Fusion-based Additive Manufacturing Research Articles

Cracking suppression in additive manufacturing of hard-to-weld nickel-based superalloy through layer-wise ultrasonic impact peening

Fusion-based additive manufacturing (AM) techniques that use high energy beams, such as laser and electron beam, to layer-wise fuse and form materials, are quickly becoming mainstream in the fabrication of performance-critical metal parts. However, the vast majority of alloys currently used in industry are not compatible with fusion-based AM. One major reason is the occurrence of high thermal stress and solidification cracking resulting from the far-from-equilibrium thermal history. In this research, a novel hybrid AM process that combines directed energy deposition (DED) and layer-wise ultrasonic impact peening (UIP) is used to mitigate residual stress formation and to suppress the occurrence of solidification cracking in a hard-to-weld Inconel 100 superalloy. A series of materials characterization techniques, including 3D topography, optical microscopy, electron backscattered diffraction, and X-ray residual stress analysis, is carried out to investigate the effect of layer-wise UIP on the additively manufactured products. Three levels of UIP force, i.e., 25 N, 50 N, and 75 N, are investigated. The results indicate that there is an inverse relationship between the crack density and applied UIP force. At the highest level of UIP force, the crack density reduces to near-zero with significantly improved surface quality. To successfully suppress detrimental residual stress, the depth of the peening-affected-zone (PAZ) must be larger than the heat-affect-zone (HAZ) of laser melting.

Understanding the grain refinement mechanisms in aluminium 2319 alloy produced by wire arc additive manufacturing

Controlling the grain size and morphology during fusion-based additive manufacturing remains a challenge for many alloys. This work investigates Titanium diboride (TiB2) nucleant particle addition to the high strength Aluminium 2319 alloy produced during wire arc additive manufacturing (WAAM). Two Cold Metal Transfer (CMT) processing conditions are explored (low heat input CMT and high heat input CMT) and the effectiveness of the grain refiner is studied. The addition of TiB2 is found to refine the average equiaxed grain size by up to 22% compared to the non-inoculated alloy. The Interdependence Model is applied to understand the grain refinement mechanisms occurring in low heat input AM technologies in alloys containing powerful growth restricting solutes such as Al 2319.

Grain refinement of 316L stainless steel through in-situ alloying with Ti in additive manufacturing

Additive manufacturing (AM) is a transformative approach to fabricating metal components in the industry. However, it is challenging for fusion-based AM to fabricate metallic materials, particularly austenitic stainless steels, which have fine equiaxed grains without the formation of undesirable intermetallic phases due to its intrinsic high thermal gradient and fast cooling rate. This work demonstrates that using in-situ alloying with Ti addition can significantly promote grain refinement (from 16.7 to 0.8 μm) without bringing undesirable intermetallic phases in selective laser melted 316L. The Ti-rich solutes at the solid/liquid interface activated heterogeneous nucleation, thus achieving grain refinement. The ultimate tensile strength (UTS) was enhanced from 704 MPa for 316L to 817 MPa for 316L-1.5Ti. The selection of grain refiner for different alloys was discussed. This methodology would be applicable to other alloys, such as Al alloys and Ti alloys, in order to tailor their microstructures and mechanical properties to suit specific engineering applications.

Effect of microstructure on fatigue crack propagation in additive manufactured nickel-based superalloy Haynes 282: an experiment and crystal plasticity study

Haynes 282 is a $$\gamma ^{\prime}$$ precipitation-strengthened nickel-based superalloy known for its exceptional high-temperature creep resistance and excellent fabricability. Recent advancements in powder-bed fusion-based additive manufacturing (PBF-AM) have enabled the fabrication of Haynes 282 with accurate, site-specific control of the grain orientation and morphology at the microscale. This ability opens up new avenues for microstructure design, to improve the material’s fatigue crack resistance and service life. This paper investigates the fatigue crack growth behavior of hybrid microstructure Haynes 282 fabricated via PBF-AM. Previous experiments revealed a higher crack propagation rate in the coarse columnar-grained microstructural regions when compared against fine-grained areas. Here, a strain gradient crystal plasticity model was adapted to study the fracture-related mechanical fields at the crack tip in the two microstructures. The simulation results showed a consistent influence of grain structure and texture on crack propagation, as was seen in the experiment. The model analysis revealed higher crack propagation driving force along crack direction in the coarse-grained sharply textured microstructure and higher driving force for crack kinking in fine-grained more diffusely textured microstructure, which is ascribed to the combined effect of yield stress, hardening rate, texture and grain morphology. The presented modeling approach will facilitate the development of the AM-based accurate microstructure design by deepening the fundamental study in AM-specific microstructure-properties relations.

