Si atom in the Al matrix of as-built SLM samples is calculated to be 8.89 at%. With the increase in the solution temperature, the solubility decreases rapidly. The artificial aging causes the further decrease of the solubility of Si atoms in the Al matrix. Upon solution heat treatment, Si atoms are rejected from the supersaturated Al matrix to form small Si particles. With increasing the solution temperature, the size of the Si particles increases, whereas their number decreases. After artificial aging, the Si particles are further coarsened. The variation in size of Si particles has a significant influence on the mechanical properties of the AlSi10Mg samples. The tensile strength decreases from 434.25±10.7 MPa for the as-built samples to 168.11±2.4 MPa, while the fracture strain remarkably increases from 5.3±0.22% to 23.7±0.84% when the as-built sample is solution-treated at 550 °C for 2 h. This study indicates that the microstructure and mechanical properties of SLM-processed AlSi10Mg alloy can be tailored by suitable solution and artificial aging heat treatments.

569 citations


Journal ArticleDOI
TL;DR: In this paper, the nano-, micro-, and macro-scale mechanical properties of selective laser melting (SLM) AlSi10Mg were examined and correlated to the generated microstructure.
Abstract: Selective laser melting (SLM) of aluminium is of research interest because of its potential benefits to high value manufacturing applications in the aerospace and automotive industries. In order to demonstrate the credibility of SLM Al parts, their mechanical properties need to be studied. In this paper, the nano-, micro-, and macro-scale mechanical properties of SLM AlSi10Mg were examined. In addition, the effect of a conventional T6-like heat treatment was investigated and correlated to the generated microstructure. Nanoindentation showed uniform hardness within the SLM material. Significant spatial variation was observed after heat treatment due to phase transformation. It was found that the SLM material's micro-hardness exceeded its die-cast counterpart. Heat treatment softened the material, reducing micro-hardness from 125±1 HV to 100±1 HV. An ultimate tensile strength (333 MPa), surpassing that of the die cast counterpart was achieved, which was slightly reduced by heat treatment (12%) alongside a significant gain in strain-to-failure (~threefold). Significantly high compressive yield strength was recorded for the as-built material with the ability to withstand high compressive strains. The SLM characteristic microstructure yielded enhanced strength under loading, outperforming cast material. The use of a T6-like heat treatment procedure also modified the properties of the material to yield a potentially attractive compromise between the material's strength and ductility making it more suitable for a wider range of applications and opening up further opportunities for the additive manufacturing process and alloy combination.

447 citations


Journal ArticleDOI
TL;DR: In this article, the authors examined the mechanical behavior of uniform and graded density SLM Al-Si10-Mg lattices under quasistatic loading and determined their effective elastic modulus and Gibson-Ashby coefficients, C1 and α, which can form the basis of new design methodologies for superior components.
Abstract: Metal components with applications across a range of industrial sectors can be manufactured by selective laser melting (SLM). A particular strength of SLM is its ability to manufacture components incorporating periodic lattice structures not realisable by conventional manufacturing processes. This enables the production of advanced, functionally graded, components. However, for these designs to be successful, the relationships between lattice geometry and performance must be established. We do so here by examining the mechanical behaviour of uniform and graded density SLM Al-Si10-Mg lattices under quasistatic loading. As-built lattices underwent brittle collapse and non-ideal deformation behaviour. The application of a microstructure-altering thermal treatment drastically improved their behaviour and their capability for energy absorption. Heat-treated graded lattices exhibited progressive layer collapse and incremental strengthening. Graded and uniform structures absorbed almost the same amount of energy prior to densification, 6.3±0.26.3±0.2 MJ/m3 and 5.7±0.25.7±0.2 MJ/m3, respectively, but densification occurred at around 7% lower strain for the graded structures. Several characteristic properties of SLM aluminium lattices, including their effective elastic modulus and Gibson-Ashby coefficients, C1 and α, were determined; these can form the basis of new design methodologies for superior components in the future.

400 citations


Journal ArticleDOI
TL;DR: In this article, the authors reviewed the findings of Bridgman and his successors from 1935 to 1988 using the HPT method and summarized their historical importance in recent advancement of materials, properties, phase transformations and HPT machine designs.
Abstract: High-pressure torsion (HPT) method currently receives much attention as a severe plastic deformation (SPD) technique mainly because of the reports of Prof. Ruslan Z. Valiev and his co-workers in 1988. They reported the efficiency of the method in creating ultrafine-grained (UFG) structures with predominantly high-angle grain boundaries, which started the new age of nanoSPD materials with novel properties. The HPT method was first introduced by Prof. Percy W. Bridgman in 1935. Bridgman pioneered application of high torsional shearing stress combined with high hydrostatic pressure to many different kinds of materials such as pure elements, metallic materials, glasses, geological materials (rocks and minerals), biological materials, polymers and many different kinds of organic and inorganic compounds. This paper reviews the findings of Bridgman and his successors from 1935 to 1988 using the HPT method and summarizes their historical importance in recent advancement of materials, properties, phase transformations and HPT machine designs.

