1 of 3

Freeman Technology Ltd
Freeman Fig 1
Powders are a bulk assembly of solids, liquids and gases which results in diverse and complex behaviour
2 of 3

Freeman Fig 2a
Comparison between shear and dynamic powder testing of two powders shows how dynamic testing can differentiate samples that shear testing classifies as identical
3 of 3

Freeman Technology Ltd
Freeman fig 2b
Comparison between shear and dynamic powder testing of two powders shows how dynamic testing can differentiate samples that shear testing classifies as identical
Recent years have seen significant progress in developing high-performance ‘3D printing’ platforms that can be offered at price points accessible to industrial users. Success is evidenced by the fact that additive manufacturing (AM) is now widely used to produce components with ‘form and fit’. The next goal is to regularly employ AM in the production of components with a precisely defined functionality. Achieving this requires greater understanding and control of the powders used in AM applications.
AM places exacting demands on powders and those at the forefront of this technology are striving to understand the impact of such demands and how to engineer powders to meet them. Reliable characterisation of powders is a critical step in advancing this understanding and valuable lessons can be learnt from other industries. This article introduces two fundamental principles of powder characterisation.
Don’t rely on a ‘single number’ approach
The idea that one number alone, such as particle size, density or angle of repose, can provide ‘pass/fail’ criteria for a powder destined for a specific application is appealing, but ultimately unrealistic. Powder behaviour is a function of complex interactions between the phases within the system, i.e. solids (particles), gas (typically entrained air) and liquid (often simply adsorbed moisture.)
The properties of each component and the interactions between them contribute to the diverse behaviour exhibited by powders. This diversity gives powders their distinct industrial value, as they can be made to behave in a variety of ways, but it also brings challenges and means that no single parametercan fully describe powder behaviour.
For example, consider selective laser sintering where success requires the following:
- Consistent, controlled dispensing to initially fill the delivery piston
- Uniform distribution by the roller, without agglomerate formation
- The release of entrained air to create a homogeneous layer of powder ready for sintering
Each stage of the process subjects the powder to different stress and flow regimes so different properties will therefore influence performance at each point. How the powder responds to gravitational and forced flow will be critical to dispensing and rolling operations and so properties such as inter-particular cohesion and mechanical interlocking will dictate performance here. In contrast the permeability of the powder will be most important in determining how entrained readily air is released.
Identifying properties that are conducive to good performance at each step is fundamental to process optimisation. The range of conditions that a powder is subjected to in any application means that a multi-faceted approach to powder characterisation must be employed.
Generate relevant data
Employing multiple test techniques is important but the data generated must also be relevant to the application. This allows powder properties to be correlated with in-process behaviour and used to predict the performance of other materials or identify optimum machine settings, such as laser intensity, for a given powder. To provide relevant data, the test technique must subject the test sample to conditions that simulate what it will be exposed to in process. In addition, the device performing the test must be sensitive enough to identify and quantify minor differences that will influence how a powder behaves
Figures 2a and 2b contrast shear cell data and dynamic powder measurements for two powders. The shear cell data (Figure 2a) classifies the powders as identical but dynamic measurements indicate a significant difference. The results suggest that the two samples may behave identically in the regime emulated by shear cell testing, i.e. when starting to flow from a static, consolidated state, and therefore may exhibit similar performance when discharged from a hopper, for example.
However, under low stress dynamic conditions, which are more representative of many AM processes, the two powders are likely to behave very differently, which may result in significant variations in efficiency and product quality. Relevant and sensitive test methods allow minor differences to be identified and quantified even in powders that are physically and chemically very similar. As an example, comparisons between fresh and recovered powder would benefit from this approach, allowing users to assess the feasibility of reusing raw materials.
Dynamic powder testing has a proven track record as a tool for process optimisation in many industries. It involves characterising a powder in motion, rather than in a static state, and provides a direct measure of powder flowability. Uniquely, dynamic properties can be measured for powder in consolidated, conditioned, aerated, and even fluidised states to directly quantify how flow is influenced by air content and stress conditions. Experience suggests that a combination of dynamic, shear and bulk property measurements delivers the necessary information to understand powder behaviour in the vast majority of processing applications, pointing the way to a valuable powder testing strategy for additive manufacturers.
Further information:
‘Powder rheology of steel powders for additive manufacturing’, by O. Lyckfeldt, Swerea IVF AB, Sweden, available at http://bit.ly/FTPRSPM (Accessed 23 Dec 2013)
‘The Characterisation of Powder and Bulk Material – a Multivariate approach using Dynamic, Shear and Bulk Property Measurements’ , by T.C. Freeman, R.E. Freeman & B. Armstrong: http://bit.ly/FTCPBM (Accessed 23 Dec 2013)