Solid-state Additive Manufacturing Research Articles

Microstructural and Mechanical Properties of a Solid-State Additive Manufactured Magnesium Alloy

Abstract In recent years, additive manufacturing (AM) has gained prominence in rapid prototyping and production of structural components with complex geometries. Magnesium alloys, which have a strength-to-weight ratio that is superior compared with steel and aluminum alloys, have shown potential in lightweighting applications. However, commercial beam-based AM technologies have limited success with magnesium alloys due to vaporization and hot cracking. Therefore, as an alternative approach, we propose the use of a near net-shape solid-state additive manufacturing process, additive friction stir deposition (AFSD), to fabricate magnesium alloys in bulk. In this study, a parametric investigation was performed to quantify the effect of process parameters on AFSD build quality including volumetric defects and surface quality in magnesium alloy AZ31B. In order to understand the effect of the AFSD process on structural integrity in the magnesium alloy AZ31B, in-depth microstructure and mechanical property characterization was conducted on a bulk AFSD build fabricated with a set of acceptable process parameters. Results of the microstructure analysis of the as-deposited AFSD build revealed bulk microstructure similar to wrought magnesium alloy AZ31 plate. Additionally, similar hardness measurements were found in AFSD build compared with control wrought specimens. While tensile test results of the as-deposited AFSD build exhibited a 20% drop in yield strength (YS), nearly identical ultimate strength was observed compared with the wrought control. The experimental results of this study illustrate the potential of using the AFSD process to additively manufacture Mg alloys for load bearing structural components with achieving wrought-like microstructure and mechanical properties.

Numerical Simulation of the Effect of Particle and Substrate Preheating on Porosity Level and Residual Stress of As-sprayed Ti6Al4V Components.

Nowadays, in the aerospace industry, additive manufacturing and repairing damaged metallic components like Ti6Al4V samples have grabbed attention. Among repairing techniques, solid-state additive manufacturing processes like cold spray are promising because of their unique benefits such as high deposition rate with almost no oxidation in the deposited materials. However, its main drawback is the level of porosity of as-sprayed samples. To increase density and inter-particle bonding, deposited particles must go through more degrees of deformation by increasing particle velocity and particle temperature. In order to increase these two parameters simultaneously, high-velocity air fuel (HVAF) can be utilized. For understanding the effect of using HVAF on particle deformation, a proper elastic-plastic finite-element-based simulation is required. The obtained outcomes show that enhancing particle velocity and providing more kinetic energy will increase particle deformation and sample density. Importantly, increasing particle temperature will seize particle deformation by thermal softening effect, i.e., enhancing as-sprayed sample density, while rising substrate temperature by preheating will soften the substrate resulting in a decrease in particle deformation.

A novel friction and rolling based solid-state additive manufacturing method: Microstructure and mechanical properties evaluation

Fusion-based metal additive manufacturing (AM) processes are becoming progressively mature in some material systems. However, the printing of high strength aluminum alloy remains a considerable challenge due to its unavoidable solidification defects. In this study, a novel solid-state additive manufacturing process, being referred to as Friction and Rolling based Additive Manufacturing (FRAM), was developed to print AA6061. The method developed here achieved heat generation and continuous material addition simultaneously, using a modified horizontal CNC machine. The results indicate that fully dense microstructure with defect-free and refined grain was successfully obtained using the FRAM method. The adjacent layers have achieved good bonding without melting because the material at the top surface of the previous layer experience a secondary friction, which can enhance the interface metallurgical bonding and eliminate the potential interface defects. The grain size of the deposited material was significantly reduced compared to that of feeding material because of dynamic recrystallization caused by severe plastic deformation. This also led to the significant increase in elongation of the deposited material. The deposited material showed ductile fracture in both horizontal and vertical directions. The proposed process has the potential to print other non-weldable engineering alloys that are sensitive to solidification defects.

