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New Materials and Understandings in Selective Laser Melting (SLM)

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A special issue of Materials (ISSN 1996-1944). This special issue belongs to the section "Manufacturing Processes and Systems".

Deadline for manuscript submissions: closed (31 July 2021) | Viewed by 43752

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Special Issue Editors

Prof. Christopher Tuck

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Guest Editor

Faculty of Engineering, University of Nottingham, Nottingham, UK
Interests: additive manufacturing; 3D printing; materials science; materials engineering; metals; polymers

Dr. Nesma Aboulkhair

E-Mail Website
Guest Editor

Faculty of Engineering, University of Nottingham, Nottingham, UK
Interests: additive manufacturing; metals, materials science; mechanical peformance; materials engineering

Special Issue Information

Dear Colleagues,

Selective laser melting (SLM) is a powder-bed fusion additive manufacturing technique used to fabricate intricate structures with unmatched degrees of complexity. Critical to this process is the feedstock material. Immense research efforts have been spent on using readily available alloys. However, in SLM, the material is irradiated with a laser beam causing rapid melting and solidification, imposing significantly different thermal experiences. Therefore, designing new alloys specifically tailored to the process or modifying available alloys is sought.

The process–material–property relationship in SLM is markedly complex. An understanding of how SLM affects the process of designing new alloys is essential to heightening the momentum in this field. The focus of this Special Issue is on approaches to developing new materials tailored to SLM and the new understandings needed to overcome the barriers to wider adoption.

It is our pleasure to invite you to submit a manuscript to this Special Issue. Full papers, communications and reviews are all welcome.

Prof. Christopher Tuck
Dr. Nesma Aboulkhair
Guest Editors

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Keywords

selective laser melting
alloy development
metallurgy
mechanical properties
melt pool dynamics
powderbed fusion.

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Published Papers (11 papers)

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Research

18 pages, 21497 KiB

Open AccessArticle

Influence of Minor Alloying Element Additions on the Crack Susceptibility of a Nickel Based Superalloy Manufactured by LPBF

by Mireia Vilanova, Mari Carmen Taboada, Ana Martinez-Amesti, Andrea Niklas, Maria San Sebastian and Teresa Guraya

Materials 2021, 14(19), 5702; https://doi.org/10.3390/ma14195702 - 30 Sep 2021

Cited by 17 | Viewed by 2405

Abstract

Inconel 738LC (IN738LC) is a nickel-based superalloy specially used in the hot section components of turbine engines. One of its main drawbacks relies on the cracking susceptibility when it is manufactured by laser powder bed fusion (LPBF). This paper analyzes the influence of minor alloying element concentration on cracking tendency of IN738LC superalloy manufactured by LPBF. For that objective, samples were manufactured using two powders, which presented different minor alloying elements concentration (Si, Zr and B). It was shown that the samples crack tendency was very different depending on the powder used for their manufacturing. In fact, the measured crack density value was 2.73 mm/mm² for the samples manufactured with the powder with higher minor alloying elements concentration, while 0.25 mm/mm² for the others. Additionally, a special emphasis has been put on elemental composition characterization in cracked grain boundaries in order to quantify possible Si or Zr enrichment. It has been also studied the differences of solidification ranges and grain structures between both samples as a consequence of different minor alloying elements concentration in order to analyze their effect on crack susceptibility. In this sense, Scheil-Gulliver simulation results have shown that samples with higher Si and Zr contents presented higher solidification range temperature. This fact, as well as an increase of the presence of high angle grain boundaries (HAGB), leaded to an increment in the crack formation during solidification. Therefore, in this research work, an understanding of the factors affecting crack phenomenon in the LPBF manufactured IN738LC was accomplished. Full article

(This article belongs to the Special Issue New Materials and Understandings in Selective Laser Melting (SLM))

