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Advanced Characterization and On-Line Process Monitoring of Additively Manufactured Materials...
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Advanced Characterization and On-Line Process Monitoring of Additively Manufactured Materials and Components

A special issue of Metals (ISSN 2075-4701). This special issue belongs to the section "Additive Manufacturing".

Deadline for manuscript submissions: closed (31 March 2022) | Viewed by 52531

Printed Edition Available!
A printed edition of this Special Issue is available here.

Special Issue Editors

Prof. Dr. Giovanni Bruno

E-Mail Website
Guest Editor
Bundesanstalt für Materialforschung und –prüfung (BAM), Berlin, Germany
Interests: neutron diffraction; residual stress; mechanical properties of materials; additive manufacturing; porous ceramics
Special Issues, Collections and Topics in MDPI journals
Dr. Christiane Maierhofer

E-Mail Website
Guest Editor
Federal Institute for Materials Research and Testing (BAM), Berlin, Germany
Interests: active thermography; additive manufacturing; welding; joint connections; composites

Special Issue Information

Dear Colleagues,

Additive manufacturing (AM) techniques have risen to prominence in many industrial sectors. This rapid success of AM is due to the freeform design, which offers enormous possibilities to the engineer, and to the reduction of waste material. Even safety-critical parts are now being produced using AM. This enthusiastic penetration of AM in our daily life is not yet paralleled by a thorough characterization and understanding of the microstructure of materials and of the internal stresses of parts. The same holds true for the understanding of the formation of defects during manufacturing. While simulation efforts are sprouting and some experimental techniques for on-line monitoring are available, still little is known about the propagation of defects throughout the life of a component (from powder to operando conditions).

This Issue aims to collect contributions about the advanced characterization of AM materials and components (especially at large-scale experimental facilities such as Synchrotron and Neutron sources), as well as efforts to liaise online process monitoring to the final product and even to the component during operation. While primarily focused on metallic materials and components, this Issue welcomes contributions dealing with all industrially relevant AM materials and components, such as (but not exhaustively) ceramic implants, polymeric parts, and geopolymers. The goal is to give an overview of advances in the understanding of the impacts of microstructure and defects on component performance and life at several length scales of both defects and parts.

Prof. Dr. Giovanni Bruno
Dr. Christiane Maierhofer
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Metals is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Thermography
  • Microstructure
  • Defects
  • Porosity
  • Mechanical properties
  • Computed tomography
  • Large-scale facilities
  • Residual stress

Published Papers (15 papers)

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Editorial

Jump to: Research, Review

3 pages, 165 KiB  
Editorial
Advanced Characterization and On-Line Process Monitoring of Additively Manufactured Materials and Components
by Giovanni Bruno and Christiane Maierhofer
Metals 2022, 12(9), 1498; https://doi.org/10.3390/met12091498 - 09 Sep 2022
Viewed by 745
Abstract
Additive manufacturing (AM) techniques have risen to prominence in many industrial sectors [...] Full article
(This article belongs to the Special Issue Advanced Characterization and On-Line Process Monitoring of Additively Manufactured Materials and Components)

