Additive manufacturing of aluminium alloy 2024 by laser powder bed fusion: microstructural evolution, defects and mechanical properties

Purpose The purpose of this study is to investigate the microstructural evolution of high-strength 2024 Al alloy prepared by the laser powder bed fusion (L-PBF) additive manufacturing (AM) route. The high-strength wrought Al alloy has typically been unsuitable for AM due to its particular solidification characteristics such as hot cracking, porosity and columnar grain growth. Design/methodology/approach In this research work, samples were fabricated using L-PBF under various laser energy densities by varying laser power and scan speed. The microstructural features that developed during the solidification are correlated with operating laser parameters. In addition, finite element modelling (FEM) was performed to understand the experimentally observed results. Findings Microstructure evolution and defect formation have been assessed, quantified and correlated with operating laser parameters. Thermal behaviour of samples was predicted using FEM to support experimental observations. An optimised combination of intermediate laser power and scan speed produced the least defects. Higher energy density increased hot tearing along the columnar grain boundaries, while lower energy density promoted void formation. From the quantitative results, it is evident that with increasing energy density, both the top surface and side wall roughness initially reduced till a minimum and then increased. Hardness and compressive strength were found to decrease with increasing power density due to stress relaxation from hot tearing. Originality/value This research work examined how L-PBF processing conditions influence the microstructure, defects, surface roughness and mechanical properties. The results indicates that complete elimination of solidification cracks can be only achieved by combining process optimisation and possible grain refining strategies.

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High strength Al alloy development for laser powder bed fusion

Journal of Micromechanics and Molecular Physics ◽

10.1142/s2424913021410010 ◽

2021 ◽

pp. 2141001

Author(s):

Jin’e Sun ◽

Baicheng Zhang ◽

Xuanhui Qu

Keyword(s):

High Strength ◽

Nucleation Site ◽

Al Alloys ◽

Hot Tearing ◽

Al Alloy ◽

Powder Bed Fusion ◽

Laser Powder Bed Fusion ◽

Alloy Development ◽

Powder Bed ◽

Columnar Grain

High strength Al alloy development is the key technique to additive manufacturing (AM) applied on lightweight of aerospace, automotive and military industry. Unlike the conventional wrought Al–Si eutectic alloys available for AM process, the strength of new developed Al alloy can be improved by in situ or additional nano-precipitated phase. This paper presents an overview of high strength Al alloys development including metallic additives, such as Zr, Sc, Mn, Cu, etc., and nanoparticle additives, such as ceramics (TiB2, TiC, LaB6 and TiN) as well as carbon nanotubes (CNTs). The addition of Zr and Sc elements significantly prevents hot tearing and enhances the strength of laser processed Al alloys because the nanoscale Al3Zr, Al3Sc and Al3 (Sc, Zr) precipitated phases generate, facilitate the heterogeneous nucleation of Al matrix and refine the microstructure. Moreover, the addition of Mn and Cu elements provides an increment in the toughness and strength of laser processed Al alloys through the superimposed effect of multi-element solid solution reinforcement and precipitation strengthening role of some Al2CuMg and Al6Mn. The growth process of Al alloy can be interrupted by the addition of nanoceramics particles as additional nucleation site which leads the columnar grain transforms to the equiaxed grain. Furthermore, the mechanism of mutual solubility of LaB6, TiB2, TiC and TiN in Al alloys is systematically studied. Finally, an assessment of the state in laser processed high strength Al alloys and the research demands for the expansion of laser powder bed fusion of Al metallic components are provided.

