The effect of three different solutionizing and annealing heat treatments on the initial superplastic microstructure of a Zn-21Al-2Cu alloy was studied. Microstructural characterization using SEM and DRX techniques revels the presence of residual grain boundaries of a high temperature beta phase (referred to as Former beta Boundaries, F beta Bs) which encompass groups of fine alpha (Al rich phase) and eta (Zn rich phase). The origin of these arrangements may most likely be due to small deviations from the eutectoid composition as consequence of the Cu addition, which preferentially promotes the precipitation of proeutectoid phase to the high temperature phase boundaries. The final domains size only are influenced by the solutionizing treatment time, while the fine phases size only is affected by the duration of the subsequent annealing process. The presence and characteristic of his F beta Bs could be import in order to explain possible microstructural changes, which lead to a reduction of the grain boundary sliding capacities of fine alpha, and eta phases that conduces at the early onset of a non-stable plastic flow previously observed in this alloy.

Atomized, pre-alloyed Ti-24Nb-4Zr-7.9Sn (wt%) powder was used to fabricate solid, prototype components by electron beam melting (EBM). Vickers microindentation hardness values were observed to average 2 GPa for the precursor powder and 2.5 GPa for the solid, EBM-fabricated products. The powder and solid product microstructures were examined by optical and electron microscopy. X-ray diffraction analyses showed that they had bcc beta-phase microstructure. However, it was found by transmission electron microscopy that the EBM-fabricated product had plate morphology with space similar to 100-200 nm. Although the corresponding selected area diffraction patterns can be indexed by beta-phase plus alpha''-martensite with orthorhombic crystal structure, the dark-field analyses failed to observe the alpha''-martensite. Such phenomenon was also found in deformed gum metals and explained by stress-induced diffusion scattering due to phonon softening.

Rene 142, a commercial, columnar grained, gas turbine airfoil Ni-based superalloy, has been fabricated from a pre-alloyed, atomized powder by additive manufacturing using electron beam melting. Monolithic components having [001] oriented, columnar grain structures exhibited a creep-optimized 59% volume fraction of cuboidal, coherent, γ′-phase precipitates averaging 275nm on the side, and with γ/γ′ channel widths ranging from 25 to 75nm. Transmission electron microscopy, utilizing bright and dark field imaging of optimally oriented γ/γ′ interfaces showed prominent misfit coherency strains as δ-fringe patterns. Corresponding hardness measurements also indicated the possibility of creep strength comparable with the commercial alloy. The notable feature of this study was the monolithic development of desirable superalloy properties without conventional, multi-step heat treatments.

The microstructures and residual hardnesses for solid components of 2-phase TiAl (Ti-48Al-2Nb-2Cr in a / o ) and Inconel 625 (Ni-19Cr-9Mo-4Nb in w / o ) fabricated by electron beam melting (EBM) were compared with a 10:1 blend of TiAl: alloy 625 pre-alloyed powders producing a complex alloy having the composition 48Ti-24Al-9Ni-8Nb-4.5Cr-4Ni (in w / o ). The blended alloy hardness (HV) reached 7.5 GPa in contrast to 1.4 GPa for the Alloy 625 and 4.0 for the 2-phase TiAl alloy. Reticulated mesh samples and stochastic foam samples prepared from the blended alloy by EBM exhibited a relative stiffness versus relative density plotted on a log-log basis consistent with other reference alloys fitted to a straight line with a slope n = 2 for ideal open cellular materials.

In this study Inconel 718 cylinders were fabricated by selective laser melting in either argon or nitrogen gas from a pre-alloyed powder. As-fabricated cylinders oriented in the build direction (z-axis) and perpendicular to the build direction (x-axis) exhibited columnar grains and arrays of γ″ (body-centered tetragonal) Ni3Nb oblate ellipsoidal precipitates oriented in a strong [200] texture determined by combined optical metallography, transmission electron microscopy, and X-ray diffraction analysis. Fabricated and hot isostatic pressed (HIP) components exhibited a more pronounced [200] columnar γ″ phase precipitate architecture parallel to the laser beam or build direction (spaced at ∼0.8μm), and a partially recrystallized fcc grain structure. Fabricated and annealed (1160°C for 4h) components were ∼50% recrystallized and the recrystallized regions contained spheroidal γ′ precipitates distributed in a dense field of fine γ″ precipitates. The γ″ precipitates were always observed to be coincident with {100} planes of the γ-fcc NiCr matrix. Some δ phase precipitates in the unrecrystallized/recrystallized interfaces and recrystallized grain boundaries were also observed in the annealed samples. The microindentation (Vickers) hardness was 3.9GPa for the as-fabricated materials, 5.7GPa for the HIP material, and 4.6GPa for the annealed material. Corresponding tensile properties were comparable with wrought Inconel 718 alloy.

