Fusion-based Additive Manufacturing Research Articles

Emergence of rapid solidification microstructure in additive manufacturing of a Magnesium alloy

Abstract Bioresorbable Mg-based alloys with low density, low elastic modulus, and excellent biocompatibility are outstanding candidates for temporary orthopedic implants. Coincidentally, metal additive manufacturing (AM) is disrupting the biomedical sector by providing fast access to patient-customized implants. Due to the high cooling rates associated with fusion-based AM techniques, they are often described as rapid solidification processes. However, conclusive observations or rapid solidification in metal AM — attested by drastic microstructural changes induced by solute trapping, kinetic undercooling, or morphological transitions of the solid-liquid interface — are scarce. Here we study the formation of banded microstructures during laser powder-bed fusion (LPBF) of a biomedical-grade Magnesium-rare earth alloy, combining advanced characterization and state-of-the-art thermal and phase-field modeling. Our experiments unambiguously identify microstructures as the result of an oscillatory banding instability known from other rapid solidification processes. Our simulations confirm that LPBF-relevant solidification conditions strongly promote the development of banded microstructures in a Mg-Nd alloy. Simulations also allow us to peer into the sub-micrometer nanosecond-scale details of the solid-liquid interface evolution giving rise to the distinctive banded patterns. Since rapidly solidified Mg alloys may exhibit significantly different mechanical and corrosion response compared to their cast counterparts, the ability to predict the emergence of rapid solidification microstructures (and to correlate them with local solidification conditions) may open new pathways for the design of bioresorbable orthopedic implants, not only fitted geometrically to each patient, but also optimized with locally-tuned mechanical and corrosion properties.

Role of nitrogen and helium gases on microstructure and anisotropic mechanical properties of cold-sprayed scalmalloy deposits

Aluminum alloys (Al) produced by fusion-based additive manufacturing techniques exhibit limited strength due to undesirable grain growth, porosity, and thermal gradient. To address these limitations, Scalmalloy (Al–Mg4.6-Sc0.72-Zr0.3 wt%) deposits of 5 mm thickness using helium and nitrogen gases were manufactured using a solid-state cold spray (CS) deposition. The microstructure of these deposits revealed a bimodal distribution of coarse and fine grains in the splat interior and jetting region, respectively, characterized by a higher degree of plastic deformation in the He-deposit. These bimodal grains resulted in a nanohardness of 1.2 GPa in the jetting region, 20% higher than that in the interior. The cumulative effect of these microstructural features resulted in a microhardness of 149 HV in the He-deposit, 1.1 times compared to that in the N2-deposit. The increased hardness is translated to a yield strength of 383 MPa and ultimate strength of 487 MPa in the He-deposit, 1.17 and 1.08 times of N2-deposit, respectively, estimated by Profilometry-based indentation plastometry (PIP). Across the hierarchical layers, profilometry-based indentation plastometry captured a consistent pile-up difference of ∼2 μm in the He-deposit and ∼3 μm in the N2-deposit on the plane perpendicular to build plane, a manifestation of in-plane anisotropy. This study advances the microstructural understanding and its relationship with observed mechanical properties, deformation behavior, and anisotropic response in high-strength Scalmalloy CS deposits.

Advancements in Additive Manufacturing for Copper-Based Alloys and Composites: A Comprehensive Review

