L. E. Murr

Metal Fabrication by Additive Manufacturing Using Laser and Electron Beam Melting Technologies

Selective laser melting (SLM) and electron beam melting (EBM) are relatively new rapid, additive manufacturing technologies which can allow for the fabrication of complex, multi-functional metal or alloy monoliths by CAD-directed, selective melting of precursor powder beds. By altering the beam parameters and scan strategies, new and unusual, even non-equilibrium microstructures can be produced; including controlled microstructural architectures which ideally extend the contemporary materials science and engineering paradigm relating structure-properties-processing-performance. In this study, comparative examples for SLM and EBM fabricated components from pre-alloyed, atomized precursor powders are presented. These include Cu, Ti-6Al-4V, alloy 625 (a Ni-base superalloy), a Co-base superalloy, and 17-4 PH stainless steel. These systems are characterized by optical metallography, scanning and transmission electron microscopy, and X-ray diffraction.

Flow visualization and residual microstructures associated with the friction-stir welding of 2024 aluminum to 6061 aluminum

The friction-stir welding (FSW) of 0.6 cm plates of 2024 Al (140 HV) to 6061 Al (100 HV) is characterized by residual, equiaxed grains within the weld zone having average sizes ranging from 1 to 15 μm, exhibiting grain growth from dynamically recrystallized grains which provide a mechanism for superplastic flow; producing intercalated, lamellar-like flow patterns. These flow patterns are visualized by differential etching of the 2024 Al producing contrast relative to 6061 Al. The flow patterns are observed to be complex spirals and vortex-like, among others, and to change somewhat systematically with tool rotation (stirring) speed between 400 and 1200 rpm; depending on tool orientation. The equiaxed grain and sub-grain microstructures are observed to vary according to estimated temperature profiles (varying from 0.6 to 0.8 TM, where TM is the absolute melting temperature) referenced to the rotating tool axis. Dislocation spirals and loops are also observed in the 2024 Al intercalation regions within the weld zones at higher speeds (>800 rpm) corresponding to slightly elevated temperatures introducing dislocation climb, and residual microhardness profiles follow microstructural variations which result in a 40% reduction in the 6061 Al workpiece microhardness and a 50% reduction in the 2024 Al workpiece microhardness just outside the FSW zone.

Effects of Strain State and Strain Rate on Deformation-Induced Transformation in 304 Stainless Steel: Part II. Microstructural Study

The γ→α transformation in 304 stainless steel was induced by plastic deformation under various conditions of strain, strain state, and strain rate, and the transformation microstructures were examined by transmission electron microscopy (TEM). The nucleation of α martensite embryos was always confined to microscopic shear band (faults, twins, and ε-martensite) intersections. In cases where shear bands consisted of bundles of intermixed faults, twins, and ε-martensite, α nucleated preferentially only within specific portions of the intersection volume. At sufficiently large strains α appeared to grow into polyhedral shapes. We postulate that growth occurs by repeated nucleation of new α embryos and coalescence of such embryos into polyhedral shapes. These shapes can grow either within an active slip plane or out of it, depending on how many shear band intersections are produced during deformation. Actual measurements of the number of intersections indicated that more intersections are formed in biaxial tension per unit effective strain than in uniaxial tension. This accounts for the more irregular, blocky α morphology observed in biaxial tension. At high strain rates we also found an increase in the number of intersections. However, adiabatic heating at large strains and high rates restricts repeated nucleation and coalescence and limits the amount of α transformation product.

Low-temperature friction-stir welding of 2024 aluminum

Solid state friction-stir welding (FSW) has been demonstrated to involve dynamic recrystallization producing ultra-fine, equiaxed grain structures to facilitate superplastic deformation as the welding or joining mechanism. However, the average residual, equiaxed, grain size in the weld zone has ranged from roughly 0.5 micron to slightly more than 10 micron, and the larger weld zone grain sizes have been characterized as residual or static grain growth as a consequence of the temperatures in the weld zone (where center-line temperatures in the FSW of 6061 Al have been shown to be as high as 480C or -0.8 T(sub M) where T(sub M) is the absolute melting temperature)). In addition, the average residual weld zone grain size has been observed to increase near the top of the weld, and to decrease with distance on either side of the weld-zone centerline, an d this corresponds roughly to temperature variations within the weld zone. The residual grain size also generally decreases with decreasing FSW tool rotation speed. These observations are consistent with the general rules for recrystallization where the recrystallized grain size decreases with increasing strain (or deformation) at constant strain rate, or with increasing strain-rate, or with increasing strain rate at constant strain; especially at lower ambient temperatures, (or annealing temperatures). Since the recrystallization temperature also decreases with increasing strain rate, the FSW process is somewhat complicated because the ambient temperature, the frictional heating fraction, and the adiabatic heating fraction )proportional to the product of strain and strain-rate) will all influence both the recrystallization and growth within the FSW zone. Significantly reducing the ambient temperature of the base metal or work pieces to be welded would be expected to reduce the residual weld-zone grain size. The practical consequences of this temperature reduction would be the achievement of low temperature welding. This study compares the residual grain sizes and microstructures in 2024 Al friction-stir welded at room temperature (about 30C and low temperature (-30C).