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A processing diagram for high-density Ti-6Al-4V by selective laser melting

Rapid Prototyping Journal ◽

10.1108/rpj-11-2017-0228 ◽

2018 ◽

Vol 24 (9) ◽

pp. 1469-1478 ◽

Cited By ~ 15

Author(s):

Yinmin (Morris) Wang ◽

Chandrika Kamath ◽

Thomas Voisin ◽

Zan Li

Keyword(s):

Mechanical Properties ◽

Energy Density ◽

Processing Parameters ◽

High Density ◽

Low Density ◽

Laser Powder Bed Fusion ◽

Content Type ◽

Powder Bed ◽

Melt Pool ◽

Hatch Spacing

Purpose Density optimization is the first critical step in building additively manufactured parts with high-quality and good mechanical properties. The authors developed an approach that combines simulations and experiments to identify processing parameters for high-density Ti-6Al-4V using the laser powder-bed-fusion technique. A processing diagram based on the normalized energy density concept is constructed, illustrating an optimized processing window for high- or low-density samples. Excellent mechanical properties are obtained for Ti-6Al-4V samples built from the optimized window. Design/methodology/approach The authors use simple, but approximate, simulations and selective experiments to design parameters for a limited set of single track experiments. The resulting melt-pool characteristics are then used to identify processing parameters for high-density pillars. A processing diagram is built and excellent mechanical properties are achieved in samples built from this window. Findings The authors find that the laser linear input energy has a much stronger effect on the melt-pool depth than the melt-pool width. A processing diagram based on normalized energy density and normalized hatch spacing was constructed, qualitatively indicating that high-density samples are produced in a region when 1 < E* < 2. The onset of void formation and low-density samples occur as E* moves beyond a value of 2. The as-built SLM Ti-6Al-4V shows excellent mechanical performance. Originality/value A combined approach of computer simulations and selected experiments is applied to optimize the density of Ti-6Al-4V, via laser powder-bed-fusion (L-PBF) technique. A series of high-density samples are achieved. Some special issues are identified for L-PBF processes of Ti-6Al-4V, including the powder particle sticking and part swelling issues. A processing diagram is constructed for Ti-6Al-4V, based on the normalized energy density and normalized hatch spacing concept. The diagram illustrates windows with high- and low-density samples. Good mechanical properties are achieved during tensile tests of near fully dense Ti-6Al-4V samples. These good properties are attributed to the success of density optimization processes.

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

Materials ◽

10.3390/ma13183927 ◽

2020 ◽

Vol 13 (18) ◽

pp. 3927

Author(s):

Eo Ryeong Lee ◽

Se Eun Shin ◽

Naoki Takata ◽

Makoto Kobashi ◽

Masaki Kato

Keyword(s):

Elastic Modulus ◽

Carbon Nanotube ◽

Energy Density ◽

Laser Power ◽

Multiwalled Carbon Nanotube ◽

Powder Bed Fusion ◽

Laser Powder Bed Fusion ◽

Multiwalled Carbon ◽

Powder Bed ◽

Scan Speed

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.

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Optimization of parameters for SLS of WC-Co

Rapid Prototyping Journal ◽

10.1108/rpj-10-2016-0168 ◽

2017 ◽

Vol 23 (6) ◽

pp. 1202-1211 ◽

Cited By ~ 11

Author(s):

Sanjay Kumar ◽

Aleksander Czekanski

Keyword(s):

