Showing papers on "Equiaxed crystals published in 2016"

Ultrafine-Grained AlCoCrFeNi2.1 Eutectic High-Entropy Alloy

Abstract: The development of microstructure and mechanical properties was investigated in a heavily cold-rolled and annealed AlCoCrFeNi2.1 high-entropy alloy. The as-cast alloy having a eutectic morphology consisting of alternate bands of ordered L1(2) and B2 phases was 90% cold-rolled. The deformed microstructure showed profuse shear banding and disordering of the L12, but no transformation of the B2 phase. A duplex microstructure consisting of ultrafine equiaxed grains (similar to 0.60 mu m) of disordered face centered cubic and B2 was observed after annealing at 800 degrees C. The annealed material showed remarkable strength-ductility combination having ultimate tensile strength similar to 1.2 GPa and elongation to failure similar to 12%.

Grain structure evolution in Inconel 718 during selective electron beam melting

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.

Effect of location on microstructure and mechanical properties of additive layer manufactured Inconel 625 using gas tungsten arc welding

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.

Wire Arc Additive Manufacturing of AZ31 Magnesium Alloy: Grain Refinement by Adjusting Pulse Frequency

Abstract: Wire arc additive manufacturing (WAAM) offers a potential approach to fabricate large-scale magnesium alloy components with low cost and high efficiency, although this topic is yet to be reported in literature. In this study, WAAM is preliminarily applied to fabricate AZ31 magnesium. Fully dense AZ31 magnesium alloy components are successfully obtained. Meanwhile, to refine grains and obtain good mechanical properties, the effects of pulse frequency (1, 2, 5, 10, 100, and 500 Hz) on the macrostructure, microstructure and tensile properties are investigated. The results indicate that pulse frequency can result in the change of weld pool oscillations and cooling rate. This further leads to the change of the grain size, grain shape, as well as the tensile properties. Meanwhile, due to the resonance of the weld pool at 5 Hz and 10 Hz, the samples have poor geometry accuracy but contain finer equiaxed grains (21 μm) and exhibit higher ultimate tensile strength (260 MPa) and yield strength (102 MPa), which are similar to those of the forged AZ31 alloy. Moreover, the elongation of all samples is above 23%.

Microstructural Evolutions During Annealing of Plastically Deformed AISI 304 Austenitic Stainless Steel: Martensite Reversion, Grain Refinement, Recrystallization, and Grain Growth

Abstract: Microstructural evolutions during annealing of a plastically deformed AISI 304 stainless steel were investigated. Three distinct stages were identified for the reversion of strain-induced martensite to austenite, which were followed by the recrystallization of the retained austenite phase and overall grain growth. It was shown that the primary recrystallization of the retained austenite postpones the formation of an equiaxed microstructure, which coincides with the coarsening of the very fine reversed grains. The latter can effectively impair the usefulness of this thermomechanical treatment for grain refinement at both high and low annealing temperatures. The final grain growth stage, however, was found to be significant at high annealing temperatures, which makes it difficult to control the reversion annealing process for enhancement of mechanical properties. Conclusively, this work unravels the important microstructural evolution stages during reversion annealing and can shed light on the requirements and limitations of this efficient grain refining approach.

Formation characteristic, microstructure, and mechanical performances of aluminum-based components by friction stir additive manufacturing

Abstract: In this study, a new solid-state technique of friction stir additive manufacturing (FSAM) based on friction stir welding (FSW) principle was used to build successfully a multilayered stack of an Al-based component. The results show that a hook stretches into the nugget zone on advancing side, while it moves upwards to the periphery on retreating side for a single-level welding. With manufacturing the second layer, the hooks bend outward significantly attributing to the extrusion of above plastic material, which can avoid the hook to stretch into the stirred zone. A transition zone (TZ) is also formed near the interface between two layers. In addition, fine equiaxed grains are observed due to the dynamic recrystallization in the whole. However, a difference in grain size still exists through the build direction and in the TZ is forming coarse band grains. A similar change occurs in the precipitate morphology, size, and distribution. Form the top to the bottom, the microhardness changes dramatically, and a maximum 115 HV at the top is obtained. The tensile strength of all the slices increases and the elongation decreases slightly in comparison of Al substrate, and the slice top has the highest mechanical properties, which is attributed to fine grains and desirable precipitate characterization.

