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Abstract
Laser powder bed fusion (LPBF) is one of the major additive manufacturing techniques that industries have adopted to produce complex metal components. The scientific and industrial literature from the past few years reveals that there is a growing demand for the development of high-strength aluminium alloys for LPBF. However, some major challenges remain for high-strength aluminium alloys, especially in relation to printability and the control of defects. Possible strategies that have been identified to achieve high strength with printability include the adaptation of existing high-strength cast and wrought alloys to LPBF, the design of new alloys specifically for LPBF, and the development of aluminium-based composites to achieve unique combinations of properties and processability. Whilst review papers exist for aluminium alloys in general for the related work up to 2019, the purpose of this paper is to review the latest developments related to high-strength aluminium alloys for LPBF up to early 2022, including alloy and process design strategies to achieve high strength without cracking. It aims to provide fresh insights into the current state-of-the-art based on a review of extensive yield strength data for a wide spectrum of aluminium alloys and tempers that have been studied and/or commercialised for LPBF.
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... Most of commercial aluminum powders for Laser Powder Bed Fusion (LPBF) or Direct Energy Deposition (DED) are based on high silicon cast alloy compositions. However, it results in limited mechanical performance of printed parts [1][2][3]. Standard wrought alloys should offer better properties, but hot cracking during processing is a current major issue. ...
... Standard wrought alloys should offer better properties, but hot cracking during processing is a current major issue. Alloy systems including exotic ingredients as scandium exhibit good properties, but the price of the powder is significantly increased, hindering their adoption in markets where low cost is a key-driver [2,3]. ...
... Compared to most of LPBF commercial aluminum alloys, they contain low amounts of Si. The effect of laser parameters was studied with the goal of reducing defects to assess the printability of selected alloys [1][2][3][4]. ...
The influence of laser parameters (energy density, hatch spacing and scanning strategy) was studied on two new high strength aluminum alloys for Laser Powder Bed Fusion (LPBF). These alloys contain Iron and low amount of Silicon compared to standard LPBF Al grades using Mg and Si as alloying elements. The energy densities and scan strategies were adapted to improve pores morphology and distribution which enabled to obtain samples with relative densities higher than 99.5% and Yield strengths higher than 350 MPa. Besides, the optimized parameters also minimized the presence and size of hot tearing cracks. Nevertheless, it was not possible to completely avoid their appearance in the case of the studied alloys which have a wide solidification interval. The printed samples were characterized by optical microscopy and tensile strength tests. Besides, twin cantilevers were printed with supports to assess the influence of the different parameters on geometrical distortion due to residual stresses.
... Furthermore, the of Sc and Zr leads to the formation of Al 3 (Sc, Zr) particles, acting as dispersoids that improve mechanical performance. This addition notably reduces the size of columnar grains, largely modifying microstructure into submicron equiaxed grains [8]. Such transformation remarkably shifts the mechanical behavior from anisotropic to isotropic [7]. ...
... Such transformation remarkably shifts the mechanical behavior from anisotropic to isotropic [7]. LPBF additively manufactured (AM) Al-Mg-Sc-Zr grade, also known as the trade name Scalmalloy ® , is a new generation within the Al-Mg alloys, featuring a bi-modal microstructure with fine and uniformly dispersed Al 3 (Sc, Zr) particles in the α-aluminum microstructure [8]. The typical microstructure of LPBF Scalmalloy® is shown in Fig. 1b [5]. ...
... The grains in an as-built A20X are relatively small and equiaxed (Fig. 1c), contrasting with the typical epitaxial growth observed in Al-Si system alloys [6] or the bi-modal feature of Scalmalloy ® [5]. This difference is attributed to TiB 2 particles being more effective grain refiners than Al 3 (Sc, Zr) because of higher coherency with the matrix lattice [8]. Ghoncheh et al. [19] have reported A20X to exhibit quasi-isotropic mechanical behavior owing to fine and equiaxed grains without a strong preferential crystallographic orientation along any direction. ...
A20X is an advanced and high-strength additive manufacturing aluminum alloy with promising applications in several fields, including aerospace and aeronautics. However, its assembling through fusion welding technologies poses challenges due to the detrimental effects of melting and solidification. Friction stir welding offers a promising solution for joining A20X, producing components with superior mechanical properties while preserving the engineered microstructures. This study investigates the influence of friction stir welding on the quality of butt joints made of 4 mm thick additively manufactured A20X plates produced by laser powder bed fusion. Different rotational (900 and 1500 rpm) and welding speeds (100 and 500 mm/min) were tested to evaluate the influence of the joining process on weld quality (mechanical strength, microstructures, welding defects, and surface roughness). Friction stir welding maintains a very fine microstructure in the welds, with only a slight reduction of the mechanical strength compared to the base material (335 MPa vs. 385 MPa on average). The hardness of the welded joints increases, attributed to local aging caused by the heat input during the joining process. Lower tool rotation and welding speed result in tunnel defects, notably reducing joint strength. 3D X-ray computed tomography reveals that the metal stirring occurring during the joining process notably reduces the intrinsic porosity of A20X. It also breaks up Ti borides and promotes the growth of Al-Cu precipitates within the stir zone. The fractographic analysis highlights the ductile behavior of A20X after welding, emphasizing the critical role of welding parameters in joint integrity.
... Furthermore, the formation of large columnar grains oriented along the heat transfer direction imparts anisotropic mechanical properties to the Al-SiMg alloys [7]. Figure 2 summarizes the yield strength for as-built AlSi10Mg alloys as found in the literature. The yield strength is the range of 150 -300 MPa for most LPBF Al-Si alloys [8], with certain aging conditions providing higher values between 275 and 325 MPa [9,10]. However, the relatively low yield strength and anisotropic mechanical properties of the Al-Si alloy system [8] do not meet the growing demand for high-strength alloys with isotropic properties. ...
... The yield strength is the range of 150 -300 MPa for most LPBF Al-Si alloys [8], with certain aging conditions providing higher values between 275 and 325 MPa [9,10]. However, the relatively low yield strength and anisotropic mechanical properties of the Al-Si alloy system [8] do not meet the growing demand for high-strength alloys with isotropic properties. This challenge has spurred significant efforts in developing an advanced generation of aluminum alloys. ...
... The introduction of small amounts of Sc and Zr leads to the formation of Al 3 (Sc, Zr) particles, acting as dispersoids that improve mechanical performance. This addition notably reduces the size of columnar grains, largely modifying microstructure into submicron equiaxed grains [8]. Such transformation remarkably shifts the mechanical behavior from anisotropic to isotropic [7]. ...
This study investigates butt friction stir welding of 4 mm thick A20X plates, which were produced using additive manufacturing through laser-powder bed fusion. The plates were joined at varying process parameters, including different welding and rotational speeds. Both the microstructure and the mechanical strength of the joints were examined, comparing joints in both as-printed and as-welded conditions.The joints welded at a welding speed of 100 mm/min and a rotational speed of 1500 rpm exhibited superior mechanical properties compared to those produced with other parameter settings. The results show that decreasing the rotational speed while increasing the welding speed leads to the formation of tunnel defects, compromising joint integrity and resulting in lower ultimate tensile strength. In contrast, the combination of 100 mm/min welding speed and 1500 rpm rotational speed produced defect-free joints, with an average ultimate tensile strength of 335 MPa and an elongation at fracture of 8.5%.
... According to various review papers, those are the most common research materials in metal PBF. The LPBF processing of aluminium (Al) alloys, however, only became increasingly important during the last decade [9,20,[48][49][50][51][52]. ...
... The best processable aluminium alloys by LPBF are aluminium-silicon-based. The most commonly studied one is AlSi10Mg, but also AlSi12 and AlSi7Mg show good processability [1,51,53]. ...
... The silicon phase in the solidified LPBF material inhibits crack initiation and propagation and reduces the overall weight of the manufactured part with its density of 2.4 g/cm 3 . Silicon as alloying element improves hardness and reduces shrinkage [1,51,53]. The addition of magnesium improves corrosion resistance and weldability [54]. ...
In the past decades, additive manufacturing (AM) has evolved from a rapid prototyping technology to a mature manufacturing process, offering significant advantages for lightweight design and specialized applications. Surface quality is crucial for the qualification of metal AM parts, particularly for load-bearing aerospace applications. Surface quality from a laser powder bed fusion (LPBF) AM process is typically characterised by agglomerations of attached powder particles, spatter, and weld or layer tracks, influenced by material, powder properties, build direction, and other factors. This work aims to provide a more comprehensive understanding and holistic description of LPBF surface quality, its formation, characterisation, and role in part functionality, using novel approaches and advanced optical measurement techniques.
The thesis is divided into three parts: "Measurement and Data Post-processing," "Surface Texture and Mechanical Properties," and "Areal Surface Features."
The first part discusses the application of optical measurements and related challenges for as-built and post-processed LPBF surfaces. Current industry practices use stylus contact measurements yielding 2D profiles, which are inadequate for LPBF's complex surface structures. Areal measurements, such as confocal microscopy and fringe projection, offer better surface coverage, reproducibility, and prevent surface damage. The transition from 2D to 3D parameters and from contact stylus to non-contact optical methods is proposed for comprehensive surface data.
The second part suggests describing AM surfaces in terms of part functionality, introducing surface texture parameters from the material ratio curve for fatigue performance, instead of traditional 2D parameters like Rt and Ra. Parameters like Svk, derived from the surface height distribution, relate well to fatigue failure modes in LPBF parts. Optimised processing parameters can achieve surface and mechanical properties comparable to post-processed and conventionally manufactured parts, potentially eliminating the need for surface post-processing.
The third part focuses on process-related surface texture characterisation, advancing functionality-based descriptions. It proposes using the particle size distribution of the processed metal powder to set pruning thresholds for feature segmentation, instead of ISO 25178's extreme value Sz. A novel approach to feature-based segmentation is developed.
These advancements will make AM more accessible and sustainable, broadening its application across various sectors with specialised functionality requirements.
... The rapid solidification rate and complex thermal cycling inherent to the PBF-LB process lead to notably different microstructures in metal alloys compared to cast or wrought material with the same composition. [2] Particular characteristics of the rapidly cooled material are the presence of non-equilibrium phases, fine microstructural features, and a supersaturated matrix [24,25]. These unique solidification conditions provide major challenges related to cracking but also opportunities related to improved heat treatments when adapting high-strength Al alloys to the PBF-LB process. ...
