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Modeling and Characterization of Cohesion in Fine Metal Powders with a Focus on Additive Manufacturing Process Simulations
Christoph Meier, Reimar Weissbach, Johannes Weinberg, Wolfgang A. Wall, A. John Hart
TL;DR
Fine metal powder cohesion strongly affects flow behavior relevant to additive manufacturing, while metallic-powder cohesion lacks direct experimental characterization. The paper develops a DEM model with calibrated cohesive interactions and finds that cohesive forces are essential for representing angle of repose and bulk powder behavior.
Problem
Fine metal powder cohesion affects flow behavior in additive manufacturing, but existing cohesion studies had not addressed metallic powders.
Method
The paper uses DEM with frictional contact, rolling resistance, and surface-energy-based cohesive forces calibrated against experimental and numerical angle-of-repose tests.
Results
Neglecting cohesive forces drastically underestimates angle of repose, while increased surface energy produces strongly increased AOR values and large cohesive agglomerates.
Takeaways & Limitations
Cohesive forces and effective surface energy must be represented for sufficient bulk-powder behavior in potential AM recoating simulations.
Takeaways & Limitations
The adhesive-force treatment assumes perfectly dry powder and does not explicitly account for capillary effects.
Abstract
from arXiv · showhide
The cohesive interactions between fine metal powder particles crucially influence their flow behavior, which is in turn important to many powder-based manufacturing processes including emerging methods for powder-based metal additive manufacturing (AM). The present work proposes a novel modeling and characterization approach for micron-scale metal powders, with a special focus on characteristics of importance to powder-bed AM. The model is based on the discrete element method (DEM), and the considered particle-to-particle and particle-to-wall interactions involve frictional contact, rolling resistance and cohesive forces. Special emphasis lies on the modeling of cohesion. The proposed adhesion force law is defined by the pull-off force resulting from the surface energy of powder particles in combination with a van-der-Waals force curve regularization. The model is applied to predict the angle of repose (AOR) of exemplary spherical Ti-6Al-4V powders, and the surface energy value underlying the adhesion force law is calibrated by fitting the corresponding angle of repose values from numerical and experimental funnel tests. To the best of the authors' knowledge, this is the first work providing an experimental estimate for the effective surface energy of the considered class of metal powders. By this approach, an effective surface energy of $0.1mJ/m^2$ is found for the investigated Ti-6Al-4V powder. This value is considerably lower than typical experimental values for flat metal contact surfaces in the range of $30-50 mJ/m^2$, indicating the crucial influence of factors such as surface roughness and chemical surface contamination on fine metal powders. More importantly, the present study demonstrates that a neglect of the related cohesive forces leads to a drastical underestimation of the AOR and, consequently, to an insufficient representation of the bulk powder behavior.
1. Introduction
Fine metal powders are important to powder-bed AM, but their cohesive behavior is insufficiently characterized for metallic systems. This work proposes a DEM model and calibration approach that represents cohesion and evaluates its effect on powder behavior.
- Metal powder behavior is central to powder-bed AM, whose complexity and sensitivity motivate experimental and numerical studies of its governing mechanisms.
- Prior DEM studies of powder recoating modeled contact, sliding friction, and rolling friction, but existing cohesion studies had not addressed metallic powders.
- Cohesiveness increases as particle size decreases because adhesive forces scale linearly with size while volume forces such as gravity scale cubically.
- The proposed DEM model combines frictional contact, rolling resistance, and cohesive particle-to-particle and particle-to-wall interactions, with cohesion defined through surface-energy-based pull-off and van-der-Waals regularization.
- The model calibrates effective surface energy by fitting numerical and experimental angle-of-repose values, showing that omitting cohesion drastically underestimates AOR and bulk powder behavior.
2. Physical and computational model
The paper models micron-scale Ti-6Al-4V powder with DEM, combining contact, frictional, rolling-resistance, and cohesive interactions. Cohesion is represented through a surface-energy-based pull-off force with a regularized van der Waals transition that accounts for roughness and contamination effects.
- Particle and contact modeling: DEM resolves spherical powder particles and their particle-to-particle and particle-to-wall mechanical interactions.The model includes frictional contact, rolling resistance, and adhesive forces.
