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Advances in Additive Manufacturing of Metals

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

Deadline for manuscript submissions: closed (31 January 2022) | Viewed by 44337

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Special Issue Editors

Prof. Dr. Katrin Wudy

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Guest Editor

Professorship of Laser-based Additive Manufacturing, Department of Mechanical Engineering, Technical University of Munich, 80333 Munich, Germany
Interests: additive manufacturing; powder bed fusion; selective laser melting; laser sintering; new process strategies

Prof. Dr. Peter Mayr

E-Mail Website
Guest Editor

Chair of Materials Engineering of Additive Manufacturing, Department of Mechanical Engineering, Technical University of Munich, 80333 Munich, Germany
Interests: metallurgy; additive manufacturing; advanced manufacturing; structure–process–property relationships

Special Issue Information

Dear Colleagues,

Additive manufacturing (AM) of metals, or rather, the layer-to-layer build-up of parts, offers new opportunities for manufacturing components with tailored properties and highly complex geometries that other traditional processes cannot match. Therefore, the interest in additively manufactured parts grows exponentially, and the technology has lately become an attractive option for advanced production. Several metallic materials including stainless steel, titanium, aluminum, and nickel-based alloys have been processed to full dense parts with excellent properties in the past. However, fundamental challenges in the field of material qualification, process monitoring and control and simulation in additive manufacturing of metallic structures still limit widespread industrial adaption.

This Special Issue of Metals aims at gathering together publications investigating advances in metal additive manufacturing including:

New materials for additive manufacturing;
Functionally graded structures;
Multimaterial parts;
Innovative process strategies;
Process monitoring and control;
Ground-breaking am systems;
Fundamental investigations on the structure–process–property relation;
Advanced material simulation.

The submission of papers on experimental work or a combination of simulation and experimental validation is welcome.

Prof. Dr. Katrin Wudy
Prof. Dr. Peter Mayr
Guest Editors

Manuscript Submission Information

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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.

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Keywords

additive manufacturing
powder bed fusion of metals
direct energy deposition
new process strategies
functionally graded structures
simulation
structure–process–property relation

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Published Papers (11 papers)

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18 pages, 7530 KiB

Open AccessArticle

Geometry Effect on Microstructure and Mechanical Properties in Laser Powder Bed Fusion of Ti-6Al-4V

by Juri Munk, Eric Breitbarth, Tobias Siemer, Norbert Pirch and Constantin Häfner

Metals 2022, 12(3), 482; https://doi.org/10.3390/met12030482 - 12 Mar 2022

Cited by 24 | Viewed by 5218

Abstract

Laser Powder Bed Fusion (LPBF) of Ti-6Al-4V enables the manufacturing of complex parts for lightweight applications. The emerging microstructure in the LPBF process and thus the mechanical properties are defined by the thermal cycles, which are locally variable for complex geometries. Predictions of local mechanical properties by simulation would reduce the development time of new applications drastically but are today not possible on part scale, so new part applications must be qualified experimentally at great effort. In this study, representative geometry sections were transferred into a simplified sample shape to mechanically characterize different geometry-dependent microstructures. In areas exposed to comparatively increased heat input over time, a lamellar

α

β

microstructure with

β

fraction up to 20% was measured in contrast to the common martensitic

α^{'}

microstructure of LPBF-manufactured Ti-6Al-4V, resulting in reduced tensile strength and fatigue life. For the first time, a correlation was successfully established between ultimate tensile strength of multiple geometries and the corresponding temperature–time cycles. With reduced computational effort by use of simplifying assumptions in the simulation, this correlation model can theoretically be applied to the part level. This work has laid the foundation for the simulation-based prediction of mechanical properties for entire parts manufactured with LPBF. Full article

(This article belongs to the Special Issue Advances in Additive Manufacturing of Metals)

