Experimental investigation on the vertical downward screw feeding with a restricted outlet

The vertical screw feeding process with a restricted outlet is analysed in this work, where the particles are fed downward onto a substrate underneath the funnel, as usually found in screw-based additive friction stir deposition (AFSD). The influence of the gap size between the funnel and the substrate, the rotational speed of the screw, and the temperature of the substrate on the screw feeding process is explored through experiment. Three kinds of particle spreading patterns are observed outside the funnel in the experiment, i.e. no-spreading, naturally-spreading, and extruded-spreading, which significantly depend on the gap size, as well as the particle flowability and stiffness. A mathematical model is developed in this work to predict how quickly the particles are likely to spread out of the funnel. The results also show that the vertical force exerted on the substrate due to screw feeding depends not only on the screw's rotational speed but also on the gap size. It is interesting that the high temperature of the substrate would prevent the particles from spreading due to the degradation of the bulk flowability. Meanwhile, the mechanical force of the substrate at high temperature is significantly affected by two competing temperature-induced effects: the strong agglomeration of particles and the softening of particles. These results indicate that screw feeding of aluminium or aluminium alloy particles in AFSD requires more careful process control and higher input energy to prevent or mitigate jamming, due to their higher thermal conductivity and stiffness compared to non-metallic materials.