Showing papers on "Equiaxed crystals published in 2017"

Novel Bimorphological Anisotropic Bulk Nanocomposite Materials with High Energy Products.

Abstract: Nanostructuring of magnetically hard and soft materials is fascinating for exploring next-generation ultrastrong permanent magnets with less expensive rare-earth elements. However, the resulting hard/soft nanocomposites often exhibit random crystallographic orientations and monomorphological equiaxed grains, leading to inferior magnetic performances compared to corresponding pure rare-earth magnets. This study describes the first fabrication of a novel bimorphological anisotropic bulk nanocomposite using a multistep deformation approach, which consists of oriented hard-phase SmCo rod-shaped grains and soft-phase Fe(Co) equiaxed grains with a high fraction (≈28 wt%) and small size (≈10 nm). The nanocomposite exhibits a record-high energy product (28 MGOe) for this class of bulk materials with less rare-earth elements and outperforms, for the first time, the corresponding pure rare-earth magnet with 58% enhancement in energy product. These findings open up the door to moving from a pure permanent-magnet system to a stronger nanocomposite system at lower costs.

Grain Structure Control of Additively Manufactured Metallic Materials.

Abstract: Grain structure control is challenging for metal additive manufacturing (AM). Grain structure optimization requires the control of grain morphology with grain size refinement, which can improve the mechanical properties of additive manufactured components. This work summarizes methods to promote fine equiaxed grains in both the additive manufacturing process and subsequent heat treatment. Influences of temperature gradient, solidification velocity and alloy composition on grain morphology are discussed. Equiaxed solidification is greatly promoted by introducing a high density of heterogeneous nucleation sites via powder rate control in the direct energy deposition (DED) technique or powder surface treatment for powder-bed techniques. Grain growth/coarsening during post-processing heat treatment can be restricted by presence of nano-scale oxide particles formed in-situ during AM. Grain refinement of martensitic steels can also be achieved by cyclic austenitizing in post-processing heat treatment. Evidently, new alloy powder design is another sustainable method enhancing the capability of AM for high-performance components with desirable microstructures.

Structure, Texture and Phases in 3D Printed IN718 Alloy Subjected to Homogenization and HIP Treatments

Abstract: 3D printing results in anisotropy in the microstructure and mechanical properties The focus of this study is to investigate the structure, texture and phase evolution of the as-printed and heat treated IN718 superalloy Cylindrical specimens, printed by powder-bed additive manufacturing technique, were subjected to two post-treatments: homogenization (1100 °C, 1 h, furnace cooling) and hot isostatic pressing (HIP) (1160 °C, 100 MPa, 4 h, furnace cooling) The Selective laser melting (SLM) printed microstructure exhibited a columnar architecture, parallel to the building direction, due to the heat flow towards negative z-direction Whereas, a unique structural morphology was observed in the x-y plane due to different cooling rates resulting from laser beam overlapping Post-processing treatments reorganized the columnar structure of a strong {002} texture into fine columnar and/or equiaxed grains of random orientations Equiaxed structure of about 150 µm average grain size, was achieved after homogenization and HIP treatments Both δ-phase and MC-type brittle carbides, having rough morphologies, were formed at the grain boundaries Delta-phase formed due to γ″-phase dissolution in the γ matrix, while MC-type carbides nucleates grew by diffusion of solute atoms The presence of (Nb078Ti022)C carbide phase, with an fcc structure having a lattice parameter a = 443 A, was revealed using Energy dispersive spectrometer (EDS) and X-ray diffractometer (XRD) analysis The solidification behavior of IN718 alloy was described to elucidate the evolution of different phases during selective laser melting and post-processing heat treatments of IN718

Strategy for Texture Management in Metals Additive Manufacturing

Abstract: Additive manufacturing (AM) technologies have long been recognized for their ability to fabricate complex geometric components directly from models conceptualized through computers, allowing for complicated designs and assemblies to be fabricated at lower costs, with shorter time to market, and improved function. Lacking behind the design complexity aspect is the ability to fully exploit AM processes for control over texture within AM components. Currently, standard heat-fill strategies utilized in AM processes result in largely columnar grain structures. Proposed in this work is a point heat source fill for the electron beam melting (EBM) process through which the texture in AM materials can be controlled. Through this point heat source strategy, the ability to form either columnar or equiaxed grain structures upon solidification through changes in the process parameters associated with the point heat source fill is demonstrated for the nickel-base superalloy, Inconel 718. Mechanically, the material is demonstrated to exhibit either anisotropic properties for the columnar-grained material fabricated through using the standard raster scan of the EBM process or isotropic properties for the equiaxed material fabricated using the point heat source fill.