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16 pages, 3957 KiB

Open AccessArticle

Investigation on the Cause-Effect Relationships between the Process Parameters and the Resulting Geometric Properties for Wire-Based Coaxial Laser Metal Deposition

by Avelino Zapata, Christian Bernauer, Christian Stadter, Cara G. Kolb and Michael F. Zaeh

Metals 2022, 12(3), 455; https://doi.org/10.3390/met12030455 - 8 Mar 2022

Cited by 23 | Viewed by 3855

Abstract

Coaxial Laser Metal Deposition with wire (LMD-w) is a valuable complement to the already established Additive Manufacturing processes in production because it allows a direction-independent process with high deposition rates and high deposition accuracy. However, there is a lack of knowledge regarding the adjustment of the process parameters during process development to build defect-free parts. Therefore, in this work, a process development for coaxial LMD-w was conducted using an aluminum wire AlMg4,5MnZr and a stainless steel wire AISI 316L. At first, the boundaries for parameter combinations that led to a defect-free process were identified. The proportion between the process parameters energy per unit length and speed ratio proved crucial for a defect-free process. Then, the influence of the process parameters on the height and width of single beads for both materials was analyzed using a regression analysis. It was shown that linear models are suitable for describing the correlation between the process parameters and the dimensions of the beads. Lastly, a material-independent formula is presented to calculate the height increment per layer needed for an additive process. For future studies, the results of this work will be an aid for process development with different materials. Full article

(This article belongs to the Special Issue Advances in Additive Manufacturing of Metals)

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16 pages, 24772 KiB

Open AccessArticle

Influences of Post-Heat Treatment on the Microstructure Evolution and Creep Properties of Ni-Based Superalloy IN718 Fabricated by Electron Beam Melting

by Thaviti Naidu Palleda, Santhosh Banoth, Yen-Ling Kuo and Koji Kakehi

Metals 2022, 12(3), 446; https://doi.org/10.3390/met12030446 - 4 Mar 2022

Cited by 3 | Viewed by 2729

Abstract

In this study, the Ni-based superalloy IN718, fabricated using an electron beam melting process, was investigated in as-built and various heat-treated conditions. The relationships between the microstructure characteristics and creep properties were elucidated. Under testing conditions of 650 °C and 650 MPa, the direct-aged specimen exhibited the lowest steady-state creep rate, at 0.15 × 10⁻⁸ s⁻¹. The superior creep resistance can be attributed to the higher volume fraction of γ’/γ”-strengthening precipitates within the grain and fine δ precipitates along the grain boundaries. Being coherent to the γ matrix, the nano-sized γ’/γ” precipitates effectively hindered the dislocation motion in the grain interior. In addition, controlled grain boundary δ precipitates inhibited grain boundary sliding and decelerated the steady-state creep strain rate during creep deformation. Full article

(This article belongs to the Special Issue Advances in Additive Manufacturing of Metals)

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18 pages, 6063 KiB

Open AccessArticle

Influence of Ring-Shaped Beam Profiles on Process Stability and Productivity in Laser-Based Powder Bed Fusion of AISI 316L

