Advances in additive manufacturing process simulation: Residual stresses and distortion predictions in complex metallic components

Powder spreading is a crucial step in the powder bed fusion process, which controls the quality of powder bed and consequently affects the quality of printed parts. To date, however, powder spreadability has received very little attention and substantial fundamental work is still needed, largely because of the lack of experimental studies. Therefore, the focus of the present study addresses the influences of powder morphology, spreading velocity and layer thickness on the powder bed topography uniformity. The experiments were conducted with a laser powder bed fusion printer and the powder layers were spread systematically and comprehensively assessed. In summary, it was found that particle sphericity and surface texture dictates the degree of impact that the spreader velocity and the layer thickness exert on the quality of powder bed topography in spread layers. The spreader velocity has substantial influence on powder bed uniformity, such that better uniformity is achieved with low spreading velocities, ≤ 80 mm/s. Powders with a wide particle distribution and containing large number of fine particles (

Influences of powder morphology and spreading parameters on the powder bed topography uniformity in powder bed fusion metal additive manufacturing

The possibility of using a variety of sensor signals acquired during metal powder bed fusion processes, to support part and process qualification and for the early detection of anomalies and defects, has been continuously attracting an increasing interest. The number of research studies in this field has been characterised by significant growth in the last few years, with several advances and new solutions compared with first seminal works. Moreover, industrial powder bed fusion (PBF) systems are increasingly equipped with sensors and toolkits for data collection, visualisation and, in some cases, embedded in-process analysis. Many new methods have been proposed and defect detection capabilities have been demonstrated. Nevertheless, several challenges and open issues still need to be tackled to bridge the gap between methods proposed in the literature and actual industrial implementation. This paper presents an updated review of the literature on in-situ sensing, measurement and monitoring for metal PBF processes, with a classification of methods and a comparison of enabled performances. The study summarises the types and sizes of defects that are practically detectable while the part is being produced and the research areas where additional technological advances are currently needed.

https://iopscience.iop.org/article/10.1088/1361-6501/ac0b6b/pdf

In-situ measurement and monitoring methods for metal powder bed fusion: an updated review

Residual stress control in the metal components by additive manufacturing (AM) has been a major challenge. To mitigate this challenge, proper selection of AM process parameters is of great importance. In this study, we investigate the influence of laser power and scanning strategies on residual stress distribution in 316L steel by a metal AM process, namely, selective laser melting (SLM). Finite element simulation and experimental verification are conducted by using the identical process parameters and part geometry to ensure that the results are indeed comparable and can shed light on the challenging issue of residual stress control. With two levels of laser power (i.e., 160 W and 200 W) and two scanning strategies (i.e., stripe scanning and chessboard scanning), four process conditions are investigated. For all four conditions, both simulation and experiment show that the tensile residual stress in the area of interest (the center area of each layer) tends to gradually increase along the depth into surface. Also, the increase of laser power from 160 W to 200 W and the adoption of stripe scanning (instead of chessboard scanning) generally lead to the increase of tensile residual stress in the area of interest. The trends are also confirmed by both simulation and experiment. In addition, the laser power increase from 160 W to 200 W appears to have more significant effect, compared with the switch of two scanning strategies.

Influence of laser power and scanning strategy on residual stress distribution in additively manufactured 316L steel

Despite the ongoing success of metal additive manufacturing and especially the selective laser melting (SLM) technology, process-related defects, distortions and residual stresses impede its usability for fracture-critical applications. In this paper, results of in situ X-ray diffraction experiments are presented that offer insights into the strain and stress formation during the manufacturing of multi-layer thin walls made from Inconel 625. Using different measuring modes and laser scanning parameters, several experimental observations are discussed to validate and extend theoretical models and simulations from the literature. As a sample is built-up layer by layer, the stress state changes continuously up until the last exposure. The localized energy input leads to a complex stress field around the heat source that involves alternating tensile and compressive stresses. The correlation of temperature and yield strength results in a stress maximum at a certain distance to the top layer. The present study demonstrates the potential of high-energy synchrotron radiation diffraction for in situ SLM research.