The layer-by-layer building of monolithic, 3D metal components from selectively melted powder layers using laser or electron beams is a novel form of 3D printing or additive manufacturing. Microstructures created in these 3D products can involve novel, directional solidification structures which can include crystallographically oriented grains containing columnar arrays of precipitates characteristic of a microstructural architecture. These microstructural architectures are advantageously rendered in 3D image constructions involving light optical microscopy and scanning and transmission electron microscopy observations. Microstructural evolution can also be effectively examined through 3D image sequences which, along with x-ray diffraction (XRD) analysis in the x-y and x-z planes, can effectively characterize related crystallographic/texture variances. This paper compares 3D microstructural architectures in Co-base and Ni-base superalloys, columnar martensitic grain structures in 17–4 PH alloy, and columnar copper oxides and dislocation arrays in copper.

This paper describes the observations of aluminum projectile (or armature) tribomaterial deposition onto copper (stator) conducting rails in an experimental solid-armature railgun system, by optical, and scanning and transmission electron microscopy. The extreme deformation at the aluminum/copper interface creates a solid-state flow regime by dynamic recrystallization which also leads to the erosion-product deposition. Melting of the low-temperature aluminum deposit also contributes to the rail damage and degradation of electromagnetic behavior. The creation of nano-grains by dynamic recrystallization allows for mixing at the aluminum/copper interface, and there is no evidence for traditional alloying.

Microstructures and a microstructural, columnar architecture as well as mechanical behavior of as-fabricated and processed INCONEL alloy 625 components produced by additive manufacturing using electron beam melting (EBM) of prealloyed precursor powder are examined in this study. As-fabricated and hot-isostatically pressed (“hipped”) [at 1393 K (1120 °C)] cylinders examined by optical metallography (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive (X-ray) spectrometry (EDS), and X-ray diffraction (XRD) exhibited an initial EBM-developed γ ″ (bct) Ni 3 Nb precipitate platelet columnar architecture within columnar [200] textured γ (fcc) Ni-Cr grains aligned in the cylinder axis, parallel to the EBM build direction. Upon annealing at 1393 K (1120 °C) (hot-isostatic press (HIP)), these precipitate columns dissolve and the columnar, γ , grains recrystallized forming generally equiaxed grains (with coherent {111} annealing twins), containing NbCr 2 laves precipitates. Microindentation hardnesses decreased from ~2.7 to ~2.2 GPa following hot-isostatic pressing (“hipping”), and the corresponding engineering (0.2 pct) offset yield stress decreased from 0.41 to 0.33 GPa, while the UTS increased from 0.75 to 0.77 GPa. However, the corresponding elongation increased from 44 to 69 pct for the hipped components.

The interfacial bonding between layers in ultrasonically consolidated structures is poor for some material combinations, resulting in relatively low bond strength. This makes resultant parts unsuitable for structural applications. This work discusses a study of the effects of post process heat treatment of ultrasonically consolidated commercially pure titanium and aluminum alloy 3003 dual-material systems. The lap shear strengths of as-consolidated specimens as well as heat treated ones were tested. The results show that there is significant improvement of the strengths of post processed specimens over the as-consolidated ones. The improvement is as a result of stress relieving of the strain hardened interface between the two materials and some interactions of the base materials across the interfacial boundaries at elevated temperatures, leading to stronger bonds. The study highlights the role of post process heat treatments for improving the mechanical properties of ultrasonically consolidated structures.

The fabrication of Cu components by additive manufacturing using electron beam melting (EBM) from low-purity, atomized Cu powder containing a high density of Cu(2)O precipitates exhibits a novel example of precipitate dislocation architecture. Such architectures are seen by optical metallography, and scanning and transmission electron microscopy, to consist generally of equiaxed precipitate dislocation cell-like arrays (1-3 mu m) in the horizontal reference plane perpendicular to the build direction with elongated or columnar-like arrays extending from similar to 12 to >60 mu m in length and corresponding spatial dimensions of 1-3 mu m. The hardnesses for these architectures ranged from similar to HV 83 to 88, in contrast to the original Cu powder microindentation hardness of HV 72 and the commercial Cu base plate hardness of HV 57. These observations illustrate the prospect for creating some form of controlled microstructural architecture by EBM parameter alternation or optimization. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Total knee replacement implants consisting of a Co-29Cr-6Mo alloy femoral component and a Ti-6Al-4V tibial component are the basis for the additive manufacturing of novel solid, mesh, and foam monoliths using electron beam melting (EBM). Ti-6Al-4V solid prototype microstructures were primarily alpha-phase acicular platelets while the mesh and foam structures were characterized by alpha'-martensite with some residual alpha. The Co-29Cr-6Mo containing 0.22% C formed columnar (directional) Cr23C6 carbides spaced similar to 2 mu m in the build direction, while HIP-annealed Co-Cr alloy exhibited an intrinsic stacking fault microstructure. A log-log plot of relative stiffness versus relative density for Ti-6Al-4V and Co-29Cr-6Mo open-cellular mesh and foams resulted in a fitted line with a nearly ideal slope, n = 2.1. A stress shielding design graph constructed from these data permitted mesh and foam implant prototypes to be fabricated for compatible bone stiffness. (C) 2011 Elsevier Ltd. All rights reserved.