Research

Jump to: Editorial, Review

21 pages, 5148 KiB  
Article
On the Registration of Thermographic In Situ Monitoring Data and Computed Tomography Reference Data in the Scope of Defect Prediction in Laser Powder Bed Fusion
by Simon Oster, Tobias Fritsch, Alexander Ulbricht, Gunther Mohr, Giovanni Bruno, Christiane Maierhofer and Simon J. Altenburg
Metals 2022, 12(6), 947; https://doi.org/10.3390/met12060947 - 31 May 2022
Cited by 11 | Viewed by 1763
Abstract
The detection of internal irregularities is crucial for quality assessment in metal-based additive manufacturing (AM) technologies such as laser powder bed fusion (L-PBF). The utilization of in-process thermography as an in situ monitoring tool in combination with post-process X-ray micro computed tomography (XCT) [...] Read more.
The detection of internal irregularities is crucial for quality assessment in metal-based additive manufacturing (AM) technologies such as laser powder bed fusion (L-PBF). The utilization of in-process thermography as an in situ monitoring tool in combination with post-process X-ray micro computed tomography (XCT) as a reference technique has shown great potential for this aim. Due to the small irregularity dimensions, a precise registration of the datasets is necessary as a requirement for correlation. In this study, the registration of thermography and XCT reference datasets of a cylindric specimen containing keyhole pores is carried out for the development of a porosity prediction model. The considered datasets show variations in shape, data type and dimensionality, especially due to shrinkage and material elevation effects present in the manufactured part. Since the resulting deformations are challenging for registration, a novel preprocessing methodology is introduced that involves an adaptive volume adjustment algorithm which is based on the porosity distribution in the specimen. Thus, the implementation of a simple three-dimensional image-to-image registration is enabled. The results demonstrate the influence of the part deformation on the resulting porosity location and the importance of registration in terms of irregularity prediction. Full article
(This article belongs to the Special Issue Advanced Characterization and On-Line Process Monitoring of Additively Manufactured Materials and Components)
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10 pages, 3297 KiB  
Article
Measuring the Depth of Subsurface Defects in Additive Manufacturing Components by Laser-Generated Ultrasound
by Zhixiang Xue, Wanli Xu, Yunchao Peng, Mengmeng Wang, Vasiliy Pelenovich, Bing Yang and Jun Zhang
Metals 2022, 12(3), 437; https://doi.org/10.3390/met12030437 - 01 Mar 2022
Cited by 7 | Viewed by 2047
Abstract
A new method to measure the depth of subsurface defects in additive manufacturing components is proposed based on the velocity dispersion analysis of Lamb waves by the wavelet-transform of laser ultrasound. Firstly, the mode-conversion from laser-generated surface waves to Lamb waves caused by [...] Read more.
A new method to measure the depth of subsurface defects in additive manufacturing components is proposed based on the velocity dispersion analysis of Lamb waves by the wavelet-transform of laser ultrasound. Firstly, the mode-conversion from laser-generated surface waves to Lamb waves caused by subsurface defects at different depths is studied systematically. Secondly, an additive manufactured 316L stainless steel sample with six subsurface defects has been fabricated to validate the efficiency of the proposed method. The measured result of the defect depth is very close to the real designed value, with a fitting coefficient of 0.98. The defect depth range for high accuracy measurement is suggested to be lower than 0.8 mm, which is enough to meet the inspection of layer thickness during additive manufacturing. The result indicates that the proposed method based on laser-generated ultrasound (LGU) velocity dispersion analysis is robust and reliable for defect depth measurement and meaningful to improve the processing quality and processing efficiency of additive/subtractive hybrid manufacturing. Full article
(This article belongs to the Special Issue Advanced Characterization and On-Line Process Monitoring of Additively Manufactured Materials and Components)
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14 pages, 1593 KiB  
Article
Relative Density Measurement of PBF-Manufactured 316L and AlSi10Mg Samples via Eddy Current Testing
by Marvin Aaron Spurek, Viet Hiep Luong, Adriaan Bernardus Spierings, Marc Lany, Gilles Santi, Bernard Revaz and Konrad Wegener
Metals 2021, 11(9), 1376; https://doi.org/10.3390/met11091376 - 31 Aug 2021
Cited by 11 | Viewed by 3352
Abstract
Powder bed fusion (PBF) is the most commonly used additive manufacturing process for fabricating complex metal parts via the layer-wise melting of powder. Despite the tremendous recent technological development of PBF, manufactured parts still lack consistent quality in terms of part properties such [...] Read more.
Powder bed fusion (PBF) is the most commonly used additive manufacturing process for fabricating complex metal parts via the layer-wise melting of powder. Despite the tremendous recent technological development of PBF, manufactured parts still lack consistent quality in terms of part properties such as dimensional accuracy, surface roughness, or relative density. In addition to process-inherent variability, this is mainly owing to a knowledge gap in the understanding of process influences and the inability to adequately control them during part production. Eddy current testing (ECT) is a well-established nondestructive testing technique primarily used to detect near-surface defects and measure material properties such as electrical conductivity in metal parts. Hence, it is an appropriate technology for the layer-wise measuring of the material properties of the fused material in PBF. This study evaluates ECT’s potential as a novel in situ monitoring technology for relative part density in PBF. Parts made from SS316L and AlSi10Mg with different densities are manufactured on a PBF machine. These parts are subsequently measured using ECT, as well as the resulting signals correlated with the relative part density. The results indicate a statistically significant and strong correlation (316L: r(8) = 0.998, p < 0.001, AlSi10Mg: r(8) = 0.992, p < 0.001) between relative part density and the ECT signal component, which is mainly affected by the electrical conductivity of the part. The results indicate that ECT has the potential to evolve into an effective technology for the layer-wise measuring of relative part density during the PBF process. Full article