Multi-layer deposition mechanism in ultra high-frequency pulsed wire arc additive manufacturing (WAAM) of NiTi shape memory alloys

Ultra high-frequency pulsed gas tungsten arc welding (UHFP-GTAW)-based Wire Arc Additive Manufacturing (WAAM) was used to fabricate thin walls of NiTi shape memory alloys. The transient heat and fluid flow are critical during fusion-based additive manufacturing, since they impact the as-built microstructure. In this work, a three-dimensional numerical model, which includes the force, surface Gauss heat source and periodic droplet transfer models, was developed to simulate the deposition of 5 layers. The gravity, buoyancy, electromagnetic, surface tension, arc pressure and arc shear stress are considered in the developed force model. An improved free surface tracing method using the volume of fluid (VOF) technique was proposed to more effectively track the free surface of the molten pool with the computational fluid dynamics (CFD) software FLUENT. The multiphysics phenomena associated to the process, namely the temperature and velocity fields of the molten pool, were studied. The model was then validated by experiments. It is revealed that the microstructure of the as-built parts is refined by the UHFP current power which induces significant vibration of the molten pool. These findings lay the foundations for optimizing the WAAM process aiming at fabricating high quality and complex NiTi parts. • Ultra high-frequency pulsed WAAM is used to promote grain refinement in as-built NiTi parts. • Development of a numerical modeling approach to understand the deposition mechanism and its effect on the molten pool. • The ultra high-frequency current promotes significant vibration of the molten pool thus inducing grain refinement. • Good agreement between experimental and modelling approaches was achieved.

The role of the solidification structure on orientation-dependent hardness in stainless steel 316L produced by laser powder bed fusion

Owing to the rapid cooling rates and directional thermal gradients involved, fusion-based additive manufacturing (AM) processes yield complex, fine solidification structures that impart anisotropic mechanical properties in materials, such as stainless steel 316L (SS316L). In this work, we present a comprehensive study of the mechanical anisotropy of SS316L produced by laser powder bed fusion using instrumented nanoindentation. We produce and test near-single crystal samples oriented along the three principal crystallographic axes—namely ⟨100⟩, ⟨110⟩, and ⟨111⟩—using a Berkovich indenter. We find that the ⟨111⟩ and ⟨100⟩ orientations exhibit the highest and the lowest hardness, respectively. To decouple the contributions of grain orientation and solidification structure to the alloy's mechanical anisotropy, we compare our experimental results against crystal plasticity finite element (CPFE) simulations. We ascribe the hardness anisotropy in SS316L to the cell spacing along the slip direction (CSSD), which is a novel metric that we introduce to account for the role of the solidification structure on plasticity. Our work provides a universal pathway to understanding the microstructure-property relationships in cubic metallic materials that exhibit solidification structures,such as those commonly imparted by fusion-based AM.

Static Compressive Behavior and Material Failure Mechanism of Trabecular Tantalum Scaffolds Fabricated by Laser Powder Bed Fusion-based Additive Manufacturing.