The role of bimodal-grained structure in strengthening tensile strength and decreasing yield asymmetry of Mg-Gd-Zn-Zr alloys

Abstract: This paper studied the role of bimodal-grained structure in strengths and asymmetry behaviors of Mg-15Gd-1Zn-0.4Zr (wt%) alloys under the as-extruded, compressive-yielded and fractured conditions. The bimodal-grained structure consisting of fine recrystallized grains and coarse un-recrystallized grains formed when the alloys were extruded with relatively low temperature. Compared with the equiaxed structure, the bimodal-grained structure possessed higher tensile strength attributed to the fine recrystallized grains plus the strong and hard un-recrystallized texture with its basal planes nearly parallel to the loading direction. However, the bimodal-grained structure weakened the ultimate compressive strength, because the un-recrystallized texture is favorable for tensile twining in the compressive test. Eventually, the aged sample with the bimodal-grained structure exhibited ultra-high tensile yield strength and ultimate tensile strength of 465 MPa and 524 MPa, respectively. Especially, the inhomogeneous bimodal-grained structure with stronger texture resulted in extraordinary low compressive-tensile yield asymmetries of 0.99 and 0.98 compared with those (from 1.09 to 1.48) of the homogeneous equiaxed structures with relatively random textures, because the fine recrystallized grains in the bimodal-grained structure undertook major deformation by slips and suppressed twining before the yield points in the mechanical tests. The relationship between the microstructures and properties allows us to produce high tensile strength Mg components by applying low extrusion temperature and obtaining bimodal-grained structure.

Grain growth during selective laser melting of a Co–Cr–Mo alloy

Abstract: Modes of solidification during selective laser melting (SLM) of metallic alloys, including Co–Cr–Mo alloy, are still not fully understood. This understanding is important in SLM to achieve acceptable properties and part reliability. Using a typical SLM condition and Co–Cr–Mo alloys, microstructures of tracks were characterized in this study. As is commonly observed, solidification starts from epitaxial growth in the boundary of melt track. Cells were found to grow immediately from the melt boundary, without forming a planar zone. This is explained by the growth velocity being sufficiently high that planar growth condition is not favorable. Epitaxial growth has been found to have two possible crystallographic orientations of : either a continuation of the same orientation as in previous track or a change of 90° to another orientation. The selection is in response to scan direction-dependent heat flux direction. The crystal growth direction in relation to heat flux direction also explains that a grain (a group of cells) after epitaxial growth could either stop or continue to the track surface. No equiaxed grains were observed, and this can be explained by the continuation of cellular growth in the whole track.

Additive Manufacturing of AlSi10Mg Alloy Using Direct Energy Deposition: Microstructure and Hardness Characterization

Abstract: This paper aims to study the manufacturing of the AlSi10Mg alloy with direct energy deposition (DED) process. Following fabrication, the macro- and microstructural evolution of the as-processed specimens was initially investigated using optical microscopy and scanning electron microscopy. Columnar dendritic structure was the dominant solidification feature of the deposit; nevertheless, detailed microstructural analysis revealed cellular morphology near the substrate and equiaxed dendrites at the top end of the deposit. Moreover, the microstructural morphology in the melt pool boundary of the deposit differed from the one in the core of the layers. The remaining porosity of the deposit was evaluated by Archimedes’ principle and by image analysis of the polished surface. Crystallographic texture in the deposit was also assessed using electron backscatter diffraction and x-ray diffraction analysis. The dendrites were unidirectionally oriented at an angle of ~80° to the substrate. EPMA line scans were performed to evaluate the compositional variation and elemental segregation in different locations. Eventually, microhardness (HV) tests were conducted in order to study the hardness gradient in the as-DED-processed specimen along the deposition direction. The presented results, which exhibited a deposit with an almost defect free structure, indicate that the DED process can suitable for the deposition of Al-Si-based alloys with a highly consolidated structure.

