Are Ejecta the Reason for Flaws in Powder Bed Fusion Additive Manufacturing? https://ift.tt/2JZjDJ1 While users around the globe are busy creating one stunning innovation after another in 3D printing—and some with the ability to change production forever—researchers are just as busy in the lab trying to perfect the technology. This means delving into the reasons why imperfections occur. Researchers from The Pennsylvania State University Applied Research Laboratory and Moog, Inc. discuss challenges presented by ejecta in their recently published paper, ‘Formation processes for large ejecta and interactions with melt pool formation in powder bed fusion additive manufacturing.’ Curiously, ejecta (particles ejected during the 3D printing process) have been noted in powder bed fusion additive manufacturing. Users have reported seeing such issues during post-process sieving but have also found surprisingly large ejecta inside built components. Investigating this issue further and finding out the reasons why such ejecta are being found could help improve quality of production in AM. The researchers set out to find out whether their hypothesis was correct, theorizing that ejecta was the result of ‘stochastic, inelastic collisions of ejecta and coalescence of partially-sintered agglomerates.’ Due to the rising popularity of laser powder bed fusion additive manufacturing (PBFAM), the research team points out a need for improved quality in some areas as parts are being created for critical applications in aerospace, medicine, and defense. PBFAM is attractive to industrial manufacturers as a technology because complex geometries that would not have been possible at all can now be produced from 3D printing.
There is not much data collected so far regarding why particles are spattered, how they form, or what effect they have on quality. In researching the presence of ejecta, the authors found that while ejecta is said to form in high-speed imaging (and not due to melt ejection) because of evaporation-driven entrainment of powder, it forms during PBFAM at 25–100 μm in size, due to the energy of ejected melt surpassing capillary pressure, resulting in larger melt droplets turning into ejecta.
Three different trains of thought have been documented regarding ejecta as the reason for flaws:
The researchers suspect that the first choice is a lesser issue in AM processes, as 2 and 3 are more feasible regarding powder height perturbations.
Findings from this study show that melt ejection is not the only reason that significant particles of ejecta are created. They can show evidence that they are formed due to stochastic, inelastic collisions of ejecta and coalescence of partially-sintered agglomerates—with stochastic collisions happening between adjacent particles as well as those that are distant. In most cases, spatter particles have no effect on melt pool geometry, according to the authors, but in some cases, there is interference. In some cases, large ejecta may be directly in the path of a laser scan too, causing it to end up in track geometry. Such interjections into the AM process offer the obvious potential for negative consequences, with verifying the true issues and investigation all speculation considered grounds for ongoing work. The authors do see a ‘likelihood’ of flaws being caused due to interaction with large ejecta.
Analysis of additive manufacturing and how to improve products has become a wide-ranging course of study for many researchers today who are fascinated with issues in technology and interested in improving processes, from reducing porosity to examining real-time in situ AM inspections, to making improvements in safety. Find out more about the effects of ejecta in AM processes here. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.
[Source / Images: ‘ Formation processes for large ejecta and interactions with melt pool formation in powder bed fusion additive manufacturing’]
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