Additively manufactured trabecular tantalum (Ta) scaffolds are promising bone repair materials for load-bearing applications due to their good pore interconnectivity. However, a thorough mechanical behavior evaluation is required before conducting animal studies and clinical research using these scaffolds. In this study, we revealed the compressive mechanical behavior and material failure mechanism of trabecular tantalum scaffolds by compression testing, finite element analysis (FEA), and scanning electron microscopy (SEM). Trabecular tantalum scaffolds with porosities of 65%, 75%, and 85% were fabricated by laser powder bed fusion-based additive manufacturing. Porosity has a significant effect on their compressive mechanical properties. As the porosity decreased from 85% to 65%, the compressive yield strength and elastic modulus increased from 11.9 MPa to 35.7 MPa and 1.1 GPa to 3.0 GPa, respectively. Compression testing results indicate that trabecular tantalum scaffolds demonstrate ductile deformation and excellent mechanical reliability. No macroscopic cracks were found when they were subjected to strain up to 50%. SEM observations showed that material failure results from tantalum strut deformation and fracture. Most microcracks occurred at conjunctions, whereas few of them appear on the struts. FEA-generated compressive stress distribution and material deformation were consistent with experimental results. Stress concentrates at strut conjunctions and vertical struts, where fractures occur during compression testing, indicating that the load-bearing capability of trabecular tantalum scaffolds can be enhanced by strengthening strut conjunctions and vertical struts. Therefore, additively manufactured trabecular tantalum scaffolds can be used in bone tissue reconstruction applications.

Permeable Leading Edges for Airfoil and Fan Noise Reduction in Disturbed Inflow

A detailed experimental study on the aerodynamic performance and noise emission of airfoils and fan blades with permeable leading edges under disturbed inflow conditions was performed. The airfoils and fan blades with permeable leading edges were made of an aluminum alloy using a powder bed fusion-based additive manufacturing process. In a first step, a wind-tunnel study was carried out. This consisted of detailed aerodynamic and aeroacoustic measurements on 16 airfoils with different permeable leading-edge designs that were performed for various flow speeds and geometric angles of attack. Based on the results from that study, unskewed fan blades with four different permeable leading-edge designs were manufactured in a second step. With the aim of reducing turbulence interaction noise of axial fans, the fans were examined with regard to their aerodynamic and acoustic properties under grid-generated turbulent inflow conditions. In a third step, a possible transfer of the observed noise emission from airfoils with permeable leading edge to that of fan blades was investigated. It was found that a notable broadband noise reduction can be transferred from airfoil applications to the sound spectra of axial fans. At the same time, the porous modifications can reduce the aerodynamic performance, and hence the fan efficiency.

Coarsening of martensite with multiple generations of twins in laser additively manufactured Ti6Al4V

Generation of inherently complex thermokinetics and thermomechanical conditions during the laser powder bed fusion-based additive manufacturing (LPBF-AM) process makes it challenging to understand the evolution of internally twinned martensite structure in Ti6Al4V. In view of this, the present study employed the finite element method based multiphysics thermokinetic and thermomechanical model in complement with experimental observations via scanning electron microscopy and transmission electron microscopy to study this occurrence. In order to understand the transient effects of thermokinetics and thermomechanics during multi-laser track treatment of LPBF-AM on evolution of the morphological features of martensite phase and twins, separate experiments involving laser surface melting with single, double, and triple laser tracks and conventional heat treatment (solutionized above β transus temperature of 1323 K followed by quenching in water at 298 K) of wrought Ti6Al4V were conducted. The major fraction of martensite laths were thinner in the water-quenched Ti6Al4V sample compared to LPBF-AM Ti6Al4V. The gradual coarsening and distortion of martensite lath seemed to occur with successive thermal cycles of single, double, and triple laser track treatments. In addition multiple generations of crystallographic twins were detected in LPBF-Ti6Al4V, which were rarely detected in solutionized and water quenched wrought Ti6Al4V. The multiple generations of twins were also distinct features of the single laser track treated wrought Ti6Al4V. The evolution of these morphologically and crystallographically diverse martensite phase structures were correlated with the computationally predicted thermokinetic and thermomechanical conditions during in-layer multi-laser track and multi-layer thermal treatments experienced during LBPF-AM process in the present study.