390 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of processing parameters on the density of the deposited Al-Cu-Mg samples was studied and it was shown that the laser energy density plays a significant role in the densification behavior of the powder during the SLM process.
Abstract: The interest for a wider range of usable materials for the technology of selective laser melting (SLM) is growing. In this work, the manufacturing of wrought Al–Cu–Mg parts using SLM technology was systematically investigated. The effect of processing parameters on the density of the deposited Al–Cu–Mg samples was studied. It shows that the laser energy density plays a significant role in the densification behavior of the Al–Cu–Mg powder during the SLM process. The laser energy density value of 340 J/mm3 is found to be the threshold, above which high density samples (99.8%) without imperfections and microcracks can be obtained. The SLMed Al–Cu–Mg part presents a unique layer-wise feature which consisted of an extremely fine supersaturated cellular-dendrites structure. The ultimate tensile strength of 402 MPa and the yield strength of 276 MPa are achieved for the SLMed Al–Cu–Mg part. The combination of grain refinement and solid solution strengthening mechanisms during SLM process are proposed to explain the high mechanical strength.

368 citations


Journal ArticleDOI
TL;DR: The mechanical properties and corrosion resistance of 316-L stainless steel fabricated using the Laser Engineered Net Shaping (LENS) technique have been studied in this paper, and the results prove that the microstructure of the SS316L fabricated using LENS is heterogeneous; its impact on the mechanical properties is visible.
Abstract: The mechanical properties and corrosion resistance of 316 L stainless steel fabricated using the Laser Engineered Net Shaping (LENS) technique have been studied. The crack-free, full density samples made using SS316L alloy powder and the LENS technique are characterized by an unusual distinct dual-phase microstructure. STEM analysis revealed a significant increase of Cr and Mo content and a decrease of Ni in the grain boundaries. Based on the Cr and Ni content (austenite stabilizing elements), the Schaeffler diagram and the EBSD results, the existence of intercellular delta ferrite on subgrain boundaries and austenite in the fine-grains are observed. The XRD patterns, in addition to the FCC austenite phase, revealed the second BCC ferrite phase. Moreover, the sigma (FeCr) phases are present in the analyzed 316 L stainless steel. The occurrence of ferrite, which does not occur in regular stainless steel fabricated using conventional metallurgical methods, improves the mechanical and corrosion properties of the LENS-fabricated sample made using 316 L stainless steel powder. The obtained results prove that the microstructure of the SS316L fabricated using LENS is heterogeneous; its impact on the mechanical properties is visible. The analyzed samples are characterized by anisotropic mechanical properties that are favorable. For both the perpendicular and parallel directions of tensile tests, samples had a ductile fracture with many dimples inside of the larger dimples. The corrosion potential of SS316L LENS and classically manufactured steel is similar. The SS316L fabricated using LENS is characterized by a relatively low value of corrosion current density, which translates into much smaller corrosion rates.

324 citations


Journal ArticleDOI
TL;DR: In this paper, a series of fully-reversed strain-controlled fatigue tests is conducted on Ti-6Al-4V specimens fabricated using Laser Engineered Net Shaping (LENS), a Direct Laser Deposition (DLD) additive manufacturing (AM) process, are investigated.
Abstract: In order for additive-manufactured parts to become more widely utilized and trusted in application, it is important to have their mechanical properties well-characterized and certified. The fatigue behavior and failure mechanisms of Ti–6Al–4V specimens fabricated using Laser Engineered Net Shaping (LENS), a Direct Laser Deposition (DLD) additive manufacturing (AM) process, are investigated in this study. A series of fully-reversed strain-controlled fatigue tests is conducted on Ti–6Al–4V specimens manufactured via LENS in their as-built and heat-treated conditions. Scanning Electron Microscopy (SEM) is used to examine the fracture surfaces of fatigue specimens to qualify the failure mechanism, crack initiation sites, and defects such as porosity. Due to the relatively high localized heating and cooling rates experienced during DLD, fabricated parts are observed to possess anisotropic microstructures, and thus, different mechanical properties than those of their traditionally-manufactured wrought counterparts. The fatigue lives of the investigated LENS specimens were found to be shorter than those of wrought specimens, and porosity was found to be the primary contributor to these shorter fatigue lives, with the exception of the heat-treated LENS samples. The presence of pores promotes more unpredictable fatigue behavior, as evidenced by data scatter. Pore shape, size, location, and number were found to impact the fatigue behavior of the as-built and annealed DLD parts. As porosity seems to be the main contributor to the fatigue behavior of DLD parts, it is important to optimize the manufacturing process and design parameters to minimize and control pore generation during the build.

