5 research outputs found
Novel Simulation-Inspired Roller Spreading Strategies for Fine and Highly Cohesive Metal Powders
When fine powders are to be used in powder bed metal additive manufacturing
(AM), a roller is typically utilized for spreading. However, the cohesive
nature of fine metal powder still presents challenges, resulting in low density
and/or inconsistent layers under sub-standard spreading conditions. Here,
through computational parameter studies with an integrated discrete
element-finite element (DEM-FEM) framework, we explore roller-based strategies
that are predicted to achieve highly cohesive powder layers. The exemplary
feedstock is a Ti-6Al-4V 0-20 um powder, that is emulated using a
self-similarity approach based on experimental calibration. The computational
studies explore novel roller kinematics including counter-rotation as well as
angular and transverse oscillation applied to standard rigid rollers as well as
coated rollers with compliant or non-adhesive surfaces. The results indicate
that most of these approaches allow to successfully spread highly cohesive
powders with high packing fraction (between 50%-60% in a single layer) and
layer uniformity provided that the angular/oscillatory, relative to the
transverse velocity, as well as the surface friction of the roller are
sufficiently high. Critically, these spreading approaches are shown to be very
robust with respect to varying substrate conditions (simulated by means of a
decrease in surface energy), which are likely to occur in LBPF or BJ, where
substrate characteristics are the result of a complex multi-physics (i.e.,
powder melting or binder infiltration) process. In particular, the combination
of the identified roller kinematics with compliant surface coatings, which are
known to reduce the risk of tool damage and particle streaking in the layers,
is recommended for future experimental investigation
Towards Additively Manufactured Metamaterials with Powder Inclusions for Controllable Dissipation: The Critical Influence of Packing Density
Particle dampers represent a simple yet effective means to reduce unwanted
oscillations when attached to structural components. Powder bed fusion additive
manufacturing of metals allows to integrate particle inclusions of arbitrary
shape, size and spatial distribution directly into bulk material, giving rise
to novel metamaterials with controllable dissipation without the need for
additional external damping devices. At present, however, it is not well
understood how the degree of dissipation is influenced by the properties of the
enclosed powder packing. In the present work, a two-way coupled discrete
element - finite element model is proposed allowing for the first time to
consistently describe the interaction between oscillating deformable structures
and enclosed powder packings. As fundamental test case, the free oscillations
of a hollow cantilever beam filled with various powder packings differing in
packing density, particle size, and surface properties are considered to
systematically study these factors of influence. Critically, it is found that
the damping characteristics strongly depend on the packing density of the
enclosed powder and that an optimal packing density exists at which the
dissipation is maximized. Moreover, it is found that the influence of
(absolute) particle size on dissipation is rather small. First-order analytical
models for different deformation modes of such powder cavities are derived to
shed light on this observation
Spatial Mapping of Powder Layer Density for Metal Additive Manufacturing via X-ray Microscopy
Uniform powder spreading is a requisite for creating consistent, high-quality
components via powder bed additive manufacturing (AM), wherein layer density
and uniformity are complex functions of powder characteristics, spreading
kinematics, and mechanical boundary conditions. High spatial variation in
particle packing density, driven by the stochastic nature of the spreading
process, impedes optical interrogation of these layer attributes. Thus, we
present transmission X-ray imaging as a method for directly mapping the
effective depth of powder layers at process-relevant scale and resolution.
Specifically, we study layers of nominal 50-250 micrometer thickness, created
by spreading a selection of commercially obtained Ti-6Al-4V, 316 SS, and
Al-10Si-Mg powders into precision-depth templates. We find that powder layer
packing fraction may be predicted from a combination of the relative thickness
of the layer as compared to mean particle size, and flowability assessed by
macroscale powder angle of repose. Power spectral density analysis is
introduced as a tool for quantification of defect severity as a function of
morphology, and enables separate consideration of layer uniformity and
sparsity. Finally, spreading is studied using multi-layer templates, providing
insight into how particles interact with both previously deposited material and
abrupt changes in boundary condition. Experimental results are additionally
compared to a purpose-built discrete element method (DEM) powder spreading
simulation framework, clarifying the competing role of adhesive and
gravitational forces in layer uniformity and density, as well as particle
motion within the powder bed during spreading
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Laser beam shape optimization: Exploring alternative profiles to Gaussian-shaped laser beams in powder bed fusion of metals
Laser-based powder bed fusion of metals (PBF-LB/M) commonly utilizes Gaussian-shaped laser
beams characterized by a high intensity at the center. However, this type of profile leads to localized high temperatures and temperature gradients. When the laser power is increased beyond
a certain threshold, the temperature inside the melt pool can reach the boiling point, causing excessive metal evaporation, hydrodynamic instabilities, and undesired effects such as keyholing.
On the other hand, ring-shaped laser beams generate a more uniform temperature distribution but
tend to produce shallower, wider, and shorter melt pools with reduced resolution compared to the
Gaussian profiles. The deep, narrow, and elongated melt pools generated by the Gaussian shapes
still have advantages for increased precision in the PBF-LB/M processes. This contribution uses
numerical optimization to generate a new laser beam shape that also leads to a deep, narrow, and
elongated melt pool, similar to a Gaussian-shaped beam, while maintaining a lower and more uniform temperature distribution inside the melt pool. The resulting optimized laser profile lowers the
maximum laser intensity by 40 % without decreasing the total laser power compared to the Gaussian profile. The more uniform distribution of temperature with a peak value of just above 3 000 â—¦C
indicates a conduction dominated process with less hydrodynamic and minimal evaporative effects.
This is expected to reduce the associated defects and improve the process stabilityMechanical Engineerin