by Jonas Grünewald, Florian Gehringer, Maximilian Schmöller and Katrin Wudy

Metals 2021, 11(12), 1989; https://doi.org/10.3390/met11121989 - 9 Dec 2021

Cited by 42 | Viewed by 6909

Abstract

A major factor slowing down the establishment of additive manufacturing processes as production processes is insufficient reproducibility and productivity. Therefore, this work investigates the influence of ring-shaped beam profiles on process stability and productivity in laser-based powder bed fusion of AISI 316L. For this purpose, the weld track geometries of single tracks and multi-track segments with varying laser power, scan speed, hatch distance, and beam profile (Gaussian profile and three different ring-shaped profiles) are analyzed. To evaluate the process robustness, process windows are identified by classifying the generated single tracks into different process categories. The influence of the beam profiles on productivity is studied by analyzing the molten cross-sectional areas and volumes per time. When using ring-shaped beam profiles, the process windows are significantly larger (up to a laser power of 1050 W and a scanning speed of 1700 mm/s) than those of Gaussian beams (laser power up to 450 W and scanning speed up to 1100 mm/s), which suggests a higher process robustness and stability. With ring-shaped beam profiles, larger volumes can be stably melted per track and time. The weld tracks created with ring-shaped profiles are significantly wider than those generated with Gaussian profiles (up to factor 2 within the process window), allowing enlargement of the hatch distances. Due to the higher scanning speeds and the enlarged hatch distances for ring-shaped beam profiles, the process can be accelerated by a factor of approximately 2 in the parameter range investigated. Full article

(This article belongs to the Special Issue Advances in Additive Manufacturing of Metals)

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16 pages, 4105 KiB

Open AccessArticle

A Novel Classification Method for Pores in Laser Powder Bed Fusion

by Natan Nudelis and Peter Mayr

Metals 2021, 11(12), 1912; https://doi.org/10.3390/met11121912 - 26 Nov 2021

Cited by 31 | Viewed by 4158

Abstract

Nowadays, additive manufacturing (AM) using laser powder bed fusion (LPBF) is acknowledged for its ability to generate near-net-shape components for various industries such as aerospace, automotive, and health industries. However, internal defects seem to be the inevitable concomitant in the current state of laser powder bed fusion of Al alloys. Hence, knowledge of the formation, different types, and morphologies of pores and their suppression is an essential element for successful future AM applications. The purpose of this research is to qualify a new approach of defect classification using X-ray tomography. In this framework, this research examined the influence of size, shape, and location of pores on crack initiation for AlSi10Mg parts produced by LPBF. For this reason, a total number of 39,228 pores detected in a cylindrical sample were categorised. Additionally, 26 selected pores of different morphology from the X-ray scan were analysed by means of finite element analysis (FEA). Moreover, fracture mechanics determinations were carried out to examine the correlations between pore characteristics and degree of stress concentration. The result is an evaluated novel pore classification method that can be used for process adjustments, quality assurance, as well as further research. Full article

(This article belongs to the Special Issue Advances in Additive Manufacturing of Metals)

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14 pages, 7970 KiB

Open AccessArticle

Experimental and Numerical Investigations of In Situ Alloying during Powder Bed Fusion of Metals Using a Laser Beam

by Andreas Wimmer, Baturay Yalvac, Christopher Zoeller, Fabian Hofstaetter, Stefan Adami, Nikolaus A. Adams and Michael F. Zaeh

Metals 2021, 11(11), 1842; https://doi.org/10.3390/met11111842 - 16 Nov 2021

Cited by 5 | Viewed by 2408

Abstract

Powder Bed Fusion of Metals using a Laser Beam (PBF-LB/M) is increasingly utilized for the fabrication of complex parts in various industrial sectors. Enabling a robust and reproducible manufacturing process is one of the main goals in view of the future success of PBF-LB/M. To meet these challenges, alloys that are specifically adapted to the process are required. This paper demonstrates the successful interplay of simulation studies with experimental data to analyze the basic phenomena of in situ alloying. The meshless Smoothed-Particle Hydrodynamics (SPH) method was employed for the numerical simulation of two-component powder systems considering both thermodynamics and fluid mechanics in the solid and the melt phase. The simulation results for the in situ alloying of stainless steel 316L blended with the aluminum alloy AlSi10Mg were enriched and validated with the data from a novel experimental test bench. The combination of both approaches can enhance the understanding of the process for in situ alloying. Therefore, future investigations of the PBF-LB/M process with multi-component powder systems can benefit from detailed numerical studies using SPH. Full article

(This article belongs to the Special Issue Advances in Additive Manufacturing of Metals)