... However, these processing methods present major limitations for industrial adoption due to low productivity and powder degradation concerns. Modification of the chemical composition to induce a fine equiaxed grain structure, particularly via the introduction of grain refining elements or particles such as Sc, Ti, Zr, TiB2, is a promising solution to render these known alloy systems printable [25]. For example, figure 5 shows the powder feedstock and PBF-LB processed microstructure of AA7075 with and without the addition of 1.2 weight percent titanium (Ti) particles smaller than 25 micrometers via powder blending. ...
... The as-built microstructures of PBF-LB Al alloys already have a supersaturated matrix and therefore require novel heat treatments to reach optimized mechanical properties [33]. Moreover, a solution heat treatment step can coarsen fine grains, and result in coarser precipitates which harm the mechanical properties compared to the as-built microstructural state [25]. Direct aging -a single-step heat treatment, notated as T4, to relieve residual stress and trigger precipitation is generally found to be a better choice for PBF-LB Al alloys [34,35,36]. ...
The present paper discusses the potential and challenges of processing metallic materials using additive manufacturing. Particular focus is given to laser powder bed fusion (PBF-LB/M) and the use of traditional alloy powders such as Al alloys and Ni-based superalloys, as well as novel materials such as metal-matrix composites. The research includes the improvement of the processability of these alloys using PBF-LB/M and optimizing material properties such as strength, creep resistance, and thermal conductivity of printed parts for various applications. Another important aspect presented within this manuscript is the digital representation of advanced manufacturing systems to improve manufacturability and enable advanced quality control. Herein, the development of a digital twin through in-situ process monitoring for the direct energy deposition process of laser metal deposition is presented. In the last part, the future of materials development for additive manufacturing is discussed, focusing on applying material computational techniques. All demonstrated examples result from the successful cooperation between the Chair of Materials Engineering of Additive Manufacturing, TUM, and its industrial and research partners.
... However, the Al alloys used most commonly for LPBF are still Al-Si and Al-Si-Mg casting alloys, which have yield strengths of less than 300 MPa [3][4][5][6]. Most commercial high-strength wrought Al alloys cannot be processed reliably by LPBF because of their high susceptibility to solidification cracking [7][8][9][10]. This has led to extensive recent research efforts to develop novel high-strength Al alloys that are more suitable for processing via LPBF (e.g., [10,11]). ...
... Most commercial high-strength wrought Al alloys cannot be processed reliably by LPBF because of their high susceptibility to solidification cracking [7][8][9][10]. This has led to extensive recent research efforts to develop novel high-strength Al alloys that are more suitable for processing via LPBF (e.g., [10,11]). One type of system that is being considered is quasicrystal-reinforced Al alloys, because these materials exhibit attractive combinations of properties including high specific strengths, high hardnesses, low friction coefficients and low surface energies [12][13][14][15][16]. ...
Aluminum metal matrix composites with dispersed quasicrystalline reinforcements exhibit attractive combinations of properties, and they could form the basis of novel alloy systems for additive manufacturing (AM) if the quasicrystalline phases can be retained or formed during the AM process. Recent results from laser glazing studies have suggested that Al-Cr-Mn-Co-Zr alloys could be good candidates for AM by laser powder bed fusion (LPBF) since the alloys form dispersions of the quasicrystalline icosahedral phase (I-phase) under appropriate conditions. Here, a series of LPBF trials has been performed using gas-atomized Al-Cr-Mn-Co-Zr alloy powder and laser parameters optimized for other high-strength Al- and Ti-based alloys. All the builds exhibited the same microstructural zones (melt pools, heat-affected zones and melt pool boundary layers) with the main differences being the relative extent of the zones and the length scale of the phases within them. The melt pool microstructures comprised coarse columnar Al grains with equiaxed I-phase dispersoids and nanoscale Al9Co2 precipitates. The heat-affected zones contained additional fine precipitates of Al3Zr, Al4(Cr,Mn), and Al45(Cr,Mn)7 phases together with finer recrystallized Al grains. The boundary layers had no I-phase but instead exhibited coarse equilibrium Al11(Cr,Mn)2 phases with leaf- or needle-like morphologies. Microindentation data revealed that the heat-affected zones and boundary layers were softer than the melt pools due to a reduction of solid solution and/or dispersion strengthening effects, and these were related to the trends in macro-hardness for the builds. These data confirm that there is significant potential for developing alloys for LPBF in this system, but that process parameters would need to be optimized to control the extent of the softer microstructural regions in the build microstructures.
... As a result, there is a growing demand for advanced materials design methods that can facilitate more efficient composition design. Recently, the calculation of phase diagrams (CALPHAD) methodology has been employed extensively in the composition design of AM-tailored materials by calculating their solidification path based on the lever rule or Scheil-Gulliver model [13,14]. Wang et al. [15] achieved the non-equilibrium solidification path of AlCuMgSi alloy and subsequently calculated the hot cracking susceptibility [16] of alloys with varying Cu, Mg and Si contents in order to develop a crack-resistant AlCuMg alloy. ...
... Similar microstructure evolution in Ti-modified AlCuMg alloys was also reported by Mair et al. [50]. As illustrated in Figure 14(b), the value of G decreased continuously from bottom to top of the molten pool, during which the G-R value bridged the boundary for full-equiaxed grains defined by Eq. (14). Therefore, the level of N in Area A failed to activate CET under extremely high G and R, which was consistent with Figure 14(b1). ...
Ti-modified AlCuMg alloys have demonstrated great potential in improving the printability and mechanical properties of conventional Al alloys processed by laser powder-bed fusion (LPBF). However, an effective material design method, which considers processing parameters and solidification microstructure, is currently absent. Accordingly, the present work proposed a comprehensive model with which the formation of hot-tearing cracks and lack-of-fusion pores could be more efficiently predicted. The hot-tearing factor was an assistant in the tailoring of crack-free Ti-modified AlCuMg alloys, while the lack-of-fusion factor suggested a volumetric energy density for better printability. This hot-tearing factor could also serve as the supplementary method for evaluating the crack susceptibility of AlCuMg alloys. The microstructure evolution and mechanical properties were also investigated to achieve an optimal processing window for these alloys, which could satisfy the dual requirements of higher mechanical properties and better formability. It was believed that this comprehensive model would facilitate the tailoring of crack-free AlCuMg alloys and the optimisation of processing parameters in a time-efficient manner with high precision.
... The production of metals, alloys, and their composites through additive manufacturing techniques has experienced significant growth in recent years [38][39][40][41][42]. The most common kind of additive manufacturing for aluminium alloys is selective laser melting (SLM) [43]. Selective laser melting (SLM), also known as laser powder bed fusion (LPBF) or direct metal laser melting (DMLM), is an AM technique devised for fusing metallic powders using a high power density laser. ...
... and magnesium content up to 0.6 wt.%, remain the primary choice for LPBF because of their exceptional castability and printability. These alloys typically demonstrate yield strengths between 150 and 300 MPa and hardness between 120Hv and 140HV [43,74,75]. Aluminium absorbs only 7% of the energy from a standard SLM laser due to its high reflectivity in the infrared band. ...
Aluminium metal matrix composites (AMMCs) hold significant interest and importance as high-performance materials with enhanced mechanical properties, finding widespread applications in the aerospace and automotive industries. SiC is a significant reinforcement in AMMCs to improve the mechanical and tribological properties. These composites are manufactured using various technological methods, including casting, powder metallurgy, and spark plasma sintering. Compared to conventional processes like casting and powder metallurgy, AMMCs produced through additive manufacturing exhibit significant differences in mechanical and tribological properties. Selective laser melting (SLM) is a suitable modern method for fabricating net-shape, fully dense parts from SiC-reinforced AMMCs. This study presents recent achievements in SiC-reinforced AMMCs fabricated via selective laser melting, divided into eight sections. Beginning with an introduction, the focus shifts to feedstock preparation for composite synthesis by SLM. Subsequent sections discuss the optimization of SLM process parameters to achieve composites with improved relative density. Following this, the fourth and fifth sections summarize the phase formation and microstructure of typical reinforcements and matrix interfaces, as well as melt pool behaviour and densification. The sixth section offers an analysis of the mechanical and tribological properties obtained from SiC-reinforced aluminium matrix composites prepared by SLM, its comparison with composites synthesized by other AM techniques (such as direct metal laser sintering and laser melt deposition), and various conventional manufacturing techniques. The inferred conclusions from the review are presented in Sect. 7, followed by a discussion on future research trends in the development of SiC-reinforced AMMCs through SLM in the final section.
... From the Ashby diagrams, tables, and figures that compile the properties and Merit Indexes of the alloys in question, in the case study of the wings spars, it can be inferred that the alloy of the Al-Mg-Sc system AA5028-H116 which was Additively Manufactured by Laser Powder Bed Fusion [35][36][37][38] or Melt-Spinning [39,40] process, compared to AA alloy 7075-T6, has 0.52% higher mechanical strength limit, 5% higher yield strength limit, fracture toughness 45.83% higher in TL orientation and 29.6% higher in LT orientation, equivalent elongation, equivalent Brinel hardness, and 166.7% higher fatigue strength limit. Thus, consequently, when analyzing the Merit Indexes table, it can be seen that the Al-Mg-Sc system alloy AA5028-H116 has the highest index E 1/2 /ρ equal to 3.19, and the highest index δy 2/3 /ρ equal to 25.313, which shows the superiority of AA5028-H116 compared to the remaining alloys in the classification concerning the mechanical properties involved in them. ...
Alloys of the Al-Mg-Sc system are possible options for use in aircraft aiming to reduce structural weight and fuel consumption, due to the demand for advanced metal alloys with better properties, but at a less attractive cost. Considering the potential of these alloys, the present work aimed to evaluate the use of Scalmalloy® in one aircraft component: wings spars, and the values of desired properties for this component were discussed. The mechanical properties of these alloys were consulted in the Aleris datasheet for alloys 5024 and 5028. Consultations were made to the data in the literature, and subsequent comparisons of values of the mechanical properties and Merit Indexes: E1/2/ρ, δy2/3/ρ, E1/2/Cmρ, δy2/3/Cmρ between Scalmalloy® and the traditional alloys, using Cambridge Engineering Selector® 2019 software. It can be seen in the results indicated in Ashby diagrams and tables produced that, for wings spars, the Al-Mg-Sc AA5028-H116, produced by additive manufacturing, has the highest index E1/2/ρ equal to 3.19 and the highest index δy2/3/ρ equal to 25.313. However, the index E1/2/Cmρ is equal to 0.17 and the index δy2/3/Cmρ is equal to 1.35. Therefore, it was found that AA5028-H116 has the potential to replace the traditional alloys, despite its higher price.