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14 pages, 2498 KiB

Open AccessArticle

Absorption of Nitrogen during Pulsed Wave L-PBF of 17-4 PH Steel

by Ben Brown, Joseph Newkirk and Frank Liou

Materials 2021, 14(3), 560; https://doi.org/10.3390/ma14030560 - 25 Jan 2021

Cited by 10 | Viewed by 2294

Abstract

In the fabrication of 17-4 PH by laser powder bed fusion (L-PBF) the well-documented occurrence of large amounts of retained austenite can be attributed to an elevated concentration of nitrogen present in the material. While the effects of continuous wave (CW) laser processing on in-situ nitrogen absorption characteristics have been evaluated, power modulated pulsed wave (PW) laser processing effects have not. In this study the effects of PW L-PBF processing of 17-4 PH on nitrogen absorption, phase composition, and mechanical performance are explored using commercially available PW L-PBF equipment and compared to samples produced by CW L-PBF. PW L-PBF samples fabricated in cover gas conditions with varying amounts of nitrogen demonstrated reduced absorption levels compared to those produced by CW L-PBF with no effects on phase composition and minimal effects on mechanical performance. Full article

(This article belongs to the Special Issue New Materials and Understandings in Selective Laser Melting (SLM))

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20 pages, 4892 KiB

Open AccessArticle

An Empirical Approach for the Development of Process Parameters for Laser Powder Bed Fusion

by Aron Pfaff, Martin Jäcklein, Max Schlager, Wilfried Harwick, Klaus Hoschke and Frank Balle

Materials 2020, 13(23), 5400; https://doi.org/10.3390/ma13235400 - 27 Nov 2020

Cited by 19 | Viewed by 2945

Abstract

For certain additive manufacturing technologies the choice of available materials is currently limited. The development of process parameters is especially elaborate for powder bed technologies. Currently, there is no common approach concerning the procedure and documentation. This work proposes a methodology for the initial development of process parameters for new L-PBF (laser powder bed fusion) alloys. Key elements are the examination of the laser–powder-bed interaction by single laser track experiments and an iterative design of experiment (DoE) approach for the development of volumetric parameters. Two types of single laser track experiments are presented and provide information regarding the laser track width and depth as well as the resulting surface roughness and melt pool classification. Based on the information gained, suitable process windows for a DoE study can be defined by avoiding parameter settings unsuitable for production or measurement. Gradually, input variables are identified and iterative steps reduce the process window in order to optimize the desired target values. Near-surface exposure parameters are developed by a one-dimensional parameter variation and metallographic investigations. The approach is primarily designed for the initial development of process parameters for new L-PBF alloys. However, the information gained can also be used to optimize established parameter sets regarding new target values (productivity, mechanical properties), optimize process parameters for specific components or for a microstructural design. Full article

(This article belongs to the Special Issue New Materials and Understandings in Selective Laser Melting (SLM))

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16 pages, 5334 KiB

Open AccessArticle

Manufacturing Aluminum/Multiwalled Carbon Nanotube Composites via Laser Powder Bed Fusion

by Eo Ryeong Lee, Se Eun Shin, Naoki Takata, Makoto Kobashi and Masaki Kato

Materials 2020, 13(18), 3927; https://doi.org/10.3390/ma13183927 - 5 Sep 2020

Cited by 9 | Viewed by 3029

Abstract

This study provides a novel approach to fabricating Al/C composites using laser powder bed fusion (LPBF) for a wide range of structural applications utilizing Al-matrix composites in additive manufacturing. We investigated the effects of LPBF on the fabrication of aluminum/multiwalled carbon nanotube (Al/MWCNT) composites under 25 different conditions, using varying laser power levels and scan speeds. The microstructures and mechanical properties of the specimens, such as elastic modulus and nanohardness, were analyzed, and trends were identified. We observed favorable sintering behavior under laser conditions with low energy density, which verified the suitability of Al/MWCNT composites for a fabrication process using LPBF. The size and number of pores increased in specimens produced under high energy density conditions, suggesting that they are more influenced by laser power than scan speed. Similarly, the elastic modulus of a specimen was also more affected by laser power than scan speed. In contrast, scan speed had a greater influence on the final nanohardness. Depending on the laser power used, we observed a difference in the crystallographic orientation of the specimens by a laser power during LPBF. When energy density is high, texture development of all samples tended to be more pronounced. Full article