(This article belongs to the Special Issue Advanced Characterization and On-Line Process Monitoring of Additively Manufactured Materials and Components)
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26 pages, 6848 KiB  
Article
Process Induced Preheating in Laser Powder Bed Fusion Monitored by Thermography and Its Influence on the Microstructure of 316L Stainless Steel Parts
by Gunther Mohr, Konstantin Sommer, Tim Knobloch, Simon J. Altenburg, Sebastian Recknagel, Dirk Bettge and Kai Hilgenberg
Metals 2021, 11(7), 1063; https://doi.org/10.3390/met11071063 - 01 Jul 2021
Cited by 17 | Viewed by 3204
Abstract
Undetected and undesired microstructural variations in components produced by laser powder bed fusion are a major challenge, especially for safety-critical components. In this study, an in-depth analysis of the microstructural features of 316L specimens produced by laser powder bed fusion at different levels [...] Read more.
Undetected and undesired microstructural variations in components produced by laser powder bed fusion are a major challenge, especially for safety-critical components. In this study, an in-depth analysis of the microstructural features of 316L specimens produced by laser powder bed fusion at different levels of volumetric energy density and different levels of inter layer time is reported. The study has been conducted on specimens with an application relevant build height (>100 mm). Furthermore, the evolution of the intrinsic preheating temperature during the build-up of specimens was monitored using a thermographic in-situ monitoring set-up. By applying recently determined emissivity values of 316L powder layers, real temperatures could be quantified. Heat accumulation led to preheating temperatures of up to about 600 °C. Significant differences in the preheating temperatures were discussed with respect to the individual process parameter combinations, including the build height. A strong effect of the inter layer time on the heat accumulation was observed. A shorter inter layer time resulted in an increase of the preheating temperature by more than a factor of 2 in the upper part of the specimens compared to longer inter layer times. This, in turn, resulted in heterogeneity of the microstructure and differences in material properties within individual specimens. The resulting differences in the microstructure were analyzed using electron back scatter diffraction and scanning electron microscopy. Results from chemical analysis as well as electron back scatter diffraction measurements indicated stable conditions in terms of chemical alloy composition and austenite phase content for the used set of parameter combinations. However, an increase of the average grain size by more than a factor of 2.5 could be revealed within individual specimens. Additionally, differences in feature size of the solidification cellular substructure were examined and a trend of increasing cell sizes was observed. This trend was attributed to differences in solidification rate and thermal gradients induced by differences in scanning velocity and preheating temperature. A change of the thermal history due to intrinsic preheating could be identified as the main cause of this heterogeneity. It was induced by critical combinations of the energy input and differences in heat transfer conditions by variations of the inter layer time. The microstructural variations were directly correlated to differences in hardness. Full article
(This article belongs to the Special Issue Advanced Characterization and On-Line Process Monitoring of Additively Manufactured Materials and Components)
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14 pages, 8303 KiB  
Article
Can Potential Defects in LPBF Be Healed from the Laser Exposure of Subsequent Layers? A Quantitative Study
by Alexander Ulbricht, Gunther Mohr, Simon J. Altenburg, Simon Oster, Christiane Maierhofer and Giovanni Bruno
Metals 2021, 11(7), 1012; https://doi.org/10.3390/met11071012 - 24 Jun 2021
Cited by 17 | Viewed by 2740
Abstract
Additive manufacturing (AM) of metals and in particular laser powder bed fusion (LPBF) enables a degree of freedom in design unparalleled by conventional subtractive methods. To ensure that the designed precision is matched by the produced LPBF parts, a full understanding of the [...] Read more.
Additive manufacturing (AM) of metals and in particular laser powder bed fusion (LPBF) enables a degree of freedom in design unparalleled by conventional subtractive methods. To ensure that the designed precision is matched by the produced LPBF parts, a full understanding of the interaction between the laser and the feedstock powder is needed. It has been shown that the laser also melts subjacent layers of material underneath. This effect plays a key role when designing small cavities or overhanging structures, because, in these cases, the material underneath is feed-stock powder. In this study, we quantify the extension of the melt pool during laser illumination of powder layers and the defect spatial distribution in a cylindrical specimen. During the LPBF process, several layers were intentionally not exposed to the laser beam at various locations, while the build process was monitored by thermography and optical tomography. The cylinder was finally scanned by X-ray computed tomography (XCT). To correlate the positions of the unmolten layers in the part, a staircase was manufactured around the cylinder for easier registration. The results show that healing among layers occurs if a scan strategy is applied, where the orientation of the hatches is changed for each subsequent layer. They also show that small pores and surface roughness of solidified material below a thick layer of unmolten material (>200 µm) serve as seeding points for larger voids. The orientation of the first two layers fully exposed after a thick layer of unmolten powder shapes the orientation of these voids, created by a lack of fusion. Full article