Double-sided laser beam welded T-joints for aluminum-lithium alloy aircraft fuselage panels: Effects of filler elements on microstructure and mechanical properties

Abstract: In the current work, T-joints consisting of 2.0 mm thick 2060-T8/2099-T83 aluminum-lithium alloys for aircraft fuselage panels have been fabricated by double-sided fiber laser beam welding with different filler wires. A new type wire CW3 (Al-6.2Cu-5.4Si) was studied and compared with conventional wire AA4047 (Al-12Si) mainly on microstructure and mechanical properties. It was found that the main combined function of Al-6.2%Cu-5.4%Si in CW3 resulted in considerable improvements especially on intergranular strength, hot cracking susceptibility and hoop tensile properties. Typical non-dendritic equiaxed zone (EQZ) was observed along welds’ fusion boundary. Hot cracks and fractures during the load were always located within the EQZ, however, this typical zone could be restrained by CW3, effectively. Furthermore, changing of the main intergranular precipitated phase within the EQZ from T phase by AA4047 to T2 phase by CW3 also resulted in developments on microscopic intergranular reinforcement and macroscopic hoop tensile properties. In addition, bridging caused by richer substructure dendrites within CW3 weld’s columnar zone resulted in much lower hot cracking susceptibility of the whole weld than AA4047.

The effect of preheating on microstructure and mechanical properties of laser solid forming IN-738LC alloy

Abstract: The microstructure and microhardness of Ni-base superalloy IN-738LC fabricated by laser solid forming (LSF) with the preheating was investigated. The results showed that with the increase of the preheating temperature (T 0 ) (25 °C, 800 °C, 900 °C and 1050 °C), the total crack length decreased significantly, the segregation of Ti and Al worsened, and the volume fraction of equiaxed grain increased, as well as the size of blocky carbides, γ-γ′ eutectic and γ′ particles. The high-angle grain boundaries, at which the continuous liquid film can be easily formed, were found to be susceptible to cracking. Both the concentration of Ti and Al and the increase of dendrite spacing contribute to the growth of carbides and γ-γ′ eutectic in the interdendrite while the size of the γ′ phases was largely affected by preheating. It was interesting to find that γ′ phase exhibited a bimodal distribution in the bottom of the deposit with the preheating at 1050 °C. The average microhardness of the deposits without preheating and with preheating at 800 °C and 900 °C is about at the same level, while the deposit with preheating at 1050 °C has the lowest average value. The crack-free deposit can be obtained when T 0 was up to 1050 °C, and its room temperature tensile properties are superior to cast IN-738LC alloy.

Hot deformation behavior and microstructure evolution of TA15 titanium alloy with nonuniform microstructure

Abstract: The flow behavior and microstructure evolution of a near α titanium alloy with nonuniform microstructure during hot deformation were studied by isothermal compression test and electron backscatter diffraction technique. It is found that the nonuniform microstructure prior to deformation consists of equiaxed α, lamellar α in the colony form and β phase, and the α colony keeps the Burgers orientation relationship with β phase. The flow stress of nonuniform microstructure exhibits significant flow softening after reaching the peak stress at a low strain, which is similar to the lamellar microstructure. Nevertheless, the existence of equiaxed α in nonuniform microstructure makes its flow stress and softening rate be lower than the lamellar microstructure. During deformation, the lamellar α undertakes most of the deformation and turns to be rotated, bended and globularized. Moreover, these phenomena exhibit significant heterogeneity due to the orientation dependence of the deformation of lamellar α. The continuous dynamic recrystallization and bending of lamellar α lead to the “fragmentation” during globularization of lamellar α. The bending of lamellar α is speculated as a form of plastic buckling, because the bending of lamellar α almost proceed in the manner of “rigid rotation” and presents opposite bending directions for the adjacent colonies.