Keywords: Al-Mg-Sc system; Cambridge Engineering Selector® 2019; Wings spars; Materials selection; Merit indexes; Scalmalloy®
... Extremely high cooling rates in both processes (10 3 to 10 8 K/s) [5,6] result in the development of residual stresses [7][8][9] and local variations in the microstructure, mechanical properties and defects formed [10][11][12]. Columnar grains, submicron size cellular structures and the texture development are commonly found in the SLM fabricated 316L [5,[13][14][15][16][17][18][19][20][21] and AlSiMg [22][23][24][25][26] alloys. The formation of a metastable martensitic phase α' is further observed in the SLM-and EBM-Ti64 alloys [27][28][29][30][31]), which increases the strength, but reduces the ductility significantly [32]. ...
Background
One potential application of additively fabricated lattice structures is in the blade containment rings of gas turbine engines. The blade containment rings are expected to be able to absorb the kinetic energy of a released blade (broken blade) in order to protect the engine parts from damaging. Metallic lattice-cored sandwich plates provide a gap (free space) between two face sheets, which helps to arrest the released blade and increases the energy absorption capability of containment rings.
Objective
The objective was to investigate numerically the projectile impact response of Body-Centered-Cubic (BCC) Electron-Beam-Melt (EBM) lattice-cored/Ti64 face sheet sandwich plates as compared with that of an equal-mass monolithic EBM-Ti64 plate.
Methods
The projectile impact simulations were implemented in LS-DYNA using the previously determined flow stress and damage models and a spherical steel impactor at the velocities ranging from 150 to 500 m s ⁻¹ . The experimental projectile impact tests on the monolithic plate were performed at two different impact velocities and the results were used to confirm the validity of the used flow stress and damage models for the monolithic plate models.
Results
Lower impact stresses were found numerically in the sandwich plate as compared with the monolithic plate at the same impact velocity. The bending and multi-cracking of the struts over a wide area in the sandwich plate increased the energy absorption and resulted in the arrest of the projectile at relatively high velocities. While monolithic plate exhibited a local bent area, resulting in the development of high tensile stresses and the projectile perforations at lower velocities.
Conclusions
The numerical impact stresses in the sandwich plate were distributed over a wider area around the projectile, leading to the fracture and bending of many individual struts which significantly increased the resistance to the perforation. Hence, the investigated lattice cell topology and cell, strut, and face sheet sizes and the lattice-cored sandwich plate was shown potentially more successful in stopping the projectiles than the equal-mass monolithic plates.
... Consequently, the production of aluminum products by SLM has primarily been limited to alloys that are highly castable or weldable. Although high-Si aluminum alloys, such as typical AlSi10Mg, are good candidates for SLM, their mechanical properties are much lower than those of high-strength aluminum alloys, such as 2xxx, 6xxx, and 7xxx alloys [14]. However, high-strength aluminum alloys are generally susceptible to severe hot tearing during SLM, which limits their widespread use [15]. ...
This study investigated the impact of TiB grain refiner additions on the microstructural evolution, hot tearing susceptibility, and mechanical properties of Al-Cu 224 alloys to enhance their processing performance during the selective laser melting (SLM) process. A simple laser surface remelting method was utilized to simulate laser-based rapid solidification. The results revealed that the addition of appropriate amounts of TiB grain refiner could completely eliminate the solidification cracks during the laser surface remelting process. The introduction of TiB2 particles in the melt pools through the TiB grain refiner addition changed the grain morphology from a coarse columnar to a fine equiaxed structure, and the grain sizes were reduced from 13 to 15 μm in the base alloys to 5.5 μm and 3.2 μm in the alloys with 0.34 wt% Ti (B-3TiB) and 0.65 wt% Ti (ZV-6TiB) additions, respectively. The hardness values of the modified B-3TiB and ZV-6TiB alloys reached 117 and 130 HV after a T6 heat treatment, which surpassed the hardness of conventional AlSi10Mg alloys by at least 15–30%. This improvement was attributed to the finer grains and nanoscale θ′/θ″ precipitates. The results demonstrate that the TiB grain refiner addition can significantly improve the processability and mechanical properties of Al-Cu 224 alloys for SLM applications, offering a promising solution to the challenge of high hot tearing susceptibility in high-strength aluminum alloys.
... Among the methods of preventing material cracking in the PBF-LB process, we can distinguish: (1) increasing the platform heating temperature, reducing in-process stresses [5,6], (2) increasing the alloying additions to obtain a eutectic structure [7,8], and (3) nano-functionalization of alloys to refine grains. Among the listed methods of hot cracking mitigation, the most popular among scientific and commercial communities, is the method of nanofunctionalizers introduction [9,10]. Typically, proposed additives are form the Al 3 X phase from aluminum, the task of which is to create homogeneous nucleation sites, allowing equiaxed grains to grow instead of elongated, epitaxial ones. ...
5xxx series aluminum alloys have good mechanical strength, high ductility, and corrosion resistance, especially to seawater. Although they are considered weldable alloys, solidification cracks occur during processing in the Powder Bed Fusion–Laser Beam Additive Manufacturing. Therefore, in this work, tantalum addition (~ 3 wt.%) was proposed as a nano-functionalizer creating homogeneous Al3Ta nucleation sites, improving processability in the presented technology and thus mitigating solidification cracking. Analysis of the produced samples showed a significant improvement in the processability of the proposed alloy, allowing to minimize the number of cracks, refining the grain from 112 µm to 2.9 µm and obtaining satisfactory ultimate tensile strength of 313 MPa, yield strength of 208 MPa, elongation of 25%. Additional attention should be paid to the analysis of the microstructure and computed tomography, which showed incomplete dissolution of the tantalum particles, forming a specific type of composite with the matrix. Despite the lack of remelting of tantalum particles, they are evenly distributed in the samples and fulfill their role as grain refiners.
... The most common Al alloys in L-PBF are the Al-Si family (e.g., AlSi10Mg), as they offer excellent weldability and printability [7]. However, these alloys exhibit relatively low yield strengths (150-300 MPa) that fall short in applications where high performance is needed [8,9]. Processing high-performance Al alloys, including 6xxx and 7xxx series, via the L-PBF process poses considerable challenges, primarily due to the steep cooling rates associated with these processes, which often lead to solidification cracking and high thermal stresses [10][11][12][13]. ...
Processing high-performance aluminum alloys, including 6xxx and 7xxx series, via laser additive manufacturing (AM) processes poses significant challenges, primarily due to the rapid cooling rates inherent in these processes, which often result in solidification cracking and metallurgical defects. This study aimed at producing dense, crack-free samples of Al6061 alloys, using the laser powder bed fusion (L-PBF) process. Taguchi’s method of design of experiments was employed to study the effects of laser power, scanning speed, and hatch spacing on the L-PBF process parameters for Al6061. Two types of samples were fabricated: cubic samples for density and microstructural analyses; and dog bone samples for tensile testing. The microstructure, density, mechanical properties, fractography, and material composition of the L-PBF Al6061 parts were investigated. Based on our experimental findings, an optimal process window is suggested, with a laser power of 200–250 W, scanning speed of 1000 mm/s, and hatch spacing of 140 µm, resulting in complete melting within the energy density range of 44–50 J/mm3. This work demonstrates that adjusting processing conditions—specifically, increasing the energy density from 25.51 J/mm3 to 44.64 J/mm3—leads to a reduction in porosity from approximately 5% to below 1%, significantly improving the density and quality of the parts fabricated using L-PBF.
... The SLM process is increasingly attractive in various industrial sectors due to its design flexibility and exceptional mechanical properties for advanced materials [1,2]. However, the production of high-integrity aluminum SLM products is primarily limited to highly castable or weldable aluminum alloys due to the challenges associated with the characteristics of the aluminum alloy itself, including a high risk of cracking, as well as a high laser reflectivity and propensity for oxidation [3][4][5]. To date, the most commonly used aluminum alloys that can easily be processed by SLM are high-Si alloys, particularly AlSi10Mg, because of their good weldability and excellent castability resulting from the presence of a large portion of low-melting point Al-Si eutectic [6,7]. ...
This study investigated the effects of various post heat treatments on the mechanical properties and microstructure evolution of an AlSi10MgMn alloy containing 0.5 wt% Mn produced by the selective laser melting process for the first time. The microstructures under different conditions were analyzed using optical microscopy, scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. In the as-manufactured (F) condition, the alloy exhibited an ultimate tensile strength (UTS) of 486 MPa, a yield strength (YS) of 299 MPa, and an elongation of 10.3 %. After a T5 treatment, the UTS and YS increased to 532 MPa and 386 MPa, respectively, resulting in a remarkable 30 % improvement in YS compared to the F state. The tensile properties achieved by the new alloy were considerably higher than those reported for conventional AlSi10Mg alloys in the F, T5, and T6 conditions. The T5 treatment promoted the precipitation of a large fraction of Si-rich nanoparticles and MgSi-based precipitates without disrupting the Si-rich network. After a T6 treatment, the Si-rich network completely disappeared, and the main strengthening phase was MgSi-based precipitates accompanied by α-Al(Mn,Fe)Si dispersoids induced by the Mn addition. Using microstructure-based constitutive models, the strengthening contributions of various microstructural components to mechanical strength in different processing conditions were analyzed.
... Aluminum alloys are widely used in many AM processes, [1][2][3][4] and material properties, microstructure, and density, and overall performance are continuously improving. [5][6][7][8][9][10] The relationship between the material processing, structure, properties, and performance is crucial to understand for component reliability, optimization, and use in any applications. The unique thermal process history that AM parts experience along with other environmental, chemical, feedstock, and geometry (i.e., mechanical stress during build) effects often lead to unpredictable or undesirable microstructure which in turn leads to compromised properties and performance or, in the best case, unexpected performance attributes (high ultimate strength with low ductility, for example). ...
Additive manufacturing has the potential to repair high value components, saving significant time and resources; however, the level of reliability and performance of additive repairs is still relatively unknown. In this work, the structure–property and performance of laser wire additive manufacturing repairs in 1100 aluminum are investigated. Two types of intentional damage are inflicted on the samples and subsequently repaired with pulsed laser deposition additive manufacturing. Quasi-static ( 10 − 3 s − 1) and high strain-rate ( 10 − 3 s − 1) mechanical testing is carried out with in situ diagnostics and post-mortem imaging. The results show that while the quasi-static strength and ductility of samples with a repaired region are lower than a pristine sample, the dynamic strength under shock loading is comparable. This work highlights both the potential utility of additive manufacturing for repair purposes, the significant risk of compromised performance of additive parts under specific conditions, and the need to test at varying strain rates to fully characterize material performance.
