Modeling and characterization of cohesion in fine metal powders with a focus on additive manufacturing process simulations

Abstract

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 micronscale 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 − 50mJ/m 2 , indicating the crucial influence of factors such as surface roughness and potential chemical surface contamination / oxidation 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.

Critical influences of particle size and adhesion on the powder layer uniformity in metal additive manufacturing

“…According to the SEM images in Figure 1, individual powder particles can be described as spherical particles in good approximation. The cohesive powder model proposed by Meier et al (2018b) relies on the discrete element method (DEM), going back to Cundall and Strack (1979), more precisely on the so-called "soft sphere" model, in order to model the bulk powder on the level of individual powder particles in a Lagrangian manner. In general, the equations of motion of an individual particle are given by the balance of linear and angular momentum:…”

“…In this context, r i j CG := r i j C − r i G represents the vector from the centroid of particle i to the point of contact with particle j. For the specific force laws underlying these interactions and the choice of model parameters, the interested reader is referred to Meier et al (2018b). Only the modeling parameters most relevant for the present study, will briefly be repeated in the following.…”

“…Since surface energies of a given material combination can easily vary by orders of magnitude as consequence of surface roughness and potential surface contamination / oxidation, it is essential to calibrate the employed model by experimentally characterizing the considered powders in terms of effective surface energy values. In the authors' recent contribution, Meier et al (2018b) determined for the first time an effective surface energy value for the considered class of metal powders, which will be employed in the present study. Moreover, also the importance of consistently considering adhesive particle interactions has been demonstrated by Meier et al (2018b) by comparing the bulk powder behavior predicted by models with and without adhesive force contributions.…”

“…In the authors' recent contribution, Meier et al (2018b) determined for the first time an effective surface energy value for the considered class of metal powders, which will be employed in the present study. Moreover, also the importance of consistently considering adhesive particle interactions has been demonstrated by Meier et al (2018b) by comparing the bulk powder behavior predicted by models with and without adhesive force contributions. This important result motivates the present work, representing the first numerical study of powder recoating processes in metal additive manufacturing based on a cohesive powder model.…”

“…The remainder of this article is structured as follows: Section 2 briefly recapitulates the essentials of the cohesive powder model proposed by Meier et al (2018b) and the choice of model parameters. In Section 3, this model is employed in order to analyze the influence of powder cohesiveness and other powder material and process parameters on the resulting powder layer quality.…”