Background
Laser powder bed fusion (LPBF) approach offers high-precision fabrication using melting-based additive manufacturing (AM) technologies, and is gradually being adopted by aerospace, automotive or even medical industry to make commercially viable parts thereby disrupting the traditional manufacturing industry. Although LPBF shows great potential in industrial applications, the process can still induce defects, such as porosities and cracks, which limit the promotion of innovative technology. In particular, the cracks in the LPBF-ed nickel-based superalloys significantly accelerate the rupture process of the component under the high-temperature and high-pressure condition. How to inhibit the formation of the cracks? Which method is more appropriate, alloy design or process control? How to explain the different crack susceptibility of different classifications of superalloys? Although previous studies have confirmed elemental segregation make important impact on the solidification cracks and liquation cracks, it is still unprecedent how the solute element partitions and transfers during the cycled melting-rapid solidification process. Up till now, structure characterisation and energy dispersive spectrometer are generally used in most of the investigations on the structure and segregation, while it is really hard to capture the crystal growth-melting and solute transport at the micrometre and even sub-micro scale using in-situ experiment at the moment. Besides, most of the numerical models either used thermal history as input for the microstructural simulation, or simulated the microstructural evolution under specific thermal-fluid conditions. However, neither the two routes could reproduced the coupled structure-composition evolution at the sub-grain scale during the cycled melting-solidification.
Introduction
Assoc. Prof. Jun Li, Prof. Mingxu Xia, and Prof. Jianguo Li, together with their collaborators, Prof. Hongbiao Dong from University of Leicester, Senior Lecture Chinnapat Panwisawas from Queen Mary University of London, and Dr. Ruiyao Zhang from Centre of Excellence for Advanced Materials, made progress on the numerical modelling of laser powder bed fusion (LPBF) process. This paper combined the microstructural evolution and the transport of multiple physical fields at sub-grain scale, and developed a two-way coupling numerical model. It demonstrated the solute transport process and the evolution of solid/liquid interfacial morphology under rapid solidification and strong melt convection during cycled melting-solidification process. The authors discussed the role the melt flow plays in the non-equilibrium solidification process. Along the solidification, the morphology of solid/liquid interface transits to ultra-fine cells, and then coarse cells (even dendrites). However, the melt flow introduces great perturbation to the microstructure and elemental distribution: it can transfer solute enriched melt to the solidification front, and dilute the solute elements rejected to the solidification front due to partitioning as well, thereby changing the evolution process of the interfacial morphology. Overall, it enlarges the (quasi) solute trapping region formed at the bottom of the melt pool.
The crack susceptibility of nickel-based superalloys was also illustrated in terms of the elemental microsegregation. Large (quasi) solute trapping regions are formed at the bottom of the melt pool when LPBF ABD-850AM and IN718, and the regions are preserved and even expanded after printing a new layer. In contrast, grain boundary strengthening elements are severely segregated in the grain boundaries in the LPBF-ed CM247LC, and no obvious solute trapping regions can be observed at the bottom of the melt pools. Except for inhibiting the formation of solidification cracks and liquation cracks, the suppression of microsegregation could also decrease the susceptibility of the solid-state cracks by reducing the differences of size and morphology among precipitated phases. The previously reported experiments also indicate the crack propagation tends to be interrupted by the solute trapping regions. Based on the discussion on the non-equilibrium behaviours, the paper proposed that increasing solidification Peclet numbers is also possible to effectively prevent the formation of cracks by suppress the microsegregation. This could be a valuable reference for the parameter design and process control of the superalloy AM of high γ′ volume fraction.
The corresponding work, entitled “Solute trapping and non-equilibrium microstructure during rapid solidification of additive manufacturing”, was published in Nature Communications.
Paper link
https://doi.org/10.1038/s41467-023-43563-x
The model was developed based on our previous works on the multi-scale simulation of the directional solidification of nickel-based superalloys:
(Acta Materialia, 2021, 206, 116620, https://doi.org/10.1016/j.actamat.2020.116620;
Acta Materialia, 2021, 215, 117043, https://doi.org/10.1016/j.actamat.2021.117043;
Metall Mater Trans A, 2023, 54, 4612–4619, https://doi.org/10.1007/s11661-023-07224-4).
Research Content
Figure 1 Solute transport and microstructural evolution during the multi-track multi-layer laser powder bed fusion process.
Figure 2 As-built microstructure and solute distribution of LPBF-ed superalloys of ABD850-AM (a-c), Inconel 718(d-f), and CM247LC(g-i).
Summary
A two-way fully coupled thermal-fluid-solutal-microstructural mathematical model is developed to understand the dynamic solute transport process and elemental segregation in AM. The results from high-fidelity simulation reveal that the non-equilibrium nature of intercellular solute segregation and cellular structures at the sub-grain scale during the multi-track multi-layer fusion and solidification of LPBF process. The predictions of melt pool, solute trapping zone, and microstructure are well verified by the reported and conducted experiments. Having demonstrated the characteristics of the solute distribution, the authors elucidate the role of melt convection on elemental segregation and propose a mechanism for the formation of solute trapping region and microstructural evolution in the printed layers. From theoretical perspectives, this work also illustrates a potential technical route of compositional control to optimising operating parameters to reduce crack density or even eliminate cracks in AM of hard-to-print superalloys. Based on the systematic analysis of the specific behaviours of rapid solidification, hard-to-print superalloy can be additively manufactured without removing essential minor elements by further accelerating the solidification to promote the solute trapping, thereby reducing the intercellular solute segregation. By comparing the solidification behaviours of different classes of superalloys, CM247LC, Inconel 718, and ABD-850AM, a qualitative estimate of the crack susceptibility (the width of the critical process window) is developed to aid in the alloy composition design of superalloys. Additionally, the proposed model framework can also be a powerful tool for extended study, such as the in-situ alloying of different elemental powders in LPBF processes for high entropy alloys or high-performance steels
Author information
Shanghai Key Laboratory of Advanced High-temperature Materials and Precision Forming, School of Material Science and Engineering, Shanghai Jiao Tong University, 200240, Shanghai, P. R. China
Dr. Neng Ren, Assoc. Prof. Jun Li*, Prof. Mingxu Xia & Prof. Jianguo Li
The research group focuses on solidification theory, controlling techniques, and process modelling.
*li.jun@sjtu.edu.cn
School of Engineering and Materials Science, Queen Mary University of London, London, E1 4NS, UK
Senior Lecture Chinnapat Panwisawas
School of Engineering, University of Leicester, Leicester, LE1 7RH, UK
Prof. Hongbiao Dong
Centre of Excellence for Advanced Materials, 523808, Dongguan, China
Dr. Ruiyao Zhang