254 citations


Journal ArticleDOI
TL;DR: In this paper, electron beam melting (EBM) was used to produce Ti-6Al-4V specimens, whose microstructure, texture, and tensile properties were fully characterized.
Abstract: Electron Beam Melting (EBM), a powder bed additive layer manufacturing process, was used to produce Ti–6Al–4V specimens, whose microstructure, texture, and tensile properties were fully characterized. The microstructure, analyzed by optical microscopy, SEM/EBSD and X-ray diffraction, consists in fine α lamellae. Numerical reconstruction of the parent β phase highlighted the columnar morphology of the prior β grains, growing along the build direction upon solidification of the melt pool. The presence of grain boundary αGB along the boundaries of these prior β grains is indicative of the diffusive nature of the β→α phase transformation. Texture analysis of the reconstructed high temperature β phase revealed a strong pole in the build direction. For mechanical characterization, tensile specimens were produced using two different build themes and along several build orientations, revealing that vertically built specimens exhibit a lower yield strength than those built horizontally. The effect of post processing, either mechanical or thermal, was extensively investigated. The influence of surface finish on tensile properties was clearly highlighted. Indeed, mechanical polishing induced an increase in ductility – due to the removal of critical surface defects – as well as a significant increase of the apparent yield strength – caused by the removal of a ~150 µm rough surface layer that can be considered as mechanically inefficient and not supporting any tensile load. Thermal post-treatments were performed on electron beam melted specimens, revealing that subtransus treatments induce very moderate microstructural changes, whereas supertransus treatments generate a considerably different type of microstructure, due to the fast β grain growth occurring above the transus. The heat treatments investigated in this work had a relatively moderate impact on the mechanical properties of the parts.

252 citations


Journal ArticleDOI
TL;DR: In this paper, two strengthening methods were investigated: inter-layer cold working and post-deposition heat treatment, and the effect of postdeposition T6 heat treatment was investigated on the as-deposited and 45-kN rolled alloys.
Abstract: Wire + Arc Additive Manufacture (WAAM) attracts great interest from the aerospace industry for producing components with aluminum alloys, particularly Al–Cu alloy of the 2000 series such as 2219 alloy. However the application is restricted by the low strength properties of the as-deposited WAAM metal. In this study two strengthening methods were investigated – inter-layer cold working and post-deposition heat treatment. Straight wall samples were prepared with 2319 aluminum alloy wire. Inter-layer rolling with loads of 15 kN, 30 kN and 45 kN were employed during deposition. The ultimate tensile strength (UTS) and yield strength (YS) of the inter-layer rolled alloy with 45 kN load can achieve 314 MPa and 244 MPa respectively. The influence of post-deposition T6 heat treatment was investigated on the WAAM alloy with or without rolling. Compared with inter-layer rolling, post-deposition heat treatment can provide much greater enhancement of the strength. After T6 treatment, the UTS and YS of both of the as-deposited and 45 kN rolled alloys exceeded 450 MPa and 305 MPa respectively, which are higher than the properties of the wrought 2219−T6 alloy. The strengthening mechanisms of this additively manufactured Al–6.3Cu alloy were investigated through microstructure analysis.