(This article belongs to the Special Issue New Materials and Understandings in Selective Laser Melting (SLM))

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16 pages, 7357 KiB

Open AccessArticle

Compressive Properties of Al-Si Alloy Lattice Structures with Three Different Unit Cells Fabricated via Laser Powder Bed Fusion

by Xiaoyang Liu, Keito Sekizawa, Asuka Suzuki, Naoki Takata, Makoto Kobashi and Tetsuya Yamada

Materials 2020, 13(13), 2902; https://doi.org/10.3390/ma13132902 - 28 Jun 2020

Cited by 30 | Viewed by 4898

Abstract

In the present study, in order to elucidate geometrical features dominating deformation behaviors and their associated compressive properties of lattice structures, AlSi10Mg lattice structures with three different unit cells were fabricated by laser powder bed fusion. Compressive properties were examined by compression and indentation tests, micro X-ray computed tomography (CT), together with finite element analysis. The truncated octahedron- unit cell (TO) lattice structures exhibited highest stiffness and plateau stress among the studied lattice structures. The body centered cubic-unit cell (BCC) and TO lattice structures experienced the formation of shear bands with stress drops, while the hexagon-unit cell (Hexa) lattice structure behaved in a continuous deformation and flat plateau region. The Hexa lattice structure densified at a smaller strain than the BCC and TO lattice structures, due to high density of the struts in the compressive direction. Static and high-speed indentation tests revealed that the TO and Hexa exhibited slight strain rate dependence of the compressive strength, whereas the BCC lattice structure showed a large strain rate dependence. Among the lattice structures in the present study, the TO lattice exhibited the highest energy absorption capacity comparable to previously reported titanium alloy lattice structures. Full article

(This article belongs to the Special Issue New Materials and Understandings in Selective Laser Melting (SLM))

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17 pages, 6919 KiB

Open AccessArticle

Selective Laser Melting of Ti-6Al-4V: The Impact of Post-processing on the Tensile, Fatigue and Biological Properties for Medical Implant Applications

by Parastoo Jamshidi, Miren Aristizabal, Weihuan Kong, Victor Villapun, Sophie C. Cox, Liam M. Grover and Moataz M. Attallah

Materials 2020, 13(12), 2813; https://doi.org/10.3390/ma13122813 - 22 Jun 2020

Cited by 86 | Viewed by 6341

Abstract

One of the main challenges in additive manufacturing (AM) of medical implants for the treatment of bone tissue defects is to optimise the mechanical and biological performance. The use of post-processing can be a necessity to improve the physical properties of customised AM processed implants. In this study, Ti-6Al-4V coupons were manufactured using selective laser melting (SLM) in two build orientations (vertical and horizontal) and subsequently post-processed using combinations of hot isostatic pressing (HIP), sandblasting (SB), polishing (PL) and chemical etching (CE). The effect of the different post-manufacturing strategies on the tensile and fatigue performance of the SLMed parts was investigated and rationalised by observing the surface topography. Vertically built samples showed higher yield strength (YS) and ultimate tensile strength (UTS) than the horizontal samples, increasing from 760.9 ± 22.3 MPa and 961.3 ± 50.2 MPa in the horizontal condition to 820.09 ± 16.5 MPa and 1006.7 ± 6.3 MPa in the vertical condition, respectively. After the HIP treatment, the ductility was substantially improved in both orientations; by 2.1 and 2.9 folds in the vertical and horizontal orientations, respectively. The vertically built samples demonstrated a superior ductility of 22% following HIP and polishing. Furthermore, chemical etching was found to be the most effective surface post-processing treatment to improve the fatigue performance after HIP, achieving the highest run-out strength of 450 MPa. Most importantly, chemical etching after HIP enhanced the cellular affinity of the surface, in addition to its good fatigue performance, making it a promising post-processing approach for bone implants where tissue integration is needed. Full article

(This article belongs to the Special Issue New Materials and Understandings in Selective Laser Melting (SLM))

