Additive manufacturing, also known as 3D printing, is a highly efficient, potentially transformative manufacturing technique. The method involves ‘printing’ intricate components to a tight specification by gradually building up powder layers which are then selectively fused together.
For process efficiency and the quality of the end product, it is critical to control the performance of the powders. The way the powder flows and packs as the layers are formed are defining aspects of this performance. Variability in feedstock can result in non-uniform layering, inconsistent bulk density, poor surface finish, and low tensile strength.
The extent to which Additive Manufacturing will shape the industrial landscape depends on the development of high-speed, precision machinery, and on the identification and consistent supply of powders able to meet the exacting demands of these machines. This focus is increasingly turning to the powders themselves and how they can be optimized in a reliable and intelligent manner.
Powder characterization plays a critical role in supporting this process, and testing techniques that can reliably measure properties that directly correlate with AM performance are also essential.
By identifying the powder properties that lead to uniform, repeatable performance of powder, new formulations can be optimized, without the significant time and financial implications associated with running samples through the process to assess suitability. It also helps to reduce the occurrence of end products that are out of specification.
There are several existing methods such as flow through a funnel, bulk density measurements, and angle of repose testing that are well-documented. However, these techniques were developed without the advantages of modern technology, and they can sometimes be too insensitive to characterize the subtle differences between powders that behave differently in process.
A universal powder tester, the FT4 Powder Rheometer® offers comprehensive, automated, and reliable measurements of bulk material characteristics. When this information is correlated with process experience, it can improve processing efficiency and aid quality control.
The FT4 specializes in the measurement of dynamic flow properties, incorporates a shear cell, and has the ability to measure bulk properties like compressibility, permeability, and density.
Please click here if you would like more information on the instrument used in this article or a quote
Quantifying Batch-to-Batch Variation in Feedstocks
The tight tolerances within which AM machines operate mean that differences between various batches of feedstocks can result in considerable variability in the quality and properties of the end product.
Any variation in performance can be avoided by screening each batch before it enters the process. However, the very subtle differences in properties that can lead to differences in performance are often undetected by conventional powder characterization techniques.
Three samples of stainless steel powder from a supplier demonstrated highly variable performance in an AM process. While Metal Powder A and Metal Powder B exhibited acceptable behavior, Metal Powder C regularly caused poor deposition and blockages, resulting in sub-standard final products.
Particle size distributions of all three samples were virtually identical and the samples demonstrated a similar response in hall flow and angle of repose tests.
However, evaluating the samples with the FT4 Powder Rheometer® highlighted a number of differences between the samples that correlated well with the process performance. The Specific Energy of the samples clearly differentiated Metal Powder C during dynamic testing, with the higher value indicating increased particle-particle friction and mechanical interlocking.
This increased resistance to flowing over itself is a common reason for blockages and other flow problems in low stress environments.
During bulk testing, an even more differentiating result was generated by the permeability test. Compared to the other samples, Metal Powder C generates a significantly higher Pressure Drop across the powder bed. This indicates that Metal Powder C is considerably less permeable than Powders A and B.
Permeability is very influential in any operation where powder is moved from one position to another, especially when gravity is the motivating force. The space vacated by the particle must be replaced by gas, and the more easily the powder can transmit this gas through the bulk, the more freely it is likely to pour, and also to release any air entrained during the pouring process.
When depositing or filling consistent densities of powder during AM applications, and when low permeability increases the amount of air retained in the bulk on deposition, it will cause poor uniformity in the layers. This can lead to imperfections in the final product that may require the product to be scrapped.
However, for the reuse to be possible...
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This information has been sourced, reviewed and adapted from materials provided by Freeman Technology.
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