234 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of thermo-mechanical processing on the evolution of microstructure and mechanical properties was investigated in an AlCoCrFeNi2.1 high entropy alloy.
Abstract: The effect of thermo-mechanical processing on the evolution of microstructure and mechanical properties was investigated in an AlCoCrFeNi2.1 high entropy alloy. For this purpose, the alloy was cold-rolled to 90% reduction in thickness and annealed at temperatures ranging from 800 °C to 1200 °C. The as-cast alloy revealed eutectic lamellar mixture of (Ni, Al) rich but Cr depleted B2 phase and Al-depleted L12 phases, having volume fractions of ~35% and 65%, respectively. Nanosized precipitates enriched in Cr and having disordered BCC structure were found dispersed inside the B2 phase. Cold-rolling resulted in progressive disordering of the L12 phase but the B2 phase maintained the ordered structure. The disordering of the L12 phase was accompanied by the evolution of ultrafine lamellar structure and profuse shear band formation. Annealing of the 90% cold-rolled material at 800 °C resulted in the formation of a duplex microstructure composed of two different phases with equiaxed morphologies, having significant resistance to grain growth up to 1200 °C. The annealed materials showed disordered FCC and precipitate-free B2 phases. This indicated that quenching of the annealed specimens to room temperature was sufficient to prevent the ordering of the L12 phase and the formation of the Cr-rich nano-precipitates which were dissolved in the B2 phase during annealing. Significant improvement in tensile properties compared to the as-cast alloy could be achieved by thermo-mechanical processing. All the specimens annealed at 800 °C to 1200 °C were having good tensile ductility over 10% as well as high tensile strength greater than 1000 MPa. These indicated that the properties of the EHEA could be successfully tailored using thermo-mechanical processing for a wide range of engineering applications.

Journal ArticleDOI
TL;DR: In situ tensile tests were performed on additively manufactured austenitic stainless steel to track damage evolution within the material as discussed by the authors, and the results showed that porosity distribution played a larger role in affecting the fracture mechanisms than measured bulk density.
Abstract: In situ tensile tests were performed on additively manufactured austenitic stainless steel to track damage evolution within the material. For these experiments Synchrotron Radiation micro-Tomography was used to measure three-dimensional pore volume, distribution, and morphology in stainless steel at the micrometer length-scale while tensile loading was applied. The results showed that porosity distribution played a larger role in affecting the fracture mechanisms than measured bulk density. Specifically, additively manufactured stainless steel specimens with large inhomogeneous void distributions displayed a flaw-dominated failure where cracks were shown to initiate at pre-existing voids, while annealed additively manufactured stainless steel specimens, which contained low porosity and randomly distributed pores, displayed fracture mechanisms that closely resembled wrought metal.

Journal ArticleDOI
TL;DR: In this paper, different scanning strategies can be used to produce either a columnar grain structure with a high texture in building direction or an equiaxed fine grained structure, and numerical simulations of the selective melting process are applied to study the fundamental mechanisms responsible for differing grain structures.
Abstract: Selective electron beam melting (SEBM) is an additive manufacturing method where complex parts are built from metal powders in layers of typically 50 µm. An electron beam is used for heating (about 900 °C building temperature) and selective melting of the material. The grain structure evolution is a result of the complex thermal and hydrodynamic conditions in the melt pool. We show how different scanning strategies can be used to produce either a columnar grain structure with a high texture in building direction or an equiaxed fine grained structure. Numerical simulations of the selective melting process are applied to study the fundamental mechanisms responsible for differing grain structures. It is shown, that the direction of the thermal gradient during solidification can be altered by scanning strategies to acquire either epitaxial growth or stray grains. We show that it is possible to locally alter the grain structure of a part, thus allowing tailoring of the mechanical properties.

Journal ArticleDOI
TL;DR: A CoCrFeNiMn high-entropy alloy (HEA) was processed by high-pressure torsion (HPT) under 6.0 GPa pressure up to 10 turns at room temperature.
Abstract: A CoCrFeNiMn high-entropy alloy (HEA) was processed by high-pressure torsion (HPT) under 6.0 GPa pressure up to 10 turns at room temperature. It is shown that there is a gradual evolution in hardness with increasing numbers of turns but full homogeneity is not achieved even after 10 turns. Microhardness measurements reveal that the material reaches a saturation hardness value of ~4.41 GPa and in this condition the microstructure shows exceptional grain refinement with a grain size of ~10 nm. An ultimate strength value of ~1.75 GPa and an elongation to fracture of ~4% were obtained in a sample processed for 5 turns. The nanostructured HEA was subjected to post-deformation annealing (PDA) at 473–1173 K and it is shown that the hardness increases slightly to 773 K due to precipitation and then decreases up to 1173 K due to a combination of recrystallization, grain growth and a dissolution of the precipitates. The formation of brittle precipitates, especially σ-phase, at 873 and 973 K significantly reduces the ductility. Short-term annealing for 10 min at 1073 K prevents grain growth and leads to a combination of high strength and good ductility including an ultimate tensile strength of ~830 MPa and an elongation to failure of ~65%.