Strain Rate Dependent Plasticity Model for Precipitate Hardened Aerospace Aluminum Alloy Produced with Solid-State Additive Manufacturing

Strain Rate Dependent Plasticity Model for Precipitate Hardened Aerospace Aluminum Alloy Produced with Solid-State Additive Manufacturing

The Effect of Anodization on the Mechanical Properties of AA6061 Produced by Additive Friction Stir-Deposition

This paper examines the impact of oxide coatings on the surfaces of feedstock material used for Additive Friction Stir-Deposition (AFS-D). The AFS-D is a solid-state additive manufacturing process that uses severe plastic deformation and frictional heating to build bulk depositions from either metallic rod or powder feedstock. Since aluminum alloys naturally form an oxide layer, it is important to determine the influence of the feedstock surface oxide layer on the resultant as-deposited microstructure and mechanical properties. In this study, three AA6061 square-rod feedstock materials were used, each with a different thickness of aluminum oxide coating: non-anodized, 10-micron thick, and 68-micron thick. Macroscale depositions were produced with these feedstock rods using the AFS-D process. Optical and electron microscopy showed that the two oxide coatings applied through anodization were efficiently dispersed during the AFS-D process, with oxide particles distributed throughout the microstructure. These oxide particles had median sizes of 1.8 and 3 μm2, respectively. The yield and tensile strengths of these materials were not measurably impacted by the thickness of the starting oxide coating. While all three feedstock material variations failed by ductile rupture, the elongation-to-failure did decrease from 68% to 55% in the longitudinal direction and from 60% to 43% in the build direction for the thickest initial oxide coating, 68 microns.

Processing-structure-property correlation in additive friction stir deposited Ti-6Al-4V alloy from recycled metal chips

Additive friction stir deposition (AFSD) is a novel thermo-mechanical solid state additive manufacturing process. AFSD enables manufacturing of near net shape, fully dense builds with refined equiaxed grain structure resulting in excellent mechanical properties. AFSD has the potential to produce ingots and components/builds from recycled metals (chips/scraps/wastes from industrial machining processes or municipal recycling centers). In the present study, recycled Ti-6Al-4V alloy chips were deposited using additive friction stir deposition. Detailed microstructural and mechanical property investigation of the deposited material fosters understanding of the effect of deposition variables on the microstructure as well as mechanical properties. The as-deposited microstructure was characterized by lamellar α/β colonies inside fine equiaxed prior β grains containing α phase at the grain boundary. Additive friction stir deposited Ti-6Al-4V alloy using consolidated recycled metal chips as raw material exhibited tensile properties better than other additive processes; thus, AFSD of recycled metal provides opportunities to produce structurally sound components with reduced energy consumption as well as reducing environmental waste.

Solid-state additive manufacturing of tantalum using high-pressure cold gas-dynamic spray

Additive manufacturing (AM) of refractory metals, including tantalum (Ta), is highly valued due to the wide application of these materials in different industrial sectors where outstanding mechanical properties at elevated temperatures are required. Among metal AM processes, newly introduced cold gas-dynamic spray or, more commonly, cold spray (CS) offers a unique opportunity for solid-state consolidation of refractory metals. This research presents the CS process as a method for additive manufacturing of Ta. Following the successful manufacturing of free-standing Ta, an extensive mechanical characterization at the macro and nano levels has been carried out to evaluate the material's structural integrity. Anisotropy in the mechanical properties, which is one of the major concerns in the AM-produced materials, was extensively analyzed at both the macro and nano levels. The produced Ta's resistance against the pre-existing crack was studied by evaluating the far-field J-integral versus the crack extension (J-R curve). The influence of microstructural characteristics and process-induced defects such as pores and micro-cracks on the Ta's mechanical and fracture properties was studied to explain the performance-microstructure linkage. The macro and nanomechanical testing results indicated an elastic modulus and ultimate tensile strength in the range of vacuum-arc melted or electron-beam melted Ta ingots following cold-working. The microstructure analysis demonstrated a mixture of ultrafine grains and highly elongated coarse grains, explaining the CS-produced Ta high strength. Moreover, excellent isotropy in the mechanical properties was observed at both the macro and nano levels. This finding distinguishes the CS process from the laser-based AM process in which mechanical properties highly depend on the build direction. On the other hand, the CS-produced Ta exhibited brittle characteristics during uniaxial tensile loading and ductile behavior during the uniaxial compression test. Also, stable crack growth accompanied by crack branching was observed in the CS-produced Ta. The crack branching and formation of secondary cracks have been identified as the mechanisms retard crack extension. Overall, this research revealed that the CS process is a promising AM technique for producing tantalum-based components.