Energy Density ◽

Process Parameters ◽

Laser Power ◽

Laser Energy ◽

Beam Diameter ◽

Content Type ◽

Powder Bed ◽

Scan Speed ◽

New Equation ◽

Hatch Spacing

Purpose WC-Co is a well-known material for conventional tooling but is not yet commercially available for additive manufacturing. Processing it by selective laser sintering (SLS) will pave the way for its commercialization and adoption. Design/methodology/approach It is intended to optimize process parameters (laser power, hatch spacing, scan speed) by fabricating a bigger part (minimum size of 10 mm diameter and 5 mm height). Microstructural analysis, EDX and hardness testing is used to study effects of process parameters. Optimized parameter is ascertained after fabricating 49 samples in preliminary experiment, 27 samples in pre-final experiment and 9 samples in final experiment. Findings Higher laser power gives rise to cracks and depletion of cobalt while higher scan speed increases porosity. Higher hatch spacing is responsible for delamination and displacement of parts. Optimized parameters are 270 W laser power, 500 mm/s scan speed, 0.04 mm layer thickness, 0.04 mm hatch spacing (resulting in energy density of 216 J/mm3) and 200°C powder bed temperature. A part comprising of small hole of 2 mm diameter, thin cylindrical pin of 0.5 mm diameter and thin wall of 2 mm width bent up to 30° angle to the base plate is fabricated. In order to calculate laser energy density, a new equation is introduced which takes into account both beam diameter and hatch spacing unlike old equation does. In order to calculate laser energy density, a new equation is formulated which takes into account both beam diameter and hatch spacing unlike old equation does. WC was not completely melted as intended giving rise to partial melting-type binding mechanism. This justified the name SLS for process in place of SLM (Selective Laser Melting). Research limitations/implications Using all possible combination of parameters plus heating the part bed to maximum shows limitation of state-of-the-art commercial powder bed fusion machine for shaping hardmetal consisting of high amount of WC (83 wt. per cent). Practical implications The research shows that microfeatures could be fabricated using WC-Co which will herald renewed interest in investigating hardmetals using SLS for manufacturing complex hard tools, molds and wear-resistance parts. Originality/value This is the first time micro features are successfully fabricated using WC-Co without post-processing (infiltration, machining) and without the help of additional binding material (such as Cu, Ni, Fe).

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Crystallization and Hardness Change of the Ti-Based Bulk Metallic Glass Manufactured by a Laser Powder Bed Fusion Process

Metals ◽

10.3390/met11071049 ◽

2021 ◽

Vol 11 (7) ◽

pp. 1049

Author(s):

Ji-Hoon Jang ◽

Hyung-Guin Kim ◽

Hwi-Jun Kim ◽

Dong-Geun Lee

Keyword(s):

Energy Density ◽

Amorphous Powder ◽

Powder Bed Fusion ◽

Laser Powder Bed Fusion ◽

Powder Bed ◽

Scan Speed ◽

3D Printed ◽

Hardness Change ◽

The Difference ◽

Printing Processes

Ti-2.5Zr-5.0Hf-37.5Cu-7.5Ni-1.0Si-5.0Sn (at.%) BMG has been successfully manufactured in amorphous powder with a size of about 25 μm (D50). Using this amorphous powder, a Ti-based BMG was manufactured by an additive manufacturing process based on a laser powder bed fusion (LPBF) technique. In 3D printing processes using amorphous powders, it is necessary and important to understand the crystallization behavior due to the difference in energy density applied to the powders. An LPBF process has been carried out with various energy density conditions to minimize the inner defects and identify the sound mechanical properties of 3D-printed BMG parts. At the lowest energy density condition (3.0 J/mm3), the most pores were generated. Even if the same energy density (3.0 J/mm3) was applied, the rapid laser movement caused many pores to form inside the material. The relatively sound 3D-printed Ti-based BMG was successfully fabricated with a size of about 5 mm × 5 mm × 3 mm. Peaks at 41° and 44° showing crystallization were observed in all conditions. The higher the laser power was, the greater each peak intensity and the more crystallization (CuTi, Ti3Cu4, etc.) was present in the BMG, and the higher the scan speed, the more the internal defects were found.

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Process Optimization and Microstructure Analysis to Understand Laser Powder Bed Fusion of 316L Stainless Steel

Metals ◽

10.3390/met11050832 ◽

2021 ◽

Vol 11 (5) ◽

pp. 832

Author(s):

Nathalia Diaz Vallejo ◽

Cameron Lucas ◽

Nicolas Ayers ◽

Kevin Graydon ◽

Holden Hyer ◽

...

