Laser powder-bed fusion additive manufacturing: Physics of complex melt flow and formation mechanisms of pores, spatter, and denudation zones
TL;DR: In this paper, the effect of the recoil pressure and Marangoni convection in laser powder bed fusion (L-PBF) of 316L stainless steel was demonstrated. And the results were validated against the experiments and the sensitivity to laser absorptivity was discussed.
About: This article is published in Acta Materialia.The article was published on 2016-04-15 and is currently open access. It has received 1649 citations till now. The article focuses on the topics: Selective laser melting & Laser power scaling.
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TL;DR: A review of the emerging research on additive manufacturing of metallic materials is provided in this article, which provides a comprehensive overview of the physical processes and the underlying science of metallurgical structure and properties of the deposited parts.
4,192 citations
Cites background from "Laser powder-bed fusion additive ma..."
...Other than a few reported cases in which a keyhole is formed during AM [34,176], the melting process typically remains in conduction mode as the penetration into the previously deposited layer only needs to be a small fraction of the layer height to ensure interlayer bonding....
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TL;DR: The potential of additive manufacturing to create alloys with unique microstructures and high performance for structural applications is demonstrated, with austenitic 316L stainless steels additively manufactured via a laser powder-bed-fusion technique exhibiting a combination of yield strength and tensile ductility that surpasses that of conventional 316L steels.
Abstract: Many traditional approaches for strengthening steels typically come at the expense of useful ductility, a dilemma known as strength-ductility trade-off. New metallurgical processing might offer the possibility of overcoming this. Here we report that austenitic 316L stainless steels additively manufactured via a laser powder-bed-fusion technique exhibit a combination of yield strength and tensile ductility that surpasses that of conventional 316L steels. High strength is attributed to solidification-enabled cellular structures, low-angle grain boundaries, and dislocations formed during manufacturing, while high uniform elongation correlates to a steady and progressive work-hardening mechanism regulated by a hierarchically heterogeneous microstructure, with length scales spanning nearly six orders of magnitude. In addition, solute segregation along cellular walls and low-angle grain boundaries can enhance dislocation pinning and promote twinning. This work demonstrates the potential of additive manufacturing to create alloys with unique microstructures and high performance for structural applications.
1,385 citations
TL;DR: In this article, the authors show that the pre-existing dislocation network, which maintains its configuration during the entire plastic deformation, is an ideal modulator that is able to slow down but not entirely block the dislocation motion.
557 citations
TL;DR: The direct visualization of the keyhole morphology and dynamics with high-energy x-rays shows that (i) keyholes are present across the range of power and scanning velocity used in laser powder bed fusion; and (ii) there is a well-defined threshold from conduction mode to keyhole based on laser power density.
Abstract: We used ultrahigh-speed synchrotron x-ray imaging to quantify the phenomenon of vapor depressions (also known as keyholes) during laser melting of metals as practiced in additive manufacturing. Although expected from welding and inferred from postmortem cross sections of fusion zones, the direct visualization of the keyhole morphology and dynamics with high-energy x-rays shows that (i) keyholes are present across the range of power and scanning velocity used in laser powder bed fusion; (ii) there is a well-defined threshold from conduction mode to keyhole based on laser power density; and (iii) the transition follows the sequence of vaporization, depression of the liquid surface, instability, and then deep keyhole formation. These and other aspects provide a physical basis for three-dimensional printing in laser powder bed machines.
523 citations
TL;DR: In this paper, a review of the literature and the commercial tools for insitu monitoring of powder bed fusion (PBF) processes is presented, focusing on the development of automated defect detection rules and the study of process control strategies.
Abstract: Despite continuous technological enhancements of metal Additive Manufacturing (AM) systems, the lack of process repeatability and stability still represents a barrier for the industrial breakthrough. The most relevant metal AM applications currently involve industrial sectors (e.g., aerospace and bio-medical) where defects avoidance is fundamental. Because of this, there is the need to develop novel in-situ monitoring tools able to keep under control the stability of the process on a layer-by-layer basis, and to detect the onset of defects as soon as possible. On the one hand, AM systems must be equipped with in-situ sensing devices able to measure relevant quantities during the process, a.k.a. process signatures. On the other hand, in-process data analytics and statistical monitoring techniques are required to detect and localize the defects in an automated way. This paper reviews the literature and the commercial tools for insitu monitoring of Powder Bed Fusion (PBF) processes. It explores the different categories of defects and their main causes, the most relevant process signatures and the in-situ sensing approaches proposed so far. Particular attention is devoted to the development of automated defect detection rules and the study of process control strategies, which represent two critical fields for the development of future smart PBF systems.
505 citations
Cites background from "Laser powder-bed fusion additive ma..."
...The spatter generation mechanism was studied in depth by Khairallah et al. (2016) and Liu et al. (2015)....
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...In SLM a plasma/metal vapour plume can change the absorptivity along the pool depth (Khairallah et al., 2016)...
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...In SLM a plasma/metal vapour plume can change the absorptivity along the pool depth (Khairallah et al., 2016) 2.3.3....
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References
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TL;DR: In this article, the development of the microstructure of the Ti-6Al-4V alloy processed by selective laser melting (SLM) was studied by light optical microscopy.
2,201 citations
TL;DR: The authors examines the characteristics and applications of 3D printing and compares it with mass customization and other manufacturing processes, and concludes that 3-D printing enables small quantities of customized goods to be produced at relatively low costs.
1,900 citations
"Laser powder-bed fusion additive ma..." refers background in this paper
...Introduction Additive manufacturing (AM) is paving the way toward the next industrial revolution (Berman, 2012)....
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...Introduction Additive manufacturing (AM) is paving the way toward the next industrial revolution (Berman, 2012)....
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TL;DR: In this paper, the high thermal gradients occurring during SLM lead to a very fine microstructure with submicron-sized cells, which can be modified to a weak cube texture along the building and scanning directions when a rotation of 90° of the scanning vectors within or between the layers is applied.
1,431 citations
TL;DR: In this article, the authors describe which types of laser-induced consolidation can be applied to what type of material, and demonstrate that although SLS/SLM can process polymers, metals, ceramics and composites, quite some limitations and problems cause the palette of applicable materials still to be limited.
1,241 citations
TL;DR: In this article, the experimental observation of keyhole-mode laser melting in a laser powder-bed fusion additive manufacturing setting for 316L stainless steel is presented, and the conditions required to transition from conduction controlled melting to keyholemode melting are identified.
981 citations