Elucidating the Effect of Additive Friction Stir Deposition on the Resulting Microstructure and Mechanical Properties of Magnesium Alloy WE43

In this work, the effect of processing parameters on the resulting microstructure and mechanical properties of magnesium alloy WE43 processed via Additive Friction Stir Deposition (AFSD), a nascent solid-state additive manufacturing (AM) process, is investigated. In particular, a parameterization study was carried out, using multiple four-layer deposits, to identify a suitable process window for a structural 68-layers bulk WE43 deposition. The parametric study identified an acceptable set of parameters with minimal surface defects and excellent consolidation for the fabrication of a bulk WE43 deposition. Microstructural, tensile, and fatigue life characterization was conducted on the bulk WE43 deposition and compared to commercially available wrought material to elucidate the process-structure-property-performance (PSPP) relationship of the AFSD process. This study shows that the bulk WE43 deposit exhibited a refined homogenous microstructure and a texture shift relative to the wrought material. However, a reduction in hardness and tensile behavior was observed in the as-deposited WE43 compared to the wrought control. Additionally, fatigue specimens extracted from the bulk deposition exhibited a decrease in life in the low-cycle regime but performed comparably to the wrought plate in the high-cycle regime. The outcomes of this study illustrate the potential of the AFSD process in additively manufactured structural load-bearing components made with magnesium alloy WE43 in the as-built condition.

The Effect of Temper Condition and Feeding Speed on the Additive Manufacturing of AA2011 Parts Using Friction Stir Deposition.

In the current study, solid-state additive manufacturing (SSAM) of two temper conditions AA2011 was successfully conducted using the friction stir deposition (FSD) process. The AA2011-T6 and AA2011-O consumable bars of 20 mm diameter were used as a feeding material against AA5083 substrate. The effect of the rotation rate and feeding speed of the consumable bars on the macrostructure, microstructure, and hardness of the friction stir deposited (FSD) materials were examined. The AA2011-T6 bars were deposited at a constant rotation rate of 1200 rpm and different feeding speeds of 3, 6, and 9 mm/min, whereas the AA2011-O bars were deposited at a constant rotation rate of 200 mm/min and varied feeding speeds of 1, 2, and 3 mm/min. The obtained microstructure was investigated using an optical microscope and scanning electron microscope equipped with EDS analysis to evaluate microstructural features. Hardness was also assessed as average values and maps. The results showed that this new technique succeeded in producing sound additive manufactured parts at all the applied processing parameters. The microstructures of the additive manufactured parts showed equiaxed refined grains compared to the coarse grain of the starting materials. The detected intermetallics in AA2011 alloy are mainly Al2Cu and Al7Cu2Fe. The improvement in hardness of AA2011-O AMPs reached 163% of the starting material hardness at the applied feeding speed of 1 mm/min. The hardness mapping analysis reveals a homogeneous hardness profile along the building direction. Finally, it can be said that the temper conditions of the starting AA2011 materials govern the selection of the processing parameters in terms of rotation rate and feeding speed and affects the properties of the produced additive manufactured parts in terms of hardness and microstructural features.

