(743f) Investigating the Influence of Processing Temperature on Powder Flowability

Authors: 
Freeman, T. - Presenter, Freeman Technology
Clayton, J., Freeman Technology
Monington, L., Freeman Technology
Numerous unit operations operate at elevated temperatures and/or require powders to be heated in preparation for processing, for example drying operations are common across numerous industries . However, predicting powder behaviour at elevated temperatures is challenging due to the complex network of factors that influence powder flow properties. Powders are bulk assemblies which consist of intricately variable particles with varying quantities of entrained gases and liquids, all with different heat capacities. These different constituent phases, as well as their interactions with each other, may be highly sensitive to variations in ambient temperature and to different extents. It is therefore evident that there is a significant demand to understand the rheological properties of powders at temperatures which are more representative of the environmental conditions within a process.

A strong understanding of the thermal deformation temperatures of powders is required to support the optimisation of in-process operating temperature. At elevated temperatures, powders may undergo thermal deformation, where irreversible changes to properties such as particle shape and surface texture may occur, in addition to particles beginning to melt and fuse together. Particle deformation can have a significant impact on the rheological properties of the powder bulk. For example, in Additive Manufacturing (AM) applications, many powders are heated prior to deposition in order to lower the temperature gradient for sintering. Changes in the flowability of powders as a result of being exposed to high temperatures can therefore have a significant impact on the performance of recycled powder and the quality of final products.

In addition to the impact of temperature on the structure of solid particles, it is also important to consider that processing at elevated temperatures can also affect liquid within the powder bed. Heating powders, for example during drying applications, can begin to evaporate liquid bridges and significantly reduce the level of capillary bonding.

The temperature can also impact on the interaction between the solid particles and gases within the powder bed, for example, gas adsorption (on to solid surfaces) is reduced at high temperatures, changing the chemical properties of the particle surfaces. Furthermore, elevated temperatures can also activate chemical reactions between surface groups, or between surface groups and entrained gases and liquids, again changing the surface chemical properties and ultimately the powder flow behaviour.

This presentation will highlight findings from a study investigating the influence of temperature on the dynamic flow properties of powders. In this study, a selection of powders was equilibrated at a range of temperatures and their rheological properties evaluated using an FT4 Powder Rheometer® (Freeman Technology Ltd, UK). Comparison with behaviour at ambient temperature was undertaken to assess the impact of exposure to elevated temperatures and to demonstrate the importance of characterising powders under conditions that are relevant to the process. As temperature-induced changes also tend to be non-reversible, the hysteresis behaviour of a range of powders were also investigated by cycling the temperature regime, with the results clearly demonstrating the importance of fully understanding the thermal history of the powder been processed.