Design approaches for printability-performance synergy in Al alloys for laser-powder bed additive manufacturing

The unprecedented increase in component design space has led to significant focus on fusion-based additive manufacturing (AM) technologies. The new design possibilities integrate features and functionalities that are not supported by conventional manufacturing, and simultaneously achieve unitization of components. However, fusion-based AM suffers from a lack of diverse alloys, as conventional high-strength Al alloys are prone to hot cracking and other defect formation during printing. New alloy design approaches are being pursued to overcome these limitations. Current strategies for design of printable Al alloys for laser-powder bed fusion (L-PBF) are primarily experimental and revolve around grain refinement and eutectic solidification (ES). Each of these strategies targets hot cracking at only a specific stage of solidification. Consequently, the processing window of the alloy shrinks and fine-tuning of the alloy microstructure becomes difficult, thus prohibiting the activation of multiple deformation mechanisms. On the other hand, strategies that integrate microstructural refinement (MR) and ES attack the problem during multiple stages of solidification. Such MR+ES integrated alloy design strategies allow widening of alloy-processing-window (printability) and activation of multiple deformation mechanisms such as back-stress strengthening and work-hardening, thus producing alloys with excellent synergy of printability and performance.

Tribocorrosion Behavior of Inconel 718 Fabricated by Laser Powder Bed Fusion-Based Additive Manufacturing

Additive manufacturing (AM) by laser powder bed fusion (LPBF) has gained significant research attention to fabricate complex 3D Inconel alloy components for jet engines. The strategic advantages of LPBF-based AM to fabricate jet components for aerospace applications are well reported. The jet components are exposed to a high degree of vibration during the jet operation in a variable aqueous environment. The combined vibration and the aqueous environment create a tribological condition that can accelerate the failure mechanism. Therefore, it is critical to understand the tribocorrosion behavior of the Inconel alloy. In the present work, tribocorrosion behavior of the LPBF fabricated standalone coating of Inconel 718 in the 3.5% NaCl aqueous solution is presented. The LPBF fabricated samples are analyzed to determine the impact of porosity, generated as a result of LPBF, on the triobocorrosion behavior of AM Inconel 718. The study includes potentiodynamic tests, cathodic polarization, along with OCP measurements. The corrosive environment is found to increase the wear by 29.24% and 49.5% without the initiation of corrosion in the case of AM and wrought Inconel 718, respectively. A corrosion accelerated wear form of tribocorrosion is observed for Inconel 718. Additionally, the corrosive environment has a significant effect on wear even when the Inconel 718 surface is in equilibrium potential with the corrosive environment and no corrosion potential scan is applied. This study provides an insight into a critical aspect of the AM Inconel components.

Design and Tailoring of Alloys for Additive Manufacturing

Additive manufacturing (AM) promises a major transformation for manufacturing of metallic components for aerospace, medical, nuclear, and energy applications. This perspective paper addresses some of the opportunities for alloy and feedstock design to achieve site-specific and enhanced properties not attainable by conventional manufacturing processes. This paper provides a brief overview of the role of powders, as well as solidification and solid-state phase transformation phenomena typically encountered during fusion-based AM. Three case studies are discussed that leverage the above to arrive at microstructure control. The first case study focuses on approaches to modify the solidification characteristics by in-situ alloying. The second case study focuses on the need for concurrent design of alloys and processing conditions to arrive at the columnar to equiaxed transition during solidification. The third case study focuses on the design of a cobalt alloy for AM, with emphasis on tailoring liquid and solid state phase transformations. The need for comprehensive knowledge of processing conditions during AM, in-situ and ex-situ probing of microstructure development under AM conditions, and post-print processing, characterization, and qualification are articulated for the design of future alloys and component geometries built by AM.

Grain refinement of stainless steel in ultrasound-assisted additive manufacturing

Metals and alloys fabricated by fusion-based additive manufacturing (AM), or 3D printing, undergo complex dynamics of melting and solidification, presenting challenges to the effective control of grain structure. Herein, we report on the use of high-intensity ultrasound that controls the process of solidification during AM of 316L stainless steel. We find that the use of ultrasound favours the columnar-to-equiaxed transition, promoting the formation of fine equiaxed grains with random crystallographic texture. Moreover, the use of ultrasound increases the number density of grains from 305 mm-2 to 2748 mm-2 despite an associated decrease in cooling rate and temperature gradient in the melt pool during AM. Our assessment of the relationship between grain size and cooling rate indicates that the formation of crystallites during AM is enhanced by ultrasound. Furthermore, the use of ultrasound increases the amount of constitutional supercooling during solidification by lowering the temperature gradient in the bulk of the melt pool, thus creating an environment that favours nucleation, growth, and survival of grains. This new understanding provides opportunities to better exploit ultrasound to control grain structure in AM-fabricated metal products.

