On the Morphology Changes of Al and Al-Cu Powder After Laser Melting
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IN the field of laser powder bed fusion (LPBF), much research has been focused on characterizing the changes that occur in the powder feedstock after laser irradiation in order to better understand the limitations of its powder recyclability.[1,2] The majority of these studies assume that all irradiated particles undergo melting and are incorporated into the melt pool of the laser track, focusing on the spatter that is ejected from the melt pool as the main source of defects in recycled powder.[3–6] However, very few consider particles on the peripheries of the laser track which have been irradiated, but not incorporated into the melt pool. If no sintering between neighboring particles occurs, this irradiated powder will end up being recycled and used in sequential builds.[7–10]
J.M. SKELTON, C.V. HEADLEY, E.J. SULLIVAN, J.M. FITZ-GERALD, and J.A. FLORO are with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903. Contact e-mail: [email protected] Manuscript submitted January 14, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Studies that focus on recycled powder are vital for the LPBF process since morphology irregularities of the particles could lead to failed builds due to uneven powder flow and spatial distribution.[11,12] Thus, a better understanding is needed on the mechanisms behind the morphological anomalies observed in recycled powder, especially those systems such as aluminum-based alloys that contain a tenacious native oxide shell that could pose a barrier to sintering.[3,13–15] Recent studies have started to categorize the different types of particle morphologies found in recycled powder. For example, Popov et al., identified thirteen different particle defects within the Ti-6Al-4V system, ranging from mechanically induced (broken particles) to agglomerates caused by spatter from the laser melt pool.[16] In order to better understand the formation of spatter-type defects within the AlSi10Mg system, Andani et al. used a highspeed camera to capture images of the laser melt pool in situ.[17] From this study, they proposed that both powder particles and liquid spatter were being jettisoned from the lasers path due to the recoil pressure of the laser. The nature of the jettisoned powder was not discussed further by Andani et al., but a similar ‘‘partially heated’’ powder is brought
to attention by Asgari et al. on their study of the same system (AlSi10Mg).[14] In this study, only the large particles separated through sieving are thought to be affected by the laser and are assumed to be agglomerates, or ‘‘condensate’’, due to their large size and fine microstructure. It was suggested that the mechanism behind the formation of these large particles and their fine microstructure was related to the heating-cooling cycles and partial sintering between particles. In contrast to this conclusion, Lutter-Gunther et al. showed by isolating spatter from the feedstock powder that agglomerated particles originate from the laser melt pool. They then categorized sp
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