Detecting keyhole pore defects and monitoring process signatures during laser powder bed fusion: A correlation between in situ pyrometry and ex situ X-ray radiography

AbstractLaser 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. Graphical Abstract

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Synchrotron Characterisation of Ultra-Fine Grain TiB2/Al-Cu Composite Fabricated by Laser Powder Bed Fusion

Acta Metallurgica Sinica (English Letters) ◽

10.1007/s40195-021-01317-y ◽

2021 ◽

Author(s):

Sheng Li ◽

Biao Cai ◽

Ranxi Duan ◽

Lei Tang ◽

Zihan Song ◽

...

Keyword(s):

Electron Microscopy ◽

Tensile Loading ◽

Fusion Pore ◽

Powder Bed Fusion ◽

Laser Powder Bed Fusion ◽

Powder Bed ◽

X Ray ◽

Random Texture ◽

Average Grain Size

AbstractIsotropy in microstructure and mechanical properties remains a challenge for laser powder bed fusion (LPBF) processed materials due to the epitaxial growth and rapid cooling in LPBF. In this study, a high-strength TiB2/Al-Cu composite with random texture was successfully fabricated by laser powder bed fusion (LPBF) using pre-doped TiB2/Al-Cu composite powder. A series of advanced characterisation techniques, including synchrotron X-ray tomography, correlative focussed ion beam–scanning electron microscopy (FIB-SEM), scanning transmission electron microscopy (STEM), and synchrotron in situ X-ray diffraction, were applied to investigate the defects and microstructure of the as-fabricated TiB2/Al-Cu composite across multiple length scales. The study showed ultra-fine grains with an average grain size of about 0.86 μm, and a random texture was formed in the as-fabricated condition due to rapid solidification and the TiB2 particles promoting heterogeneous nucleation. The yield strength and total elongation of the as-fabricated composite were 317 MPa and 10%, respectively. The contributions of fine grains, solid solutions, dislocations, particles, and Guinier–Preston (GP) zones were calculated. Failure was found to be initiated from the largest lack-of-fusion pore, as revealed by in situ synchrotron tomography during tensile loading. In situ synchrotron diffraction was used to characterise the lattice strain evolution during tensile loading, providing important data for the development of crystal-plasticity models.

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Cooling dynamics of two titanium alloys during laser powder bed fusion probed with in situ X-ray imaging and diffraction

Materials & Design ◽

10.1016/j.matdes.2020.108987 ◽

2020 ◽

Vol 195 ◽

pp. 108987

Author(s):

Nicholas P. Calta ◽

Vivek Thampy ◽

Duncan R.C. Lee ◽

Aiden A. Martin ◽

Rishi Ganeriwala ◽

...

Keyword(s):

Titanium Alloys ◽

Powder Bed Fusion ◽

Laser Powder Bed Fusion ◽

Powder Bed ◽

X Ray ◽

X Ray Imaging

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Characterizing the effects of laser control in laser powder bed fusion on near-surface pore formation via combined analysis of in-situ melt pool monitoring and X-ray computed tomography

Additive Manufacturing ◽

10.1016/j.addma.2021.102372 ◽

2021 ◽

Vol 48 ◽

pp. 102372

Author(s):

F.H. Kim ◽

H. Yeung ◽

E.J. Garboczi

Keyword(s):

Pore Formation ◽

Powder Bed Fusion ◽

Laser Powder Bed Fusion ◽

Near Surface ◽

Powder Bed ◽

X Ray ◽

Melt Pool ◽

X Ray Computed ◽

Laser Control

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Linking In Situ Melt Pool Monitoring to Melt Pool Size Distributions and Internal Flaws in Laser Powder Bed Fusion

Metals ◽

10.3390/met11111856 ◽

2021 ◽

Vol 11 (11) ◽

pp. 1856

Author(s):

Claudia Schwerz ◽

Lars Nyborg

Keyword(s):

Near Infrared ◽

In Situ Monitoring ◽

Ex Situ ◽

Process Conditions ◽

Powder Bed Fusion ◽

Laser Powder Bed Fusion ◽

Powder Bed ◽

Melt Pool ◽

Melt Pool Size

In situ monitoring of the melt pools in laser powder bed fusion (LPBF) has enabled the elucidation of process phenomena. There has been an increasing interest in also using melt pool monitoring to identify process anomalies and control the quality of the manufactured parts. However, a better understanding of the variability of melt pools and the relation to the incidence of internal flaws are necessary to achieve this goal. This study aims to link distributions of melt pool dimensions to internal flaws and signal characteristics obtained from melt pool monitoring. A process mapping approach is employed in the manufacturing of Hastelloy X, comprising a vast portion of the process space. Ex situ measurements of melt pool dimensions and analysis of internal flaws are correlated to the signal obtained through in situ melt pool monitoring in the visible and near-infrared spectra. It is found that the variability in melt pool dimensions is related to the presence of internal flaws, but scatter in melt pool dimensions is not detectable by the monitoring system employed in this study. The signal intensities are proportional to melt pool dimensions, and the signal is increasingly dynamic following process conditions that increase the generation of spatter.