Development of a material model for predicting extreme deformation and grain refinement during cold spraying

Fusion-based additive manufacturing techniques such as selective laser melting, electron beam freeform fabrication cause solidification problems such as grain coarseness and high porosity. As a new technique, cold spraying (CS) can overcome such melting-induced drawbacks. In this study, a material model using dislocation dynamics was developed specifically for the CS process for describing the following five nanosecond-scale physical phenomena: strain hardening, normal-range strain rate hardening, ultra-high strain rate hardening, thermal softening and grain size evolution. A single Cu microparticle impact test was conducted, and a good agreement between experimental and model-predicted microparticle deformations was observed, indicating high model accuracy. The corresponding finite element model was established, and the individual effects of the above phenomena were discussed in detail to show that material deformation is mainly controlled by ultra-high strain rate hardening while jetting is controlled by thermal softening. Additionally, both simulated and actual grain size distributions indicated that grain refinement occurs only near the microparticle-substrate interface (mainly at the interface edge). Thus, the newly developed model could accurately reproduce the dynamic deformation behaviors of impacting particles and correctly predict grain refinement (particularly due to dynamic recrystallization).

Grain Refinement of Alloys in Fusion-Based Additive Manufacturing Processes

One of the less desirable aspects of fusion-based additive manufacturing is the propensity for coarse columnar grain structures crossing build layers to form. This paper initially attempts to explain the reason for the formation of columnar grain structures in terms of the high thermal gradients typically observed during solidification and the alloy compositions that are typically used which promote epitaxial growth. Successful approaches to the grain refinement of titanium alloys using alloying elements that produce constitutional supercooling are discussed along with the difficulty with nucleant additions. Much of the grain-refining technology already used in aluminium casting is shown to also be applicable to additive manufacturing, although the novelty of the effective use of nanoparticles as nucleants is highlighted. It is also shown that for other alloy systems for which there is a lack of grain-refining technology using chemical means, mechanical means, such as ultrasonic treatment, can be effective across a wide range of alloys. Finally, consideration is given to the difficulties and the possible solutions of producing parts layer by layer. In particular, the importance of understanding nucleation in solidification conditions characterized by high cooling rates and thermal gradients; the importance of melt dynamics; and how previous layers could provide possibilities for refinement of the subsequent layer are highlighted.

Immunological response triggered by metallic 3D printing powders

Powder bed fusion-based Additive Manufacturing (3D printing) methods, particularly Selective Laser Melting, are increasingly being employed for customized medical implants. However, partially-melted residual metal powder is difficult to fully remove from the as-printed materials, and the existence of loose powder is another potential defect in the as-printed implant lattice. Understanding what effect loose powder and partially remelted powders have on biocompatibility requires a detailed study of the immunological response associated with the presence of metal powder particles, and the present study is designed to address this critically important concern. Typical powders for implant materials, including 316 L stainless steel, Ti, and Ti-6Al-4 V, with a range of particle sizes (15–100 μm) have been investigated in this study. Macrophage cells were selected to determine the immune response to metallic powder exposure and the cell responses were evaluated using confocal laser scanning microscopy and enzyme-linked immunosorbent assay analysis. Real-time monitoring of the cell reaction to the metal powders, cell viability, and secretion behavior of the inflammatory cytokines was employed for the different metal powders with various particle sizes. The main findings from the study are: (a) low concentrations (e.g. 1 × 104 particles/mL) of metal powders were not found to affect the cell viability, (b) in high particle concentrations (e.g. 1 × 106 particles/mL), the particle size was observed to be a significant factor in determining the cell reaction, and (c) the 316 L powder triggerd macrophages to secret the most pro-inflammatory cytokines and this tends to induce higher inflammatory reaction than the Ti and Ti-6Al-4 V powders. The underlying reasons are discussed from the perspectives of powder surface chemistry and powder morphology, which were investigated by detailed X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy studies. A comparison between the metal powder for 3D printing and conventional wear debris has also been briefly addressed. The results answer some critical questions relating to additively manufactured medical materials and lay a good foundation for better and more systematic assessment of the biological response of metal implants containing residual powders for potential orthopedic applications.