Journal ArticleDOI
TL;DR: In this paper, the design and development of ductile and strong refractory single-phase high-entropy alloys (HEAs) for high temperature applications, based on NbTaV with addition of Ti and W, were reported.
Abstract: This study reports the design and development of ductile and strong refractory single-phase high-entropy alloys (HEAs) for high temperature applications, based on NbTaV with addition of Ti and W. Assisted by CALPHAD modeling, a single body-centered cubic solid solution phase was experimentally confirmed in the as-cast ingots using X-ray diffraction and scanning electron microscopy. The observed elemental segregation in each alloy qualitatively agrees with CALPHAD prediction. The Vickers microhardnesses (and yield strengths) of the alloys are about 3 (and 3.5–4.4) times that those estimated from the rule of mixture. While NbTaTiVW shows an impressive yield strength of 1420 MPa with fracture strain of 20%, NbTaTiV exhibits exceptional compressive ductility at room temperature.

Journal ArticleDOI
TL;DR: In this paper, selective laser melting (SLM) was used to fabricate tensile specimens using Hastelloy-X pre-alloyed powder and they were compared with ones post processed by heat treatments (HT), hot isostatic pressing (HIP) and a combination of both (Hip+HT).
Abstract: Selective laser melting (SLM) was used to fabricate tensile specimens using Hastelloy-X pre-alloyed powder. Mechanical behaviour at room temperature, normal and parallel to building direction, was investigated. Furthermore, as-fabricated tensile samples were compared with ones post processed by heat treatments (HT), hot isostatic pressing (HIP) and a combination of both (HIP+HT). Yield strength (YS), ultimate tensile strength (UTS) and elongation to failure (e f ) were analysed and explained based on the microstructure evolution. Dendrites and molten pool boundaries are mainly responsible for the observed anisotropy in e f of horizontal and vertical samples in the as-fabricated condition. After their dissolution by HT an increase in e f was observed. The columnar grain structure also contributes to the observed anisotropy in e f , inducing more ductile and cleavage like fracture surfaces in vertical and horizontal samples, respectively. The removal of porosity after HIP and HIP+HT yields a positive effect on e f . HIP or HT after SLM reduces the YS due to recovery processes such as dislocation density reduction and rearrangement of these dislocations in subgrain boundaries. Carbides of the type M x C y were partially segregated at the grain boundaries after HIP with detrimental effect on e f .

Journal ArticleDOI
TL;DR: In this paper, the influence of the manufacturing process of the 316L grade austenitic steel on the microstructure and the resulting material properties were investigated, and the mechanical properties of cast and solution annealed, as well as steel powder densified by hot-isostatic pressing (HIP), selective laser melting (SLM), and SLM+HIP, were compared.
Abstract: Besides the chemical composition, the manufacturing route primarily determines a material's properties. In this work, the influence of the manufacturing process of the 316 L grade austenitic steel on the microstructure and the resulting material properties were investigated. Thus, the microstructure and mechanical properties of cast and solution annealed, as well as steel powder densified by hot-isostatic pressing (HIP), selective laser melting (SLM) and SLM+HIP, were compared. A SLM parameter study illustrates that the porosity of SLM-densified specimens can be reduced with direction of a higher exposure time and a smaller point distance. With an additional treatment by HIP, the porosity scarcely changes, while cracks are reduced. The mechanical properties were investigated depending on the manufacturing process, and the influence of the sample build up by SLM was examined. High mechanical values have been obtained; in particular, the yield strength in the SLM-densified condition is much higher than in cast or HIP condition, as a result of the smaller grain size.

Journal ArticleDOI
TL;DR: In this paper, the anisotropy of Young's modulus with the texture of the material was measured using electron backscatter diffraction (EBSD) and the results showed that the applied laser scanning strategies allow to tailor the crystallographic texture locally.
Abstract: Selective laser melting (SLM) is an emerging technology of additive manufacturing, which is used to directly produce metallic parts from thin powder layers. This study aims at correlating laser scanning strategies with the resulting textures and corresponding anisotropy of the elastic behavior of bulk materials. Tensile test specimens made of the γ’-containing Ni-base superalloy IN738LC were built with the loading direction oriented either parallel (z-specimens) or perpendicular to the build-up direction (xy-specimens). Their bulk mechanical properties were determined at room temperature and at 850 °C. Specimens were investigated in the ‘as-built’ condition and after recrystallization heat treatment. SEM-based electron backscatter diffraction (EBSD) was applied to measure their crystallographic preferred orientations (texture) and to correlate the anisotropy of Young's modulus with the texture of the material. It is shown that the applied laser scanning strategies allow to tailor the crystallographic texture locally. The possibility to switch from transverse anisotropic to transverse isotropic properties and reverse is demonstrated for triple layered tensile samples. A recrystallization heat treatment reduces the degree of crystallographic texture and thus the elastic anisotropy by abundant annealing twinning. Predictions of Young's modulus calculated from the measured textures compare well with the data from tensile tests.