Cu reticulated mesh and stochastic open cellular foams were fabricated by additive manufacturing using electron beam melting. Fabricated densities ranged from 0.73 g/cm(3) to 6.67 g/cm3. The precursor Cu powder contained Cu2O precipitates and the fabricated components contained arrays of Cu2O precipitates and interconnected dislocation microstructures having average spacings of similar to 2 mu m, which provide hardness values similar to 75% above commercial Cu products. Plots of stiffness (Young's modulus) versus density and relative stiffness versus relative density were in very close agreement with the Gibson-Ashby model for open cellular foams. These open cellular structure components exhibit considerable potential for novel, complex, multi-functional electrical and thermal management systems, especially complex, monolithic heat exchange devices. (C) 2011 Elsevier B.V. All rights reserved.

In this paper, we examine prospects for the manufacture of patient-specific biomedical implants replacing hard tissues (bone), particularly knee and hip stems and large bone (femoral) intramedullary rods, using additive manufacturing (AM) by electron beam melting (EBM). Of particular interest is the fabrication of complex functional (biocompatible) mesh arrays. Mesh elements or unit cells can be divided into different regions in order to use different cell designs in different areas of the component to produce various or continually varying (functionally graded) mesh densities. Numerous design elements have been used to fabricate prototypes by AM using EBM of Ti-6Al-4V powders, where the densities have been compared with the elastic (Young) moduli determined by resonant frequency and damping analysis. Density optimization at the bone–implant interface can allow for bone ingrowth and cementless implant components. Computerized tomography (CT) scans of metal (aluminium alloy) foam have also allowed for the building of Ti-6Al-4V foams by embedding the digital-layered scans in computer-aided design or software models for EBM. Variations in mesh complexity and especially strut (or truss) dimensions alter the cooling and solidification rate, which alters the α -phase (hexagonal close-packed) microstructure by creating mixtures of α / α ′ (martensite) observed by optical and electron metallography. Microindentation hardness measurements are characteristic of these microstructures and microstructure mixtures ( α / α ′) and sizes.

Ti–6Al–4V open cellular foams were fabricated by additive manufacturing using electron beam melting (EBM). Foam models were developed from CT-scans of aluminum open cellular foams and embedded in CAD for EBM. These foams were fabricated with solid cell structures as well as hollow cell structures and exhibit tailorable stiffness and strength. The strength in proportion to the measured microindentation hardness is as much as 40% higher for hollow cell (wall) structures in contrast to solid, fully dense EBM fabricated components. Plots of relative stiffness versus relative density were in good agreement with the Gibson–Ashby model for open cellular foam materials. Stiffness or Young's modulus values measured using a resonant frequency-damping analysis technique were found to vary inversely with porosity especially for solid cell wall, open cellular structure foams. These foams exhibit the potential for novel biomedical, aeronautics, and automotive applications.

The microstructures and mechanical behavior of simple, as-fabricated, solid geometries (with a density of 8.4 g/cm3), as-fabricated and fabricated and annealed femoral (knee) prototypes, and reticulated mesh components (with a density of 1.5 g/cm3) all produced by additive manufacturing (AM) using electron beam melting (EBM) of Co-26Cr-6Mo-0.2C powder are examined and compared in this study. Microstructures and microstructural issues are examined by optical metallography (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD), while mechanical properties included selective specimen tensile testing and Vickers microindentation hardness (HV) and Rockwell C-scale hardness (HRC) measurements. Orthogonal (X-Y) melt scanning of the electron beam during AM produced unique, orthogonal and related Cr23C6 carbide (precipitate) arrays (a controlled microstructural architecture) with dimensions of ~2 μm in the build plane perpendicular to the build direction, while connected carbide columns were formed in the vertical plane, parallel to the build direction, with microindentation hardnesses ranging from 4.4 to 5.9 GPa, corresponding to a yield stress and ultimate tensile strength (UTS) of 0.51 and 1.45 GPa with elongations ranging from 1.9 to 5.3 pct. Annealing produced an equiaxed fcc grain structure with some grain boundary carbides, frequent annealing twins, and often a high density of intrinsic {111} stacking faults within the grains. The reticulated mesh strut microstructure consisted of dense carbide arrays producing an average microindentation hardness of 6.2 GPa or roughly 25 pct higher than the fully dense components.