(This article belongs to the Special Issue Advanced Characterization and On-Line Process Monitoring of Additively Manufactured Materials and Components)
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17 pages, 6735 KiB  
Article
Radiographic Visibility Limit of Pores in Metal Powder for Additive Manufacturing
by Gerd-Rüdiger Jaenisch, Uwe Ewert, Anja Waske and Alexander Funk
Metals 2020, 10(12), 1634; https://doi.org/10.3390/met10121634 - 04 Dec 2020
Cited by 2 | Viewed by 2459
Abstract
The quality of additively manufactured (AM) parts is determined by the applied process parameters used and the properties of the feedstock powder. The influence of inner gas pores in feedstock particles on the final AM product is a phenomenon which is difficult to [...] Read more.
The quality of additively manufactured (AM) parts is determined by the applied process parameters used and the properties of the feedstock powder. The influence of inner gas pores in feedstock particles on the final AM product is a phenomenon which is difficult to investigate since very few non-destructive measurement techniques are accurate enough to resolve the micropores. 3D X-ray computed tomography (XCT) is increasingly applied during the process chain of AM parts as a non-destructive monitoring and quality control tool and it is able to detect most of the pores. However, XCT is time-consuming and limited to small amounts of feedstock powder, typically a few milligrams. The aim of the presented approach is to investigate digital radiography of AM feedstock particles as a simple and fast quality check with high throughput. 2D digital radiographs were simulated in order to predict the visibility of pores inside metallic particles for different pore and particle diameters. An experimental validation was performed. It was demonstrated numerically and experimentally that typical gas pores above a certain size (here: 3 to 4.4 µm for the selected X-ray setup), which could be found in metallic microparticles, were reliably detected by digital radiography. Full article
(This article belongs to the Special Issue Advanced Characterization and On-Line Process Monitoring of Additively Manufactured Materials and Components)
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26 pages, 6042 KiB  
Article
Experimental Determination of the Emissivity of Powder Layers and Bulk Material in Laser Powder Bed Fusion Using Infrared Thermography and Thermocouples
by Gunther Mohr, Susanna Nowakowski, Simon J. Altenburg, Christiane Maierhofer and Kai Hilgenberg
Metals 2020, 10(11), 1546; https://doi.org/10.3390/met10111546 - 20 Nov 2020
Cited by 26 | Viewed by 4144
Abstract
Recording the temperature distribution of the layer under construction during laser powder bed fusion (L-PBF) is of utmost interest for a deep process understanding as well as for quality assurance and in situ monitoring means. While having a notable number of thermal monitoring [...] Read more.
Recording the temperature distribution of the layer under construction during laser powder bed fusion (L-PBF) is of utmost interest for a deep process understanding as well as for quality assurance and in situ monitoring means. While having a notable number of thermal monitoring approaches in additive manufacturing (AM), attempts at temperature calibration and emissivity determination are relatively rare. This study aims for the experimental temperature adjustment of an off-axis infrared (IR) thermography setup used for in situ thermal data acquisition in L-PBF processes. The temperature adjustment was conducted by means of the so-called contact method using thermocouples at two different surface conditions and two different materials: AISI 316L L-PBF bulk surface, AISI 316L powder surface, and IN718 powder surface. The apparent emissivity values for the particular setup were determined. For the first time, also corrected, closer to real emissivity values of the bulk or powder surface condition are published. In the temperature region from approximately 150 °C to 580 °C, the corrected emissivity was determined in a range from 0.2 to 0.25 for a 316L L-PBF bulk surface, in a range from 0.37 to 0.45 for 316L powder layer, and in a range from 0.37 to 0.4 for IN718 powder layer. Full article
(This article belongs to the Special Issue Advanced Characterization and On-Line Process Monitoring of Additively Manufactured Materials and Components)
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17 pages, 6409 KiB  
Article
Use of X-ray Computed Tomography for Assessing Defects in Ti Grade 5 Parts Produced by Laser Melting Deposition
by Diana Chioibasu, Sabin Mihai, Muhammad Arif Mahmood, Mihail Lungu, Ioana Porosnicu, Adrian Sima, Cosmin Dobrea, Ion Tiseanu and Andrei C. Popescu
Metals 2020, 10(11), 1408; https://doi.org/10.3390/met10111408 - 23 Oct 2020
Cited by 13 | Viewed by 3445
Abstract
Laser Melting Deposition (LMD) is a metal printing technique that allows for the manufacturing of large objects by Directed Energy Deposition. Due to its versatility in variation of parameters, the possibility to use two or more materials, to create alloys in situ or [...] Read more.
