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

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 17µm), 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 particleto-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. While the present study focuses on a rigid blade recoating mechanism, the proposed simulation framework can be applied to and the general results can be transferred to systems based on alternative recoating tools such as soft blades, rakes or rotating rollers.

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

“…A comparison with the (theoretical) case γ = 0 revealed that a neglect of cohesive forces in the powder model leads to a drastic underestimation of the AOR and, consequently, to an insufficient description of the bulk powder behavior when employing the model in potential applications such as AM powder recoating simulations [53]. Eventually, the present study demonstrated that the resulting angle of repose is more sensitive concerning the magnitude of surface energy as compared to the magnitude of other powder material parameters such as particle stiffness, friction coefficient or coefficient of restitution.…”

“…Research work going on in parallel focuses on the verification of the fitted surface energy values on particle level based on atomic force microscope (AFM) experiments, and on the application of the proposed powder model for analyzing the powder recoating process in metal additive manufacturing processes [53].…”

“…These findings are expected to have considerable implications in applications based on fine metal powders e.g. concerning the powder layer uniformity in metal AM processes such as selective laser melting, electron beam melting or binder jetting [53]. Based on the proposed calibration procedure, the powder model can easily be tailored to alternative powder materials.…”

Characterization of IN 625 recycled metal powder used for selective laser melting

“…Depending on the processing conditions, different other powder morphologies can be obtained (like irregular, satellite, "splat cap", elongated, broken, agglomerated etc.) [1][2][3][4][5][6]. Smooth spherical powders are the most desired for AM processes because they ensure a better flowability compared to other morphologies, a higher flow rate is achieved by a reduced inter-particular friction and low risk of mechanical interlocking [6].…”

Collaborative Optimization on Density and Surface Roughness of 316L Stainless Steel in Selective Laser Melting

“…However, there are more than 130 parameters [3] of SLM which may have significant impacts on the forming properties (such as density and surface roughness) of the parts, including the diameter of the laser beam, laser power, scanning speed, hatch spacing, scanning strategy, layer thickness and so on [4][5][6][7]. Mumtaz [8], Song [9] and Dadbakhsh [10] investigated the effects of processing parameters including laser power, scanning speed on surface roughness, and revealed that higher laser power tended to reduce top surface roughness.…”