... Composite materials [2] and advanced lightweight metal alloys [3][4][5] have emerged as promising solutions, with the latter favored for their more cost-effective production processes. Specifically, advanced aluminum alloys have been developed with unique chemical compositions and microstructures that are highly effective for weight reduction at high temperatures, offering both performance benefits and cost savings [6]. Among these, high-strength and heattreatable aluminum-scandium alloys stand out. ...
The development of high-strength advanced additively manufactured (AM) aluminum alloys is driven by the need for weight reduction in complex-shaped structural applications. In this context, heat-treatable aluminum-scandium alloy, known commercially as Scalmalloy®, offers high strength and lightness, also at high temperatures, due to solution strengthening Al3(Sc, Zr) particles. A widespread diffusion of such AM alloy is also related to welding technologies that could preserve its engineered microstructure. This study investigates the microstructural and mechanical properties of butt friction stir welding (FSW) joints of LPBF Scalmalloy® plates under different welding settings. Joint performance was evaluated under quasi-static and cyclic loading conditions. Porosity in Scalmalloy® and welds was assessed using 3D X-ray computed tomography. An aging heat treatment assessed the extent of precipitation hardening in the FSW joints. Results show that metal stirring during FSW notably reduced the intrinsic porosity of Scalmalloy®, decreasing the equivalent pore diameter from about 200 μm to 60 μm in the welded joints. Under quasi-static loading, welded specimens failed at the interface between the thermo-mechanically affected and the stir zones on the advancing side. The aging heat treatment improved the mechanical strength of Scalmalloy® from approximately 400 to 480 MPa, albeit at the expense of ductility (elongation at fracture decreased from 16 to 4%). The higher heat input and stirring developed at a low welding speed reduced lazy S defects but limited the effectiveness of subsequent aging. In fatigue testing, welded joints consistently failed within the aged base material due to the intrinsic porosity of Scalmalloy®.
In the metal matrix composites (MMCs) reinforced with nanoparticles, the enhancement of composite performance is often closely associated with the types of nanoparticles. It is vital to explore the influences and differences of various nanoparticles on the microstructure evolution and mechanical properties of highly alloyed aluminum-based composites. This study is the first to employ hot pressing for preparing Al-Zn-Mg-Cu composite materials reinforced with SiC and Al 2 O 3 nano-ceramic particles, respectively. By employing hot extrusion and T6 heat treatment techniques, a systematic investigation was conducted on the effects of the addition of nano-ceramic particles on the microstructure evolution and mechanical properties of the Al-Zn-Mg-Cu composites, and a comparative analysis was performed on the two types of composite materials. The experimental results demonstrated that the Mg atoms in the α-Al matrix were consumed by the addition of SiC/Al 2 O 3 nano-ceramic particles, and thereby the grain size primary η’ (MgZn 2 ) strengthening phase was refined, leading to an enhanced overall mechanical performance of the Al-Zn-Mg-Cu composite. In particular, the Mg 2 Si phase was generated by the addition of SiC nanoceramic particles, through the interfacial reactions, and thus with the synergistic effects of precipitation strengthening and Orowan strengthening, the ultimate compressive strength and compressibility of Al-Zn-Mg-Cu composite can be increased to 844 MPa and 27.5%, respectively. On the other hand, the addition of Al 2 O 3 nanoceramic particles can bring about the formation of an oxygen-rich phase and refined precipitates at grain boundaries. Accordingly, the elongation is increased to 43.5% while a high ultimate compressive strength in the composites can be maintained. It is anticipated that nanoceramic particle-reinforced Al-based metal composites have significant potential for achieving both high strength and exceptional ductility for the application in the industry field.
The additive manufacturing of metal matrix composites (MMCs) using laser powder bed fusion (LPBF) is gaining considerable attention for its ability to produce high‐performance materials with intricate geometries. However, incorporating reinforcement such as diamond (D) particles poses challenges to the melting and solidification behavior of the powders, potentially affecting print quality. In this study, the laser irradiation of AlSi10Mg powder mixed with 5 vol% of uncoated D particles is investigated across varying processing parameters. Dense (97%) and crack‐free parts are successfully produced using high laser powers (300 and 400 W) and low laser scanning speeds (300 and 400 mm s⁻¹). It is shown that the energy needed for proper melting of the powder surpasses that required for printing pure AlSi10Mg. Scanning transmission electron microscopy coupled with energy‐dispersive X‐ray spectroscopy uncovers a direct interfacial reaction between the molten aluminum (Al) and the D reinforcement, forming Al carbide at the Al–D interface. Moreover, Al composites processed under optimal energy density exhibit an enhanced Young's modulus. It is highlighted that optimizing LPBF processing parameters is crucial to achieve superior material properties in MMCs, while controlled matrix–reinforcement interactions offer the potential for tailored properties.
Ultra-fine-grained (UFG) and nanotwinned (NT) materials are anticipated to exhibit exceptional resistance to irradiation due to their significant volume fraction of grain boundaries. However, a notable drawback is their susceptibility to grain coarsening at elevated temperatures, which significantly limits their practical application as irradiation-resistant materials, particularly in high-temperature environments. In this study, an AlSi10Mg alloy, prepared using laser powder bed fusion (LPBF), underwent post-processing via the KOBO extrusion method, resulting in an ultra-fine-grained microstructure with an enhanced fraction of coincident site lattice (CSL) twin boundaries. The investigation was conducted in three phases. The first phase involved modelling radiation damage to gain insights into the expected behaviour of the microstructures under irradiation. The second phase included a comprehensive analysis of the microstructures of both as-built and KOBO-processed samples using light, scanning, and transmission electron microscopy. This analysis revealed an ultra-fine-grained microstructure with a mean grain size of approximately 0.8 µm and an increase in the fraction of CSL boundaries from 30% in the as-built state to 42% following KOBO extrusion. In the third phase, the thermal stability of both samples was assessed through annealing experiments conducted for 1 h across a temperature range of 300–500 °C, with 50 °C intervals. To further explore the impact of the nanotwinned microstructure on thermal stability, irradiation experiments were conducted using 60 keV He⁺ ions to a dose of 5 × 101⁷ ions cm⁻2 at 130 °C. The results indicated an improved irradiation resistance in the KOBO-processed sample, as evidenced by a thinner sponge-like structure formation upon Ar⁺-ion irradiation compared to the as-built counterpart.
This additive manufacturing (AM) work is focused on developing a new combination of aluminium–copper (Al–Cu) alloys using direct ink writing (DIW) methods for automotive, aerospace, and electrical industry components. The combination of adding Cu in Al at various percentages ranging from 20 to 80% is applied in this work to evaluate the synergistic properties. The microstructure, mechanical properties, and chemical composition of the thermally post-processed parts were studied by Vickers microhardness, X-ray diffraction (XRD), optical microscope, and scanning electron microscope (SEM) with energy dispersive X-ray analysis (EDX). The investigation reveals that the mechanical property, microstructure, and chemical composition depend upon the physical inclusions of Cu with Al. Our results show that 40% Cu composition possesses crucial traits like the hardness of 656 HV due to the formation of AlCu intermetallics without any unwanted oxides.
Herein, the effects of cooling rate on primary silicon (Si) phases in laser powder bed fusion (PBF‐LB/M) processed hypereutectic Al–Si alloy are investigated. These alloys are particularly in demand for automotive and electronic applications, thanks to their excellent wear and thermal properties. Nevertheless, when processed by conventional methods like casting with comparatively lower cooling rates, the coarse primary Si phases are responsible for increasing brittleness and inducing crack propagation. The refinement of the primary silicon phases is aimed to be achieved through the PBF‐LB/M process, which offers a high cooling rate. The primary Si phases are observed under three different thermal substrate plate conditions—untempered, with cooling and heating, and additionally with varying volume energy density. Furthermore, the impact of primary Si size on the hardness of the fabricated samples is evaluated. The findings show that a faster cooling rate has a notable effect on refining the primary Si size. Then, hardness is directly affected by the primary Si size, with larger Si resulting in decreased hardness.
In this paper, micro-arc oxidation (MAO) coating is prepared on SLM AlSi10Mg alloy to improve its corrosion performance. The microstructure, composition, and electrochemical properties of the substrate and the coating at different oxidation times are analyzed. The results show that the MAO coating is mainly composed of Al, γ-Al2O3, α-Al2O3, and amorphous phases. At an oxidation time of 10 min, the coating achieves its maximum density. Additionally, the corrosion current density of the coating decreases by 5 orders of magnitude, the resistance value increases by 3 orders of magnitude, and the corrosion resistance is the best. Furthermore, the coating on the XY-plane exhibits better corrosion resistance compared to the coating on the XZ-plane.
In this paper, the influence of oxygen concentration in the building chamber on the melt pool behavior and the mechanical properties of the built part obtained via a metal-based powder bed fusion with a laser beam (PBF-LB/M), which is one of an additive manufacturing (AM) method, was experimentally investigated. The metal powders used were two types of aluminum alloy powders; Al-Si-Mg and Al-Mg-Sc. The amount of fumes and spatter particles scattered from laser-irradiated area was calculated when the oxygen concentration was varied from 10 to 10000 ppm, and their chemical composition was evaluated. In addition, the mechanical properties of the built part obtained in different oxygen concentration and their chemical composition were also evaluated. The results showed that the amount of oxygen in the building chamber was one of the principal factor affecting the melt pool behavior. The amount of fumes scattered during the building of Al-Mg-Sc powder was significantly increased when the oxygen concentration in the building chamber was below 5000 ppm, and the amount of Mg composition included in fumes was extremely larger than the virgin powder. Aluminum alloy powders are oxygen-active, so fume generation must be minimized. It was noted that the suitable oxygen concentration during the building of Al-Mg-Sc powder in the PBF-LB/M was ranged from 3000 to 5000 ppm in order to obtain the excellent mechanical properties such as high relative density and elongation while minimizing the amount of fume generation.
Additive manufacturing, better known as 3D printing, is an innovative manufacturing technique which allows the production of parts, with complex and challenging shapes, layer by layer mainly through melting powder particles (metallic, polymeric, or composite) or extruding material in the form of wire, depending on the specific technique. Three-dimensional printing is already widely employed in several sectors, especially aerospace and automotive, although its large-scale use still requires the gain of know-how and to overcome certain limitations related to the production process and high costs. In particular, this innovative technology aims to overtake some of the shortcomings of conventional production methods and to obtain many additional advantages, such as reduction in material consumption and waste production, high level of customisation and automation, environmental sustainability, great design freedom, and reduction in stockpiles. This article aims to give a detailed review of the state of scientific research and progress in the industrial field of metal additive manufacturing, with a detailed view to its potential use in civil engineering and construction. After a comprehensive overview of the current most adopted additive manufacturing techniques, the fundamental printing process parameters to achieve successful results in terms of quality, precision, and strength are debated. Then, the already existing applications of metal 3D printing in the field of construction and civil engineering are widely discussed. Moreover, the strategic potentiality of the use of additive manufacturing both combined with topological optimisation and for the eventual repair of existing structures is presented. It can be stated that the discussed findings led us to conclude that the use of metal additive manufacturing in the building sector is very promising because of the several benefits that this technology is able to offer.