Copper-based materials have long been used for their outstanding thermal and electrical conductivities in various applications, such as heat exchangers, induction heat coils, cooling channels, radiators, and electronic connectors. The development of advanced copper alloys has broadened their utilization to include structural applications in harsh service conditions found in industries like oil and gas, marine, power plants, and water treatment, where good corrosion resistance and a combination of high strength, wear, and fatigue tolerance are critical. These advanced multi-component structures often have complex designs and intricate geometries, requiring extensive metallurgical processing routes and the joining of the individual components into a final structure. Additive manufacturing (AM) has revolutionized the way complex structures are designed and manufactured. It has reduced the processing steps, assemblies, and tooling while also eliminating the need for joining processes. However, the high thermal conductivity of copper and its high reflectivity to near-infrared radiation present challenges in the production of copper alloys using fusion-based AM processes, especially with Yb-fiber laser-based techniques. To overcome these difficulties, various solutions have been proposed, such as the use of high-power, low-wavelength laser sources, preheating the build chamber, employing low thermal conductivity building platforms, and adding alloying elements or composite particles to the feedstock material. This article systematically reviews different aspects of AM processing of common industrial copper alloys and composites, including copper-chrome, copper-nickel, tin-bronze, nickel-aluminum bronze, copper-carbon composites, copper-ceramic composites, and copper-metal composites. It focuses on the state-of-the-art AM techniques employed for processing different copper-based materials and the associated technological and metallurgical challenges, optimized processing variables, the impact of post-printing heat treatments, the resulting microstructural features, physical properties, mechanical performance, and corrosion response of the AM-fabricated parts. Where applicable, a comprehensive comparison of the results with those of their conventionally fabricated counterparts is provided.

Additive friction stir deposition of AA7050 achieving forging-like tensile properties

Owing to hot cracking, printing aerospace grade AA7050 is challenging via fusion-based additive manufacturing. Using a deformation-based additive process, here we show that forging-like tensile properties are achieved in additively manufactured AA7050 without chemical modification for the first time (excluding lubricant on feed-rod surfaces) — YS: 430 MPa; UTS: 491 MPa; elongation: 11.2% with T74 temper.

Metallurgy and Solidification Microstructure Control of Fusion-Based Additive Manufacturing Fabricated Metallic Alloys: A Review

Metallurgy and Solidification Microstructure Control of Fusion-Based Additive Manufacturing Fabricated Metallic Alloys: A Review

The Influence of the Deposition Speed during Friction Screw Extrusion Additive Manufacturing of AA6060

Solid-state additive manufacturing may solve critical issues typically arising during fusion-based additive manufacturing of high-strength aluminium alloys. In this work, the recently introduced Friction Screw Extrusion Additive Manufacturing (FSEAM) process was employed to manufacture wall-like rectangular builds of AA6060 T6 deposited with deposition speeds from 300 mm/min to 500 mm/min. All builds were manufactured at a tool rotation rate of 400 rpm with 1 mm layer thickness. The volumetric supply rates were adjusted to maintain constant build width. Solid builds were formed without major defects over the full range of deposition speeds. The process generated sufficient normal force and heat at all deposition speeds which resulted in manufacturing of defect free builds. The resulting average grain size was consistently below 5 micrometer throughout all builds independent of deposition speed or location through the height. Microhardness measurements revealed a decrease in hardness from a feedstock value of 80 HV to around 50 HV in all manufactured builds. Tensile tests in the building direction showed consistent results for all the samples as a result of defect-free parts, demonstrating a tensile strength of approximately 150 MPa, yield strength of 100 MPa, and uniform elongation of 12-15%. The fracture surfaces revealed large amounts of dimples at all deposition speeds in line with the high degree of plastic deformation preceding fracture observed from the tensile tests. The obtained results indicated that FSEAM is a promising process for solid-state additive manufacturing of aluminium alloys.

Delineating dislocation structures and residual stresses in additively manufactured alloys

Dislocation structures and residual stresses have tremendous impact over the properties of alloys produced by fusion-based additive manufacturing (AM) processes. Yet, their origin is still unclear. We quantify and map, for the first time, the spatial distribution of dislocations, including those classified as geometrically necessary dislocations (GND), lattice strains, and type-III residual stresses in body-centered cubic (BCC) Ti-42Nb alloy and face-centered cubic (FCC) stainless steel (SS) 316L using a combination of electron channeling contrast imaging and cross correlation-based electron backscatter diffraction. Our findings reveal distinct residual stresses, lattice strains, and dislocation arrangement along cell walls in both alloys. In particular, Ti-42Nb demonstrates no GND network in cell walls, while SS316L exhibits a high GND density. We show that the Ti-42Nb alloy primarily accommodates solidification strains and cooling stresses elastically while SS316L accommodates them by elasto-plastic deformation and builds GNDs. Our work establishes relationships between dislocations and residual stresses in BCC and FCC LPBF alloys and highlights that not all LPBF-manufactured materials contain a dense dislocation network in their microstructures. These findings can also be extended to other AM alloys, therefore enabling a better control over their properties and performance.