Microstructural evolution and tensile behavior of Ti2AlNb alloys based α2-phase decomposition

Abstract: The formation mechanism of the fine plate-like O-phases within α2-phases and tensile behavior of an isothermally forged Ti–22Al–25Nb (at%) orthorhombic alloy at 1040 °C during heat treatment were investigated. The investigation indicated that the alloys were heat-treated in O+B2 phase region after α2+B2 phase region isothermally forging, the equiaxed α2-phase was not stable and decomposed into O+α2 phases. The α2 phases formed during isothermal forging process have higher concentration of Nb and begun to decompose during O+B2 phase region heat treatment. And then the α2 phases separated into Niobium-lean and Niobium-rich regions through the Niobium diffusion: α2→α2 (Nb-lean)+O (Nb-rich). Nb-rich regions with composition similar to Ti2AlNb transformed to the O-phase, while the Nb-lean regions remained untransformed and retained the α2-phase. The deformation behavior and fracture mechanism of Ti–22Al–25Nb alloy at room temperature were discussed. The deformation behavior and microstructural evolution of this alloy at different temperatures and stain rates were also investigated using uniaxial tensile test.

Control of solidification in laser powder bed fusion additive manufacturing using a diode laser fiber array

Abstract: A method of method of forming or repairing a superalloy article having a columnar or equiaxed or directionally solidified or amorphous or single crystal microstructure includes emitting a plurality of laser beams from selected fibers of a diode laser fiber array corresponding to a pattern of a layer of the article onto a powder bed of the superalloy to form a melt pool; and controlling a temperature gradient and a solidification velocity of the melt pool to form the columnar or single crystal microstructure.

Microstructural and Mechanical Properties of Friction Stir Welded Nickel-Aluminum Bronze (NAB) Alloy

Abstract: In this study, the applicability of friction stir welding to cast NAB alloy (i.e., C95800) with a thickness of 9 mm has been investigated. The joint performance was determined by conducting optical microscopy, microhardness measurements, and mechanical testing (e.g., tensile and Charpy impact tests). The effect of stir intensity on joint performance was also determined. A grain refinement (equiaxed fine grain structure) as well as evolution of a Widmanstatten structure was achieved within the stir zone of all the joints produced. Thus, all of the joints produced exhibited higher proof stress (i.e., between 512 and 616 MPa) than that of the base material, i.e., 397 MPa. On the other hand, only half of the specimens exhibited higher tensile strength values than that of the base plate (i.e., 794 MPa), whereas the other specimens displayed lower tensile strength than the base plate due to the existence of weld defects, namely cold bonding and/or tunnel defect.

Microstructure and high temperature oxidation resistance of Ti-Ni gradient coating on TA2 titanium alloy fabricated by laser cladding

Abstract: To improve the high temperature oxidation resistance of TA2 titanium alloy, a gradient Ni-Ti coating was laser cladded on the surface of the TA2 titanium alloy substrate, and the microstructure and oxidation behavior of the laser cladded coating were investigated experimentally. The gradient coating with a thickness of about 420–490 µm contains two different layers, e.g. a bright layer with coarse equiaxed grain and a dark layer with fine and columnar dendrites, and a transition layer with a thickness of about 10 µm exists between the substrate and the cladded coating. NiTi, NiTi 2 and Ni 3 Ti intermetallic compounds are the main constructive phases of the laser cladded coating. The appearance of these phases enhances the microhardness, and the dense structure of the coating improves its oxidation resistance. The solidification procedure of the gradient coating is analyzed and different kinds of solidification processes occur due to the heat dissipation during the laser cladding process.