In fusion‐based metal additive manufacturing (MAM), the high‐intensity energy input leads to serious evaporation, but how evaporation induces composition evolution and variation and further impacts microstructure and mechanical properties remain a knowledge gap. Here a model integrating composition evolution with molten pool dynamics is developed to reproduce temperature‐ and composition‐dependent evaporative losses and subsequent transport during laser melting. Together with comprehensive experimental characterizations and tests, the simulation results illustrate varying evaporation rates of different elements altering compositions, resulting in a 3D cirrus‐shaped concentration distribution, which significantly impacts the mechanical properties. The simulations reproduce the detailed composition evolution from surface evaporation to molten pool transport and reveal underlying mechanisms relating the composition, temperature, fluid flow, and cracking, which is challenging to observe experimentally. This study elucidates the critical role of evaporation‐induced composition evolution in determining microstructure and mechanical properties. In future alloy design for MAM, integrating initial composition and manufacturing parameters is imperative, where composition evolution simulation offers valuable guidance.
Laser material processing like laser-beam welding or powderbed fusion (LPB-F) of Al-alloys have demonstrated many options for light weight structures & sustainable by function driven designs. Scalmalloy ® , an AlMgSc material concept developed by Airbus, commercialized by PAG APWorks, offers outstanding good material properties in LPB-F (Rm ≥ 500 MPa) but suffers on LPB-F process stability due to complex interactions of laser energy (its physical conversion into melting heat) with its physical bulk material absorption & reflection propensities. Replacing Mg by Cr enables a new and promising Al-material concept (ScanCromAl ®). First investigations at Airbus Central R&T showed that AlCrSc alloys can offer a very good strength-ductility property mix owing an unexpected solidification micro structure in comparison to AlMgSc.
The high Fe-containing AlSi12 alloy was processed by additive manufacturing of laser powder bed fusion (LPBF) to understand the features of microstructures and mechanical properties under as-fabricated condition. The Fe impurity was found to be beneficial for mechanical property enhancement in the LPBFed samples. The parameters including the combination of laser power of 200 W, scanning speed of 1110 mm/s, hatch spacing of 0.15 mm, layer thickness of 0.03 mm and laser volumetric energy density of 40 J/mm³ were optimized to achieve a high relative density of 99.7%. The as-LPBFed AlSi12FeMn alloy was featured by a high density of significantly refined spherical α-Al(Fe,Mn)Si phase (10-50 nm), which was coherent with the Al matrix. Meanwhile, the as-LPBFed AlSi12FeMn alloy can deliver superior mechanical properties including the yield strength of 305 MPa, the ultimate tensile strength of 485 MPa and the fracture strain of 6.1%. The improved mechanical properties are attributed to synergistic strengthening mechanisms, including solid solution strengthening, grain boundary strengthening and precipitation strengthening. Moreover, the formation of high-density stacking faults (SFs) and Lomer-Cottrell locks (LCs) in localized regions can also offer strengthening in the as-LPBFed AlSi12FeMn alloy.
Aluminum (Al) and its alloys are the second most used materials spanning industrial applications in automotive, aircraft and aerospace industries. To comply with the industrial demand for high-performance aluminum alloys with superb mechanical properties, one promising approach is reinforcement with ceramic particulates. Laser powder-bed fusion (LPBF) of Al alloy powders provides vast freedom in design and allows fabrication of aluminum matrix composites with significant grain refinement and textureless microstructure. This review paper evaluates the trends in in situ and ex situ reinforcement of aluminum alloys by ceramic particulates, while analyzing their effect on the material properties and process parameters. The current research efforts are mainly directed toward additives for grain refinement to improve the mechanical performance of the printed parts. Reinforcing additives has been demonstrated as a promising perspective for the industrialization of Al-based composites produced via laser powder-bed fusion technique. In this review, attention is mainly paid to borides (TiB2, LaB6, CaB6), carbides (TiC, SiC), nitrides (TiN, Si3N4, BN, AlN), hybrid additives and their effect on the densification, grain refinement and mechanical behavior of the LPBF-produced composites.
High strength aluminum alloys, especially those that are age-hardenable, such as 2xxx series, 6xxx series, and 7xxx series, are widely used as structural materials in transport and aerospace industries due to their good mechanical properties. However, additive manufacturing of high strength aluminum alloys is challenging due to their susceptibility to hot tearing. In this work, a systematic study has been conducted in an attempt to eliminate hot tearing in laser powder bed fusion (PBF-LB/M) manufacturing of Al2139 alloy through selecting process parameters and the addition of an AlTiB grain refiner. It was found that hot tearing in Al2139 during PBF-LB/M can be reduced or eliminated by increasing volumetric energy density. Furthermore, grain refinement by AlTiB addition shows a clear effect in reducing hot tearing, even though the refined grains remain predominantly columnar rather than equiaxed. The effect of increasing volumetric energy density on hot tearing of Al2139 during PBF-LB/M was analyzed by thermomechanical finite-element simulation, which showed that the reduction in hot tearing with increasing energy density is associated with a decrease in the thermal residual stress. However, it was also shown that there can be a substantive loss of Mg due to evaporation at high energy densities. From a hot tearing model based on solidification thermodynamics, the loss of Mg on hot tearing in Al2139 was estimated to reduce hot tearing by up to 10%, compared to the initial powder composition. With the selected PBF-LB/M parameters, crack-free tensile specimens were fabricated, with and without the addition of AlTiB. Compared with their wrought or cast counterparts, the PBF-LB/M Al2139 and Al2139-AlTiB specimens show lower yield strength but better ductility, which can be attributed to the loss of Mg during PBF-LB/M.
This study presents the microstructure and mechanical properties of an additive manufactured recyclable Al-2Fe alloy fabricated by laser powder bed fusion (LPBF). The microstructure evolution of the Al-2Fe alloy aged at elevated temperatures and its impact on the mechanical properties were systematically investigated. Firstly, the single melt scanning experiment verified that Al-Al6Fe anomalous eutectic structure was initially formed from the liquid phase. Secondly, the as-built Al-2Fe contains nano-size and rod-shaped Al6Fe particles which uniformly dispersed in a supersaturated Al matrix. The Al6Fe particles with an average diameter of ~89 nm shows a coherent crystallographic relationship with the matrix as (046)Al6Fe // (2¯02¯)Al, and [032¯]Al6Fe // [1¯11]Al. The ultimate tensile strength and total elongation of 287.0 MPa and 12.0% were obtained in the ultrafine as-built Al-2Fe, respectively. Aging at elevated temperatures made the Al6Fe particles coarsened and transformed into Al13Fe4 particles at 350 °C. The amount of the total Al-Fe intermetallic compounds (Al6Fe+Al13Fe4) increased with the aging temperature due to the precipitation of the supersaturated Fe atoms. It exhibits a slow increase in grain size and a decrease in dislocation density in the Al-2Fe samples with the increase of aging temperature. All Al6Fe particles were transformed into coarse Al13Fe4 particles at 550 °C. Thirdly, the quantitative analysis indicates that the dominant contribution to the yield strength derives from the dispersion strengthening mechanism by creation of fine Al6Fe and Al13Fe4 particles. The results in this work demonstrate that ultrafine microstructure can be obtained in a simple binary Al-Fe alloy as a result of the extremely high cooling rate during LPBF process, which offers a design strategy towards high-strength recyclable aluminum alloys.
To achieve sustainability across the product life cycle, attention to the production process is a prerequisite. As a result of technological advancements, innovation and inventions in production methods are in full swing. Production methods that enable mass customisation (MC) are one of the recent developments in the production domain. This study aims to empirically explore the sustainability impact of two MC-oriented production methods, namely, additive manufacturing (i.e., Selective Laser Sintering) and subtractive manufacturing (Computer Numerical Control Milling) within two complete production lines (i.e., from raw material to assembly) for a wearable product. In the context of the triple bottom line framework, the production lines are analysed from an economic, environmental, and social standpoint. A Discrete-Event Simulation (DES) is used to quantify and compare both production systems with their inherent variability in a dynamic setting of fluctuating order volume and diversity. The findings of the simulation are qualitatively evaluated using expert interviews. This study provides a detailed insight into several sustainability trade-offs in production systems where additive and subtractive manufacturing are involved.
To exploit the full potential of the additive layer manufacturing technique it is necessary to adapt the material to the process via a smart alloy design strategy. To this end, in order to derive and investi- gate various material concepts, the microstructural evolution of Sc-modified Al alloys was studied during the course of their production by laser powder bed fusion. Adding Mg as the main element (Al-4.4Mg- 0.8Sc-0.3Zr-0.5Mn) generates an already-familiar bimodal microstructure. In contrast, if Cr is added as the main element (Al-2.6Cr-0.7Sc-0.3Zr), epitaxial grain growth takes place across several weld tracks, resulting in a distinct texture; and adding Ti as the main element (Al-1Ti-1Sc-0.4Zr) produces a uniform ultrafine-grained microstructure. The differences between these microstructures arise from interactions of the grain growth restriction factors and the solute with the primary precipitation structure. Thus, the precise manipulation of key metallurgical factors leads to novel materials which can be tailor-made for certain requirements.
Using the calculation of phase diagrams approach and Scheil solidification modeling, the Al-2.5Mg-1.0Ni-0.4Sc-0.1Zr alloy was designed, intentionally with an extraordinarily high cracking susceptibility, making it prime for solidification cracking during laser powder bed fusion. This study demonstrates the ability to mitigate even the most extreme solidification cracking tendencies in aluminum alloys with only minor alloying additions of Sc and Zr, 0.5 wt.% max. Furthermore, by employing a simple direct ageing heat treatment, good tensile mechanical properties were observed with a yield strength of 308 MPa, an ultimate tensile strength of 390 MPa, and a total elongation of 11%.
