17 research outputs found
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Thermomechanical Modeling of Successive Material Deposition in Layered Manufacturing
Residual stress build-up due to successive deposition of superheated molten metal onto
metal substrates is modeled for application to layered manufacturing methods. This work is
specifically applied to microcasting, which is a deposition process used within shape deposition
manufacturing. One-dimensional thennal and mechanical models are used to predict temperature
and stress evolution related to two physical phenomena. First, the effect of thennal cycling by
newly deposited material on stress states in previously deposited and cooled layers is investigated.
Here, deposited molten metal solidifies and cools to room temperature before new molten metal is
deposited. For this case, predicted stress distributions as a function of depth are relatively
uncomplicated and can be related to residual stress-induced part tolerance loss. In the second case,
the effect of localized preheating by previously deposited material is investigated. In this model,
molten metal is successively deposited at a rate comparable to that used to deposit individual
droplets in the microcasting process. Results indicate that although preheating by previously
deposited material strongly affects transient stresses, final stress states are not substantially altered.Mechanical Engineerin
Drying colloidal systems: laboratory models for a wide range of applications
The drying of complex fluids provides a powerful insight into phenomena that take place on time and length scales not normally accessible. An important feature of complex fluids, colloidal dispersions and polymer solutions is their high sensitivity to weak external actions. Thus, the drying of complex fluids involves a large number of physical and chemical processes. The scope of this review is the capacity to tune such systems to reproduce and explore specific properties in a physics laboratory. A wide variety of systems are presented, ranging from functional coatings, food science, cosmetology, medical diagnostics and forensics to geophysics and art
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Control of Residual Thermal Stresses in Shape Deposition Manufacturing
Layer-level thermal and mechanical modeling ofthe microcasting stage of ShaPe Deposition
Manufacturing is presented. Thermo-mechanical models of carbon steel deposited onto a carbon
steel substrate are described. Mechanics modeling addresses the issue of residual stress build-up.
The effects of substrate heating and bending constraint on the build-up of residual stresses are
shown. Results show that thermal cycling from newly applied droplets drastically changes the
stress state in the top of the substrate. Originally unstressed regions go through an inelastic
compression-tension stress cycle. Residual stresses reach values that may cause yielding in carbon
steel. Moderate heating of the substrate above room temperature prior to droplet deposition
reduces stresses significantly. Bending constraint during part manufacture allows partial relaxation
of stresses as the constraint is removed.Mechanical Engineerin
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Measurement of Residual Stresses in Parts Created by Shape Deposition Manufacturing
Residual stress build-up is a concern in any solid freeform fabrication process involving
successive deposition of uncured or molten material, due to differential contractions caused by
solidification or curing. The most detrimental effect of residual stresses is typically part warping,
which can lead to unacceptable losses in part tolerance. In many processes residual stress build-up
is a fundamental barrier to the consistent manufacture of high-quality artifacts. In this paper, two
methods of measuring residual stresses in parts created by Shape Deposition Manufacturing (SDM)
with microcasting are described. First, a technique for measuring warping in deposited plateshaped
specimens is detailed, which can be used to determine residual stress resultants as well as
to quantify gross effects of processing changes on residual stress magnitudes. Next, x-ray
diffraction procedures are described by which residual stresses in deposited layers can be measured
at discrete in-plane locations as a function of depth. Measured results for 308L stainless steel
deposits determined from each method are interpreted in the context of residual stress modeling
results obtained numerically in a separate research effort. The measured results provide insight
into the effects on residual stress of both the material deposition path and the discrete droplet-bydroplet
nature of the microcasting deposition process. The insights provided here may also be
applicable to other processes involving successive material deposition.Mechanical Engineerin
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Processing, Thermal and Mechanical Issues in Shape Deposition Manufacturing
An overview of Shape Deposition Manufacturing (SDM) is presented, detailing
manufacturing, thermal and mechanical issues of concern in making it a commercially viable
method for creating arbitrarily shaped three-dimensional metal parts. SDM is a layered
manufacturing process which combines the benefits of solid freeform fabrication and other
processing operations, such as multi-axis CNC machining. This manufacturing process makes
possible the fabrication of multi-material layers, structures of arbitrary geometric complexity,
artifacts with controlled microstructures, and the embedding of electronic components and sensors
in conformal shape structures. Important issues toward the production of high quality objects are
the creation of inter-layer metallurgical bonding through substrate remelting, the control of cooling
rates of both the substrate and the deposition material, and the minimization of residual thermal
stress effects. Brief descriptions of thermal and mechanical modeling aspects of the process are
given. Because SDM involves molten metal deposition, an understanding of thermal aspects of the
process is crucial. Current thermal modeling of the process is centered on the issue of localized
remelting of previously deposited material by newly deposited molten droplets. Residual stress
build-up is inherent to any manufacturing process based on successive deposition of molten
material. Current mechanics modeling is centered on the issues of residual stress build-up and
residual stress-driven debonding between deposited layers.Mechanical Engineerin
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Measurement and Modeling of Residual Stress-Induced Warping in Direct Metal Deposition Processes
Tolerance loss due to residual stress-induced warping is a major concern in solid freeform
fabrication (SFF) processes. An understanding of how residual stresses develop and how they
lead to tolerance loss is a key issue in advancing these processes. In this paper, results are
presented from warping experiments on plate-shaped specimens created by microcasting and
welding processes used in Shape Deposition Manufacturing (SDM). Results from these
experiments give insight into differences between the two processes, the role of preheating and
insulating conditions during manufacture and the influence of deposition path on magnitudes and
distributions of warping displacements. Results are then compared to predictions from two types
of residual stress models. While the models effectively predict warping magnitudes and the effects
of various thermal conditions, they are unable to capture some of the more subtle trends in the
experiments. Results from the experiments and numerical models suggest that a combination of
initial substrate preheating and part insulation can be applied to SDM and similar SFF processes to
limit warping deflections, which is substantially simpler than active control of part temperatures
during manufacture. Results also suggest that 3-D mechanical constraints are important in
achieving precise control of warping behavior in SFF processes.Mechanical Engineerin