Additive Layer Manufacturing of metal parts is, in my opinion, an incredible feat of engineering. Using high precision electron beams or lasers, moving at high speeds, to selectively melt layer-upon-layer of metal, 10’s of microns thick. This technique has the capability to manufacture parts with novel functionality using much less material compared with traditional techniques such as subtractive manufacturing. But it has its foibles, not least is the excess of unused material from Powder Bed Fusion processes.

Figure 1. The powder bed process for additive layer manufacturing

Commercial constraints

At around £200-300 per kilo, premium alloy powders are expensive and are estimated to contribute up to 1/3rd of the total build cost. Therefore, in order to realise the commercial potential of powder bed fusion processes the powder needs to be recycled as much as possible. However, there is the risk of impacting build quality with continuous powder re-use, leading to potential flaws in the manufactured part and risk of premature failure – a further unwanted expense but also a major safety risk depending on end-use application.

Although there is no standard approach for determining whether a powder is suitable for re-use there are number of analytical techniques that can be used to assess powder quality and that’s where Malvern Panalytical can help.

Packing and flow characteristics

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Figure 2. Powder flow to form a high packing density powder bed and the effect on melt pools. Image adapted from Y.S. Lee and W. Zhang, Mesoscopic simulation of heat transfer and fluid flow in laser powder bed additive manufacturing, 26th Solid Freeform Fabrication Symposium, Austin, Texas, 2015

In Powder Bed Fusion, successive layers of powder are spread across the build platform to form a powder bed which is then melted at specific locations to form a single tier of the 3D structure. It follows, that any inhomogeneities in the powder bed would lead to inhomogeneities in the final part. In practical terms, the packing density of the powder should be high enough to supply the required mass of metal to the layer being fused and avoid unwanted porosity. Furthermore, that metal powder needs to spread evenly and consistently across the build platform 1000’s of times to ensure a uniform component. These two properties, packing density and powder flow, are controlled by particle size and shape which can be measured using laser diffraction and automated imaging. You can learn more of this in our whitepaper.

Our latest application note, Characterizing the particle size and shape of metal powders for additive layer manufacturing, discusses how the Morphologi 4 Automated Imaging system can be used to investigate morphological differences of metal powders due to recycling and how we can classify and compare samples based on their shape, as shown below.

Figure 3. Particle classifications based on shape parameters. The classifications range from highly spherical to agglomerated, which are indicative of flowability and packing density.

This shows how particle characteristics change during recycling and how it is possible to use Automated Imaging to optimize the atomization process. Wall Colmonoy, for example, have been using Morphologi 4 to benchmark and improve their current processes and products.

See the instrument in action:

Structural integrity and elemental composition

So, by controlling particle morphology we can help ensure that the correct mass of material can get from the supply hopper to the build plate and that we form a dense powder bed with minimum voidage. But we also need to ensure that the metal or alloys we use have the correct elemental composition and that the built component has the appropriate microstructure with little or no residual stress. Malvern Panalytical’s X-ray fluorescence (XRF) and X-ray diffraction solutions (XRD) solutions are commonly used for this purpose and you can find out more from our webinar on using X-rays to see inside your powdered metal materials and processes.