A TEM study of precipitation and related microstructures in friction-stir-welded 6061 aluminium

Residual microstructures, including dynamic recrystallization and grain growth structures and a wide range of precipitation phenomena associated with a friction-stir-weld in a thin 6061-T6 aluminium plate have been systematically investigated utilizing light metallography and transmission electron microscopy. In this rather remarkable process, a hard steel head pin rotating at 400 r.p.m. was advanced into a solid 6061-aluminium plate at a traverse velocity of approximately 2 mm s−1 to produce a solid-phase weld in its trailing side. Maximum work-piece temperatures did not exceed 425°C and there was no melt evidence. Dynamic recrystallization associated with the solid-state plastic flow therefore seems to provide the process mechanism. Weld zone hardnesses averaged roughly 55 Vickers hardness number (VHN) in contrast to the base plate or work-piece hardness of 110 VHN. Precipitation microstructures ranged from continuous to discontinuous coherent zones (∼2 nm thick) coincident with {1 0 0} planes, semicoherent and non-coherent needles and plates characteristic of Widmanstatten structures coincident with {1 1 0} planes, and a range of homogeneous precipitate particles often intermixed with these microstructures in the effective heat-affected zone (HAZ′) connecting the friction-stir-weld zone with the unaltered work piece microstructures.

A model for the formation of annealing twins in F.C.C. metals and alloys

Abstract A model for the formation of annealing twins in f.c.c. metals and alloys is proposed. According to it, annealing twin formation proceeds in two stages: initiation and propagation. Initiation takes place at grain boundary ledges. After ‘popping out’ of the grain boundary, the twin grows into the grain by the migration of the noncoherent twin boundary, that can be represented by an array of partial Shockley dislocations with total Burgers vector equal to zero. Since the twin ‘pops out’ of the bondary and grows into the grain, the model does not require associated migration of the existing grain boundaries. The twinned region is separated from the grain by means of two parallel coherent twin boundaries. There are two relative orientations of adjacent grains for which the model can operate: 1. (a) when they are at twin orientation (but the boundary is not a coherent twin boundary) and 2. (b) when they are amenable to forming a ‘special’ boundary. Experimental evidence supporting the model is presented.

Fundamental studies of the contribution of galvanic interaction to acid-bacterial leaching of mixed metal sulfides

Abstract A systematic study has been made towards understanding the role of galvanic interactions in the leaching of sulfide minerals both in the absence and presence of bacteria. When two sulfide minerals are in contact with each other in an acid-aqueous solution, the mineral lower in the electromotive series dissolves rapidly while the one higher in the series is galvanically protected. To ascertain the magnitude of galvanic interaction when chalcopyrite (CuFeS 2 ) and pyrite (FeS 2 ) are in contact, potentiodynamic polarization measurements were carried out in 1 M H 2 SO 4 and in Bryner and Anderson medium. The individual rest potentials of CuFeS 2 and FeS 2 were found to be 0.52 V and 0.63 V [vs. Standard Hydrogen Electrode (SHE)] respectively. The mixed potential of the CuFeS 2 : FeS 2 couple was found to be 0.56 V and 0.28 V (vs. SHE) in 1 M H 2 SO 4 and in Bryner and Anderson medium respectively. The corrosion current (5 μ A/cm 2 ) was calculated from the polarization curves. These results were found to be in agreement with the actual leaching rates of copper from contacting large mineral specimens composing the CuFeS 2 /FeS 2 system. Both powdered samples as well as large mineral specimens of CuFeS 2 , FeS 2 and ZnS were used to observe the effect of galvanic interactions on the process of leaching. Chalcopyrite and pyrite powders, as pulp mixtures, were leached under a variety of experimental conditions to optimize their ratio (total pulp density) and size fraction for efficient metal extraction by way of maximum contact with each other. A CuFeS 2 :FeS 2 ratio of 5:5 (g/g) and size fraction of −200 mesh was found to be the most preferable when leaching was carried out in the presence of T. ferrooxidans . The effect of temperature and thermophilic bacteria on the rate of leaching was also studied. The presence of pyrite in the optimized quantities considerably enhanced the rate of copper dissolution which was increased, by a factor of 2, to 15. Under these experimental conditions, E h increased from 338 mV to 580 mV while pH dropped from 2.30 to 1.56. In order to have a definite control over surface area of the ores and a constant contact between two sulfide ores, large crystalline-mineral samples of CuFeS 2 , FeS 2 and ZnS were leached singly as well as in intimate contact (coupled galvanically). This also made it possible to observe the surfaces of the specimens as the leaching proceeded in the presence of galvanic interaction. Coupled CuFeS 2 /FeS 2 , ZnS/FeS 2 and CuFeS 2 /FeS 2 /ZnS systems showed improved metal dissolution as compared to leaching of single large specimens. Scanning electron microscope observations showed that there were no major changes on pyrite surfaces when in galvanic contact with chalcopyrite and/or sphalerite. These observations have been supported by chemical data and by energy-dispersive X-ray analysis.