Enhanced Relative Slip Distance in Gas-Tungsten-Arc-Welded Al 0.5 CoCrFeNi High-Entropy Alloy

Abstract: Gas-tungsten-arc-welded (GTAW) Al0.5CoCrFeNi high-entropy alloy (HEA) was analyzed using scanning electron microscopy (SEM), microhardness, and tensile testing. The weld metal having refined equiaxed and elongated columnar dendritic microstructure experienced 6.38 pct reduction in strength and marginally reduced hardness compared to the base metal (BM). Lower work hardening with enhanced relative slip distance, which was observed through the Kocks–Mecking plot and slip distance–true strain plots, was attributed to the reduced bcc fraction in the weld.

Microstructures and mechanical properties of a high-strength Mg-3.5Sm-0.6Zn-0.5Zr alloy

Abstract: Microstructures and mechanical properties of a high-strength extruded Mg-3.5Sm-0.6Zn-0.5Zr alloy were thoroughly investigated. The results indicate that the dominant intermetallic phase in both as-cast and extruded samples is not the widely reported Mg 41 Sm 5 phase but Mg 3 Sm phase. After extrusion, the microstructures are composed of fine equiaxed dynamically recrystallized (DRXed) grains, coarse elongated unDRXed grains with ultrafine sub-grains, disintegrated intermetallic particles, and numerous small dynamically precipitated particles. Also, there are amounts of particularly dislocations in the Mg matrix after extrusion. Finally, numerous nano-scale plate β′ particles and lamellae β″ particles homogeneously precipitated in the matrix during artificial aging treatments. Therefore, the studied alloy exhibits very high ultimate tensile strength and yield strength, whose maximum values at room temperature are approximately 427 MPa and 416 MPa, respectively, much greater than most conventional Mg–RE based alloys with comparative or even much higher RE additions.

Influence of welding pass on microstructure and toughness in the reheated zone of multi-pass weld metal of 550 MPa offshore engineering steel

Abstract: The objective of this paper is to study the influence of thermal cycles produced by the later welding passes on the properties of the sub-regions in reheated zone of multi-pass weld metal for a 550 MPa grade offshore engineering steel. A Gleeble-3500 simulator was applied to simulate microstructural evolution in sub-regions of the reheated zone and its effect on the properties. The results indicated that the reheating process changed the prior austenitic morphology from columnar structure to equiaxed structure and similar columnar structure with quasi-polygonal ferrite (QPF) or blocky M-A (martensite/austenite) constituent distributed on the grain boundaries while the matrix microstructure (acicular ferrite) changed slightly. Charpy impact results indicated that WM region (as-deposited) had the highest impact energy. However, the actual impact sample showed lower impact energy because the machined notch contained one or more brittle reheated zones. In these brittle reheated zones, the necklace-type M-A constituent was hard phase decorating prior columnar or equiaxed austenite grain boundaries, which yielded stress concentration significantly and was mainly responsible for lower toughness of the entire weld metal. Fortunately, this deterioration in toughness could be reduced as decomposition of necklace-type M-A constituent occurred due to later welding passes.

Mechanical properties of additive manufactured nickel alloy 625

Abstract: The mechanical, metallurgical and corrosion properties of Alloy 625 produced using the laser powder bed fusion (L-PBF) manufacturing process were investigated and compared with typical performance of the alloy produced using conventional forging processes. Test specimens were produced near net shape along with several demonstration pieces that were produced to examine the geometric complexity that could be achieved with the process. The additively manufactured specimens exhibited strength, fracture toughness and impact toughness that was equal to or better than properties typically achieved for wrought product. There was no evidence of stress corrosion cracking susceptibility in 3.5% NaCl solution at stress intensities up to 70 ksi-in1/2 after 700 h exposure. The microstructure was equiaxed in the plane of the powder bed build platform (X–Y) and exhibited a columnar shape in the Z direction although there was not any significant evidence of anisotropy in the mechanical properties.