Laser powder bed fusion (LPBF) was used to horizontally print A205 (Al—Cu—Mg—Ag—Ti—B) metal powder. The samples have been cut in various directions to analyze their phases, grain morphology, size, crystallographic orientation, and distribution of elements via X-ray diffraction (XRD), electron backscatter diffraction (EBSD), and X-ray energy dispersive spectroscopy (XEDS) in a transmission electron microscope (TEM). To monitor deformation behavior and strain distribution, several uniaxial tensile tests accompanied by digital image correlation (DIC) technique were carried out. The results showed that the as-built microstructure fully contains fine equiaxed grains with no strong preferential crystallographic orientation. The XEDS elemental maps confirmed the solute trapping phenomenon taking place during the LPBF. The stress-strain curves obtained by the uniaxial tensile testing revealed a quasi-isotropic behavior in terms of building direction; moreover, the DIC strain contour maps confirmed yield-point phenomenon and Lüders bands propagation during uniaxial tensile testing. Finally, a time/temperature interdependence grain growth model was successfully applied based on no diffusion in solid and partial mixing in liquid through the entire melt pool.
The Sc-containing Al-Mn alloy system produced by additive manufacturing (AM) has recently presented exciting new opportunities to achieve a step-change in mechanical properties, but its processability remains unclear. In this work, the optimum processing window for the Al-Mn-Mg-Sc-Zr alloy fabricated by selective laser melting (SLM) has been established for the first time. The window covers the range of processing parameters that can lead to a good combination of part density, strength, ductility, and processability. The alloys fabricated within this optimized processing window of SLM have the material relative density more than 99.8% with less porosity. Moreover, all these alloys have the yield strength exceeding 430 MPa and the ductility of over 17%. Further microstructural examinations suggest that such excellent mechanical properties are associated with a bimodal grain architecture. Also, a high number density of intermetallic particles has been detected in these two-grain regions. They are confirmed to be Al3Sc and Mn(Fe)-rich quasicrystal. Most of these particles distributing along grain boundaries are expected to pin the grain boundaries and contribute to the high strength of this alloy. The findings will provide an essential basis for achieving exceptional mechanical performance and intricate geometry designs of the alloy using AM.
Selective laser melting (SLM) is a powder bed fusion type metal additive manufacturing process which is being applied to manufacture highly customised and value-added parts in biomedical, defence, aerospace, and automotive industries. Aluminium alloy is one of the widely used metals in manufacturing parts in SLM in these sectors due to its light weight, high strength, and corrosion resistance properties. Parts used in such applications can be subjected to severe dynamic loadings and high temperature conditions in service. It is important to understand the mechanical response of such products produced by SLM under different loading and operating conditions. This paper presents a comprehensive review of the latest research carried out in understanding the mechanical properties of aluminium alloys processed by SLM under static, dynamic, different build orientations, and heat treatment conditions with the aim of identifying research gaps and future research directions.
In this work, crack-free samples with a relative density of 99.5 ± 0.1% were produced from a gas-atomized Al–Cu–Ag–Mg–Ti–TiB2 powder via laser powder-bed fusion. The homogeneous equiaxed microstructure without preferred grain orientation shows the α-Al grains’ mean size to be 0.64 μm ± 0.26 μm TiB2 particles with sizes of several tens of nm up to 1.5 μm were observed in the as-built component. Small TiB2 particles of up to approx. 200 nm are located within the α-Al grains, which show a semi-coherent interface to the α-Al phase. Larger TiB2 particles of up to 1.5 μm accumulate in the liquid between the growing α-Al grains during solidification and inhibit grain growth. Al2Cu phase is precipitated at the α-Al grain boundaries. Coarse Al2Cu precipitates, which are slightly enriched with silver and magnesium, are also observed within the grains preferentially precipitated on small pores and TiB2 particles. The novel fine-grained microstructure results in the as-built state in a tensile strength of 401 ± 2 MPa and total elongation at fracture of 17.7 ± 0.8%.
The effects of inoculation treatment with LaB 6 nanoparticles (0-2 wt% additions) on the microstructural evolution and mechanical performance in a selective laser melted AlSi10Mg alloy were comprehensively investigated. The addition of 0.2-0.5 wt% LaB 6 nanoparticles was identified to be optimal to achieve substantial grain refinement , microstructural homogeneity and thus remediation in the mechanical property anisotropy in the AlSi10Mg alloy. The substantial grain refinement was attributed to the coherent Al/LaB 6 interfaces, which facilitated the heterogeneous nucleation of Al on the LaB 6 nanoparticles during solidification. Increasing the LaB 6 addition up to 2 wt% only marginally further refined the equiaxed grains, which can be understood in terms of the concept of nucleation free zone formed in the liquid at front of the growing solid-liquid interfaces. The LaB 6 nanoparticles within the nucleation free zone could not be activated to be nucleants for α-Al. As a result, random orientation relationships between LaB 6 nanoparticles within the nucleation free zone and the Al matrix were determined. Those excessive LaB 6 nanoparticles weakened the melt pool boundaries, and therefore deteriorated the longitudinal ductility of the SLMed AlSi10Mg alloy.
Laser-Based Powder bed Fusion (LBPF) process or commonly known as Selective Laser Melting (SLM) has made significant progress since its inception. Initially, conventional materials like 316L, Ti6Al4V, and IN-718 were fabricated using the SLM process. However, it was inevitable to explore the possible fabrication of the second most popular structural material after Fe-based alloys/steel, the Al-based alloys by SLM. Al-based alloys exhibit some inherent difficulties due to the following factors: the presence of surface oxide layer, solidification cracking during melt cooling, high reflectivity from the surface, the high thermal conductivity of the metal, poor flowability of the powder, low melting temperature, etc. Researchers have overcome these difficulties to fabricate successfully the different Al-based alloys by SLM. However, there exists no review dealing with the fabrication of different Al-based alloys by SLM, their fabrication issues, microstructure and their correlation with properties in detail. Hence, the present review attempts to introduce the SLM process followed by a detailed discussion about the processing parameters that form the core of the alloy development process. This is followed by the current research status on the processing of Al-based alloys and microstructure evaluation (including defects, internal stresses, etc.), which are dealt with individual Al-based series. The mechanical properties of these alloys were discussed in detail followed by the other important properties like tribological properties, fatigues properties, etc. Lastly, an outlook is given at the end of this review.
The Al-7Si-0.6Mg alloy with added rare earth erbium (Er) was prepared by laser powder bed fusion (LPBF) technology, and the effect of heat treatment on the microstructure and mechanical properties was studied. The addition of Er introduces Al3Er to the molten pool, which can effectively hinder the epitaxial growth of the columnar crystal. The effect of rapid cooling and the addition of Er can also modify eutectic Si particles, resulting in an ultrafine eutectic network cellular structure. Moreover, the as-built samples have an excellent tensile strength exceeding 438 MPa and a ductility of over 8%; furthermore, such excellent mechanical properties are associated with small grains, a continuous Si network, and extremely oversaturated solid solubility. After heat treatment, LPBF-processed alloy samples exhibit superior mechanical properties, especially after direct age treatment, which can mainly be attributed to the precipitation strengthening effect introduced by nanosized precipitates.
Grain refinement is critical to surpassing the bottlenecks of inherent hot tearing of high-strength aluminum alloys fabricated by additive manufacturing (AM). In this study, a synergistic grain-refining strategy including heterogeneous nucleation, solute-driven growth restriction and nanoparticle-induced growth restriction was introduced to control the microstructure of Al-Zn-Mg-Cu alloys during the laser powder bed fusion (LPBF) process. Crack-free Al-Zn-Mg-Cu alloys with significantly refined grains were safely fabricated via LPBF by coincorporation of TiC and TiH2 particles. In-situ L12-Al3Ti particles were produced to promote the heterogeneous nucleation. The grain growth was restricted by adding Ti solute, while introduced TiC nanoparticles (NPs) improved the density of heterogeneous nucleation sites and blocked grain growth physically. The resultant elimination of columnar grains and hot cracks in the (1 wt.%) TiC- and (0.8 wt.%) TiH2-modified Al-Zn-Mg-Cu alloy resulted in excellent ultimate tensile strength (UTS) of 593 ± 24 MPa, yield strength (YS) of 485 ± 41 MPa and elongation (EL) of 10.0% ± 2.5% under the T6 condition. This study provides new insights into improving the grain microstructure and mechanical properties of high-strength aluminum alloys during LPBF.
Defect formation is a critical challenge for powder-based metal additive manufacturing (AM). Current understanding on the three important issues including formation mechanism, influence and control method of metal AM defects should be updated. In this review paper, multi-scale defects in AMed metals and alloys are identified and for the first time classified into three categories, including geometry related, surface integrity related and microstructural defects. In particular, the microstructural defects are further divided into internal cracks and pores, textured columnar grains, compositional defects and dislocation cells. The root causes of the multi-scale defects are discussed. The key factors that affect the defect formation are identified and analyzed. The detection methods and modeling of the multi-scale defects are briefly introduced. The effects of the multi-scale defects on the mechanical properties especially for tensile properties and fatigue performance of AMed metallic components are reviewed. Various control and mitigation methods for the corresponding defects, include process parameter control, post processing, alloy design and hybrid AM techniques, are summarized and discussed. From research aspect, current research gaps and future prospects from three important aspects of the multi-scale AM defects are identified and delineated.
Coarse column grain and hot cracking severely hinder the application of AA7075 aluminum alloy fabricated by selective laser melting (SLM). In this work, a novel grain refiner consisting of submicron TiH2 and amorphous B particles was developed to control such features in SLM-fabricated AA7075 alloy. Crack-free and dense (TiH2+B)/AA7075 alloy was fabricated under optimal SLM processing. Results show that the addition of TiH2 and B particles can convert coarse columnar grains into homogeneous equiaxed grains, decrease the average grain size from 29.1 μm to 2.3 μm, and consequently eliminate the hot-cracking in (TiH2+B)/AA7075 alloy. The remarkable grain refinement is mainly attributed to heterogeneous nucleation promoted by a combination of in-situ formed L12-Al3Ti and TiB2 nanoparticles. After T6 heat treatment, the (TiH2+B)/AA7075 alloy behaves superior mechanical properties(tensile strength of ∼582 MPa and ductility of ∼12.8%) to most previously reported SLM-fabricated 7075 alloys. Synergetic effects of crack elimination, grain refinement, and precipitation strengthening are responsible for the excellent mechanical properties.
