(380ac) Identifying the Impact of Processing on the Physical Properties of AM Powders and Their Subsequent Reuse

Whilst the term 3D printing is used as a synonym for all additive manufacturing process, additive manufacturing specifically covers a multitude of processes which vary the method of layer manufacturing. Powder Bed Fusion requires layers of powder to be built up, which are then selectively fused together, using either a laser or electron beam, depending on the process and substrate been utilised

Typically, the stock powder is contained in a hopper or reservoir and a blade or roller is used to spread a discrete amount of powder over the build platform, delivering a 0.1mm / 1-2 particles layer of powder which is then fused to the previous layer. The process is then repeated until the entire model is completed. Once complete any loose, unfused powder is removed. Whilst this process is suitable for a wide range of powders, it is typically used with common metals, for example, Stainless Steel, Titanium, Aluminium and thermoplastic polymers including nylon and polypropylene.

The volume of the actual component is typically considerably less than the volume of powder deposited during the build process, resulting in a large volume of loose powder at the end of the printing process. In order to make the process economically viable this material is usually recovered and reused. The recycled material is mixed with virgin material in order to achieve acceptable process performance and final product, with the required ratio of virgin to recycled material varying across different instruments.

The effect of continued reuse can affect both the chemical and physical properties of the powder, for example, powder exiting an AM machine may contain splatter from the melt pool in the form of larger particles, or the powder surface may be contaminated. Importantly it’s not just material close to fabrication that’s affected by going through process, with the process itself influencing the powder properties. How the physical and chemical changes impact the powder-bed fusion process is less well understood although previous studies have indicated changes in the powder bulk behaviour – including the flowability which may explain why the recycled material exhibits unacceptable process behaviour.

In order to better understand both the changes to the powder physical properties and their subsequently influence on the process performance, samples of virgin and recycled materials alongside mixtures with known process performance where characterised using a wide range of techniques including particle size analysis and surface area measurements. The powder bulk properties including flowability where also characterised using powder rheology. By comparing the particle and powder properties as well as their relative performance it was possible to identify both the key changes to the powder physical properties resulting from the initial processing of the virgin material and the resultant changes in the bulk properties, thereby providing an understanding of why the recycled powders exhibit poor process performance.