Additive manufacturing (AM) is a new paradigm for the design and production of high-performance components for aerospace, medical, energy, and automotive applications. This review will exclusively cover directed energy deposition (DED)-AM, with a focus on the deposition of powder-feed based metal and alloy systems. This paper provides a comprehensive review on the classification of DED systems, process variables, process physics, modelling efforts, common defects, mechanical properties of DED parts, and quality control methods. To provide a practical framework to print different materials using DED, a process map using the linear heat input and powder feed rate as variables is constructed. Based on the process map, three different areas that are not optimized for DED are identified. These areas correspond to the formation of a lack of fusion, keyholing, and mixed mode porosity in the printed parts. In the final part of the paper, emerging applications of DED from repairing damaged parts to bulk combinatorial alloys design are discussed. This paper concludes with recommendations for future research in order to transform the technology from “form” to “function,” which can provide significant potential benefits to different industries.

https://www.mdpi.com/2079-6412/9/7/418/pdf

State of the Art in Directed Energy Deposition: From Additive Manufacturing to Materials Design

Mechanistic models for additive manufacturing of metallic components

Reliable computational models of metal additive manufacturing will assist in optimising part quality, and are likely to play a role in component qualification. A key component of these models will be a detailed simulation of flow and heat transfer in and around the melt pool formed as the powder bed is melted. This paper reviews the burgeoning literature concerning melt pool simulation. The physical theory underlying the current benchmark models is first presented and the main approximations and assumptions discussed. The individual capabilities of the leading simulation groups around the world are listed in detail. Publications by less prominent research groups are also summarised. Finally, the overall status of melt pool simulation and the implications for model development are discussed.

Simulation of Melt Pool Behaviour during Additive Manufacturing: Underlying Physics and Progress

Melt flow plays a critical role in laser metal additive manufacturing, yet the melt flow behavior within the melt pool has never been explicitly presented. Here, we report in-situ characterization of melt-flow dynamics in every location of the entire melt pool in laser metal additive manufacturing by populous and uniformly dispersed micro-tracers through in-situ high-resolution synchrotron x-ray imaging. The location-specific flow patterns in different regions of the melt pool are revealed and quantified under both conduction-mode and depression-mode melting. The physical processes at different locations in the melt pool are identified. The full-field melt-flow mapping approach reported here opens the way to study the detailed melt-flow dynamics under real additive manufacturing conditions. The results obtained provide crucial insights into laser additive manufacturing processes and are critical for developing reliable high-fidelity computational models.

In-situ full-field mapping of melt flow dynamics in laser metal additive manufacturing

https://hal.archives-ouvertes.fr/hal-03059610/document

Reducing hot tearing by grain boundary segregation engineering in additive manufacturing: example of an AlxCoCrFeNi high-entropy alloy

Internal flow behaviour during melt-pool-based metal manufacturing remains unclear and hinders progression to process optimisation. In this contribution, we present direct time-resolved imaging of melt pool flow dynamics from a high-energy synchrotron radiation experiment. We track internal flow streams during arc welding of steel and measure instantaneous flow velocities ranging from 0.1 m s−1 to 0.5 m s−1. When the temperature-dependent surface tension coefficient is negative, bulk turbulence is the main flow mechanism and the critical velocity for surface turbulence is below the limits identified in previous theoretical studies. When the alloy exhibits a positive temperature-dependent surface tension coefficient, surface turbulence occurs and derisory oxides can be entrapped within the subsequent solid as result of higher flow velocities. The widely used arc welding and the emerging arc additive manufacturing routes can be optimised by controlling internal melt flow through adjusting surface active elements. Understanding what happens to the liquid in melt pools during welding and metal-based additive manufacturing remains a challenge. Here, the authors directly image internal melt pool dynamics using synchrotron radiation to show surface tension affects flow speed, orientation and surface turbulence.

/pdf/revealing-internal-flow-behaviour-in-arc-welding-and-4un8cf314n.pdf

Revealing internal flow behaviour in arc welding and additive manufacturing of metals

This paper was presented at the XIth MCWASP (Modelling of Casting, Welding and Solidification) Conference, Opio, France, 28 May to 2 June 2006, and published in the Proceedings.

Simulation of molten steel refining in a gas-stirred ladle using a coupled CFD and thermodynamic model

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Revealing internal flow behaviour in arc welding and additive manufacturing of metals

Simulation of molten steel refining in a gas-stirred ladle using a coupled CFD and thermodynamic model