Laser Melting Deposition (LMD) is a metal printing technique that allows for the manufacturing of large objects by Directed Energy Deposition. Due to its versatility in variation of parameters, the possibility to use two or more materials, to create alloys in situ or produce multi-layer structures, LMD is still being scientifically researched and is still far from industrial maturity. The structural testing of obtained samples can be time consuming and solutions that can decrease the samples analysis time are constantly proposed in the scientific literature. In this manuscript we present a quality improvement study for obtaining defect-free bulk samples of Ti6Al4V under X-Ray Computed Tomography (XCT) by varying the hatch spacing and distance between planes. Based on information provided by XCT, the experimental conditions were changed until complete elimination of porosity. Information on the defects in the bulk of the samples by XCT was used for feedback during parameters tuning in view of complete removal of pores. The research time was reduced to days instead of weeks or months of samples preparation and analysis by destructive metallographic techniques. Full article
(This article belongs to the Special Issue Advanced Characterization and On-Line Process Monitoring of Additively Manufactured Materials and Components)
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15 pages, 17967 KiB  
Article
Separation of the Formation Mechanisms of Residual Stresses in LPBF 316L
by Alexander Ulbricht, Simon J. Altenburg, Maximilian Sprengel, Konstantin Sommer, Gunther Mohr, Tobias Fritsch, Tatiana Mishurova, Itziar Serrano-Munoz, Alexander Evans, Michael Hofmann and Giovanni Bruno
Metals 2020, 10(9), 1234; https://doi.org/10.3390/met10091234 - 14 Sep 2020
Cited by 21 | Viewed by 3938
Abstract
Rapid cooling rates and steep temperature gradients are characteristic of additively manufactured parts and important factors for the residual stress formation. This study examined the influence of heat accumulation on the distribution of residual stress in two prisms produced by Laser Powder Bed [...] Read more.
Rapid cooling rates and steep temperature gradients are characteristic of additively manufactured parts and important factors for the residual stress formation. This study examined the influence of heat accumulation on the distribution of residual stress in two prisms produced by Laser Powder Bed Fusion (LPBF) of austenitic stainless steel 316L. The layers of the prisms were exposed using two different border fill scan strategies: one scanned from the centre to the perimeter and the other from the perimeter to the centre. The goal was to reveal the effect of different heat inputs on samples featuring the same solidification shrinkage. Residual stress was characterised in one plane perpendicular to the building direction at the mid height using Neutron and Lab X-ray diffraction. Thermography data obtained during the build process were analysed in order to correlate the cooling rates and apparent surface temperatures with the residual stress results. Optical microscopy and micro computed tomography were used to correlate defect populations with the residual stress distribution. The two scanning strategies led to residual stress distributions that were typical for additively manufactured components: compressive stresses in the bulk and tensile stresses at the surface. However, due to the different heat accumulation, the maximum residual stress levels differed. We concluded that solidification shrinkage plays a major role in determining the shape of the residual stress distribution, while the temperature gradient mechanism appears to determine the magnitude of peak residual stresses. Full article
(This article belongs to the Special Issue Advanced Characterization and On-Line Process Monitoring of Additively Manufactured Materials and Components)
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15 pages, 6489 KiB  
Article
Residual Stress and Microstructure of a Ti-6Al-4V Wire Arc Additive Manufacturing Hybrid Demonstrator
by Tatiana Mishurova, Benjamin Sydow, Tobias Thiede, Irina Sizova, Alexander Ulbricht, Markus Bambach and Giovanni Bruno
Metals 2020, 10(6), 701; https://doi.org/10.3390/met10060701 - 26 May 2020
Cited by 14 | Viewed by 4351
Abstract
Wire Arc Additive Manufacturing (WAAM) features high deposition rates and, thus, allows production of large components that are relevant for aerospace applications. However, a lot of aerospace parts are currently produced by forging or machining alone to ensure fast production and to obtain [...] Read more.
Wire Arc Additive Manufacturing (WAAM) features high deposition rates and, thus, allows production of large components that are relevant for aerospace applications. However, a lot of aerospace parts are currently produced by forging or machining alone to ensure fast production and to obtain good mechanical properties; the use of these conventional process routes causes high tooling and material costs. A hybrid approach (a combination of forging and WAAM) allows making production more efficient. In this fashion, further structural or functional features can be built in any direction without using additional tools for every part. By using a combination of forging basic geometries with one tool set and adding the functional features by means of WAAM, the tool costs and material waste can be reduced compared to either completely forged or machined parts. One of the factors influencing the structural integrity of additively manufactured parts are (high) residual stresses, generated during the build process. In this study, the triaxial residual stress profiles in a hybrid WAAM part are reported, as determined by neutron diffraction. The analysis is complemented by microstructural investigations, showing a gradient of microstructure (shape and size of grains) along the part height. The highest residual stresses were found in the transition zone (between WAAM and forged part). The total stress range showed to be lower than expected for WAAM components. This could be explained by the thermal history of the component. Full article
(This article belongs to the Special Issue Advanced Characterization and On-Line Process Monitoring of Additively Manufactured Materials and Components)