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17 pages, 9081 KiB

Open AccessArticle

Effect of Heat Treatment on Gradient Microstructure of AlSi10Mg Lattice Structure Manufactured by Laser Powder Bed Fusion

by Mulin Liu, Naoki Takata, Asuka Suzuki and Makoto Kobashi

Materials 2020, 13(11), 2487; https://doi.org/10.3390/ma13112487 - 29 May 2020

Cited by 23 | Viewed by 4243

Abstract

The present study addressed the effect of heat treatment process on microstructure of an AlSi10Mg lattice structure with a body-centered cubic unit cell manufactured via laser powder bed fusion (LPBF). The as-manufactured lattice specimen exhibited a unique cellular structure composing of primary α-Al phases bounded by α-Al/Si eutectic microstructure. A gradient microstructure (continuous microstructural changes) was found in the node and strut portions composed of the lattice specimen. The microstructure appears more equiaxed and coarser with approaching the bottom surface of both portions. The continuous microstructural changes contributed to a variation in hardness measured at different locations in the as-manufactured lattice specimen. Si particles finely precipitate in the primary α-Al phases, and eutectic Si particle coarsening occurs at an elevated temperature of 300 °C. The microstructural coarsening is more pronounced at a higher temperature. A number of significantly coarsened Si particles and a stable Fe-containing intermetallic phase (β-AlFeSi) were observed at all locations in 530 °C solution-treated specimen. The homogenous microstructure results in a constant hardness value independent of the location in the lattice specimen. These results provide new insights to control the compressive properties of the AlSi10Mg lattice structure manufactured via LPBF by subsequent heat treatment processes. Full article

(This article belongs to the Special Issue New Materials and Understandings in Selective Laser Melting (SLM))

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17 pages, 9020 KiB

Open AccessFeature PaperArticle

Heat Treatments and Critical Quenching Rates in Additively Manufactured Al–Si–Mg Alloys

by Leonhard Hitzler, Stephan Hafenstein, Francisca Mendez Martin, Helmut Clemens, Enes Sert, Andreas Öchsner, Markus Merkel and Ewald Werner

Materials 2020, 13(3), 720; https://doi.org/10.3390/ma13030720 - 5 Feb 2020

Cited by 20 | Viewed by 3418

Abstract

Laser powder-bed fusion (LPBF) has significantly gained in importance and has become one of the major fabrication techniques within metal additive manufacturing. The fast cooling rates achieved in LPBF due to a relatively small melt pool on a much larger component or substrate, acting as heat sink, result in fine-grained microstructures and high oversaturation of alloying elements in the α-aluminum. Al–Si–Mg alloys thus can be effectively precipitation hardened. Moreover, the solidified material undergoes an intrinsic heat treatment, whilst the layers above are irradiated and the elevated temperature in the built chamber starts the clustering process of alloying elements directly after a scan track is fabricated. These silicon–magnesium clusters were observed with atom probe tomography in as-built samples. Similar beneficial clustering behavior at higher temperatures is known from the direct-aging approach in cast samples, whereby the artificial aging is performed immediately after solution annealing and quenching. Transferring this approach to LPBF samples as a possible post-heat treatment revealed that even after direct aging, the outstanding hardness of the as-built condition could, at best, be met, but for most instances it was significantly lower. Our investigations showed that LPBF Al–Si–Mg exhibited a high dependency on the quenching rate, which is significantly more pronounced than in cast reference samples, requiring two to three times higher quenching rate after solution annealing to yield similar hardness results. This suggests that due to the finer microstructure and the shorter diffusion path in Al–Si–Mg fabricated by LPBF, it is more challenging to achieve a metastable oversaturation necessary for precipitation hardening. This may be especially problematic in larger components. Full article

(This article belongs to the Special Issue New Materials and Understandings in Selective Laser Melting (SLM))