Cold Spray Additive Manufacturing: Microstructure Evolution and Bonding Features

ConspectusMost metal additive manufacturing (AM) methods involve the melting or sintering of feedstock powder or wire using an energy source (laser, electron beam, or electric arc). Solid-state AM, sometimes also known as non-beam-based AM, is a process in which the deposited material does not melt and is built up layer-by-layer, typically through severe plastic deformation. Initially considered to be a coating technique, by virtue of its high deposition rate, cold spray additive manufacturing (CSAM) is attractive as a solid-state AM technology. In the CSAM process, metal powder particles are impacted onto a substrate at a supersonic velocity and relatively low temperature. The CSAM process reduces or eliminates many problems associated with melting or beam-based AM methods, making the CSAM technically attractive for a wide range of applications, such as in the aerospace, automobile, marine, biomedical, machinery, and energy sectors.In this Account, the author briefly reviews the setup of a cold spray system and discusses the strengths and drawbacks of CSAM compared with thermal sprays and beam-based AM processes, as well as applications in relevant industries. The author summarizes the bonding mechanisms proposed for the cold spray process. The focus of this Account is to review the microstructure evolution of several typical model metals (copper, nickel, aluminum, and titanium) during the cold spray process. The author shows a large variety of microstructure characteristics (recrystallized grains, annealing twins, shear bands, submicron-sized grains, deformation twins, and nanometer-sized grains) in cold-sprayed copper, dynamic recrystallization of cold-sprayed nickel, and the formation of refined grains even below 10 nm in size in cold-sprayed aluminum. The magnitude of the stacking fault energy of as-sprayed materials considerably influences the microstructure after cold spray and postprocessing. Due to the relatively low thermal conductivity and ductility of titanium, cold spraying of titanium shows both a high deposition efficiency and relatively high porosity of as-sprayed parts, as well as a heterogeneous microstructure. Moreover, the Account introduces the postprocessing heat treatment and nanoindentation characterization of cold-sprayed materials. A fundamental understanding of microstructure evolution during and after the cold spray process is essential for optimal mechanical properties. Lastly, the author provides a perspective on applying old lessons and taking advantage of new techniques and materials, including powder characterization, hybrid additive manufacturing, machine learning for searching process windows, nanomechanical testing, and emerging alloys (high-entropy alloys, nanocrystalline alloys, and quasicrystals), to advance the research and applications of CSAM.

A Review of Solid-State Additive Manufacturing Processes

A Review of Solid-State Additive Manufacturing Processes

Nanostructure Evolution in AA7075 Alloy Produced by Solid State Additive Manufacturing – Additive Friction Stir - Deposition

Nanostructure Evolution in AA7075 Alloy Produced by Solid State Additive Manufacturing – Additive Friction Stir - Deposition

Synchrotron X-ray diffraction studies of the phase-specific deformation in additively manufactured Ni–CrC composites

Cold spray is a promising solid-state additive manufacturing process for the fabrication of metal matrix composites in the forms of coatings, and more recently, bulk materials, particularly when deposition of two phases is needed with minimal alterations of the chemistry and phase composition. While prior studies have reported improved mechanical performance in cold sprayed composites, a fundamental understanding of the response and contribution of individual constituent phases to the collective mechanical properties of the composite is yet to be developed. In this work, we use in-situ synchrotron X-ray diffraction to study the strain partitioning between the soft and hard phases in cold sprayed Ni–CrC bulk composites. We conduct stepwise uniaxial tensile loading experiments and use high energy X-ray diffraction at multiple load steps to measure phase-specific strains and stresses upon loading. We find neither plasticity in the Ni phase nor a significant load transfer or redistribution between Ni and CrC phases during the loading process and attribute them to a premature failure governed by the presence of defects.

The effect of solutionizing and artificial aging on the microstructure and mechanical properties in solid-state additive manufacturing of precipitation hardened Al–Mg–Si alloy

In this study, the effect of post-deposition heat treatments on an Al–Mg–Si alloy processed with additive friction stir deposition (AFS-D), a solid-state additive manufacturing process, is examined. Results reveal that wrought-like microstructure and mechanical properties are achievable in AFS-D builds through the application of a post-deposition heat treatment.