Journal ArticleDOI
TL;DR: In this article, the microstructure and mechanical properties are affected by the location within the manufactured wall component, and the results are further explained in detail through the weld pool behavior and temperature field measurement.
Abstract: Additive layer manufacturing (ALM), using gas tungsten arc welding (GTAW) as heat source, is a promising technology in producing Inconel 625 components due to significant cost savings, high deposition rate and convenience of processing. With the purpose of revealing how microstructure and mechanical properties are affected by the location within the manufactured wall component, the present study has been carried out. The manufactured Inconel 625 consists of cellular grains without secondary dendrites in the near-substrate region, columnar dendrites structure oriented upwards in the layer bands, followed by the transition from directional dendrites to equiaxed grain in the top region. With the increase in deposited height, segregation behavior of alloying elements Nb and Mo constantly strengthens with maximal evolution in the top region. The primary dendrite arm spacing has a well coherence with the content of Laves phase. The microhardness and tensile strength show obvious variation in different regions. The microhardness and tensile strength of near-substrate region are superior to that of layer bands and top region. The results are further explained in detail through the weld pool behavior and temperature field measurement.

Journal ArticleDOI
TL;DR: Inconel 718 specimens were additively manufactured via selective laser melting (SLM) and subjected to different post-process heat treatments as discussed by the authors, and HIP was employed in order to reduce the porosity typically present after SLM.
Abstract: Inconel 718 specimens were additively manufactured via selective laser melting (SLM) and subjected to different post-process heat treatments. Performance under monotonic and cyclic loading in the low-cycle fatigue (LCF) regime was investigated at room temperature (RT). Hot isostatic pressing (HIP) was employed in order to reduce the porosity typically present after SLM. Prior functional encapsulation with a Ni-20Cr coating applied by cathodic arc deposition (Arc-PVD) was applied to achieve full densification. The results show that after Arc-PVD and HIP, the mechanical properties are not only affected by reduced porosity but also by substantial microstructural changes. Precipitates, i.e. Laves phase, already present in the as-built condition can be dissolved by solution heat treatment. HIP leads to recrystallization and thus significantly changes microstructural appearance. Elongated grains and sub-micron sized cell structures stemming from SLM processing are eliminated by HIP. Aging leads to evolution of strengthening γ″ precipitates. Concurrently, δ phase forms upon HIP and aging. In comparison to aged conditions that were not subjected to HIP, microstructure upon HIP and aging results in improved ductility under monotonic loading, however, concomitantly deteriorates fatigue properties at RT.

Journal ArticleDOI
TL;DR: In this article, the effects of strain, strain rate and deformation temperature on the subgrain structures, local and cumulative misorientations and twinning phenomena of an IN718 superalloy were investigated.
Abstract: The hot deformation behavior of an IN718 superalloy was studied by isothermal compression tests under the deformation temperature range of 950–1100 °C and strain rate range of 0.001–1 s−1 up to true strains of 0.05, 0.2, 0.4 and 0.7. Electron backscattered diffraction (EBSD) technique was employed to investigate systematically the effects of strain, strain rate and deformation temperature on the subgrain structures, local and cumulative misorientations and twinning phenomena. The results showed that the occurrence of dynamic recrystallization (DRX) is promoted by increasing strain and deformation temperature and decreasing strain rate. The microstructural changes showed that discontinuous dynamic recrystallization (DDRX), characterized by grain boundary bulging, is the dominant nucleation mechanism in the early stages of deformation in which DRX nucleation occurs by twining behind the bulged areas. Twin boundaries of nuclei lost their ∑3 character with further deformation. However, many simple and multiple twins can be also regenerated during the growth of grains. The results showed that continuous dynamic recrystallization (CDRX) is promoted at higher strains and large strain rates, and lower temperatures, indicating that under certain conditions both DDRX and CDRX can occur simultaneously during the hot deformation of IN718.

Journal ArticleDOI
TL;DR: In this paper, the creep properties of a polycrystalline nickel-based superalloy produced via selective laser melting were investigated, and it was shown that the additively manufactured material showed superior creep strength compared to conventional cast and wrought material.
Abstract: The creep properties of a polycrystalline nickel-based superalloy produced via selective laser melting were investigated in this study. All heat treatment conditions of the additively manufactured material show superior creep strength compared to conventional cast and wrought material. The process leads to a microstructure with fine subgrains. In comparison to conventional wrought material no Niobium rich δ phase is necessary to control the grain size and thus more Niobium is available for precipitation hardening and solid solution strengthening resulting in improved creep strength.