- Particle and contact modeling: The normal contact interaction uses a linear spring-dashpot law that applies compressive forces only during particle overlap.The model relates elastic and damping parameters to particle material properties and collision behavior.
- Particle and contact modeling: Tangential interactions use a Coulomb friction law with spring-dashpot regularization and a numerical stick-slip return-mapping procedure.Normal and tangential stiffnesses are coupled through Poisson’s ratio rather than selected independently.
- Rolling resistance: Rolling resistance represents dissipation from viscous or elastic losses, plastic deformation, and deviations from ideal spherical particle shape.The rolling-resistance coefficient is approximated using material and restitution information rather than treated solely as an independent unknown.
- Adhesive forces: The adhesive force model is parameterized by surface energy and combines a surface-energy-based pull-off force with a regularized van der Waals force curve.A finite transition interval improves numerical conditioning, while changes in surface energy substantially affect bulk powder behavior; roughness and contamination reduce the applicability of ideal smooth-surface values.
3. Model calibration
The DEM model is calibrated by matching numerical and experimental funnel-test angles of repose for spherical Ti-6Al-4V powders. The calibration identifies an effective surface energy of 0.1mJ/m^2 and shows that cohesion dominates bulk powder behavior relative to several other parameters.
- Surface-energy calibration: 0.1mJ/m^2 matches the ≈41° mean experimental AOR for the medium-sized powder at a=a0/4.This value is adopted as the experimentally fitted effective surface energy γ0.
- Numerical–experimental comparison: Increasing adhesion or decreasing particle size increases the angle of repose and powder-pile height, with experiments and simulations showing good qualitative agreement.The comparison covers different surface energies, particle sizes, and problem scalings.
- Numerical–experimental comparison: Coarse-powder numerical and experimental AOR values agree well, whereas fine-powder values vary more strongly and experiments slightly exceed the numerical counterparts.For the fine powder, the 2.4-fold diameter reduction corresponds to an approximately 5.8-fold increase in effective surface energy.
- Cohesion and rolling resistance: Neglecting cohesion drastically underestimates AOR, while increased rolling resistance changes AOR less than the cohesive-force contribution for these highly spherical particles.The study notes that the employed velocity-proportional rolling resistance contributes nothing in static equilibrium.
- Parameter sensitivity: AOR is less sensitive to doubling the penalty parameter or increasing friction and restitution by 1.5 than to varying surface energy.The authors therefore recommend calibrating surface energy with bulk powder experiments while using literature values for less-sensitive parameters.
- Effective surface energy: The fitted 0.1mJ/m^2 surface energy is two orders of magnitude below typical 30–50mJ/m^2 measurements for flat metallic contacts.The authors associate the lower effective value with factors including surface roughness and chemical contamination or oxidation.
4. Conclusion
The study calibrates a cohesive DEM model for micron-scale Ti-6Al-4V powders and shows that surface energy strongly controls bulk powder behavior. Cohesion omission drastically underestimates AOR, while increased effective surface energy produces stronger agglomeration and higher AOR.
- The proposed DEM approach calibrates effective surface energy by fitting numerical and experimental AOR values for Ti-6Al-4V powder.The model includes frictional contact, rolling resistance, and adhesive forces, with cohesion represented through a surface-energy-based adhesion law.
- Particle-size and surface-energy variations produced equivalent quasi-static bulk behavior under the study’s tested scaling relationship.The simulations compared γ = γ0/4 and γ = 4γ0 with powders having mean diameters of 2d̄0 and d̄0/2.
- A fourfold surface energy increase caused adhesive forces to dominate gravity by almost two orders of magnitude, producing strongly increased AOR values and large cohesive agglomerates.
- Neglecting cohesive forces drastically underestimates AOR and yields an insufficient description of bulk behavior in applications such as AM powder recoating.The resulting AOR is more sensitive to surface energy than to particle stiffness, friction coefficient, or coefficient of restitution.
- Future work will verify fitted surface-energy values with AFM particle-level experiments and apply the model to metal-AM powder recoating.