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Additive manufacturing (AM), free form fabrication, direct digital manufacturing, or 3D printing is rapidly developing as a disruptive technology for superior-quality component manufacturing. The technology allows the agile manufacturing of multipart components, whilst significantly decreasing the material waste seen with computer numerical control (CNC) techniques.
Manufacturers are already making investments in AM as a methodology of the future. The US Navy sees AM as having high potential as it will enable the manufacture of essential high-quality aircraft components at-sea or on-site with little time lag allowing their aircraft to be serviced precisely and quickly.
Additive Manufacturing with Metallic Powders
The American Society for Testing and Materials (ASTM) defines additive manufacturing (AM) as‘a process of joining materials in order to make objects from 3D model data, typically layer upon layer, as opposed to subtractive manufacturing methodologies’. The definition is mostly appropriate to varied types of material including ceramics, polymers, metals, biological systems, and composites.
Even though AM has been available for two decades, it is only recently that it has started to develop as a key commercial manufacturing technology. AM empowers distributed manufacturing and the manufacturing of parts-on-demand, while providing the potential to decrease cost, any carbon footprint, and energy consumption.
Additive manufacturing is capable of revolutionizing the global parts manufacturing and logistics landscape. However, to manufacture at superior quality, the process has extremely specialized demands for powder traceability and build management. To completely exploit AM materials, particularly metal powders, they must be of a consistently superior quality and Micromeritics has the instrumentation and standards to offer this assurance.
The industry has not yet completely developed acceptable standards for metallic powders employed in AM, which means high standard analytical technology is a requirement for the provision of quality assurance on the metal powders used in manufacturing.
Important Particle Parameters for Additive Manufacturing
Metal particles have a number of vital parameters which not only impact the additive build processes but also affect the properties of the final component. This includes the chemical and physical properties of the raw material, which have to be known and characterized in order to optimize the build processes and the outcome.
Powder properties include the particle size distribution and related powder density (tap and bulk density) including flowability, which can directly affect the potential to produce layers during the ‘printing’ process. In addition, these parameters are also capable of affecting the thermal and optical properties of the particles. Powder layer properties largely depend on the powder density and flowability.
In the present development of AM, especially where extremely thin powder layers are needed with exceptional layering properties, there is a tendency to use fine powders to enhance the scan speed and eventually the microstructure, surface qualities and component density. However, this does increase the risk of processing metallic powders with insufficient flowability and can result in a bad layer quality. Inter-particle forces such as Van der Waals forces and moisture can impact the behavior of powders, which tend to agglomerate the finer they are, leading to poor powder flowability.
Characterizing AM Particle Size and Distribution
Generally, it is vital to characterize powders in a manner that is as close to the manufacturing process as possible. In that respect, a good flowability of a powder is needed but in addition, the particle sizes and their distribution will have to be optimal for AM.
Particle size distribution has a direct effect on flowability and the potential to provide a uniform powder bed density. Laser diffraction is the accepted analytical technique for determining particle size and distribution, and it uses the angle and intensity of scattered light (refracted and diffracted light) in order to provide data on particle size and distribution.
The Micromeritics Saturn DigiSizer II uses digital technology including a high-resolution charge coupled device (CCD) detector for detecting scattered laser light and providing a high resolution, repeatable scattering particle sizing technique.
Characterizing AM Particle Shape
In addition, the shape of powder particles can considerably influence both flow properties and bulk packing of the metal powder feedstock being employed in AM. Spherical particles are more likely to arrange and then pack together more efficiently than equivalent non-symmetrical particles. The spherical shape also eases the flowability of metal powders and guarantees the consistency of powder layers in the AM powder bed system.
Particle shape is also a strong influence toward powder bed packing density and eventually the apparent density of the final components. Irregular shaped particles are capable of lowering the final component density and leading to an increase in porosity and possible mechanical failure during the service cycle of the part. The Micromeritics/Particulate Systems Particle Insight is a dynamic image analyzer, capable of capturing and characterizing projected images of particles in motion to offer critical shape information and also additional data about particle size.
As images are captured, statistical data for all measurement parameters are calculated and then recorded, and up to 28 different shape parameters can be analyzed and reported in real-time.
Developing AM Powder Standards
Defining the needed properties of the metal feedstock powder employed in AM processes is vital for industry’s confidence in powder selection and the potential to produce consistent components with predictable and known properties. The ASTM have worked tirelessly in creating these standards and have produced a guide for characterization (ASTM F3049 - 14 Standard Guide for Characterizing Properties of Metal Powders Used for Additive Manufacturing Processes).
This interim forma details the standards developed thus far for particle shape and size and the analytical technology used. This guide is a starting point for the development of a set of precise standard test methods that will concentrate on each individual property vital for the performance of metal-based AM systems.
Processing defects, for instance micro-porosity or issues with surface finish, are understood to govern the fatigue properties of AM produced alloys. Certification and qualification have been continually identified as a challenge to the general adoption of AM structurally critical parts as the present procedure is too prolonged and too costly.
Presently, AM is perfect in small production lots in which the greater cost of AM raw materials is balanced by the reduction in fixed costs associated with standard manufacturing. There is a value on the flexibility and speed of AM as it is perfect for just-in-time manufacturing and the US military are already extremely interested. However, the economic viability of huge production AM will eventually depend profoundly on the reduction of reoccurring costs, that is, the quality and cost of the starting materials employed in AM fabrication.
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2. Spierings, A.B., Voegtlin, M., Bauer, T. et al., Powder flowability characterisation methodology for powder-bed-based metal additive manufacturing, Prog Addit Manuf (2016) 1: 9. https://doi.org/10.1007/s40964-015-0001-4
3. Mellin, P., Lyckfeldt, O., Harlin, P., Brodin, H., Blom, H., Strondl, A. Evaluating flowability of additive manufacturing powders, using the Gustavsson flow meter, Metal Powder Report, Volume 72, Issue 5, September 2017
4. E. Dan Hirleman, V. Oechsle, N. A. Chigier, "Response Characteristics of Laser Diffraction Particle Size Analyzers: Optical Sample Volume Extent And Lens Effects," Optical Engineering 23(5), 235610 (1 October 1984)
This information has been sourced, reviewed and adapted from materials provided by Micromeritics Instrument Corporation.
For more information on this source, please visit Micromeritics Instrument Corporation.