Numerous studies on laser powder bed fusion (LPBF) have already demonstrated the evolution of out-of-equilibrium microstructures with metastable phases. In the present work, a self-designed, pre-alloyed Al-Cu-Ti-Ag-Mg alloy is processed using LPBF. The solidification path, which is necessary to achieve sufficient supercooling to exceed the critical nucleation supercooling (ΔTn) required for heterogeneous nucleation on L12 Al3Ti nuclei, is derived from the microstructure. This unique microstructure can be divided into two areas: Area 1, with a thickness in the building direction of 5–10 µm, solidifies first and forms on the bottom of the semicircular melting pool. It is dominated by columnar α-Al grains, which contain numerous precipitated cube-shaped Al-Cu-Ti-Ag nanoparticles. During the solidification of Area 1, the constitutional supercooling (ΔTCS) and the thermal supercooling (ΔTtherm) gradually increase. The Ti and Al atoms in the residual melt react to form numerous primary L12 Al3Ti particles, which are activated for heterogeneous nucleation and serve as nuclei for α-Al grain growth once ΔTtotal (ΔTCS + ΔTtherm) exceeds ΔTn. Area 2, formed by heterogeneous grain refinement, occupies the remaining part of the melting pool and consists of fine equiaxed α-Al grains. The cube-shaped Al-Cu-Ti-Ag nanoparticles precipitated from the supersaturated α-Al in Area 1 cannot be observed in Area 2. The novel alloy with a fine-grained microstructure exhibits a tensile strength of 475 ± 7 MPa in combination with an elongation to fracture of 8.7 ± 0.5%.
The microstructural and strength evolution of an additively manufactured Al-8.6Cu-0.5Mn-0.9Zr alloy upon aging at 300, 350, and 400 °C is investigated. The strengthening phases of the alloy evolve significantly upon aging, with breakdown and spheroidization of the interconnected θ-Al2Cu network, dissolution of metastable θ'-Al2Cu precipitates, and precipitation of nanometric L12-Al3Zr from a matrix supersaturated in Zr. In the peak-aged states, the alloy displays a favorable combination of strength and ductility, with a room-temperature yield strength of 314–341 MPa and ductility of 11–13%. The measured yield strengths for microstructures with different aging treatments are compared to predictions of yield strengths from grain boundary, solid solution, and particle strengthening contributions. The observed strain hardening behavior is related to fundamental precipitate and dislocation interactions. Comparison between predicted and measured strength values indicates a continued need for strengthening models specifically developed for the heterogeneous microstructures of additively manufactured alloys.
Existing additively manufactured aluminum alloys exhibit poor creep resistance due to coarsening of their strengthening phases and refined grain structures. In this paper, we report on a novel additively manufactured Al-10.5Ce-3.1Ni-1.2Mn wt.% alloy which displays excellent creep resistance relative to cast high-temperature aluminum alloys at 300–400°C. The creep resistance of this alloy is attributed to a high volume fraction (∼35%) of submicron intermetallic strengthening phases which are coarsening-resistant for hundreds of hours at 350°C. The results herein demonstrate that additive manufacturing provides opportunities for development of creep-resistant aluminum alloys that may be used in bulk form in the 250–400°C temperature range. Pathways for further development of such alloys are identified.
Readily available high strength Al-alloys for laser power bed fusion (LPBF) typically require post-processing such as mechanical working or heat treatment to evolve strengthening phases. Through the calculation of phase diagrams (CALPHAD) and a method based on Scheil solidification, the Al-8wt.%Ce-10wt.%Mg alloy was evaluated for LPBF to be microstructurally favorable to yield components with full volumetric density, without the need for post-processing. Through an exhaustive LPBF parametric study, documentation of defects was performed for the use of a wide range of laser power and scan speed combinations, i.e., varying the energy density, followed by microstructural characterization with X-ray diffraction, optical and electron microscopy, and mechanical testing with both Vickers hardness and quasi-static uniaxial tension. Overall, defects were observed with use of high and low laser powers and scan speeds, but a processing window for LPBF was observed at both high and low laser powers, where >99% volumetric density was achieved. Besides defects, decreasing the scan speed, i.e., increasing the energy density, resulted in decreases in the concentration of Mg, down to a maximum of 4.5 wt.% due to vaporization. Moreover, a decrease in Mg led to a lower Vickers hardness, attributed to a decrease in the solid solutioning of Mg in the α-Al matrix. Samples selected for tensile mechanical testing were produced with a LPBF laser power of 200 W traversing at 800 mm/s, which yielded a density > 99% and Mg concentration of approximately 7 wt.%. A yield strength and ultimate tensile strength of 377 and 468 MPa, respectively, was found for those produced with the power of 200 W. However, the ductility of the alloy was less than 2% elongation at failure. A fine (< 1 μm) sub-grain cellular solidification structure was found distributed throughout the α-Al matrix, with ribbons of the eutectic Al11Ce3 intermetallic distributed along the intercellular boundaries. The high strengths achieved in this alloy were attributed to contribution from Orowan dislocation looping of the Al11Ce3 phase, the high degree of solid solutioning of Mg in the α-Al matrix, and cellular boundary strengthening due to impeding dislocation movement.
Grain refinement is an effective way to avoid hot cracking of non-weldable high strength aluminum alloys during additive manufacturing. In the present work, primary Al3Nb with the tetragonal D022 structure was discovered to be an extremely effective inoculant for Al-Zn-Mg-Cu alloys manufactured by selective laser melting (SLM). The columnar crystals were transformed into fine equiaxed crystals with an average grain size of ~1.9μm through adding 1.5 wt.% Nb nanoparticles, eliminating the defects such as cracks and porosity. The high number density of primary Al3Nb and the coherent interface with high lattice matching between the primary Al3Nb and the Al matrix are the main reasons for grain refinement. The Time-Temperature-Transformation (TTT) curves of the precipitation kinetics curves of primary Al3X (X=Nb, Zr, Ti, Sc) show that primary Al3Nb has the advantage in grain refinement compared with Al3Sc and Al3Ti. Due to grain refinement and defect elimination, the 1.5 wt.% Nb-modified Al-Zn-Mg-Cu alloy after T6 heat treatment exhibits excellent mechanical properties, with a tensile strength of ~ 505±12 MPa, and an elongation after fracture of ~ 12.3±1.3%.
The addition of a sufficient amount of the potent heterogeneous nucleating agent CaB6 enables the fabrication of crack-free specimens from the solidification-crack susceptible high-strength 2024 (Al–Cu–Mg) aluminum (Al) alloy using laser powder bed fusion (LPBF). The present work investigates the effects of varying addition contents of CaB6 nanoparticles (0.0–2.0 wt%) on the alloys' solidification behavior as well as the specimens’ solidification-crack volume, microstructure, and mechanical properties.
The findings of X-ray microscopy (XRM) analyses on LPBF specimens and in-situ differential fast scanning calorimetry (DFSC) analyses on single powder particles at LPBF-like high heating and cooling rates reveal decreasing crack volumes with decreasing solidification supercooling. A CaB6 content of equal to or greater than 0.5 wt% effectively suppresses solidification cracking. 1.0 wt% is defined as the optimum CaB6 content in terms of mechanical properties. With this content an average grain size of 0.77 μm, an ultimate tensile strength (UTS) of 478 ± 4 MPa and an elongation (A) of 13.2 ± 0.1% are achieved. When the CaB6 content is further increased, the alloy's average grain size asymptotically approaches a minimum size of ∼0.7 μm for the given process parameters. This value corresponds to the nucleation-free zone (NFZ), within which the CaB6 nanoparticles present are not activated as nucleating agents, resulting in deposition along the grain boundaries.
The properties of modified conventional wrought aluminum alloys cannot be significantly enhanced by normal post-heat treatment in that the fine-grained strengthening, arising from high cooling rate in SLM, is underutilized. In this work, compared with the normal T6 heat treatment, a novel simple direct aging regime was proposed to maintain the grain-boundary strengthening and to utilize the precipitation strengthening of secondary Al3Zr. It was found that a heterogeneous grain structure, which consisted of ultrafine equiaxed (∼0.82 μm) and columnar (∼1.80 μm) grains at the bottom and top of molten pool, respectively, was formed in the SLM processed sample. After direct aging (DA), the ultrafine grains were maintained and a mass of spherical coherent L12-Al3Zr particles with a mean radius of approximately 1.15 nm was precipitated. In contrast, after solution treatment and aging (STA), a significant grain coarsening occurred in the equiaxed grain region. Meanwhile, the coarsening L12-Al3Zr particles, nano-sized S′ phases and GPB zones were detected in the STA sample. This subsequently induced that the yield strength of the DA sample (∼435 MPa) was higher than that of the STA sample (∼402 MPa) owing to the grain boundary strengthening and precipitation strengthening. Both the STA and DA samples exhibited a higher strength than that of the other SLMed Al-Cu-Mg series alloys; this was comparable to that of the wrought AA2024-T6 alloy (∼393 MPa). Both the STA and DA samples exhibited a higher strength than that of the other SLMed Al-Cu-Mg series alloys; this was comparable to that of the wrought AA2024-T6 alloy (∼393 MPa).
In the metallic components fabricated by the emerging selective laser melting (SLM) technology, most strategies used for strengthening the materials sacrifice the ductility, leading to the so-called strength-ductility trade-off. In the present study, we report that the strength and ductility of materials can be enhanced simultaneously by introducing nanoparticles, which can break the trade-off of the metallic materials. In the case of in-situ nano-TiB2 decorated AlSi10Mg composites, the introduced nanoparticles lead to columnar-to-equiaxed transition, grain refinement and texture elimination. With increasing content of nanoparticles, the strength increases continually. Significantly, the ductility first increases and then decreases. Our results show that the ductility is controlled by the competition between the crack-induced catastrophic fracture and ductile fracture associated with dislocation activities. The first increase of ductility is mainly attributed to the suppression of crack-induced catastrophic fracture when TiB2 nanoparticles present. With the further increase of TiB2 nanoparticles, the subsequent decrease of ductility is mainly controlled by dislocation activities. Thus, the materials will exhibit the optimum strength and ductility combination in a certain range of TiB2 nanoparticles. This study clarifies the physical mechanism controlling ductility for nano-TiB2 decorated AlSi10Mg composites, which provides the insights for the design of structural materials.
