Electrons injected into the build envelope during powder bed electron-beam additive manufacturing can accumulate on the irradiated particles and cause them to repel each other. Under certain ...conditions, these electrostatic forces can grow so large that they drive the particles out of the build envelope in a process known as “smoking”. In the present work, we investigate the causes of powder bed charging and smoking during electron-beam additive manufacturing. In the first part of the paper, we characterize the surface chemistry of a common feedstock material—gas-atomized Ti-6Al-4V powder—and find that a thick, electrically insulating oxide overlayer encapsulates the particles. Based on these experimental results, we then formulate an analytical model of powder bed charging in which each particle is approximated as a capacitor, where the particle and its substrate are the electrodes and the oxide overlayer is the dielectric. Using this model, we estimate the charge distribution in the powder bed, the electrostatic forces acting on the particles, and the conditions under which the powder bed will smoke. It is found that the electrical resistivity of the oxide overlayer strongly influences the charging behavior of the powder bed and that a high resistivity promotes charge accumulation and consequent smoking. This analysis suggests new quality control and process design measures that can help suppress smoking.
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Cracking during sintering is a common problem in powder processing and is usually caused by constraint that prevents the sintering material from shrinking in one or more directions. Different factors ...influence sintering‐induced cracking, including temperature schedule, packing density, and specimen geometry. Here we use the discrete element method to directly observe the stress distribution and sinter‐cracking behavior in edge notched panels sintered under a uniaxial restraint. This geometry allows an easy comparison with traditional fracture mechanics parameters, facilitating analysis of sinter‐cracking behavior. We find that cracking caused by self‐stress during sintering resembles the growth of creep cracks in fully dense materials. By deriving the constrained densification rate from the appropriate constitutive equations, we discover that linear shrinkage transverse to the loading axis is accelerated by a contribution from the effective Poisson's ratio of a sintering solid. Simulation of different notch geometries and initial relative densities reveals conditions that favor densification and minimize crack growth, alluding to design methods for avoiding cracking in actual sintering processes. We combine the far‐field stress and crack length to compute the net section stress, finding that it characterizes the stress profile between the notches and correlates with the sinter‐crack growth rate, demonstrating its potential to quantitatively describe sinter‐cracking.
Forced chemical mixing during extensive straining requires that the constituent phases co-deform, and is therefore a sensitive function of their mechanical properties, particularly strength. To ...develop a quantitative understanding of such phase strength effects on co-deformation and steady-state chemical mixity during severe deformation processing, we studied several tungsten–transition metal couples with a range of differences in strength during a process of mechanical alloying in a high-energy ball mill. Changes in the powders’ microstructures, mechanical properties and chemical mixing revealed two distinct behaviors: alloys either chemically homogenized or remained dual phase, depending on the relative strengths of the base alloying elements. A kinetic Monte Carlo simulation of mechanical alloying that accounts for a phase strength mismatch reproduced the experimentally observed behaviors, and provides quantitative insight into the combination of material and processing parameters that control mechanical mixing.
This paper introduces the chain lattice, a hierarchical porous structure comprising two interpenetrating cellular solids. One constituent toughens the material and prevents catastrophic localized ...failure while the other serves as a porous matrix which densifies to absorb energy during tensile loading. Through tension testing, we demonstrate 3D-printed plastic chain lattices that exhibit delocalized damage and an order of magnitude increment in strain-to-failure over the fully dense base material. These experiments validate a micromechanics-based model of tensile specific energy absorption, which we then use in a parametric study on the effects of chain geometry and matrix properties on tensile behavior. We find that ceramic chain lattices can achieve an order of magnitude improvement in tensile specific energy absorption over the fully dense material, in line with the improvement seen when forming monolithic ceramics into fiber-reinforced ceramic matrix composites. The experiments and analysis highlight the ability of the chain lattice to impart damage-tolerance to 3D-printable materials that are normally brittle and flaw-sensitive.
