Powder Bed Fusion (PBF) techniques constitute a family of Additive Manufacturing (AM) processes, which are characterised by high design flexibility and no tooling requirement. This makes PBF techniques attractive to many modern manufacturing sectors (e.g. aerospace, defence, energy and automotive) where some materials, such as Nickel-based superalloys, cannot be easily processed using conventional subtractive techniques. Nickel-based superalloys are crucial materials in modern engineering and underpin the performance of many advanced mechanical systems. Their physical properties (high mechanical integrity at high temperature) make them difficult to process via traditional techniques. Consequently, manufacture of nickel-based superalloys using PBF platforms has attracted significant attention. To permit a wider application, a deep understanding of their mechanical behaviour and relation to process needs to be achieved. The motivation for this paper is to provide a comprehensive review of the mechanical properties of PBF nickel-based superalloys and how process parameters affect these, and to aid practitioners in identifying the shortcomings and the opportunities in this field. Therefore, this paper aims to review research contributions regarding the microstructure and mechanical properties of nickel-based superalloys, manufactured using the two principle PBF techniques: Laser Powder Bed Fusion (LPBF) and Electron Beam Melting (EBM). The ‘target’ microstructures are introduced alongside the characteristics of those produced by PBF process, followed by an overview of the most used building processes, as well as build quality inspection techniques. A comprehensive evaluation of the mechanical properties, including tensile strength, hardness, shear strength, fatigue resistance, creep resistance and fracture toughness of PBF nickel-based superalloys are analysed. This work concludes with summary tables for data published on these properties serving as a quick reference to scholars. Characteristic process factors influencing functional performance are also discussed and compared throughout for the purpose of identifying research opportunities and directing the research community toward the end goal of achieving part integrity that extends beyond static components only.

Powder Bed Fusion of nickel-based superalloys: A review

The effect of the process gas during laser powder bed fusion (L-PBF) was investigated using high-speed shadowgraphy while melting Ti-6Al-4V powder under high purity argon, helium, and a mixture of both, on a laboratory-scale machine. These recordings reveal that the generation of incandescent spatters can be reduced by at least 60% using pure helium and by ∼30% using addition of helium to argon in comparison to the use of traditional argon. The quantity of colder spatters appeared unaffected by the change of process gas. Different configurations of gas flow versus laser scanning direction were investigated and revealed that fumes and spatters are less accumulated at the laser spot with helium addition. Furthermore, the use of the argon–helium mixture proved to be as efficient as pure argon in the dragging and extraction of the fumes. Shadowgraphs revealed the more rapid expansion of fumes in helium-rich atmospheres, limiting the accumulation of scattering objects close to the laser spot and thus melt pool instability. These results were correlated to process snapshots on an industrial-scale system, confirming the reduction of hot spatter generation. Finally, the findings put in evidence the more rapid cooling of spatters with helium addition to the process gas – a promising aspect to limit powder bed degradation during L-PBF. In addition, the use of mixtures of helium and argon would be economically interesting compared to pure helium, typically more expensive than the traditionally used argon.

Reduction of incandescent spatter with helium addition to the process gas during laser powder bed fusion of Ti-6Al-4V

As well as preventing oxidation, a cross-flow of inert gas over the powder bed extracts process by-products, making it critical for process repeatability and build quality during the additive manufacturing of metallic parts. In this study, high-magnification schlieren imaging was employed to visualise the interactions between the cross-flow, laser plume and individual powder particles for the first time. Novel 3D multiphysics simulations of the cross-flow, including the vapour jet and laser plume, were used to calculate energy, momentum and species transport over the processed area in order to understand and quantify the observed interactions. During processing, metallic fumes as well as a significant number of airborne powder particles and agglomerates were observed to remain close to the powder bed, over the scanned area. These particles interact in a range of stochastic collisions that affect the build consistency. Refractive index gradients were formed in the atmosphere above the island scans, which extended many tens of mm beyond the processing area. The rate of extraction of these by-products, regulated by the velocity of the cross-flow, was related to the quality of the builds. A cross-flow perpendicular to the laser scan direction was observed to remove airborne particles from adjacent laser scans most effectively, limiting laser-particle and particle-particle interactions. The model demonstrated that the refractive index gradients away from the vapour jet are due to the metal concentration rather than the temperature of the heated gas.

https://pure.hw.ac.uk/ws/files/42790509/1_s2.0_S2214860420310629_main.pdf

Visualisation and numerical analysis of laser powder bed fusion under cross-flow

The powder motion in laser powder bed fusion (LPBF) processes causes defect and variability issues in the built products. It has been reported that the ambient pressure has a significant influence on the powder motion, but the physical effects of the ambient pressure on the gas flow, powder-gas interaction, and powder behavior are not quantitatively understood. In this work, we have developed the first three-dimensional multiphysics model for LPBF to simulate the molten pool dynamics, depression zone evolution, gas flow structure, and powder motion in a fully coupled manner. The model enables the first quantitative investigation of the gas flow, powder-gas interaction, and powder behavior in LPBF with different ambient pressure levels, all of which are difficult to measure by experiments. The simulation results show a consistent gas flow structure for all different pressure levels, but the gas flow parameters (temperature, velocity, Reynolds number, and Knudsen number) vary significantly with the ambient pressure. Four powder-gas interaction modes are defined by the gas flow around the particle and the gas-induced forces on the particle, and the interaction modes, individually or collectively, control the motion of each particle. With the changes in the ambient pressure and the gas flow parameters, the significance of the four modes to the powder motion varies, and the powder behavior (temperature, force, velocity, and ejection angle) becomes different. A new strategy is proposed to mitigate the powder motion based on the modeling results.

Quantitative investigation of gas flow, powder-gas interaction, and powder behavior under different ambient pressure levels in laser powder bed fusion

Inconel superalloys are one of the main classes of materials with good potential for production using the additive manufacturing process of laser powder bed fusion (PBF-LB), which has a broad range of applications in the transportation and energy sectors. Thus, they have received considerable attention in the scientific literature, with researchers aiming to better understand their capabilities and limitations in recent decades. This review is based on around 340 publications spanning the past 14 years and highlights the most significant findings in the field along with the current challenges and research goals resulting from them. A contextualization is followed by a succinct microstructural review, which introduces the topics analyzed. The effects of the processing parameters, heat treatment and machining on the materials and their composites are evaluated and the performance of materials is compared in terms of their mechanical, corrosion and tribological behavior. Publications are mapped according to their research focus in order to reveal the current research trends in the literature. • Review of more than 330 experimental publications of the last 14 years. • Processing, post processing and performance aspects are covered. • Similarities and contrast between the behavior of alloys are displayed. • Research gaps and current challenges are identified. • Maps linking each reviewed publication and its research focus are provided. 

Socona Traore

Papers

Influence of gas atmosphere (Ar or He) on the laser powder bed fusion of a Ni-based alloy