Abstract
Additive manufacturing (AM) is going through an exponential growth, due to its enormous potential for rapid manufacturing of complex shapes. One of the manufacturing methods is based on powder processing, but its major bottleneck is associated with powder spreading, as mechanical arching adversely affects both product quality and speed of production. Here we analyse transient jamming of gas-atomised metal powders during spreading. These particles are highly frictional, as they have asperities and multiple spheres and are prone to jamming in narrow gaps. Therefore their detailed characterisations of mechanical properties are critical to be able to reliably predict the jamming frequency as influenced by powder properties and process conditions. Special methods have been used to determine the physical and mechanical properties of gas-atomised stainless steel powders. These properties are then used in numerical simulations of powder spreading by the Discrete Element Method. Particle shape is reconstructed for the simulations as a function of particle size. The characteristic size D90 by number (i.e. the particle size, based on the projected-area diameter, for which 90% of particles by number are smaller than this value) is used as the particle dimension accountable for jamming. Jamming is manifested by empty patches over the work surface. Its frequency and period have been characterised as a function of the spreader gap height, expressed as multiple of D90. The probability of formation of empty patches and their mean length, the latter indicating jamming duration, increase sharply with the decrease of the gap height. The collapse of the mechanical arches leads to particle bursts after the blade. The frequency of jamming for a given survival time decreases exponentially as the survival time increases.
Highlights
Additive manufacturing (AM) has been widely explored in a wide range of applications, such as aerospace, biology, medicine and architecture [1,2,3,4,5]
As the powder is spread by a blade or roller with a small gap height, mechanical arching and transient jamming are more prevalent [14]
We report on our analysis of the effect of gap height on particle dynamics and transient jamming based on numerical simulations using Discrete Element Method (DEM)
Summary
Additive manufacturing (AM) has been widely explored in a wide range of applications, such as aerospace, biology, medicine and architecture [1,2,3,4,5]. There are several manufacturing methods, ranging from 3D printing from a liquid-based feed [9,10] to powder-based process [11,12], in which typically 10–50 μm particles are spread over a particle bed (already partially sintered/melted) by a roller or blade, resulting in a particle layer with thickness of a few particle diameters. The particle properties and simulation conditions in these works did not fully represent those of real particles in AM, as the particle shape with high fidelity or mechanical properties of particles (including Young's Modulus, interfacial surface energy, coefficient of sliding friction and so on) were not always used in the simulations They mainly focussed on the volume fraction and roughness as well as micro-structure of the final particle layer, and did not address particle dynamics in the region near the blade, which gives rise to jamming. Each particle is approximated by a number of overlapping spheres of different sizes, following the multi-sphere approximation approach of Pasha etal. [28]
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