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25 pages, 11815 KiB

Open AccessArticle

Thermophysical Properties of Electric Arc Plasma and the Wire Melting Effect with Lanthanum and Sulfur Fluorides Addition in Wire Arc Additive Manufacturing

by Sergey G. Parshin and Peter Mayr

Metals 2021, 11(11), 1756; https://doi.org/10.3390/met11111756 - 1 Nov 2021

Cited by 1 | Viewed by 2602

Abstract

Achieving a higher quality in wire arc additive manufacturing (WAAM) is a result of the development of welding metallurgy, the development of filler wires, and the control of the thermophysical properties of the electric arc. In this paper, the authors developed composite wires for WAAM with a Ni-LaF₃, Ni-LaB₆ coating. The addition of LaF₃, LaB₆, and SF₆ increases specific heat, thermal conductivity, enthalpy, and degree of plasma ionization, which leads to the increase in the transfer of heat from the arc plasma to the wire and to the change in the balance of forces during wire melting. The increase in the Lorentz electromagnetic force and the decrease in the surface tension force made it possible to reduce the droplet diameter and the number of short circuits during wire melting. The change in the thermophysical properties of the plasma and droplet transfer with the addition of LaF₃, LaB₆, and SF₆ made it possible to increase the welding current, penetration depth, accuracy of the geometric dimensions of products in WAAM, reduce the wall thickness of products, and refine the microstructure of the weld metal using G3Si1, 316L, AlMg5Mn1Ti, and CuCr0.7 wires. Full article

(This article belongs to the Special Issue Advances in Additive Manufacturing of Metals)

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15 pages, 3350 KiB

Open AccessArticle

Influence of Pulsed Exposure Strategies on Overhang Structures in Powder Bed Fusion of Ti6Al4V Using Laser Beam

by Jonas Grünewald, Pirmin Clarkson, Ryan Salveson, Georg Fey and Katrin Wudy

Metals 2021, 11(7), 1125; https://doi.org/10.3390/met11071125 - 15 Jul 2021

Cited by 9 | Viewed by 3513

Abstract

Manufacturing structures with low overhang angles without support structures is a major challenge in powder bed fusion of metals using laser beam (PBF-LB/M). In the present work, various test specimens and parameter sets with continuous wave (cw) and pulsed exposure are used to investigate whether a reduction of downskin roughness and overhang angle can be achieved in PBF-LB/M of Ti6Al4V. Starting from cw exposure, the limits of overhang angle and surface roughness at the downskin surface are investigated as a reference. Subsequently, the influence of laser power, scanning speed, and hatch distance with fixed pulse duration (τ_pulse = 25 µs) and repetition rate (υ_rep = 20 kHz) on surface roughness R_a is investigated. Pulsed exposure strategies enable the manufacturing of flatter overhang angles (≤20° instead of ≥25°). Furthermore, a correlation between the introduced volume energy density and the downskin roughness can be observed for pulsed exposure. As the reduction in volume energy density causes an increase in porosity, the combination of pulsed downskin exposure and commercial cw infill exposure is investigated. The larger the gap in volume energy density between the infill area and downskin area, the more challenging it is combining the two parameter sets. By combining cw infill and pulsed downskin exposure, flatter overhang structures cannot be manufactured, and a reduction in roughness can be achieved. Full article

(This article belongs to the Special Issue Advances in Additive Manufacturing of Metals)

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18 pages, 6459 KiB

Open AccessArticle

Numerical Investigation of Thermo-Mechanical Field during Selective Laser Melting Process with Experimental Validation

