Exploration of improved, roller-based spreading strategies for cohesive powders in additive manufacturing via coupled DEM-FEM simulations

TL;DR

This work demonstrates that roller-based spreading of highly cohesive fine powders in additive manufacturing can achieve high-density, uniform powder layers when roller kinematics and surface friction are carefully tuned. By combining DEM with FEM (DEM-FEM) and calibrating a Ti-6Al-4V 0-20µm powder via self-similarity, the authors identify a critical surface velocity range where counter-rotation and rotational oscillation yield packing fractions above $50\%$ for layers near $2\times D_{90}$. A robust process window emerges, showing resilience to substrate adhesion variations, and rotational oscillation—especially with rubber-coated rollers—offers practical advantages over strict counter-rotation by mitigating runout issues. The findings indicate thicker layers further improve packing and suggest experimental validation of oscillatory modes for robust, high-throughput powder recoating in LPBF and BJ.

Abstract

Spreading of fine (D50 <=20um) powders into thin layers typically requires a mechanism such as a roller to overcome the cohesive forces between particles. Roller-based spreading requires careful optimization and can result in low density and/or inconsistent layers depending on the characteristics of the powder feedstock. Here, we explore improved, roller-based spreading strategies for highly cohesive powders using an integrated discrete element-finite element (DEM-FEM) framework. Powder characteristics are emulated using a self-similarity approach based on experimental calibration for a Ti-6Al-4V 0-20um powder. We find that optimal roller-based spreading relies on a combination of surface friction of the roller and roller kinematics that impart sufficient kinetic energy to break cohesive bonds between powder particles. However, excess rotation can impart excessive kinetic energy, causing ejection of particles and a non-uniform layer. Interestingly, the identified optimal surface velocities for counter-rotation as well as rotational oscillation are very similar, suggesting this quantity as the critical kinematic parameter. When these conditions are chosen appropriately, layers with packing fractions beyond 50% are predicted for layer thicknesses as small as ~2 times D90 of the exemplary powder, and the layer quality is robust with respect to substrate adhesion over a 10-fold range. The latter is an important consideration given the spatially varying substrate conditions in AM due to the combination of fused/bound and bare powder regions. As compared to counter-rotation, the proposed rotational oscillation is particularly attractive because it can overcome practical issues with mechanical runout of roller mechanisms. In particular, the application to rubber-coated rollers, which promises to reduce the risk of tool damage and particle streaking, is recommended for future investigation.

Figures (15)

• Figure 1: Simulation setup with roller, fully extended powder reservoir piston with 42,000 particles and powder reservoir
• Figure 2: Top view of Ti-6Al-4V 0-20 $\mu m$ powder spread with a blade a) experimentally and b) computationally
• Figure 3: Simulated relationships between mean packing fraction of powder layer and normalized cohesion, for a traverse velocity of $50 \frac{mm}{s}$: (a) spreading with a 90$^{\circ}$ blade (based on data from PENNY2022Blade); (b) spreading with a roller, without rotation (based on data from PENNY2022Blade), and with counter-rotation at 500 rpm; Vertical lines indicate representative cohesion levels for typical LPBF and BJ powders with noted size distributions.
• Figure 4: Simulated relationships between mean packing fraction and: (a) traverse velocity $v$ for different rotational velocities $\omega$ and roller-powder friction coefficient $\mu = 0.8$; (b) rotational velocity $\omega$ for different friction coefficients $\mu$ and traverse velocity $v=25\frac{mm}{s}$; (c) rotational velocity $\omega$ for different traverse velocities $v$ and roller-powder friction coefficient $\mu = 0.8$
• Figure 5: Visualization of counter-rotational spreading with $\mu = 0.4$, both in the side-view during spreading and top-view of the powder layer after spreading. Gap under the roller highlighted on red background. (A) v=$50 \frac{mm}{s}$, $\omega=0$ rpm; (B) v=$25 \frac{mm}{s}$, $\omega=-500$ rpm; (C) v=$5 \frac{mm}{s}$, $\omega=-1000$ rpm