Focus Variation Measurement and Prediction of Surface Texture Parameters Using Machine Learning in Laser Powder Bed Fusion

Abstract The powder bed fusion-based additive manufacturing process uses a laser to melt and fuse powder metal material together and creates parts with intricate surface topography that are often influenced by laser path, layer-to-layer scanning strategies, and energy density. Surface topography investigations of as-built, nickel alloy (625) surfaces were performed by obtaining areal height maps using focus variation microscopy for samples produced at various energy density settings and two different scan strategies. Surface areal height maps and measured surface texture parameters revealed the highly irregular nature of surface topography created by laser powder bed fusion (LPBF). Effects of process parameters and energy density on the areal surface texture have been identified. Machine learning methods were applied to measured data to establish input and output relationships between process parameters and measured surface texture parameters with predictive capabilities. The advantages of utilizing such predictive models for process planning purposes are highlighted.

Mechanical and Microstructural Characterization of Arc-Welded Inconel 625 Alloy.

The objective of this work was to verify a relatively new fusion-based additive manufacturing (AM) process to produce a high-temperature aerospace material. The nickel-based superalloy Inconel 625 (IN625) was manufactured by an arc-based AM technique. Regarding microstructure, typical columnar-oriented dendritic structure along the building direction was present, and epitaxial growth was visible. The mechanical behavior was characterized by a combination of quasi-static tensile and compression tests, whereas IN625 showed high yield and ultimate tensile strength with a maximum fracture strain of almost 68%. Even quasi-static compression tests at room and elevated temperatures (650 °C) showed that compression strength only slightly decreased with increasing temperature, demonstrating the good high-temperature properties of IN625 and opening new possibilities for the implementation of arc-based IN625 in future industrial applications.

Steels in additive manufacturing: A review of their microstructure and properties

Today, a large number of different steels are being processed by Additive Manufacturing (AM) methods. The different matrix microstructure components and phases (austenite, ferrite, martensite) and the various precipitation phases (intermetallic precipitates, carbides) lend a huge variability in microstructure and properties to this class of alloys. This is true for AM-produced steels just as it is for conventionally-produced steels. However, steels are subjected during AM processing to time-temperature profiles which are very different from the ones encountered in conventional process routes, and hence the resulting microstructures differ strongly as well. This includes a very fine and highly morphologically and crystallographically textured microstructure as a result of high solidification rates as well as non-equilibrium phases in the as-processed state. Such a microstructure, in turn, necessitates additional or adapted post-AM heat treatments and alloy design adjustments. In this review, we give an overview over the different kinds of steels in use in fusion-based AM processes and present their microstructures, their mechanical and corrosion properties, their heat treatments and their intended applications. This includes austenitic, duplex, martensitic and precipitation-hardening stainless steels, TRIP/TWIP steels, maraging and carbon-bearing tool steels and ODS steels. We identify areas with missing information in the literature and assess which properties of AM steels exceed those of conventionally-produced ones, or, conversely, which properties fall behind. We close our review with a short summary of iron-base alloys with functional properties and their application perspectives in Additive Manufacturing.

Junction growth and interdiffusion during ultrasonic additive manufacturing of multi-material laminates

Ultrasonic additive manufacturing is a promising approach for making net-shaped multi-material laminates from material combinations difficult to process with fusion-based additive manufacturing techniques. The properties of these multi-material laminates depend sensitively on the interface between the constituents, which can be decorated with pores as well as thin intermetallic layers. Here, we develop process models for junction growth and interdiffusion during ultrasonic additive manufacturing of dissimilar metals. These process models are validated against published experimental data, then used to generate process diagrams which reveal that high normal loads and high sonotrode velocities can reduce intermetallic growth while maintaining strong interlayer bonding.