Revealing Particle-Scale Powder Spreading Dynamics in Powder-Bed-Based Additive Manufacturing Process by High-Speed X-Ray Imaging

Powder spreading is a key step in the powder-bed-based additive manufacturing process, which determines the quality of the powder bed and, consequently, affects the quality of the manufactured part. However, powder spreading behavior under additive manufacturing condition is still not clear, largely because of the lack of particle-scale experimental study. Here, we studied particle-scale powder dynamics during the powder spreading process by using in-situ high-speed high-energy x-ray imaging. Evolution of the repose angle, slope surface speed, slope surface roughness, and the dynamics of powder clusters at the powder front were revealed and quantified. Interactions of the individual metal powders, with boundaries (substrate and container wall), were characterized, and coefficients of friction between the powders and boundaries were calculated. The effects of particle size on powder flow dynamics were revealed. The particle-scale powder spreading dynamics, reported here, are important for a thorough understanding of powder spreading behavior in the powder-bed-based additive manufacturing process, and are critical to the development and validation of models that can more accurately predict powder spreading behavior

Ultrafast X-Ray Imaging of Laser-Metal Additive Manufacturing Processes

The high-speed synchrotron X-ray imaging technique was synchronized with a custom-built laser-melting setup to capture the dynamics of laser powder-bed fusion processes in situ. Various significant phenomena, including vapor-depression and melt-pool dynamics and powder-spatter ejection, were captured with high spatial and temporal resolution. Imaging frame rates of up to 10 MHz were used to capture the rapid changes in these highly dynamic phenomena. At the same time, relatively slow frame rates were employed to capture large-scale changes during the process. This experimental platform will be vital in the further understanding of laser additive manufacturing processes and will be particularly helpful in guiding efforts to reduce or eliminate microstructural defects in additively manufactured parts

Characterization of Bulk to Thin Wall Mechanical Response Transition in Powder Bed AM

In the development of powder bed AM process parameters, the characterization of mechanical properties is generally performed through relatively large mechanical test samples that represent a bulk response. This provides an accurate representation of mechanical properties for equivalently sized or larger parts. However as feature size is reduced, mechanical properties transition from a standard bulk response to a thin wall response where lower power border scans and surface roughness have a larger effect. This study identifies this threshold between bulk and thin wall for 304L SS on the Selective Laser Melting (SLM) platform and Ti-6Al-4V on the Electron Beam Melting (EBM) platform. A possible method for improving those properties and shifting the transition from bulk to thin wall response to smaller wall thicknesses was investigated. Mechanical testing and fractography was performed on samples to characterize the effect of wall thickness.Mechanical Engineerin

Evolution of AISI 304L Stainless Steel Part Properties Due to Powder Recycling in Laser Powder-Bed Fusion

Laser Powder-Bed Fusion (L-PBF), often called selective laser melting (SLM), is a powder-bed fusion process in Additive Manufacturing (AM) that uses a laser beam to selectively fuse layers of powder into near net-shape components with little porosity. However, inconsistencies in the part properties due to the presence of defects in as-built components have hindered the widespread adoption of L-PBF for industrial applications motivating researchers to study the sources of variation for quality control purposes. A critical area suspected of creating variation in the part properties is the feedstock, where batch-to-batch differences as well as changes in the powder properties with reuse have the potential to affect performance. During processing, laser spatter and condensate form and deposit into the powder-bed surrounding the built parts. These particulates, collectively known as ejecta, differ morphologically and chemically from the virgin powder, potentially compromising reusability. In this study, 304L stainless steel powder was recycled for a total of 5 times through a systematic approach aimed at accelerating powder reuse to reveal its influence on both the tensile properties and impact toughness. Through analysis of variance (ANOVA), it was found that tensile properties did not change with reuse, while the impact toughness showed a steady decline revealing the differences in static and dynamic part properties due to powder reuse

Revealing particle-scale powder spreading dynamics in powder-bed-based additive manufacturing process by high-speed x-ray imaging

Abstract Powder spreading is a key step in the powder-bed-based additive manufacturing process, which determines the quality of the powder bed and, consequently, affects the quality of the manufactured part. However, powder spreading behavior under additive manufacturing condition is still not clear, largely because of the lack of particle-scale experimental study. Here, we studied particle-scale powder dynamics during the powder spreading process by using in-situ high-speed high-energy x-ray imaging. Evolution of the repose angle, slope surface speed, slope surface roughness, and the dynamics of powder clusters at the powder front were revealed and quantified. Interactions of the individual metal powders, with boundaries (substrate and container wall), were characterized, and coefficients of friction between the powders and boundaries were calculated. The effects of particle size on powder flow dynamics were revealed. The particle-scale powder spreading dynamics, reported here, are important for a thorough understanding of powder spreading behavior in the powder-bed-based additive manufacturing process, and are critical to the development and validation of models that can more accurately predict powder spreading behavior

Transient Dynamics of Powder Spattering in Laser Powder Bed Fusion Additive Manufacturing Process Revealed by In-Situ High-Speed High-Energy X-Ray Imaging

Powder spattering is a major cause of defect formation and quality uncertainty in the laser powder bed fusion (LPBF) additive manufacturing (AM) process. It is very difficult to investigate this with either conventional characterization tools or modeling and simulation. The detailed dynamics of powder spattering in the LPBF process is still not fully understood. Here, we report insights into the transient dynamics of powder spattering in the LPBF process that was observed with in-situ high-speed high-energy x-ray imaging. Powder motion dynamics, as a function of time, environment pressure, and location, is presented. The moving speed, acceleration, and driving force of powder motion that are induced by metal vapor jet/plume and argon gas flow are quantified. A schematic map showing the dynamics and mechanisms of powder motion during the LPBF process as functions of time and pressure is constructed. Potential ways to mitigate powder spattering during the LPBF process are discussed and proposed, based on the revealed powder motion dynamics and mechanisms

In-Situ Characterization of Pore Formation Dynamics in Pulsed Wave Laser Powder Bed Fusion

Laser powder bed fusion (LPBF) is an additive manufacturing technology with the capability of printing complex metal parts directly from digital models. Between two available emission modes employed in LPBF printing systems, pulsed wave (PW) emission provides more control over the heat input compared to continuous wave (CW) emission, which is highly beneficial for printing parts with intricate features. However, parts printed with pulsed wave LPBF (PW-LPBF) commonly contain pores, which degrade their mechanical properties. In this study, we reveal pore formation mechanisms during PW-LPBF in real time by using an in-situ high-speed synchrotron x-ray imaging technique. We found that vapor depression collapse proceeds when the laser irradiation stops within one pulse, resulting in occasional pore formation during PW-LPBF. We also revealed that the melt ejection and rapid melt pool solidification during pulsed-wave laser melting resulted in cavity formation and subsequent formation of a pore pattern in the melted track. The pore formation dynamics revealed here may provide guidance on developing pore elimination approaches

Publisher Correction: Pore Elimination Mechanisms during 3D Printing of Metals (Nature Communications, (2019), 10, 1, (3088), 10.1038/S41467-019-10973-9)

The original version of this Article contained an error in Fig. 4. The x-axis labels in Fig. 4a, b were incorrectly labelled \u27Diameter (mm)\u27, rather than the correct \u27Diameter (µm)\u27. This has been corrected in both the PDF and HTML versions of the Article