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19 pages, 9763 KiB  
Article
Effect of Stress-Relieving Heat Treatment on the High Strain Rate Dynamic Compressive Properties of Additively Manufactured Ti6Al4V (ELI)
by Amos Muiruri, Maina Maringa, Willie du Preez and Leonard Masu
Metals 2020, 10(5), 653; https://doi.org/10.3390/met10050653 - 18 May 2020
Cited by 9 | Viewed by 2474
Abstract
A study was undertaken on the compressive high strain rate properties and deformation behaviour of Direct Metal Laser-Sintered (DMLS) Ti6Al4V (ELI) parts in two separate forms: as-built (AB) and stress relieved (SR). The high strain rate compression tests were carried out using a [...] Read more.
A study was undertaken on the compressive high strain rate properties and deformation behaviour of Direct Metal Laser-Sintered (DMLS) Ti6Al4V (ELI) parts in two separate forms: as-built (AB) and stress relieved (SR). The high strain rate compression tests were carried out using a Split Hopkinson Pressure Bar test system at ambient temperature. The average plastic strain rates attained by the system were 400 s−1 and 700 s−1. Comparative analyses of the performance (flow stresses and fracture strains) of AB and SR specimens were carried out based on the results obtained at these two plastic strain rates. Microstructural analyses were performed to study the failure mechanisms of the deformed specimens and fracture surfaces. Vickers microhardness test values were obtained before and after high strain rate compression testing. The results obtained in both cases showed the strain rate sensitivity of the stress-relieved samples to be higher in comparison to those of as-built ones, at the same value of true strain. Full article
(This article belongs to the Special Issue Advanced Characterization and On-Line Process Monitoring of Additively Manufactured Materials and Components)
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19 pages, 5915 KiB  
Article
In-Situ Defect Detection in Laser Powder Bed Fusion by Using Thermography and Optical Tomography—Comparison to Computed Tomography
by Gunther Mohr, Simon J. Altenburg, Alexander Ulbricht, Philipp Heinrich, Daniel Baum, Christiane Maierhofer and Kai Hilgenberg
Metals 2020, 10(1), 103; https://doi.org/10.3390/met10010103 - 09 Jan 2020
Cited by 89 | Viewed by 8424
Abstract
Among additive manufacturing (AM) technologies, the laser powder bed fusion (L-PBF) is one of the most important technologies to produce metallic components. The layer-wise build-up of components and the complex process conditions increase the probability of the occurrence of defects. However, due to [...] Read more.
Among additive manufacturing (AM) technologies, the laser powder bed fusion (L-PBF) is one of the most important technologies to produce metallic components. The layer-wise build-up of components and the complex process conditions increase the probability of the occurrence of defects. However, due to the iterative nature of its manufacturing process and in contrast to conventional manufacturing technologies such as casting, L-PBF offers unique opportunities for in-situ monitoring. In this study, two cameras were successfully tested simultaneously as a machine manufacturer independent process monitoring setup: a high-frequency infrared camera and a camera for long time exposure, working in the visible and infrared spectrum and equipped with a near infrared filter. An AISI 316L stainless steel specimen with integrated artificial defects has been monitored during the build. The acquired camera data was compared to data obtained by computed tomography. A promising and easy to use examination method for data analysis was developed and correlations between measured signals and defects were identified. Moreover, sources of possible data misinterpretation were specified. Lastly, attempts for automatic data analysis by data integration are presented. Full article
(This article belongs to the Special Issue Advanced Characterization and On-Line Process Monitoring of Additively Manufactured Materials and Components)
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13 pages, 4601 KiB  
Article
Exploring the Correlation between Subsurface Residual Stresses and Manufacturing Parameters in Laser Powder Bed Fused Ti-6Al-4V
by Tatiana Mishurova, Katia Artzt, Jan Haubrich, Guillermo Requena and Giovanni Bruno
Metals 2019, 9(2), 261; https://doi.org/10.3390/met9020261 - 22 Feb 2019
Cited by 38 | Viewed by 3921
Abstract
Subsurface residual stresses (RS) were investigated in Ti-6Al-4V cuboid samples by means of X-ray synchrotron diffraction. The samples were manufactured by laser powder bed fusion (LPBF) applying different processing parameters, not commonly considered in open literature, in order to assess their influence on [...] Read more.
Subsurface residual stresses (RS) were investigated in Ti-6Al-4V cuboid samples by means of X-ray synchrotron diffraction. The samples were manufactured by laser powder bed fusion (LPBF) applying different processing parameters, not commonly considered in open literature, in order to assess their influence on RS state. While investigating the effect of process parameters used for the calculation of volumetric energy density (such as laser velocity, laser power and hatch distance), we observed that an increase of energy density led to a decrease of RS, although not to the same extent for every parameter variation. Additionally, the effect of support structure, sample roughness and LPBF machine effects potentially coming from Ar flow were studied. We observed no influence of support structure on subsurface RS while the orientation with respect to Ar flow showed to have an impact on RS. We conclude recommending monitoring such parameters to improve part reliability and reproducibility. Full article
(This article belongs to the Special Issue Advanced Characterization and On-Line Process Monitoring of Additively Manufactured Materials and Components)
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Review