Friction stir powder additive manufacturing of Al 6061/FeCoNi and Al 6061/Ni metal matrix composites: Reinforcement distribution, microstructure, residual stresses, and mechanical properties

Fusion based additive manufacturing (FBAM) of second, sixth, and seventh series Al alloys and their metal matrix composites (MMC) is difficult due to their higher thermal conductivity and solidification related problems namely porosity, cracks, thermal distortion, and formation of undesired phases. This paper presents friction stir powder additive manufacturing (FSPAM) process as a promising alternative to overcome these problems in producing multi-layer depositions of Al 6061 based MMCs namely Al 6061/6 wt%FeCoNi and Al 6061/6 wt%Ni. Their microstructure, distribution and elemental mapping of reinforcement particles, phase analysis, residual stresses of the MMCs and their correlation with microhardness, tensile strength, and fretting wear characteristics are investigated. Material accumulation on their advancing side was minimized by changing tool rotation direction in consecutive layers which produced smoother surfaces on both sides of their deposition. FSPAM made multi-layer depositions of Al 6061/FeCoNi and Al 6061/Ni MMCs have uniform distribution of reinforcement particles, good bonding between layers without cracks and defects, refined and equiaxed grains facilitated by dynamic recrystallization and pinning effect of reinforcement particles, compressive residual stresses of 39 and 48 MPa, no formation of deleterious intermetallic compounds due to absence of melting of matrix and reinforcement, and bowl-shaped substrate-deposition interface. Microhardness and ultimate tensile strength of the MMCs improved by 11.3% and 22.3%, and 30.5% and 31.5% respectively than Al 6061 alloy depositions, their wear resistance enhanced significantly, but % elongation reduced. This study proves FSPAM to be a potential alternative to FBAM processes for better quality multi-layer deposition of Al alloy-based MMCs.

In-situ observation of powder spreading in powder bed fusion metal additive manufacturing process using particle image velocimetry

Providing a dense powder bed is necessary to ensure the rationality of the final product prepared by powder bed fusion-based additive manufacturing under broad building parameters. In this study, the powder spreading mechanism was elucidated using particle image velocimetry and discrete element simulations. The particle flow regimes in the spreading process were identified based on the particle displacement: alignment, rotation, and deposition. The alignment regime was dominant in gas-atomized stainless steel 304 powder piles, whereas the rotation regime was dominant in plasma rotating electrode processed stainless steel 304 powder piles. A high-fidelity spreading simulation model was developed to clarify the critical factors resulting in the different flow regimes of different stainless steel 304 powders. The dominant rotational regime of the plasma rotating electrode processed powder was substituted for alignment by increasing the cohesive force. The particle supply in the spreading process was further suppressed by increasing the cohesive force, owing to the formation of a strong force arch and agglomeration. The high cohesive force in the gas-atomized powder was mainly attributed to the electrostatic force caused by the thick oxide film. Therefore, it was proven that the oxide film thickness is a key factor in determining the powder spreading mechanism and powder bed quality in the powder bed fusion additive manufacturing process.

Potential and challenges for large-scale near-net-shaping of 7xxx aerospace grade aluminum via additive friction stir deposition

This letter examines the potential and challenges of using additive friction stir deposition for large-scale near-net-shaping of 7xxx Al alloys, particularly AA7075 and AA7050. Compared to fusion-based additive manufacturing, which is unable to print these alloys without feedstock modification or composition change, additive friction stir deposition is an emerging solid-state additive process with a thermomechanical processing nature that can render fully-dense AA7075 and AA7050 in the as-deposited state. More importantly, wrought-like tensile properties have been demonstrated in the deposited AA7075 based on the original composition. The process is well-suited for large-scale printing due to its high build rate and scalability. Significant challenges nevertheless exist in process and post-process control, in addition to the impurity from lubrication.

