Researchers from the University of Washington are exploring further complexities of material science in metal printing, outlining their findings in the recently published ‘Electron beam additive manufacturing of Ti6A14V: Evolution of powder morphology and part microstructure with powder reuse.’
Concerned with quality for aerospace applications, the authors designed this study to address ‘fundamental issues’ concerning the use of powder bed fusion in additive manufacturing—mainly in re-use. As additive manufacturing continues to increase in popularity for industrial users, the benefits become more apparent—from the ability to create complex and optimized geometries to using significantly fewer materials that are stronger and more durable, yet light in weight. As is generally the case though, challenges exist, such as slow processing times.
“The opportunities for metal AM to be used for manufacturing components with stress-critical applications has caused need for greater understanding of the process,” state the authors.
During the process of EBM powder bed fusion, the characteristics and quality of powder are critical as it is sequentially deposited, sintered, and melted. There are the obvious benefits of reusing powder to cut down on waste of materials, but the effects on mechanical properties of parts are of concern—and an area that the authors point out has previously not been researched fully—and when it has, reuse cycles have involved ‘relatively low exposure time of the powder during the build process.’
“There are many concerns that remain to be addressed in powder reuse for metal AM,” state the researchers, pointing out the necessity for an analysis of part microstructure.
Researching powder reuse in EBM AM of Ti6Al4V, the authors performed 30 build cycles. They also investigated the following:
- Transformations in powder chemistry
- Particle size distribution with reuse
- Effects on the built metal part
“The novelty of this investigation regarding powder reuse is in the large number of reuse cycles achieved for EBM AM, the extensive evaluation of powder that was performed, and the complimentary evaluation of the part microstructure,” stated the authors.
Grade 5 Titanium alloy (Ti6Al4V) was used, with the 30 builds performed over six months on an ARCAM A2X Electron Beam AM system, from ‘b1 for the first build performed with 50 kg of virgin powder and subsequently up to b30 for the final build.’
During the study and ongoing builds, the research team noted that particles did change in size, and both surface texture and morphology changed also as the powder was reused.
Staircase and pyramid part samples were used in this study to examine how powder reuse affected the printed Ti6Al4V microstructure. Both samples were cross-sectioned and mounted during evaluation, polished, then etched by immersion.
“In comparing responses for the two geometries, the primary difference in microstructure between the pyramids and staircases is the broader spread of alpha volume and lath thickness in the pyramid, along with the increase in alpha lath thickness with powder reuse,” stated the researchers in discussion.
[Source / Images – ‘Electron beam additive manufacturing of Ti6A14V: Evolution of powder morphology and part microstructure with powder reuse’]
“One of the most marked changes in powder with reuse was the increase in surface deformation to the particles and physical damage. The particles changed shape with reuse cycles from spherical geometry to increasingly deformed with surface dimples and irregular shape,” concluded the researchers. “This aspect of the powder degradation appears to result from the mechanical aspects of the recycling and part extraction process. In addition, there was an increase in fractured particles, partially fused particles and recast particles with powder reuse.”
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