Journal ArticleDOI
TL;DR: In this paper, the evolution of phases in modified 9Cr-1Mo P91 steel and their effects on microstructural stability and mechanical properties have been studied for specimens that were subjected to different thermal heat treatment conditions.
Abstract: To achieve high thermal efficiency, modern day thermal power plants operate at higher operating temperature and pressure which necessitates use of steels with high creep rupture strength such as modified 9Cr-1Mo steels. In the present study, the evolution of phases in modified 9Cr-1Mo P91 steel and their effects on microstructural stability and mechanical properties have been studied for specimens that were subjected to different thermal heat treatment conditions. The main focus has been to study the effect of heat treatment temperature ranging from 623 K to 1033 K (350–760 °C) on P91 steel. Further, the effect of furnace cooling, water quenching, tempering at 1273 K (1000 °C) and austenitizing on the mechanical properties and microstructure has been studied. The techniques used for material characterization were scanning electron microscopy (SEM), optical microscopy (OM) and X-ray diffraction. For low tempering temperature, i.e. 623 K (350 °C), M 23 C 6 , M 3 C, M 7 C 3, and MX precipitates have been observed with high yield strength (YS), tensile strength (UTS), hardness and low toughness. In the high temperature range, 923–1033 K (650–760 °C), fine MX, M 7 C 3 , M 23 C 6 , M 2 X, and M 3 C precipitates have been observed with low YS, UTS, hardness and high toughness. The steel tempered at 1033 K (760 °C) was observed to be having best combination of YS, UTS, hardness, toughness and ductility.

Journal ArticleDOI
TL;DR: In this article, the microstructures of the ball-milled powders and GNP/Al5083 composites were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and transmission electron microscope.
Abstract: Graphene is considered as an excellent reinforcement due to its excellent physical and mechanical properties. In this work, milling balls of small diameter were used to reduce the collision energy during the ball milling process, and graphene nanoplates (GNP) reinforced Al5083 alloy matrix composites were successfully fabricated by ball milling, hot pressing and hot extrusion. The microstructures of the ball-milled powders and GNP/Al5083 composites were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. The results showed that the GNP could remain after ball milling. However, the Al4C3 phase was found in bulk GNP/Al5083 composites, suggesting that some of the GNP reacted with Al and formed Al4C3 during the consolidation process. Tensile tests revealed that both the yield strength and ultimate tensile strength of the 1.0 wt.%GNP/Al5083 composite were increased by 50% compared with pure Al5083 fabricated under the same procedure. The relevant strengthening mechanisms of the composites were discussed.

Journal ArticleDOI
TL;DR: In this article, the authors examined the evolution of geometrically necessary dislocation (GND) structure following tensile deformation in a commercially produced dual phase steel, DP 590.
Abstract: The present investigation examined the evolution of geometrically necessary dislocation (GND) structure following tensile deformation in a commercially produced dual phase steel, DP 590. GND measurements were made using electron back scatter diffraction (EBSD). The average GND density increased with imposed macroscopic strain, however the rate of increased slowed with increasing strain. GND density was found to be influenced by the ferrite grain size and orientation of the ferrite grains. Small ferrite grains generally had a higher GND density. For this steel the highest GND density was measured for {011}[111] orientations. Analysis of these data using the classical Ashby model for GND content shows that GND density is increasing linearly with strain. The discrepancy between measured and predicted GND density is attributed to the plastic deformation of martensite reducing the requirement of compatibility between ferrite and martensite and dynamic recovery of the dislocation structures decreasing the rate of GND storage with strain.

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TL;DR: In this article, the elastic properties of as-cast TiHfZrTaNb high entropy alloy were investigated by ultrasound measurements, yielding G=C44=28 GPa and C11=172 GPa effective isotropic elastic constants, allowing computation of the Young's modulus E, the bulk modulus B and Poisson ratio ν of about 78.5 GPa, 134.6GPa and 0.402.
Abstract: Elastic properties of as-cast TiHfZrTaNb high entropy alloy were investigated by ultrasound measurements, yielding G=C44=28 GPa and C11=172 GPa effective isotropic elastic constants, allowing computation of the Young’s modulus E, the bulk modulus B and Poisson ratio ν of about 78.5 GPa, 134.6 GPa and 0.402, respectively. A Pugh ratio G/B as lower as 0.208 and high positive Cauchy pressure (C12–C44)=80 GPa were calculated, suggesting a ductile behavior. Tensile tests were carried out on specimens taken along the ingot diameter to address micro-segregations effect on the macroscopic behavior. More specifically, micro-segregations were addressed at a smaller scale via nanohardness measurements. Given the observed low deviations from both tensile and nanoindentation experiments, micro-segregations influence was concluded to be negligible. The necking and fracture surface investigations revealed multiples slip bands, grains boundary distortions, mixture of shallow and profound dimples of varying sizes, all of which characterize a high tensile ductility behavior, in line with elasticity measurements prediction. Post-mortem EBSD investigations revealed lattice distortions mainly at grain boundaries vicinity as a consequence of dislocations accumulation.