Keyword(s):

Stainless Steel ◽

Energy Density ◽

Cellular Structure ◽

316L Stainless Steel ◽

Powder Bed Fusion ◽

Laser Powder Bed Fusion ◽

Powder Bed ◽

Wide Range ◽

Scan Speed ◽

Hatch Spacing

The microstructural development of 316L stainless steel (SS) was investigated over a wide range of systematically varied laser powder bed fusion (LPBF) parameters, such as laser power, scan speed, hatch spacing and volumetric energy density. Relative density, melt pool width and depth, and the size of sub-grain cellular structure were quantified and related to the temperature field estimated by Rosenthal solution. Use of volumetric energy density between 46 and 127 J/mm3 produced nearly fully dense (≥99.8%) samples, and this included the best parameter set: power = 200 W; scan speed = 800 mm/s; hatch spacing = 0.12 mm; slice thickness = 0.03; energy density = 69 J/mm3). Cooling rate of 105 to 107 K/s was estimated base on the size of cellular structure within melt pools. Using the optimized LPBF parameters, the as-built 316L SS had, on average, yield strength of 563 MPa, Young’s modulus of 179 GPa, tensile strength of 710 MPa, and 48% strain at failure.

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Processability and Optimization of Laser Parameters for Densification of Hypereutectic Al–Fe Binary Alloy Manufactured by Laser Powder Bed Fusion

Crystals ◽

10.3390/cryst11030320 ◽

2021 ◽

Vol 11 (3) ◽

pp. 320

Author(s):

Wenyuan Wang ◽

Naoki Takata ◽

Asuka Suzuki ◽

Makoto Kobashi ◽

Masaki Kato

Keyword(s):

Energy Density ◽

Binary Alloy ◽

Process Window ◽

Process Conditions ◽

Powder Bed Fusion ◽

Laser Powder Bed Fusion ◽

Powder Bed ◽

Melt Pool ◽

Deposited Energy ◽

Scan Speed

Centimeter-sized samples of hypereutectic Al–15 mass% Fe alloy were manufactured by a laser powder bed fusion (L-PBF) process while systematically varying laser power (P) and scan speed (v). The effects on relative density and melt pool depth of L-PBF-manufactured samples were investigated. In comparison with other Al alloys, a small laser process window of P = 77–128 W and v = 0.4–0.8 ms−1 was found for manufacturing macroscopically crack-free samples. A higher v and P led to the creation of macroscopic cracks propagating parallel to the powder-bed plane. These cracks preferentially propagated along the melt pool boundaries decorated with brittle θ-Al13Fe4 phase, resulting in low L-PBF processability of Al–15%Fe alloy. The deposited energy density model (using P·v−1/2) would be useful for identifying the optimum L-PBF process conditions towards densification of Al–15%Fe alloy samples, in comparison with the volumetric energy density (using P·v−1), however, the validity of the model was reduced for this alloy in comparison with other alloys with high thermal conductivities. This is likely due to inhomogeneous microstructures having numerous coarsened θ–Al13Fe4 phases localized at melt pool boundaries. These results provide insights into achieving sufficient L-PBF processability for manufacturing dense Al–Fe binary alloy samples.

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Development of a Novel High-Temperature Al Alloy for Laser Powder Bed Fusion

Metals ◽

10.3390/met11010035 ◽

2020 ◽

Vol 11 (1) ◽

pp. 35

Author(s):

Filippo Belelli ◽

Riccardo Casati ◽

Martina Riccio ◽

Alessandro Rizzi ◽

Mevlüt Y. Kayacan ◽

...

Keyword(s):