Variously non-isotropic arrays of reticulated mesh geometries and isotropic foam structures of Ti-6Al-4V have been fabricated by additive manufacturing (AM) using electron beam melting (EBM). These arrays consist of complex, monolithic prototypes which include various mesh geometries and foam structures with a range of densities, including fully dense geometries. Density and stiffness variations have been fabricated for aeronautics/aerospace applications and these complex arrays provide unique energy or impact absorption features, thermal management, stiffness and strength in sandwich cores, and excellent corrosion resistance. These monolithic mesh arrays have been fabricated using geometrical unit cells whose bases and size variations can allow density, porosity, stiffness, and strength tailoring to fabricate multifunctional materials prototypes. Foam components have been fabricated from CAD models based on CT-scans of common aluminum alloy foams. The microstructures of these prototypes have been characterized by optical and electron microscopy.

Porous coatings, notably thin, porous bead coatings, sintered mesh arrays, thermal-spray coatings and metallic foams have been incorporated into biomedical devices and appliances for several decades to improve bone compatibility, stability, and bone ingrowth. In this research program, we are concerned with the fabrication of reticulated mesh arrays as integral components of monolithic products using Ti-6Al-4V powder to build complex, 3D structures by electron beam melting (EBM). Utilizing software capable of building lattice-truss or cellular lattices with high symmetry, 3D-periodic reticulated arrangements such as hip stem and knee component prototypes have been fabricated with complete mesh arrays or with solid stems with a surrounding mesh structure completely fabricated as a monolithic product. The 3D mesh structures begin with so-called lattice elements which can be designed, computed, and attached to a CAD program for additive layered manufacturing by EBM. Mesh arrays with cortical bone density (1.9 g/cm3) can be fabricated with various lattice-truss structures and truss dimensions tailored to stress-strain and stiffness properties to optimize porous bone-replacement implants, including craniofacial replacements, etc. Mesh-to-mesh structures and functionally graded structures are also explored. Metallographic analysis of these structures using optical and electron microscopies illustrate their microstructural characteristics in association with measured mechanical properties such as microindentation hardness which can be related linearly to residual stress.

Rapid prototype (RP) manufacturing using Ti-6Al-4V powder and electron beam melting (EBM) has presented the prospects of microstructure-property control within small volumes sand linear dimensions of <1 mm. Utilizing optical and electron microscopy (SEM and TEM), it has been demonstrated that alpha (hcp) acicular platelet dimensions and dislocation substructures within these platelets, composing simple build geometries, can be varied with concomitant variations in hardness, tensile strength, and elongation. These structure-property variations occur by thermal differences as a consequence of beam current, focus, and scan rate or scan sequencing, In addition, during layer building various defects can be created by beam tripping and related phenomena. These include spherical or irregular voids ranging from a few microns to tens of microns in diameter as well as porous zones of even larger dimensions which result from non-melting or local variations in sintering. Examples of these build-related defects will be described.

Intermetallic, γ-TiAl, equiaxed, small-grain (∼2μm) structures with lamellar γ/α2-Ti3Al colonies with average spacing of 0.6μm have been fabricated by additive manufacturing using electron beam melting (EBM) of precursor, atomized powder. The residual microindentation (Vickers) hardness (HV) averaged 4.1GPa, corresponding to a nominal yield strength of ∼1.4GPa (∼HV/3), and a specific yield strength of 0.37GPacm3g−1 (for a density of 3.76gcm−3), in contrast to 0.27GPacm3g−1 for EBM-fabricated Ti–6Al–4V components. These results demonstrate the potential to fabricate near net shape and complex titanium aluminide products directly using EBM technology in important aerospace and automotive applications.

The microstructures and microstructure evolution associated with adiabatic shear band (ASB) formation in ballistic plugging in thick (2.5cm) Ti–6Al–4V targets impacted by cylindrical, 4340 steel projectiles (2.0cm in height) at impact velocities ranging from 633m/s to 1027m/s (just above the ballistic limit) were investigated by optical and transmission electron microscopy. ASB width increased from 10μm to 21μm as the velocity increased. ASB evolution was accompanied by the evolution of dark deformation bands composed of α′ martensite platelets which increased in density with increasing impact velocity. The corresponding Vickers microindentation hardness also increased from HV 619 to HV 632 in contrast to the surrounding matrix microindentation hardness of HV 555. These deformation bands were not necessarily precursors to ASB formation. The ASB average Vickers microindentation hardness was essentially constant at HV 645, a 16% increase over the matrix. This constant microindentation hardness was characterized by a consistent DRX grain structure which varied from equiaxed, defect-free grains (∼2μm diameter) to heavily dislocated, equiaxed grains. Cracks nucleating and propagating within the ABSs were observed to increase from 8% to 87% of the ASB length with increasing impact velocity.