Process-structure-property relations for as-deposited solid-state additively manufactured high-strength aluminum alloy

Solid-state additive manufacturing methods provide innovative solutions to circumvent problems associated with materials susceptible to hot cracking by avoiding liquid solid phase transformations. In this work, the process parameter influence on microstructural evolution and mechanical response of a fully dense aluminum alloy 7050 (AA7050) component manufactured via a rapid, solid-state additive manufacturing process known as Additive Friction Stir Deposition (AFS-D) was quantified for the first time. Three sections (starting dwell, transient, crossover of roads) of the deposition that exhibit differing thermomechanical processing histories were evaluated for the resulting microstructure and mechanical response. The microstructural characterization was performed on the as-deposited AA7050 via Electron Backscatter Diffraction (EBSD), Transmission electron microscopy (TEM), optical microscopy, and Scanning Electron Microscopy (SEM). The microstructural characterization revealed refined constituent particles and grains throughout the as-deposited AA7050 microstructure. Furthermore, quasi-static tensile experiments were conducted in both the build and transverse directions, in order to quantify the orientation influence on tensile properties of the as-deposited AA7050 build. Spatially dependent tensile properties were observed in the material due to heat input variation coarsening of secondary phases towards the initial layers of the AFS-D build. Post-mortem analysis revealed that voids nucleated and coalesced from the overgrowth of the strengthening precipitates present in the material, resulting in fracture.

A meshfree computational framework for the numerical simulation of the solid-state additive manufacturing process, additive friction stir-deposition (AFS-D)

In this work, a fully coupled thermo-mechanical meshfree approach is developed for the first time to simulate a solid-state layer-by-layer additive manufacturing process, Additive Friction Stir-Deposition (AFS-D). The meshfree method in this present work uses a Lagrangian reference frame, which permits tracking of material point history. A solid mechanics formulation is used, allowing the resolution of both elastic and plastic strains. An explicit dynamics time stepping scheme is used to ensure that the code is robust for the large level of non-linearity native to the AFS-D process. In this present work, a description of the meshfree method will first be described. Then a new thermo-mechanical joining contact algorithm will be introduced. Following that, a description of the experimental setup for the AFS-D model calibration experimental one layer deposition cases is explained. Subsequently, the simulation model and results for three different parameter sets will be detailed and compared against the experimental results. Finally, temperature and strain rate gradients are revealed across the entire deposition elucidating spatial-temporal flow phenomena in the AFS-D process.

Microstructure and Mechanical Property Evaluation of Aluminum F357 Powder for Cold Spray Applications

Cold spray (CS) is a solid-state additive manufacturing process that can create high-density, high-performance deposits for many applications. Aluminum alloys have been the target of recent CS research due to their excellent mechanical behavior when cold-sprayed. The introduction of new Al alloys to CS is of interest as unique properties may be achieved, especially when powder is heat-treated prior to deposition. In this study, three batches of Al F357 powder—a control as-atomized, heat-treated at 230 °C for 75 min and heat-treated at 385 °C for 6 h—were studied. Scanning electron microscopy revealed microstructural modifications after heat treatment in the powder and sprayed deposits. X-ray diffraction highlighted the presence of eutectic Si, Mg2Si and SiO2 in all specimens. Energy-dispersive spectroscopy showed the spheroidization of Si during the 385 °C heat treatment, with an increase in the fraction of Mg2Si during both treatments. Nanohardness and microhardness testing showed a decrease in hardness with thermal processing in both the powder and sprayed deposits. Tensile testing demonstrated ductile behavior from both heat-treated powders, but significantly higher strengths from the 230 °C treatment. These results suggest Al F357 may be used in CS when appropriate powder heat treatments are applied.

Solid-state cladding on thin automotive sheet metals enabled by additive friction stir deposition