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TL;DR: The tensile property of Al 0.5 CoCrFeNi high entropy alloys heat-treated at 650-°C for 834 MPa, 1220 MPa for 650-C/8-h heat-treated condition, which was attributed to the nano-sized B2 phase in the interdendritic region as mentioned in this paper.
Abstract: The tensile property of Al 0.5 CoCrFeNi high entropy alloys heat-treated at 650 °C for 0.5–8 h respectively was executed. The yield strength and ultimate tensile strength reached to 834 MPa, 1220 MPa for 650 °C/8 h heat-treated condition, which was attributed to the nano-sized B2 phase in the interdendritic region and precipitates in the dendritic region.

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TL;DR: In this article, selective electron beam melting (SEBM) was employed for fabricating equiatomic AlCoCrFeNi HEA specimens, and their microstructures and mechanical properties were evaluated by comparing them with those of a conventionally cast specimen.
Abstract: Because of their superior properties, high-entropy alloys (HEAs) are considered promising novel structural materials that can substitute conventional alloys. From the viewpoint of future applications, it is important to explore methods for producing complex shaped products with HEAs. In this study, selective electron beam melting (SEBM) was employed for fabricating equiatomic AlCoCrFeNi HEA specimens, and their microstructures and mechanical properties were evaluated by comparing them with those of a conventionally cast specimen. Both cast and SEBM specimens dominantly consisted of a nano-lamellar mixture of disordered body-centered-cubic (BCC) and B2 (ordered BCC) phases. The face-centered cubic (FCC) phase was also precipitated at the grain boundaries of the B2/BCC mixture phases on the SEBM specimens. The fraction of the FCC phase at the bottom part of the SEBM specimen was higher than that at the top part. The preheating procedure—a process unique to SEBM—is responsible for the precipitation of the FCC phase, because of the long-term exposure at sufficiently high temperatures. As a result, the hardness of the SEBM specimens gradually decreased as we approached the bottom part of the specimens due to the increased fraction of the FCC phase, which had lower hardness than the B2/BCC phases. Further, the SEBM specimen exhibited much higher plastic deformability than the cast specimen, without significant loss of strength.

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TL;DR: In this paper, pristine graphene (PG) prepared by intercalation and exfoliation of graphite, with negligible oxygen-containing functional groups, much less defects and higher electrical conductivity than reduced graphene oxide (rGO), exhibits better performance than rGO as additives for the enhancement of the strength of metal composites.
Abstract: Graphene nanosheets have shown great potential in enhancing the strength of metal composites. In previous researches, reduced graphene oxide (rGO) are usually used as the additive. Here, we demonstrate that pristine graphene (PG) prepared by intercalation and exfoliation of graphite, with negligible oxygen-containing functional groups, much less defects and higher electrical conductivity than rGO, exhibits better performance than rGO as additives for the enhancement of the strength of metal composites. Surface modification of PG and Cu was conducted to enhance the interaction between two components, resulting in homogeneous distribution of PG in Cu matrix. The PG/Cu composite exhibits yield strength σ 0.2 and 5% compression strength up to 172 and 228 MPa, respectively, which is a 90% and 81% promotion comparing to pure Cu, while its electrical conductivity still stays at 84.2% IACS. As to rGO/Cu composite, yield strength σ 0.2 and 5% compression strength is 156 and 208 MPa, respectively, and its electrical conductivity is 73.4% IASC. Such significant improvement on strength can be explained by the two-dimensional geometry and high crystallinity of PG whose high strength and modulus effectively constrain the movement of dislocations.

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TL;DR: In this article, the Portuguese Foundation of Science and Technology through the projects EXCL/EMS-TEC/0460/2012, UID/EEA/04436/2013 and by the FCT grant SFRH/BPD/112111/2015.
Abstract: This work was supported by the Portuguese Foundation of Science and Technology through the projects EXCL/EMS-TEC/0460/2012, UID/EEA/04436/2013 and by the FCT grant SFRH/BPD/112111/2015.