A precise control of the Mg content in Al–Mg alloys is essential to obtain predictable mechanical properties but the processing of Al–Mg alloys often suffers Mg losses due to Mg evaporation and oxidation. A new high strength Al–Mg–Sc alloy designed for LPBF (laser powder bed fusion) processing has been developed here, where Mg losses are effectively prevented by the addition of a low amount of calcium. A LPBF processing window which results in built parts with a 99.7% relative density and no detectable loss of Mg has been identified. The as-built microstructure of the new Calciscal® alloy, studied by transmission electron microscopy, is found to comprise areas of fine equiaxed grains and areas of coarser grains, with many Al4Ca precipitates present at the grain boundaries and within the coarser grains. After a subsequent ageing of 1 h at 375 °C, the tensile strength of Calciscal® is increased by 44% compared to the as-built condition and reaches 522 ± 2 MPa. The increase in strength observed in the heat-treated condition comes from the additional precipitation of numerous finely dispersed Sc-rich precipitates. This high strength, combined with a good ductility, makes Calciscal® competitive to other Al alloys and suitable for structural applications. Moreover, Calciscal® shows very reproducible tensile properties thanks to the Ca addition which leads to a better control and less variations of the Mg content in the alloy.
For additive manufactured aluminum alloys, the inferior mechanical properties along the building direction have been a serious weakness. In this study, an optimized heat treatment was developed as a simple and effective solution. The effects of direct aging on microstructure and mechanical properties along the building direction of AlSi10Mg samples produced via selective laser melting (SLM) were investigated. The results showed that, compared with the conventional heat treatment at elevated temperatures, direct aging at temperatures of 130-190°C could retain the fine grain microstructure of SLM samples and promote further precipitation of Si phase, however, the growth of pores occurred during direct aging. With increasing aging temperature, while finer cell structures were obtained, more and larger pores were developed, resulting in decreased density of the samples. Two types of pore formation mechanisms were identified. Considering the balance between the refinement of cell structure and the growth of pores, aging at 130°C was determined as the optimized heat treatment for SLM AlSi10Mg samples. The tensile strength along the building direction of the 130°C aged sample was increased from 403 MPa to 451 MPa, with relatively high elongation of 6.5%.
Grain refinement is effective in restraining hot tearing, reducing anisotropy, eliminating defects, improving processability, and enhancing the mechanical properties of high-strength aluminum components additively manufactured by laser powder bed fusion (LPBF). However, achieving the desired strength and ductility in LPBF-fabricated high-strength aluminum alloys post grain refinement is a predominant challenge. We have therefore designed and developed a novel hybrid grain refiner (solute/ceramic nanoparticles) which can effectively refine grains and enhance the mechanical properties of LPBF-fabricated high-strength aluminum alloys. Adding a Ti/TiN hybrid grain refiner to the LPBF-fabricated 7050 alloy can produce ultrafine grains with an average size of 775 nm, resulting in an ultimate tensile strength and ductility of up to 408–618 MPa and 13.2–8.8%, respectively. These tensile properties are comparable to those of conventional wrought 7XXX alloys. During LPBF processing, the hybrid grain refiner exhibited interesting synergistic grain refinements and strengthening mechanisms between the solute and the ceramic nanoparticles. During solidification, not only in-situ particles formed by the chemical reaction of the solute in liquid Al and the externally added ceramic nanoparticles can act as the nuclei of α-Al respectively, but also solute can inhibit the agglomeration of ceramic nanoparticles to promote their nucleation efficiency. Moreover, the strength can be further improved by doping the solute atoms at the ceramic nanoparticle/Al interface. The improvement in elongation benefited from the uniform dispersion of the various particles.
Using laser powder bed fusion (L-PBF), we designed and fabricated specimens of a heat-resistant Al-4mass%Cr-1.5mass%Zr alloy (Al-4Cr-1.5Zr). The Al-4Cr-1.5Zr L-PBF specimen achieved high relative density greater than 99.9% by optimizing the laser scan conditions. The hardness and tensile and 0.2% proof stresses of the Al-4Cr-1.5Zr L-PBF specimens significantly increased after aging heat treatments, and the peak-aged specimens exhibited excellent high-temperature strength. The L-PBF specimens consisted of coarse columnar and fine equiaxed crystal grains owing to heterogeneous nucleation by the primarily crystallized Al3Zr phase. In the peak-aged L-PBF specimen, a finely dispersed Al3Zr granular phase on the order of single nanometers was precipitated in the α-Al matrix by aging. Meanwhile, the chromium was supersaturated in an α-Al solid solution even after peak aging, and partially precipitated into coarse granular Al-Cr-based compounds. These multi-scale microstructural observation results revealed that the dominant strengthening factors in the Al-4Cr-1.5Zr L-PBF specimens were precipitation hardening by the fine L12-Al3Zr phase having a strong crystallographic consistency with the α-Al matrix, solid-solution hardening with chromium, dispersion hardening due to the Al-Cr-based compounds, and hardening by the crystal grain refinement mainly with the fine equiaxed grains.
In the development of high-strength aluminium alloys tailored specifically to additive manufacturing (AM), L12-Al3X-forming elements have been proven to be particularly effective alloying additions, reducing the susceptibility of aluminium alloys to solidification cracking by grain refinement and increasing the strength by forming secondary nanoscale precipitates. In the present work, we employ laser powder bed fusion (L-PBF) to examine the feasibility of using Ti as the main strengthening, L12-forming element for the design of a well-processable, precipitation-strengthened model alloy. Al-1.76Ti and Al-2.51Ti (wt%) alloys with and without ternary additions of Ni and Si are fabricated during processing from powder mixtures. Crack-free microstructures are produced, which display alternating fine- and coarse-grained regions. During heat treatment, one of the investigated alloys, Al-2.51Ti-0.7Si, exhibits an ageing response due to the formation of nanoscale, metastable L12-(Al,Si)3Ti precipitates, reaching a peak hardness of 97 HV. This demonstrates that L-PBF-processed Ti-containing aluminium alloys can exhibit a precipitation hardening response similar to alloys containing Sc or Zr. In the other Al-Ti-based materials tested, no significant hardness increase was found. This discrepancy is ascribed to the effects Si exerts on the precipitation process. First, the addition of Si increases the thermodynamic driving force for homogeneous nucleation of L12-Al3Ti precipitates. Secondly, Si-Ti co-clustering is observed by atom probe tomography, which is expected to enhance the otherwise sluggish diffusion of Ti. The Al-2.51Ti-0.7Si alloy developed in this work can in the future be further strengthened by adding additional alloying elements and by optimising the heat treatment.
In order to manage both the strength and ductility of Al–Si alloys additively manufactured by laser powder bed fusion (L-PBF), an attempt was made to introduce nanoscale precipitates within the α-Al (fcc) supersaturated solid solution using the L-PBF manufactured Al–12%Si alloy by artificial aging treatments. The aging treatment at 120°C improved both the tensile strength and ductility (total elongation) of the L-PBF manufactured specimen. The simultaneous strength–ductility enhancement is due to the homogeneously distributed nanoscale Si precipitates that enhanced the strain hardening. These results provide new insights for overcoming the strength-ductility trade-off in the L-PBF manufactured Al alloys.
The Al-Mn-Sc-based alloys specific for additive manufacturing (AM) have been recently developed and can reach ultrahigh strength and adequate elongation. However, these alloys commonly exhibit non-uniform plasticity during tensile deformation, which is a critical issue hindering their wider application. In this work, the origin of this non-uniform plasticity of the alloys produced by laser powder bed fusion (LPBF) has been systematically investigated for the first time. The results show that the loss of uniform plasticity in the alloy originates from microstructural regions containing equiaxed fine-grains (FGs) (∼650 nm in size) at the bottom of the melt pools. In micro-tensile tests, the strength of these FG regions can reach a peak of ∼630 MPa. After this, an apparent yield drop occurs, followed by rapid strain softening. This FG behavior is associated with intermetallic particles along grain boundaries and a lack of uniform mobile dislocations during deformation. The columnar coarse-grain (CG) regions in the remaining melt pools show uniform plasticity and moderate work hardening. Furthermore, the quantitative calculations indicate that the solid solution strengthening in these two regions is similar. Nevertheless, secondary Al3Sc precipitates contribute to ∼260 MPa strength in the FG, compared to 310 MPa in the CG due to their different number density. In addition, grain boundary strengthening can reach 230 MPa in the FG region; nearly double the CG region value.
Selective laser melting (SLM) provides optimized lightweight structures for aircraft and space applications. However, the strength of the current SLMed aluminum alloys is still lower than that of the traditional high-performance aluminum alloys. This study presents an ultra-high-strength Al-Mn-Mg-Sc-Zr aluminum alloy specifically designed for SLM by increasing the (Mg + Mn) and (Sc + Zr) content simultaneously based on the rapid solidification characteristics of the SLM process. The alloy exhibits good SLM processability with a minimum porosity of 0.23%. After aging at 300 °C, the strength of the alloy was effectively improved, and the anisotropy of mechanical properties was reduced. Additionally, the tensile yield strength and ultimate tensile strength of the alloy reached 621 ± 41 MPa and 712 ± 28 MPa, respectively; these values are superior to those of most SLMed aluminum alloys reported previously. Multiple strengthening mechanisms including solid solution strengthening, precipitation strengthening and grain refinement strengthening contribute to the high strength of the present alloys.
Additive manufacturing (AM) of metallic alloys for structural and functional applications has attracted significant interest in the last two decades as it brings a step change in the philosophy of design and manufacturing. The ability to design and fabricate complex geometries not amenable to conventional manufacturing, and the potential to reduce component weight without compromising performance, is particularly attractive for aerospace and automotive applications. This has culminated in rapid progress in AM with Ti- and Ni-based alloys. In contrast, the development of AM with Al-alloys has been slow, despite their widespread adoption in industry owing to an excellent combination of low density and high strength-to-weight ratio. Research to date has focused on castable and weldable AlSiMg-based alloys (which are less desirable for demanding structural applications), as well as on the development of new AM-specific AlMgSc alloys (based on 5xxx series). However, high strength wrought Al-alloys have typically been unsuitable for AM due to their unfavourable microstructural characteristics under rapid directional solidification conditions. Nevertheless, recent research has shown that there is promise in overcoming the associated challenges. Herein, we present a review of the current status of AM with Al-alloys. We primarily focus on the microstructural characteristics, and on exploring how these influence mechanical properties. The current metallurgical understanding of microstructure and defect formation in Al-alloys during AM is discussed, along with recent promising research exploring various microstructural modification methodologies. Finally, the remaining challenges in the development of AM with high-strength Al-alloys are discussed.
Cross-scale coordination
Laser-based additive manufacturing has the potential to revolutionize how components are designed. Gu et al. suggest moving away from a strategy that designs and builds components in a serial manner for a more wholistic method of optimization for metal parts. The authors summarize several key developments in laser powder bed fusion and directed energy deposition and outline a number of issues that still need to be overcome. A more integrated approach will help to reduce the number of steps required for fabrication and expand the types of structures available for end-use components.
Science , abg1487, this issue p. eabg1487