Experimental observation of stress formation during selective laser melting using in situ X-ray diffraction

In Laser Powder Bed Fusion (LPBF), the highly localized energy input by the laser leads to high-temperature gradients. Combined with the inherent cycles of re-melting and solidification of the material, they can result in high mechanical stresses. These stresses can cause distortion and cracking within the component. In situ diffraction experiments with high-energy synchrotron radiation allow an analysis of the lattice spacing during the LPBF process and provide insight into the dynamics of stress generation and texture evolution. In this work, an LPBF system for the purpose of synchrotron x-ray diffraction experiments during the manufacturing process of multi-layer components with simple geometries is described. Moreover, results from diffraction experiments at the HEMS beamline P07 at PETRA III, DESY, Hamburg, Germany, are presented. Components with a length of ls = 20 mm and a width of ws = 2.5 mm consisting of 100 layers with a layer thickness of Δz = 50 µm were produced using the nickel-base alloy Inconel 625 as the powder material. Diffraction experiments were carried out in situ at sampling rates of f = 10 Hz with a synchrotron radiation beam size of 750 × 70 µm2. The presented experimental setup allows for the observation of arbitrary measuring positions in the sample in the transmission mode while gathering full diffraction rings. Thus, new possibilities for the observation of the dynamic evolution of strains, stresses, and textures during the LPBF process are provided.

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A laser powder bed fusion system for in situ x-ray diffraction with high-energy synchrotron radiation.

Laser powder bed fusion is an additive manufacturing process that employs highly focused laser radiation for selective melting of a metal powder bed. This process entails a complex heat flow and thermal management that results in characteristic, often highly textured microstructures, which lead to mechanical anisotropy. In this study, high-energy X-ray diffraction experiments were carried out to illuminate the formation and evolution of microstructural features during LPBF. The nickel-base alloy Inconel 625 was used for in situ experiments using a custom LPBF system designed for these investigations. The diffraction patterns yielded results regarding texture, lattice defects, recrystallization, and chemical segregation. A combination of high laser power and scanning speed results in a strong preferred crystallographic orientation, while low laser power and scanning speed showed no clear texture. The observation of a constant gauge volume revealed solid-state texture changes without remelting. They were related to in situ recrystallization processes caused by the repeated laser scanning. After recrystallization, the formation and growth of segregations were deduced from an increasing diffraction peak asymmetry and confirmed by ex situ scanning transmission electron microscopy.

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In situ microstructure analysis of Inconel 625 during laser powder bed fusion

Internal Stress Evolution and Subsurface Phase Transformation in Titanium Parts Manufactured by Laser Powder Bed Fusion – An in situ X-ray Diffraction Study

The combination of laser powder bed fusion (LPBF), known for its geometrical freedom and accuracy, and the nozzle-based laser metal deposition process (LMD), known for its high build-up rates, has great potential to reduce the additive manufacturing times for large metallic parts. For the industrial application of the LPBF-LMD hybrid process chain, it is necessary to investigate the influence of the LMD process on the LPBF substrate. In addition, the build plate material also has a significant impact on the occurrence of distortion along the additive manufacturing process chain. In the literature, steel build plates are often used in laser-based additive manufacturing processes of Inconel 718, since a good metallurgical bonding can be assured whilst reducing costs in the production and restoration of the build plates. This paper examines the distortion caused by LMD material deposition and the influence of the build plate material along the hybrid additive manufacturing process chain. Twin cantilevers are manufactured by LPBF and an additional layer is subsequently deposited with LMD. The distortion is measured in the as-built condition as well as after heat treatment. The effect of different LMD hatch strategies on the distortion is determined. The experiments are conducted using the nickel-base alloy Inconel 718. The results show a significant influence of LMD path strategies on distortion, with shorter tool paths leading to less distortion. The remaining distortion after heat treatment is considerably dependent on the material of the build plate.

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Effects on the distortion of Inconel 718 components along a hybrid laser-based additive manufacturing process chain using laser powder bed fusion and laser metal deposition

Erwin Krohmer

Papers

Experimental observation of stress formation during selective laser melting using in situ X-ray diffraction

A laser powder bed fusion system for in situ x-ray diffraction with high-energy synchrotron radiation.

In situ microstructure analysis of Inconel 625 during laser powder bed fusion

Internal Stress Evolution and Subsurface Phase Transformation in Titanium Parts Manufactured by Laser Powder Bed Fusion – An in situ X-ray Diffraction Study

Effects on the distortion of Inconel 718 components along a hybrid laser-based additive manufacturing process chain using laser powder bed fusion and laser metal deposition