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14 pages, 5092 KiB

Open AccessArticle

The Effects of Feature Sizes in Selectively Laser Melted Ti-6Al-4V Parts on the Validity of Optimised Process Parameters

by Chinmay Phutela, Nesma T. Aboulkhair, Christopher J. Tuck and Ian Ashcroft

Materials 2020, 13(1), 117; https://doi.org/10.3390/ma13010117 - 26 Dec 2019

Cited by 57 | Viewed by 5654

Abstract

Ti-6Al-4V is a popular alloy due to its high strength-to-weight ratio and excellent corrosion resistance. Many applications of additively manufactured Ti-6Al-4V using selective laser melting (SLM) have reached technology readiness. However, issues linked with metallurgical differences in parts manufactured by conventional processes and SLM persist. Very few studies have focused on relating the process parameters to the macroscopic and microscopic properties of parts with different size features. Therefore, the aim of this study was to investigate the effect of the size of features on the density, hardness, microstructural evolution, and mechanical properties of Ti-6Al-4V parts fabricated using a fixed set of parameters. It was found that there is an acceptable range of sizes that can be produced using a fixed set of parameters. Beyond a specific window, the relative density decreased. Upon decreasing the size of a cuboid from (5 × 5 × 5 mm) to (1 × 1 × 5 mm), porosity increased from 0.3% to 4.8%. Within a suitable size range, the microstructure was not significantly affected by size; however, a major change was observed outside the acceptable size window. The size of features played a significant role in the variation of mechanical properties. Under tensile loading, decreasing the gauge size, the ultimate and yield strengths deteriorated. This investigation, therefore, presents an understanding of the correlation between the feature size and process parameters in terms of the microscopic and macroscopic properties of Ti-6Al-4V parts manufactured using SLM. This study also highlights the fact that any set of optimized process parameters will only be valid within a specific size window. Full article

(This article belongs to the Special Issue New Materials and Understandings in Selective Laser Melting (SLM))

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12 pages, 1939 KiB

Open AccessArticle

Processability of Atypical WC-Co Composite Feedstock by Laser Powder-Bed Fusion

by Mohaimen Al-Thamir, D. Graham McCartney, Marco Simonelli, Richard Hague and Adam Clare

Materials 2020, 13(1), 50; https://doi.org/10.3390/ma13010050 - 20 Dec 2019

Cited by 10 | Viewed by 2377

Abstract

Processing of tool materials for cutting applications presents challenges in additive manufacturing (AM). Processes must be carefully managed in order to promote the formation of favourable high-integrity ‘builds’. In this study, for the first time, a satelliting process is used to prepare a WC_M-Co (12 wt.% Co) composite. Melting trials were undertaken to evaluate the consolidation behaviour of single tracks within a single layer. Tracks with continuous and relatively uniform surface morphology were obtained. These features are essential for high-quality AM builds in order to encourage good bonding between subsequent tracks within a layer which may reduce porosity within a 3D deposition. This study elucidates the formation of track irregularities, melting modes, crack sensitivity, and balling as a function of laser scanning speed and provides guidelines for future production of WC_M-Co by laser powder-bed fusion. Full article

(This article belongs to the Special Issue New Materials and Understandings in Selective Laser Melting (SLM))

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15 pages, 1470 KiB

Open AccessFeature PaperArticle

Defect Prevention in Selective Laser Melting Components: Compositional and Process Effects

by Hossein Eskandari Sabzi and Pedro E. J. Rivera-Díaz-del-Castillo

Materials 2019, 12(22), 3791; https://doi.org/10.3390/ma12223791 - 18 Nov 2019

Cited by 48 | Viewed by 4769

Abstract

A model to predict the conditions for printability is presented. The model focuses on crack prevention, as well as on avoiding the formation of defects such as keyholes, balls and lack of fusion. Crack prevention is ensured by controlling the solidification temperature range and path, as well as via quantifying its ability to resist thermal stresses upon solidification. Defect formation prevention is ensured by controlling the melt pool geometry and by taking into consideration the melting properties. The model’s core relies on thermodynamics and physical analysis to ensure optimal printability, and in turn offers key information for alloy design and selective laser melting process control. The model is shown to describe accurately defect formation of 316L austenitic stainless steels reported in the literature. Full article

(This article belongs to the Special Issue New Materials and Understandings in Selective Laser Melting (SLM))

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