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In-Situ XRD Study of Phase Transformation Kinetics in a Co-Cr-W-Alloy Manufactured by Laser Powder-Bed Fusion

Crystals ◽

10.3390/cryst11020176 ◽

2021 ◽

Vol 11 (2) ◽

pp. 176

Author(s):

Patrick Hegele ◽

Jonas von Kobylinski ◽

Leonhard Hitzler ◽

Christian Krempaszky ◽

Ewald Werner

Keyword(s):

Residual Stresses ◽

Sample Surface ◽

Treatment Temperature ◽

Ex Situ ◽

Patient Specific ◽

Powder Bed Fusion ◽

Laser Powder Bed Fusion ◽

Corrosion Properties ◽

Powder Bed

The additive manufacturing process of laser powder-bed fusion (L-PBF) is an increasingly popular approach for patient-specific production of dental frameworks made from Co-Cr alloys. Macroscopically, frameworks produced in this way exhibit high anisotropy especially in Young’s modulus, and are missing standardized requirements. Microscopically, pronounced texture and high residual stresses are characteristic. To reduce resulting detrimental effects, the as-built (AB) parts are heat treated. Dependent on the treatment temperature, effects like the transformation of the γ-phase matrix in the AB condition to ϵ-phase, precipitation, stress relief, and grain growth were observed. While the existence of these processes was established in the past, little is known about their kinetics. To fill this gap, these effects were studied with in-situ X-ray diffraction (XRD) methods in isothermal heat treatments (HTs) at four different sample surface temperatures TS reaching from 650∘C to 900∘C. Furthermore, room temperature ex situ XRD and SEM/EDS measurements completed the analysis. An evaluation of the datasets, with single peak fitting and QXRD methods, yielded the following results. In the HTs below a certain threshold, a γ-to-ϵ transformation was observed in the sample bulk and close to the sample surface. In the latter case, evidence for a partially strain-induced transformation related to oxide formation was present. Above this threshold and possibly slightly below, σ- and Laves-phase precipitated. Additionally, peak profile evolutions hinted at a drop of inter- and intragranular stresses within the first 30 to 60 min. Therefore, an HT of about 30 to 60 min slightly above the threshold is proposed as optimal for reducing residual stresses while retaining a predominantly single-phased microstructure, possibly superior in corrosion properties and likewise in bio-compatibility.

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

Advanced Engineering Materials ◽

10.1002/adem.202001502 ◽

2021 ◽

Author(s):

Felix Schmeiser ◽

Erwin Krohmer ◽

Norbert Schell ◽

Eckart Uhlmann ◽

Walter Reimers

Keyword(s):

Phase Transformation ◽

Diffraction Study ◽

Internal Stress ◽

Powder Bed Fusion ◽

Stress Evolution ◽

Laser Powder Bed Fusion ◽

X Ray Diffraction ◽

Powder Bed ◽

X Ray

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An investigation on the suitability of modern nondestructive testing methods for the inspection of specimens manufactured by laser powder bed fusion

SN Applied Sciences ◽

10.1007/s42452-021-04685-3 ◽

2021 ◽

Vol 3 (7) ◽

Author(s):

Cara G. Kolb ◽

Katja Zier ◽

Jan-Carl Grager ◽

Andreas Bachmann ◽

Tobias Neuwirth ◽

...

Keyword(s):

Empirical Studies ◽

Ex Situ ◽

Powder Bed Fusion ◽

Laser Powder Bed Fusion ◽

Detection Capability ◽

Powder Bed ◽

X Ray ◽

Active Infrared Thermography ◽

X Ray Computed ◽

Nondestructive Testing Methods

AbstractLaser powder bed fusion (L-PBF) is increasingly used to fabricate functional parts used in safety-relevant applications. To ensure that the sophisticated part specifications are achieved, 100% quality inspections are performed subsequent to the buildup process. However, knowledge about the detectability of defects in L-PBF parts using NDT methods is limited. This paper analyzes the suitability of NDT techniques in an ex situ environment, in particular active infrared thermography, neutron grating interferometry (nGI), X-ray computed tomography, and ultrasonic testing for the examination of L-PBF parts made from Inconel 718. Based on a test specimen with artificially inserted defects with varying dimensions and depths, these NDT techniques were compared in terms of their attainable resolution and thus defect detection capability. The empirical studies revealed that nGI shows the highest resolution capability. It was possible to detect defects with a diameter of 100–200 m at a depth of 60–80 $${\upmu } \hbox {m}$$ μ m . The results are discussed with regard to their relevance for the examination of L-PBF parts and thus not only contribute to a better understanding of the potential of the NDT techniques in comparison but also assist stakeholders in additive manufacturing in evaluating the suitability of the NDT techniques investigated.

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