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In order to properly control and optimize processing methods, these materials must be accurately characterized, often with particle characteristics (e.g., particle size, chemical composition, or morphology) or bulk powder behavior (e.g., flowability, mixture stability, density, or electrostatic properties) .
However, when it comes to analyzing the physical behavior of bulk powders, most of the techniques commonly used in quality control or R&D laboratories are based on long-standing and often outdated measurement techniques.
GranuTools has been updating these technologies over the past decade to meet the requirements of contemporary production departments and R&D laboratories.
The measurement process has become increasingly automated, and the development of rigorous initialization methods has made it easier than ever to obtain interpretable and reproducible results.Image analysis technology is also at the heart of improving measurement accuracy.
Various industries already use GranuTools instruments in many different fields.For example, food processing, additive manufacturing, pharmaceuticals and bulk material handling.
The use of powder-based additive manufacturing when processing metal powders is already common.This technique is of increasing interest for the manufacture of parts whose structures are often too complex to be achieved using standard machining.
A number of powder-based additive technologies have been developed; for example, dripping of binder, local fusion of particles in a powder bed, or continuous deposition of molten metal.
Regardless of the technology used, powder must be transported through the various process stages in the printer.As the powder flows through the printer, multiple contacts with the belt material and the particles themselves will cause an electrical charge to build up inside the powder.This is due to the triboelectric effect.
As charge density increases, electrostatic-related degradation of powder properties may occur, leading to processability issues.A thorough understanding of the different process stages that lead to charge build-up enables continuous improvement in the field of powder-based additive manufacturing.
In the studies outlined here, GranuCharge has been used to study the electrostatic charging of metal powders as they pass through various parts of a laser metal deposition (LMD) machine.
Powder samples are collected at different locations during the powder delivery stage to facilitate assessment of their effect on charge build-up.The study also aimed to highlight the effect of powder dispenser speed on any charge build-up.
During the flow, electrostatic charges develop inside the powder.This is a result of the triboelectric effect, which is essentially the exchange of charges when two solids come into contact.
When powders flow within equipment (eg, silos, mixers, or conveyor belts), triboelectric effects will occur at the contact between the particles themselves, as well as at the contact between the particles and the equipment.
Therefore, both the nature of the material used to construct the device and the properties of the powder used are important parameters.
The GranuCharge instrument (Figure 1) automatically measures and precisely quantifies the electrostatic charge generated by powders during flow and in contact with selected materials.
The process involves the flow of a powder sample inside a vibrating V-shaped tube, falling into a Faraday cup attached to an electrometer.An electrometer is used to measure any charge the powder acquires as it flows inside the V-tube.Use a rotating or vibrating device to periodically feed the V-tube to ensure repeatable results.
The study described here used a BEAM Modulo 400 LMD machine.The powder passes through different delivery stages within the machine (Figure 2), initially exiting the powder dispenser and then being delivered to the nozzle.
The powder then flows through the nozzle and is melted by the laser before being deposited.Each of these stages has the potential for tribocharging, resulting in an increase in powder charge density.
Powder charge density was measured using GranuCharge at three process stages of interest: dispenser output, pre-nozzle, and post-nozzle.
During each measurement, the powder was poured directly into the GranuCharge cup through the machine tube (Figure 3).To reposition the Faraday Cup outside of GranuCharge, an extension is used.
Accurately measure the total charge and mass arriving in the cup.The dispenser speed was initially set to 0.68 rpm, 20% of the maximum speed of 3.4 rpm.The speed was varied to assess its effect on charge build-up within the powder.
Figure 4 illustrates the powder charge density measured at each stage of the process.At the output of the dispenser, the powder had a charge density of 0.39 nC/g.This indicates that the flow through the dispenser caused the build-up of charge inside the powder.
However, the powder charge density just before the nozzle is significantly reduced.This means that charge dissipation occurs during transport through the pipe between the dispenser and the nozzle.Finally, the flow through the nozzle continues to dissipate the charge, resulting in a slightly negative charge density at the output.
It is worth noting that the delivery of powder through pipes and nozzles offers some benefits in terms of tribocharging, as the process aids in the dissipation of static charges.It is clear that the distributor is part of the process responsible for charge accumulation.
The effect of the dispenser on powder charging was further investigated.Measured over the dispenser speed range.Figure 5 shows the charge density measured using GranuCharge.
Data is provided for several dispenser speeds ranging from 10% to 30% of the maximum dispenser speed (3.4 rpm).
These results show that the dispenser speed has a considerable effect on the charge build-up inside the powder – the higher the dispenser speed, the higher the charge density of the powder at the dispenser output.Figure 5 shows the best linear fit to this data.
A linear evolution of the charge density versus distributor velocity was obtained for the investigated velocity range.Therefore, powders processed at high dispenser speeds are expected to exhibit electrostatic-related powder performance degradation.
Figure 5: Effect of dispenser speed on powder charge density.The dashed line represents the best linear fit to the data.Image credit: Granutools
The GranuCharge instrument was used to study the effect of powder transport on the charge build-up of Inconel 718 powder in the LMD process.The charge density of the powder was measured at the output of the dispenser, after transport between the dispenser and the nozzle, and after flow through the nozzle.
The GranuCharge instrument facilitates precise investigation of the electrostatic charging of powders at various points throughout the process.This leads to the potential for new optimization perspectives in avoiding electrostatic charging throughout the process.
Granular tool.(May 19, 2021).Investigating powder electrostatics during laser metal deposition.AZOM.Retrieved May 26, 2022 from https://www.azom.com/article.aspx?ArticleID=20431.
Granular tool.”Investigating Powder Electrostatics During Laser Metal Deposition”.AZOM.May 26, 2022..
Granular tool.”Investigating Powder Electrostatics During Laser Metal Deposition”.AZOM.https://www.azom.com/article.aspx?ArticleID=20431.(Accessed 26 May 2022).
Granular tool.2021. Study of powder electrostatics during laser metal deposition.AZoM, accessed May 26, 2022, https://www.azom.com/article.aspx?ArticleID=20431.
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Post time: May-26-2022