Production of Non-Oxide Ceramic Granulated Feedstock for Powder Bed Additive Manufacturing
Case study
Powder bed fusion (PBF) is a type of additive manufacturing used in industry due to low material waste and cost. Layers of powdered material are melted or sintered using a laser or electron beam to create 3D-printed parts. Non-oxide ceramics are particularly challenging to sinter, requiring fine reactive powders to promote effective sintering. Unfortunately, fine powders (< 5 μm) have poor flowability and so cannot conventionally be utilised in powder bed additive processes.
The challenge
Chemical, mineral, and pharmaceutical industries require powder homogeneity in products to reduce separation and improve flowability of different components. Particle size, shape, and the density of particles are often the main parameters which are manipulated during processing to improve the quality of powder blends. Milling, spray drying, and granulation are commonly used processing methods used to achieve particles with an optimum size and shape.
In ceramics, factors such as size, shape, and density of particles play a role in segregation of powder blends. Lack of homogeneity can impact the characteristics of produced components such as reduced quality and concentrated sections of stress/lower strength which increases the likelihood of failure.
What we delivered
Prior to processing, the raw materials chosen (non-oxide powder 1 and 2), had drastically different particle sizes and shape, as displayed in the graph below. Although both powders had a similar median particle size (2.92μm in comparison to 3.23μm), the 90th percentile of the particle distribution (D 90) in powder 2 was 15.2x larger than powder 1- this contributed to a large degree of separation when mixed. To avoid this, the materials were granulated to produce a homogenous blend of a desired particle size, which would not separate and would display improved flowability. The particle size of produced granules, made with both non-oxide powders 1 and 2, is also shown in the graph below.
Granulation is a mechanical process used to increase the physical size, shape, and quality of powders. Liquid granulation uses binder that is dispersed in a liquid medium to act as an adhesive to agglomerate and form large irregular particle aggregates which are reformed into spherical granules using shearing and compaction forces.
An Eirich mixer, which is a high-speed shear mixer, was used to combine binder and powder at speeds of 1500-2400 RPM. The Eirich mixer has an internal temperature monitor and has a maximum operating speed of 3000 RPM. Lucideon achieved granules sized from 20-500μm. The optimum size for PBF was found to be between 50-150μm so the granules were separated by size using an Endecotts Octagon 200 sieve shaker. Overall, a yield of 22.4% of granulated material was produced within the desired size fraction, which can be further reprocessed to obtain higher yields.
The produced granules were pyrolyzed, to remove organic binder under an argon atmosphere. Pyrolyzed granules were analysed for their shape using a Sympatec QICPIC Dynamic Image Analyser (DIA) and size on a Malvern HYDRO EV.
The graph below represents the aspect ratio of granules in comparison to raw material. Aspect ratio is a shape descriptor that refers to the ratio of a particle width and height. It is a measure of elongation of particles where a perfect sphere would have an aspect ratio of 1. Therefore, an increased aspect ratio is beneficial. The average, or D 50, of manufactured ceramic granules had an increased aspect ratio of 0.80, in comparison to the raw materials of 0.68 and 0.71. Sphericity is beneficial for granules used in AM due to increased flowability, which was studied using a Hall of Flow and Carney Flow tests. Good flowability of powders creates a homogenous powder bed for PBF which leads to consistent layer quality and good packing density. Both raw ceramic powders failed to flow entirely while the granulated material presented moderate flow through the Hall test- yielding a flow rate of 64 s/50g, which is a significant improvement.
Granules were also characterised using a Keyence VHX-7000 Digital microscope. Observations into the granules and microstructure confirmed the spherical nature of the granules as shown below.
Additionally, the homogenous nature of the granules is also seen due to the lack of visible separation of the raw materials. This is another overall improvement to the initial components that suffered from drastically different particle size, particle size distribution and particle shape. Such improvements are necessary to facilitate PBF and create better quality prints.
Value to the client
Two non-oxide powders were granulated to create a blend with an increased particle size and sphericity. This is beneficial to the non-oxide blend for reduced separation and enhanced flowability for powder bed fusion, which led to a successful print achieved using granulated feedstock. This method can be used for various materials and industries where granulation of advanced ceramics is deemed necessary.
*This work was supported and partly funded by UK Research and Innovation through the Strength in places Fund Programme. Midlands Advanced Ceramics for Industry 4.0.