Outlook on texture evolution in additively manufactured stainless steels: Prospects for hydrogen embrittlement resistance, overview of mechanical, and solidification behavior

Realizing application specific manufacture with fusion-based additive manufacturing (F-BAM) processes requires understanding of the physical phenomena that drive evolution of microstructural attributes, such as texture. Current approaches for understanding texture evolution in F-BAM are majorly considerate of the phenomena occurring only during solidification. This hinders the comprehensive understanding and control of texture during F-BAM. In this perspective article, we discuss several physical phenomena occurring during and after solidification that can determine texture in F-BAM processed stainless steels (SS). A crystal plasticity-coupled hydrogen adsorption-diffusion modeling framework is also leveraged to demonstrate the prospects of grain boundary engineering with F-BAM for enhanced hydrogen embrittlement resistance of SS. Implications of varying thermokinetics in F-BAM for solidification behavior of SS are discussed. Additionally, microstructural attributes that are key to high temperature mechanical performance of SS are highlighted. Considerations as outlined in this perspective article will enable grain boundary engineering and application specific microstructural design of SS with F-BAM.Graphical abstract

Solidification of Al12Si Melt Pool in Laser Powder Bed Fusion

An understanding of the solidification characteristics of the melt pool during laser powder bed fusion, L-PBF, metal printing is essential to ensure the manufacture of sound parts. This is the main theme of this article where it is attempted to unlock the solidification black box through calculation/measurement of the main solidification parameters of cooling rate, growth rate, temperature gradient, and undercooling. The very small melt pool size and its rapid cooling hinder the application of conventional tools to extract such information. A knowledge of these parameters and their effect on the microstructure and morphology of constituent phases could help to better control the L-PBF process and indeed all the fusion-based additive manufacturing routes, to fabricate high-quality parts. Al12Si alloy solidification during the L-PBF fabrication route is examined to highlight the extreme changes in solidification parameters and their effect on the morphology and size of eutectic Si. This is because the morphology of Si controls the mechanical properties of the finished part. The Al–Si eutectic has a divorced architecture attributed to the difficulty associated with Si growth resulted from rapid solidification. The generation of a temperature gradient of about 7700 °C/mm resulted in cooling rates in the range of 1.1 × 106 °C/s and a growth rate of 140 mm/s. Such values were then used to predict the morphology of eutectic Si based on the traditional approach.

Accelerating Thermal Simulations in Additive Manufacturing by Training Physics-Informed Neural Networks With Randomly Synthesized Data

Abstract The temperature history of an additively manufactured part plays a critical role in determining process–structure–property relationships in fusion-based additive manufacturing (AM) processes. Therefore, fast thermal simulation methods are needed for a variety of AM tasks, from temperature history prediction for part design and process planning to in situ temperature monitoring and control during manufacturing. However, conventional numerical simulation methods fall short in satisfying the strict requirements of time efficiency in these applications due to the large space and time scales of the required multiscale simulation. While data-driven surrogate models are of interest for their rapid computation capabilities, the performance of these models relies on the size and quality of the training data, which is often prohibitively expensive to create. Physics-informed neural networks (PINNs) mitigate the need for large datasets by imposing physical principles during the training process. This work investigates the use of a PINN to predict the time-varying temperature distribution in a part during manufacturing with laser powder bed fusion (L-PBF). Notably, the use of the PINN in this study enables the model to be trained solely on randomly synthesized data. These training data are both inexpensive to obtain, and the presence of stochasticity in the dataset improves the generalizability of the trained model. Results show that the PINN model achieves higher accuracy than a comparable artificial neural network trained on labeled data. Further, the PINN model trained in this work maintains high accuracy in predicting temperature for laser path scanning strategies unseen in the training data.

A Comparison of Solidification Structures and Submicroscale Cellular Segregation in Rapidly Solidified Stainless Steels Produced via Two-Piston Splat Quenching and Laser Powder Bed Fusion.