Tensile Strength ◽

High Temperature ◽

Elevated Temperatures ◽

High Strength ◽

Al Alloy ◽

Powder Bed Fusion ◽

Laser Powder Bed Fusion ◽

Powder Bed ◽

Solidification Cracks ◽

Manufacturing Technologies

The number of available materials for Laser Powder Bed Fusion is still limited due to the poor processability of many standard alloys. In particular, the lack of high-strength aluminium alloys, widely used in aerospace and automotive industries, remains a big issue for the spread of beam-based additive manufacturing technologies. In this study, a novel high-strength aluminium alloy for high temperature applications having good processability was developed. The design of the alloy was done based on the chemical composition of the widely used EN AW 2618. This Al-Cu-Mg-Ni-Fe alloy was modified with Ti and B in order to promote the formation of TiB2 nuclei in the liquid phase able to stimulate heterogeneous nucleation of grains and to decrease the hot cracking susceptibility of the material. The new Al alloy was manufactured by gas atomisation and processed by Laser Powder Bed Fusion. Samples produced with optimised parameters featured relative density of 99.91%, with no solidification cracks within their microstructure. After aging, the material revealed upper yield strength and ultimate tensile strength of 495 MPa and 460 MPa, respectively. In addition, the alloy showed tensile strength higher than wrought EN AW 2618 at elevated temperatures.

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Melt Pool Features Analysis Using a High Speed Coaxial Monitoring System for Laser Powder Bed Fusion of Ti-6Al-4V Grade 23

10.21203/rs.3.rs-1128009/v1 ◽

2021 ◽

Author(s):

Aditi Thanki ◽

Louca Goossens ◽

Agusmian Partogi Ompusunggu ◽

Mohamad Bayat ◽

Abdellatif Bey-Temsamani ◽

...

Keyword(s):

Energy Density ◽

Laser Power ◽

High Speed ◽

Powder Bed Fusion ◽

Laser Powder Bed Fusion ◽

Porosity Formation ◽

Powder Bed ◽

Binary Images ◽

Melt Pool ◽

Scan Speed

Abstract In laser powder bed fusion (LPBF), defects such as pores or cracks can seriously affect the final part quality and lifetime. Keyhole porosity, being one type of porosity defects in LPBF, results from excessive energy density which may be due to changes in process parameters (laser power and scan speed) and/or result from the part’s geometry and/or hatching strategies. To study the possible occurrence of keyhole pores, experimental work as well as simulations were carried out for optimum and high volumetric energy density conditions in Ti-6Al-4V grade 23. By decreasing the scanning speed from 1000 mm/s to 500 mm/s for a fixed laser power of 170 W, keyhole porosities are formed and later observed by X-ray computed tomography. Melt pool images are recorded in real-time during the LPBF process by using a high speed coaxial Near-Infrared (NIR) camera monitoring system. The recorded images are then pre-processed using a set of image processing steps to generate binary images. From the binary images, geometrical features of the melt pool and features that characterize the spatter particles formation and ejection from the melt pool are calculated. The experimental data clearly show spatter patterns in case of keyhole porosity formation at low scan speed. A correlation between the number of pores and the amount of spatter is observed. Besides the experimental work, a previously developed, high fidelity finite volume numerical model was used to simulate the melt pool dynamics with similar process parameters as in the experiment. Simulation results illustrate and confirm the keyhole porosity formation by decreasing laser scan speed.

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On the Relevance of Volumetric Energy Density in the Investigation of Inconel 718 Laser Powder Bed Fusion

Materials ◽

10.3390/ma13030538 ◽

2020 ◽

Vol 13 (3) ◽

pp. 538 ◽

Cited By ~ 6

Author(s):

Fabrizia Caiazzo ◽

Vittorio Alfieri ◽

Giuseppe Casalino

Keyword(s):

Surface Roughness ◽

Energy Density ◽

Process Parameters ◽

Vickers Hardness ◽

Powder Bed Fusion ◽

Processing Conditions ◽

Laser Powder Bed Fusion ◽

Powder Bed ◽

Fractional Density ◽

Experimental Variation

Laser powder bed fusion (LPBF) can fabricate products with tailored mechanical and surface properties. In fact, surface texture, roughness, pore size, the resulting fractional density, and microhardness highly depend on the processing conditions, which are very difficult to deal with. Therefore, this paper aims at investigating the relevance of the volumetric energy density (VED) that is a concise index of some governing factors with a potential operational use. This paper proves the fact that the observed experimental variation in the surface roughness, number and size of pores, the fractional density, and Vickers hardness can be explained in terms of VED that can help the investigator in dealing with several process parameters at once.

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