Jump to: Editorial, Research

34 pages, 32678 KiB  
Review
Diffraction-Based Residual Stress Characterization in Laser Additive Manufacturing of Metals
by Jakob Schröder, Alexander Evans, Tatiana Mishurova, Alexander Ulbricht, Maximilian Sprengel, Itziar Serrano-Munoz, Tobias Fritsch, Arne Kromm, Thomas Kannengießer and Giovanni Bruno
Metals 2021, 11(11), 1830; https://doi.org/10.3390/met11111830 - 13 Nov 2021
Cited by 18 | Viewed by 4038
Abstract
Laser-based additive manufacturing methods allow the production of complex metal structures within a single manufacturing step. However, the localized heat input and the layer-wise manufacturing manner give rise to large thermal gradients. Therefore, large internal stress (IS) during the process (and consequently residual [...] Read more.
Laser-based additive manufacturing methods allow the production of complex metal structures within a single manufacturing step. However, the localized heat input and the layer-wise manufacturing manner give rise to large thermal gradients. Therefore, large internal stress (IS) during the process (and consequently residual stress (RS) at the end of production) is generated within the parts. This IS or RS can either lead to distortion or cracking during fabrication or in-service part failure, respectively. With this in view, the knowledge on the magnitude and spatial distribution of RS is important to develop strategies for its mitigation. Specifically, diffraction-based methods allow the spatial resolved determination of RS in a non-destructive fashion. In this review, common diffraction-based methods to determine RS in laser-based additive manufactured parts are presented. In fact, the unique microstructures and textures associated to laser-based additive manufacturing processes pose metrological challenges. Based on the literature review, it is recommended to (a) use mechanically relaxed samples measured in several orientations as appropriate strain-free lattice spacing, instead of powder, (b) consider that an appropriate grain-interaction model to calculate diffraction-elastic constants is both material- and texture-dependent and may differ from the conventionally manufactured variant. Further metrological challenges are critically reviewed and future demands in this research field are discussed. Full article
(This article belongs to the Special Issue Advanced Characterization and On-Line Process Monitoring of Additively Manufactured Materials and Components)
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