Fusion-based additive manufacturing techniques leverage rapid solidification (RS) conditions to create parts with complex geometries, unique microscale/nanoscale morphological features, and elemental segregation. Three custom composition stainless steel alloys with varying chromium equivalence to nickel equivalence ratio (Creq/Nieq) between 1.53 and 1.95 were processed using laser powder bed fusion (LPBF) and/or two-piston splat quenching (SQ) to produce solidification rates estimated between 0.4 and 0.8 m/s. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized to collect high-resolution images, electron backscatter diffraction (EBSD) phase identification, and measure cellular segregation. Similar features were observed in both LPBF and SQ samples including phase and microstructure, nanoscale oxide particles, cell size, and segregation behavior. However, dislocation pileup was observed along the cell boundaries only in the LPBF austenite solidified microstructure. Targeted adjustment of the SQ feedstock Cr and Ni concentrations, within the ASTM A240 specification for 316L resulted in no observable impact on the cell size, oxide particle size, or magnitude of segregation. Also, the amount of Ni segregation in the ferrite solidified microstructures did not significantly differ, regardless of Cr/Nieq or processing technique. SQ is demonstrated as capable of simulating RS rates and microstructures similar to LPBF for use as an alternative screening tool for new RS alloy compositions.

Quantitative study on the correlation between microstructure and mechanical properties of additive friction stir deposited 6061-T6 Al-Mg-Si alloy

Additive friction stir deposition (AFSD) technology is attractive for its ability to create freeform and fully dense structures without melting and solidification. Hence, AFSD is an alternative to fusion-based additive manufacturing technology. However, the quantitative relationship between mechanical properties and microstructure has not been established yet. The purpose of the research is to establish a quantitative relationship between the microstructure and mechanical properties of AFSD-ed 6061-T6 aluminum alloy. In this study, 6061-T6 aluminum alloy feedstock was processed using AFSD technology and then subjected to post-deposited heat treatment (PDHT). The microstructure evolution and mechanical properties of the feedstock, as-deposited and PDHT-ed 6061-T6 aluminum alloy specimens, were investigated for a comparative study. The microstructure was analyzed using electron backscatter diffraction (EBSD), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Hardness testing and tensile testing were conducted for the feedstock, as-deposited, and PDHT-ed specimens. The strengthening mechanisms were discussed. It was found that precipitation strengthening was the main strengthening mechanism for feedstock and PDHT-ed specimens, which contributed to 69% and 83% of the total yield strength, respectively. Whereas grain refinement strengthening was the main strengthening mechanism for the as-deposited specimen, which occupied 68% of the total yield strength. Finally, the relationship between microstructure and mechanical properties of additive friction stir-deposited 6061-T6 Al–Mg–Si alloy was quantified.

Reprint of: Bi-metal structures fabricated by extrusion-based sintering-assisted additive manufacturing

The manufacturing of bi-metal structure has received much interest because it provides compensated properties for a single metal alloy, meeting the harsh requirements for components used in key industries like aerospace, defense, energy, etc. Additive manufacturing (AM) has been extensively applied in building multi-material structures because of its ability to vary material type and composition in a layer-by-layer mode. However, fusion-based AM technologies usually induce interfacial cracks and delamination resulted from the large mismatch of coefficient of thermal expansion (CTE) between dissimilar materials, while the solid-state AM methods generate a large number of pores at the interface, requiring post-fabrication heat treatment. In this study, we aim to build high-quality 17–4 PH stainless steel and nickel alloy bi-metal structure with material extrusion AM method, followed by debinding and sintering processes. The microstructure and mechanical properties of the bi-metal structure were thoroughly investigated. It was revealed that small pores were distributed in the whole part, and no brittle intermetallic phase but slightly larger pores were formed at the interface. The material transition zone was relatively small in thickness, exhibiting low bonding strength and low hardness but a ductile deformation behavior. The completion of this study provides a pioneering analysis of bi-metal structures built by extrusion-based sintering-assisted AM, and shows a great promise for further adoption of this technology in a variety of industrial applications.

Effect of powder morphology on flowability and spreading behavior in powder bed fusion additive manufacturing process: A particle-scale modeling study

Improving the quality of the powder bed is of great interest in the powder bed fusion-based additive manufacturing (PBF-AM) process to reduce micro-defects in the final product. We investigated the influence of particle morphology on flowability and spreading behavior using a discrete element method (DEM) simulation with multi-sphere modeling. The addition of non-spherical particles effectively decreased the flowability under gravity and rotation, resulting in a transition from rolling flow to cascading flow in the rotating condition. In the powder spreading process, the quality of the powder bed was decreased by adding elongated particles, owing to the frequent force-arch formation and additional wall effect caused by strong particle interlocking. Furthermore, the high spreading velocity facilitated loose powder packing owing to the frequent force arch and wide rearrangement area. In conclusion, spherical particles with a low spreading velocity are desirable for improving the quality of the powder bed, such as packing density and surface roughness, in the PBF-AM process.