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Grain boundaries always have a positive excess free energy that drives grain growth and structural evolution during high-temperature heat treatments. Grain boundaries can, however, become trapped in ...a metastable configuration because of the geometry or composition of the specimens in which they are contained. For example, second phase dispersoids can pin grain boundaries in a polycrystal, and thermal grooves can pin grain boundaries in thin films and wires. In these examples of frustrated grain boundary systems, further reduction in the grain boundary area is energetically preferred, but grain boundary migration is arrested because energy is required to create additional grain boundary surface area and de-pin from the obstacle. In this work, we consider an idealized class of frustrated grain boundaries, namely curved grain boundaries in geometrically complex bicrystals. By calculating the energy landscape of the grain boundary configuration space, we show that the local stability of these curved grain boundaries is a sensitive function of the bicrystal shape. We also assess the trajectory of curved grain boundaries in bicrystals grown via directional solidification to determine which grain boundary configurations are physically realizable. By comparing these two analyses, we find that a curved grain boundary may be thermodynamically metastable but kinematically inaccessible, highlighting the general importance of thermodynamics and kinematics in descriptions of frustrated grain boundary systems encountered in a wide range of different materials.
In this work, we examine the macroscale and fine-scale shock responses of interpenetrating phase composites comprising a body-centered cubic steel lattice embedded in an aluminum matrix. Through ...plate impact simulations, we find that the complex mesoscale geometry reduces shock velocity relative to monolithic constituents, slowing and spreading the shock front via reflection and redirection. The periodicity of the mesoscale composite geometry is also reflected by quasi-steady-wave behavior. On the fine-scale, we can predict several aspects of the oscillatory pressure and longitudinal velocity responses by tracking internal wave reflections. We also observe that the post-shock maximum temperature increases with structural openness and temperature hotspots form at interfaces parallel to the shock direction. The findings in this work provide novel structure–property linkages in the dynamic response of architectured interpenetrating phase composites.
The nucleation and growth of chimney pores during powder-bed electron-beam additive manufacturing is investigated using in situ infrared thermography and micro-computed tomography. The chimney pores ...are found to nucleate heterogeneously at dimples on the side surfaces of additively manufactured components, and to grow through a molten-film rupture process. Further, these nucleation and growth processes are found to be strongly influenced by the beam diameter. Several strategies for suppressing the formation of chimney pores are discussed in light of these results.
Ultrasonic additive manufacturing has been used to fabricate laminated composites of commercially pure aluminum and a nanocrystalline nickel–cobalt (nc-NiCo) alloy. The nc-NiCo alloy would not weld ...to itself but readily welded to aluminum. Thus, by alternating between foils of nc-NiCo and Al, we achieved multi-material laminates with strong interlayer bonding. Electron microscopy showed that the nanoscale grain structure of the nc-NiCo was preserved during deposition and that the nc-NiCo/Al weld interface was decorated with comminuted surface oxides as well as Al–Ni–Co intermetallics. These findings are considered in light of process models of junction growth, interdiffusion, and grain growth, which together reveal how the different pressure- and temperature dependences of these phenomena give rise to a range of processing conditions that maximize bonding while minimizing coarsening and intermetallic formation. This analysis quantitatively demonstrates that using a soft, low melting point interlayer material decouples junction growth at the weld interface from grain growth in the nc-NiCo, expanding the range of optimal processing conditions.
Graphical abstract
•A novel CNC deformation process for fabricating 3D wireframe structures is presented.•The process, termed Bend-Forming, combines CNC wire bending with mechanical joints to construct reticulated ...structures from a spool of feedstock, with rapid build times compared to other additive manufacturing techniques.•A key component of the process is a path planning framework which uses Euler paths and geometrical computations to convert an arbitrary 3D wireframe structure into fabrication instructions for a CNC wire bender.•The error stack-up of the Bend-Forming process is also analyzed to enable fabrication of precise structures.
This paper presents a computer numerical control (CNC) deformation process, termed Bend-Forming, for fabricating 3D wireframe structures. The process relies on the combination of CNC wire bending with mechanical joints to construct reticulated structures from wire feedstock. A key component of the process is a path planning framework which uses Euler paths and geometrical computations to derive fabrication instructions for arbitrary 3D wireframe geometries. We demonstrate the process by fabricating exemplary structures on the order of 1 m, including reticulated columns, shells, and trusses, with rapid build times compared to other additive manufacturing techniques. The structures fabricated herein contain defects which result in residual stress and imperfect geometries. To determine the tolerances needed to fabricate accurate structures, we develop a model of error stack-up for Bend-Forming, using fabrication defects in feed length, bend and rotate angle, and strut curvature. We find that for tetrahedral trusses fabricated with Bend-Forming, defects in feed length and strut curvature have a large effect on the surface precision and stiffness of the truss, respectively, and are thus important tolerances to control to achieve structural performance metrics. Overall, Bend-Forming is a versatile and low-power process that is well suited for a wide-range of applications, from rapid prototyping of wireframe structures to in-space manufacturing.
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