Influence of cumulative strain on microstructure and mechanical properties of multi-directional forged 2A14 aluminum alloy

Abstract: 2A14 aluminum alloy was subjected to multidirectional forging (MDF) with a constant strain rate at 350 °C, to different cumulative strain. The effects of cumulative strain on microstructure and mechanical properties of 2A14 aluminum alloy were investigated. The inhomogeneity of deformation during MDF was improved and grains of the alloy were gradually refined by increasing cumulative strain, while dislocation density first increased and then decreased with increasing cumulative strain in the range 0.4–7.2. After subsequent T6 aging, fine and equiaxed recrystallization grains with large-angle grain boundaries were more easily formed at relatively higher cumulative strain, and the optimized mechanical properties were achieved when cumulative strain was up to 3.6, with ultimate tensile strength of 505 MPa, yield strength of 435 MPa, Vickers hardness of 178 Hv and elongation of 16.8% at room temperature. The improvement in mechanical properties of the alloy was attributed to the presence of fine recrystallized grains which are uniformly distributed and high dislocation density, as well as precipitation strengthening.

Effect of a small addition of Ti on the Fe-based coating by laser cladding

Abstract: In this study, the coatings were fabricated on a low carbon steel substrate by laser cladding using the Fe-based powders mixed with different Ti content. The effects of the Ti and its content on the microstructure, phase compositions and microhardness of the clad layers are studied systematically. The results show that, the addition of Ti into Fe-based alloy powder changes the morphology and microstructure of the coatings obviously. The equiaxed grains or dendritics appear instead of dendrite structures, and the microstructure becomes refined and uniform. In addition, with the increase of Ti content, the dilution rate of the coatings goes up and the size of crystal phase in the deposits becomes larger, which makes the hardness of high-doped layers decrease sharply. Meanwhile, the reasons for these differences are analyzed in detail. At last, by comparison, the Fe-based mixed with 1 wt.% Ti coating has the most desired performance. It has high hardness which is not sensitive to the scanning speed under the condition that the ratio of the powder feed rate to the scanning rate is constant. And the hardness distribution through the clad layers is much stable.

Equiaxed Ti-based Composites With High Strength And Large Plasticity Prepared By Sintering And Crystallizing Amorphous Powder

Abstract: High-performance titanium alloys with an equiaxed composite microstructure were achieved by sintering and crystallizing amorphous powder. By introducing a second phase in a β-Ti matrix, series of optimized Ti–Nb–Fe–Co–Al and Ti–Nb–Cu–Ni–Al composites, which have a microstructure composed of ultrafine-grained and equiaxed CoTi2 or (Cu,Ni)Ti2 precipitated phases surrounded by a ductile β-Ti matrix, were fabricated by sintering and crystallizing mechanically alloyed amorphous powder. The as-fabricated composites exhibit ultra-high ultimate compressive strength of 2585 MPa and extremely large compressive plastic strain of around 40%, which are greater than the corresponding ones for most titanium alloys. In contrast, the alloy fabricated by sintering and crystallizing Ti–Zr–Cu–Ni–Al amorphous powder, which possesses significantly higher glass forming ability in comparison with the Ti–Nb–Fe–Co–Al and Ti–Nb–Cu–Ni–Al alloy systems, exhibits a complex microstructure with several intermetallic compounds and a typical brittle fracture feature. The deformation behavior and fracture mechanism indicate that the ultrahigh compressive strength and large plasticity of the as-fabricated equiaxed composites is induced by dislocations pinning effect of the CoTi2 or (Cu,Ni)Ti2 second phases and the interaction and multiplication of generated shear bands in the ductile β-Ti matrix, respectively. The results obtained provide basis guidelines for designing and fabricating titanium alloys with excellent mechanical properties by powder metallurgy.