Impacts of residual 3D printing metal powders on immunological response and bone regeneration: an in vivo study

Residual powder is a defect in powder bed fusion-based additive manufacturing (3D printing), and it is difficult to completely remove it from as-printed materials. In addition, it is not necessary to apply 3D printed implants with residual powder in the clinic. The immunological response triggered by the residual powder is an important area of study in medical research. To further understand the possible immunological reactions and hidden dangers caused by residual powders in vivo, this study compared the immunological reactions and osteolysis caused by typical powders for four implant materials: 316 L stainless steel, CoCrMo, CP-Ti, and Ti-6Al-4V (particle size range of 15–45 μm), in a mouse skull model. Furthermore, the possible immunological responses and bone regeneration induced by the four 3D printed implants with residual powder in a rat femur model were compared. In the mouse skull model, it was found that the 316L-S, CoCrMo-S, and especially the 316L-M powders, upregulated the expression of pro-inflammatory factors, increased the ratio of RANKL/OPG, and activated more functional osteoclasts, resulting in more severe bone resorption compared with those in other groups. In the rat femur model, which is more suitable for clinical practice, there is no bone resorption in implants with residual powders, but they show good bone regeneration and integration ability because of their original roughness. The results indicate that the expressions of inflammatory cytokines in all experimental groups were the same as those in the control group, showing good biological safety. The results answered some critical questions related to additively manufactured medical materials in vivo and indicated that as-printed implants may have great potential in future clinical applications.Graphical

Effect of rotational speed and feed rate on microstructure and mechanical properties of 6061 aluminum alloy manufactured by additive friction stir deposition

Additive friction stir deposition (AFSD) is attractive for its ability to create freeform and fully dense structures without melting and solidification. Hence, additive friction stir deposition is an alternative to fusion-based additive manufacturing technology. In this study, the influence of the AFSD parameter (i.e., rotational speed and feed rate) on the mechanical properties and microstructure of 6061 aluminum alloy is investigated. The as-deposited 6061 aluminum alloy exhibits a relatively homogeneous microstructure with extensive equiaxed grains. Compared to the feedstock material, the ultimate tensile strength of the as-deposited 6061 aluminum alloy decreased to 65% from 320 to 210 MPa. The results of electron backscatter diffraction indicate that continuous dynamic recrystallization occurs during the AFSD process. Furthermore, it is evidence that the grain size and ultimate tensile strength are positively correlated with feed rate and rotational speed, whereas the elongation at break decreases with the increase in feed rate and rotational speed.

Bi-metal structures fabricated by extrusion-based sintering-assisted additive manufacturing

The manufacturing of bi-metal structure has received much interest because it provides compensated properties for a single metal alloy, meeting the harsh requirements for components used in key industries like aerospace, defense, energy, etc. Additive manufacturing (AM) has been extensively applied in building multi-material structures because of its ability to vary material type and composition in a layer-by-layer mode. However, fusion-based AM technologies usually induce interfacial cracks and delamination resulted from the large mismatch of coefficient of thermal expansion (CTE) between dissimilar materials, while the solid-state AM methods generate a large number of pores at the interface, requiring post-fabrication heat treatment. In this study, we aim to build high-quality 17–4 PH stainless steel and nickel alloy bi-metal structure with material extrusion AM method, followed by debinding and sintering processes. The microstructure and mechanical properties of the bi-metal structure were thoroughly investigated. It was revealed that small pores were distributed in the whole part, and no brittle intermetallic phase but slightly larger pores were formed at the interface. The material transition zone was relatively small in thickness, exhibiting low bonding strength and low hardness but a ductile deformation behavior. The completion of this study provides a pioneering analysis of bi-metal structures built by extrusion-based sintering-assisted AM, and shows a great promise for further adoption of this technology in a variety of industrial applications.