• Additive friction stir deposition enables good cladding on 1.4 mm-thin substrates. • The reinforced structure is characterized by good formability and high strength. • The mechanical forces from the tool cause no local buckling in the thin substrates. • The residual stress and mismatch strain are first quantified for this new process. • The maximum tensile stress is around 40 MPa and the mismatch strain is 7.37 × 10 - 4 . Additive friction stir deposition is an innovative solid-state additive manufacturing process that creates near-net-shape 3D components based on deformation bonding rather than melting and solidification. Here, we employ this process for selective cladding on thin Al-Mg-Si sheet metals and evaluate its feasibility for automotive manufacturing based on the cladding quality and substrate distortion. We show that under optimal conditions, additive friction stir deposition can produce high-quality cladding without surface or interface porosity even if the substrate is as thin as 1.4 mm. In addition, the high strength and good formability of the original Al-Mg-Si substrate are preserved in the post-cladding reinforced structure, which exhibits an ultimate tensile strength of 250 MPa and an elongation of 30 %. Despite the mechanical forces imposed by the tool during deposition, no local buckling or wrinkling is observed in the thin substrate. During cooling, however, in situ monitoring shows that mild, global substrate distortion gradually develops. Upon unclamping, the cladding-on-plate system reconfigures itself to minimize the potential energy, leading to an anticlastic curvature. Based on the curvature measurement and classical lamination theory, this work provides the first quantification of the residual stress caused by this newly-developed additive technology, in which the maximum tensile stress is estimated to be around 40 MPa in the rectangular reinforced structure and the interface mismatch strain is 7.37 × 10 −4 . Both values are low thanks to the solid-state nature of the process.

Solid-state additive manufacturing of aluminum and copper using additive friction stir deposition: Process-microstructure linkages

Among metal additive manufacturing technologies, additive friction stir deposition stands out for its ability to create freeform and fully-dense structures without melting and solidification. Here, we employ a comparative approach to investigate the process-microstructure linkages in additive friction stir deposition, utilizing two materials with distinct thermomechanical behavior—an Al-Mg-Si alloy and Cu—both of which are challenging to print using beam-based additive processes. The deposited Al-Mg-Si is shown to exhibit a relatively homogeneous microstructure with extensive subgrain formation and a strong shear texture, whereas the deposited Cu is characterized by a wide distribution of grain sizes and a weaker shear texture. We show evidence that the microstructure in Al-Mg-Si primarily evolves by continuous dynamic recrystallization, including geometric dynamic recrystallization and progressive lattice rotation, while the heterogeneous microstructure of Cu results from discontinuous recrystallization during both deposition and cooling. In Al-Mg-Si, the continuous recrystallization progresses with an increase of the applied strain, which correlates with the ratio between the tool rotation rate Ω and travel velocityV. Conversely, the microstructure evolution in Cu is found to be less dependent on Ω, instead varying more with changes to V. This difference originates from the absence of Cu rotation in the deposition zone, which reduces the influence of tool rotation on strain development. We attribute the distinct process-microstructure linkages and the underlying mechanisms between Al-Mg-Si and Cu to their differences in intrinsic thermomechanical properties and interactions with the tool head.

Solid-state additive manufacturing of porous Ti-6Al-4V by supersonic impact

Additive manufacturing of functional metallic parts based on layer-by-layer melting and solidification suffers from the detrimental effects of high-temperature processing such as large residual stresses, poor mechanical properties, unwanted phase transformations, and part distortion. Here we utilize the kinetic energy of powder particles to form a solid-state bonding and overcome the challenges associated with the high temperature processing of metals. Specifically, we accelerated powders to supersonic impact velocities (~600 m/s) and exploited plastic deformation and softening due to high strain rate dynamic loading to 3D print Ti-6Al-4V powders at temperatures (800 °C, 900 °C) well below their melting point (1626 °C). By using processing conditions below the critical powder impact velocity and controlling the surface temperature, we created mechanically robust, porous metallic deposits with spatially controlled porosity (apparent modulus 51.7 ± 3.2 GPa, apparent compressive yield strength 535 ± 35 MPa, porosity 30 ± 2%). When the mechanical properties of solid-state 3D printed Ti-6Al-4V were compared to those fabricated by other additive manufacturing techniques, the compressive yield strength was up to 42% higher. Post heat treatment of solid-state printed porous Ti-6Al-4V modified the mechanical behavior of the deposit under compressive loading. Additionally, the 3D printed porous Ti-6Al-4V was shown to be biocompatible with MC3T3-E1 SC4 murine preosteoblast cells, indicating the potential biomedical applications of these materials. Our study demonstrates a single-step, solid-state additive manufacturing method for producing biocompatible porous metal parts with higher strength than conventional high temperature additive manufacturing techniques.