by Lan Li and Frank Liou

Metals 2021, 11(7), 1003; https://doi.org/10.3390/met11071003 - 23 Jun 2021

Cited by 7 | Viewed by 2762

Abstract

In this study, thermo-mechanical simulation was conducted to predict thermal and stress behavior in Selective Laser Melting (SLM). Temperature-dependent material properties for processed material 304L stainless steel were incorporated into the model in order to capture the change from powder to fully dense solid stainless steel. Temperature and thermal stress history were tracked under conditions of different parameter sets which were designed to reduce defect formation. The thermal model predicted the temperature history for multi-track scans under different process parameters, such as laser power, effective scanning speed and hatch spacing. Subsequently, the corresponding melt-pool size, solidification rate and temperature gradients could be calculated from simulated temperature data. These three parameters from the simulation were compared with experimental melt pool size, grain structure and cell spacing data obtained from a Renishaw AM250. The experimental data were also used to determine unknown simulation parameters required by the continuum model, e.g., the optical penetration depth and thermal conductivity multiplier for the molten region. This allowed the simulation model to accurately predict melt pool size and solidification structure of SLM 304L stainless steel. Simulated stress showed that the subsequent thermal cyclic melting in successive scanned tracks resulted in alternating compressive and tensile thermal stresses. This work will provide insight for studying microstructure morphology, residual stress and deformations in the SLM process of 304L stainless steel. Full article

(This article belongs to the Special Issue Advances in Additive Manufacturing of Metals)

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15 pages, 34486 KiB

Open AccessArticle

Carbon Particle In-Situ Alloying of the Case-Hardening Steel 16MnCr5 in Laser Powder Bed Fusion

by Matthias Schmitt, Albin Gottwalt, Jakob Winkler, Thomas Tobie, Georg Schlick, Karsten Stahl, Ulrich Tetzlaff, Johannes Schilp and Gunther Reinhart

Metals 2021, 11(6), 896; https://doi.org/10.3390/met11060896 - 31 May 2021

Cited by 7 | Viewed by 4047

Abstract

The carbon content of steel affects many of its essential properties, e.g., hardness and mechanical strength. In the powder bed fusion process of metals using a laser beam (PBF-LB/M), usually, pre-alloyed metal powder is solidified layer-by-layer using a laser beam to create parts. A reduction of the carbon content in steels is observed during this process. This study examines adding carbon particles to the metal powder and in situ alloying in the PBF-LB/M process as a countermeasure. Suitable carbon particles are selected and their effect on the particle size distribution and homogeneity of the mixtures is analysed. The workability in PBF-LB is then shown. This is followed by an evaluation of the resulting mechanical properties (hardness and mechanical strength) and microstructure in the as-built state and the state after heat treatment. Furthermore, potential use cases like multi-material or functionally graded parts are discussed. Full article

(This article belongs to the Special Issue Advances in Additive Manufacturing of Metals)

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12 pages, 3498 KiB

Open AccessCase Report

Capability of Multi-Material Laser-Based Powder Bed Fusion—Development and Analysis of a Prototype Large Bore Engine Component

by Matthias Schneck, Max Horn, Maik Schindler and Christian Seidel

Metals 2022, 12(1), 44; https://doi.org/10.3390/met12010044 - 25 Dec 2021

Cited by 28 | Viewed by 3922

Abstract

Additive Manufacturing (AM) allows the manufacturing of functionally graded materials (FGM). This includes compositional grading, which enables the allocation of desired materials corresponding to local product requirements. An upcoming AM process for the creation of metal-based FGMs is laser-based powder bed fusion (PBF-LB/M) utilized for multi-material manufacturing (MM). Three-dimensional multi-material approaches for PBF-LB/M are stated to have a manufacturing readiness level (MRL) of 4 to 5. In this paper, an advancement of multi-material technology is presented by realizing an industry-relevant complex part as a prototype made by PBF-LB/M. Hence, a multi-material injection nozzle consisting of tool steel and a copper alloy was manufactured in a continuous PBF-LB/M process. Single material regions showed qualities similar to the ones resulting from mono-material processes. A geometrically defined transition zone between the two materials was achieved that showed slightly higher porosity than mono-material regions. Nevertheless, defects such as porosity, cracks, and material cross-contamination were detected and must be overcome in further MM technology development. Full article

(This article belongs to the Special Issue Advances in Additive Manufacturing of Metals)

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