Fabrication of Metal and Alloy Components by Additive Manufacturing: Examples of 3D Materials Science

Objective This paper provides a brief review of relatively new additive manufacturing technologies for the fabrication of unusual and complex metal and alloy products by laser and electron beam melting. A number of process features and product microstructures are illustrated utilizing 3D optical and transmission electron microscope image compositions representing examples of 3D materials science. Methods Processing methods involving electron beam melting (EBM) and a process referred to as direct metal laser sintering (DMLS), often called selective laser melting (SLM) are described along with the use of light (optical) microscopy (OM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) to elucidate microstructural phenomena. Results Examples of EBM and SLM studies are presented in 3D image compositions. These include EBM of Ti-6Al-4V, Cu, Co-base superalloy and Inconel 625; and SLM of 17-4 PH stainless steel, Inconel 718 and Inconel 625. Conclusions 3D image compositions constituting 3D materials science provide effective visualization for directional solidification-related phenomena associated with the EBM and SLM fabrication of a range of metals and alloys, especially microstructures and microstructural architectures.

Next generation orthopaedic implants by additive manufacturing using electron beam melting

This paper presents some examples of knee and hip implant components containing porous structures and fabricated in monolithic forms utilizing electron beam melting (EBM). In addition, utilizing stiffness or relative stiffness versus relative density design plots for open-cellular structures (mesh and foam components) of Ti-6Al-4V and Co-29Cr-6Mo alloy fabricated by EBM, it is demonstrated that stiffness-compatible implants can be fabricated for optimal stress shielding for bone regimes as well as bone cell ingrowth. Implications for the fabrication of patient-specific, monolithic, multifunctional orthopaedic implants using EBM are described along with microstructures and mechanical properties characteristic of both Ti-6Al-4V and Co-29Cr-6Mo alloy prototypes, including both solid and open-cellular prototypes manufactured by additive manufacturing (AM) using EBM.

Carbon nanotubes, nanocrystal forms, and complex nanoparticle aggregates in common fuel-gas combustion sources and the ambient air

Aggregated multiwall carbon nanotubes (with diameters ranging from ∼3 to 30nm) and related carbon nanocrystal forms ranging in size from 0.4 to 2 μm (average diameter) have been collected in the combustion streams for methane/air, natural gas/air, and propane gas/air flames using a thermal precipitator. Individual particle aggregates were collected on carbon/formvar-coated 3mm nickel grids and examined in a transmission electron microscope, utilizing bright-field imaging, selected-area electron diffraction analysis, and energy-dispersive X-ray spectrometry techniques. The natural gas and propane gas sources were domestic (kitchen) stoves, and similar particle aggregates collected in the outdoor air were correspondingly identified as carbon nanocrystal aggregates and sometimes more complex aggregates of silica nanocrystals intermixed with the carbon nanotubes and other carbon nanocrystals. Finally, and in light of the potential for methane-series gas burning as major sources of carbon nanocrystal aggregates in both the indoor and outdoor air, data for natural gas consumption and corresponding asthma deaths and incidence are examined with a degree of speculation regarding any significance in the correlations.