Flowability of Hard Metal Granules

The mass flow rate of granules through an orifice and a flat-bottomed hopper can be
estimated with the Beverloo law, considering the properties of the granules, such as the bulk density and granule diameter. The Beverloo Law contains two fitting parameters C and k, which must be determined anew for any granular materials. Understanding the influences on C and k is the key to optimising granules’ properties and reaching an ideal granules’ flow through an orifice. This study aims to understand the influence of C and k on the flow of hard metal granules. This influence is studied by performing mass flow rate, angle of repose (AOR), and rheological measurements for various materials with changing powder properties. A new technique for AOR measurements has been developed to minimise the influence of avalanching and heap skewness by advanced image processing. The outcome shows that the AOR is no longer sensitive to decentring of the base plate. The findings show that the AOR
increases with a decrease in the mean diameter. This increase is attributed to increasing attractive forces, expressed by the granular Bond number, measured with an atomic force microscope (AFM). The attractive forces also influence the mass flow rate through an orifice. Reducing the mean diameter increases the mass flow rate, reaching a maximum. Beyond the maximum, the mass flow rate decreases by the increasing dominance of the attractive forces. Studying the influence on C and k shows that the fitting parameter k is affected by the attractive forces between granules, while the fitting parameter C is linked to the friction coefficient calculated from the AOR. These relationships are considered to propose a modified empirical model to estimate the mass flow rate through orifices for hard metal granules.
Rheology measurements showed the applicability of 3D-printed rotor geometries. Plate cup measurements revealed the existence of a linear-viscoelastic regime for hard metal granules in amplitude sweep tests.