Critical Influences of Particle Size and Adhesion on the Powder Layer Uniformity in Metal Additive Manufacturing (1804.06822v2)

Abstract: The quality of powder layers, specifically their packing density and surface uniformity, is a critical factor influencing the quality of components produced by powder bed metal additive manufacturing (AM) processes, including selective laser melting, electron beam melting and binder jetting. The present work employs a computational model to study the critical influence of powder cohesiveness on the powder recoating process in AM. The model is based on the discrete element method (DEM) with particle-to-particle and particle-to-wall interactions involving frictional contact, rolling resistance and cohesive forces. Quantitative metrics, namely the spatial mean values and standard deviations of the packing fraction and surface profile field, are defined in order to evaluate powder layer quality. Based on these metrics, the size-dependent behavior of exemplary plasma-atomized Ti-6Al-4V powders during the recoating process is studied. It is found that decreased particle size / increased cohesiveness leads to considerably decreased powder layer quality in terms of low, strongly varying packing fractions and highly non-uniform surface profiles. For relatively fine-grained powders (mean particle diameter $17 \mu m$), it is shown that cohesive forces dominate gravity forces by two orders of magnitude leading to low quality powder layers not suitable for subsequent laser melting without additional layer / surface finishing steps. Besides particle-to-particle adhesion, this contribution quantifies the influence of mechanical bulk powder material parameters, nominal layer thickness, blade velocity as well as particle-to-wall adhesion. Finally, the implications of the resulting powder layer characteristics on the subsequent melting process are discussed and practical recommendations are given for the choice of powder recoating process parameters.

• The paper demonstrates that cohesive forces in fine powders (≈17 µm) significantly reduce packing density and layer uniformity.
• The paper finds that using a layer thickness two to three times the maximum particle diameter optimizes packing quality.
• The paper reveals that adjusting blade velocity and reducing particle-to-blade adhesion improves surface uniformity.

Critical Influences of Particle Size and Adhesion on the Powder Layer Uniformity in Metal Additive Manufacturing

The paper investigates the effects of particle cohesiveness on powder layer uniformity in metal additive manufacturing (AM) processes through discrete element method (DEM) simulations. The focus is on plasma-atomized Ti-6Al-4V powders subjected to various process parameters influencing the recoating process. Importantly, this work highlights how powder particle size and adhesion forces significantly impact the packing density and surface uniformity of powder layers, both crucial for the quality of the end product in AM.

Key Findings

1. Cohesiveness and Particle Size:
   • Cohesive forces become predominant in fine-grained powders, with a mean diameter of 17 µm experiencing cohesive forces that exceed gravity by two orders of magnitude. This condition results in low packing fractions and high surface non-uniformity.
   • The paper suggests a direct correlation between particle size and cohesive behavior, influencing the uniformity and density of the powder layer.
2. Layer Thickness:
   • As the layer thickness increases, so does the uniformity and packing density, particularly for less cohesive powders. The ideal nominal layer thickness should be two to three times the maximal particle diameter for optimal results.
3. Blade Velocity:
   • Higher velocities can detrimentally affect layer uniformity due to dynamic post-flow effects reducing mean layer thickness. However, decreasing powder flowability, for instance, through higher inter-particle friction, can help counteract these negative effects.
4. Adhesion Dynamics between Components:
   • Reducing particle-to-blade adhesion improves surface uniformity. With high cohesive powders, powder-to-substrate adhesion significantly impacts the resulting layer quality.
5. Recommendations for Manufacturing:
   • For cohesive powders, strategies like customizing blade materials to reduce adhesion or increasing substrate friction could enhance spreadability and layer quality.

Implications

The computational findings underline the intricate dependency of powder layer quality on physical interactions at microscopic scales, directly impacting macro-level AM outcomes. Fine-tuning parameters such as particle size distribution, layer thickness, and blade velocity can lead to process optimizations, yielding superior products with fewer defects. Particularly, addressing the challenges posed by highly cohesive powders can lead to significant improvements in the consistency and reliability of metal AM processes.

Future Directions

For future developments in AM, further research could explore alternative recoating tools and analyze different materials to diversify its applications. Experimental validation of the simulation results could offer additional insights into refining AM technologies, ensuring that more robust, efficient processes can be devised. Additionally, extending the analysis to include multi-layer interactions and more complex geometries would enrich understanding and applicability across various AM setups.

In conclusion, this paper makes a substantive contribution to understanding and improving powder-based metal additive manufacturing, paving the way for more effective implementations by modifying powder and process parameters.