Electron Beam Freeform Fabrication Research Articles

Mechanism of carbide transformation and its impact on the mechanical properties of electron beam freeform fabricated TiC-γ'/Ni-Fe-Cr composite during aging treatment

The inherent instability of the microstructure in nickel matrix composites fabricated via additive manufacturing limits their application in high-temperature environments. This research proposed and prepared a Ni-Fe-Cr matrix composite reinforced with in-situ micron-size TiC and nano-size γ' phases using electron beam freeform fabrication (EBF3) with nickel-based powder-cored wire. To attain a stable microstructure at high temperature, the deposited composite was subjected to aging treatment for different durations. The mechanism of carbide transformation from TiC to Cr23C6 and its influence on the mechanical properties were thoroughly investigated. After aging heat treatment, the γ' particles coarsened from around 10 nm to over 100 nm. TiC phase partially decomposed and transformed into Cr23C6 carbides through the TiC+γ→Cr23C6+γ′ reaction. A distinct Cr23C6/γ'/γ transition layer was formed around the TiC phase. The ideal work of adhesion of the phase interfaces was calculated by first-principles calculations, and the γ' (200)/Cr23C6 (200) interface exhibits the lowest value. With prolonged aging time, the ongoing reaction leads to an increased proportion of mixed carbides, exhibiting a continuous aggregated distribution along grain boundaries. This phenomenon adversely affects the toughness due to weak bonding at the γ'/Cr23C6 interface. As a result, the ultimate tensile strength of the aging treated composites initially increased and then decreased, while the elongation continuously decreased with the extension of aging time.

Effect of aging time on precipitates and mechanical properties of the electron beam freeform fabricated Ni-Fe-Cr alloys reinforced by γ’ and TiC phases

Utilizing a synergistic multi-phase and multi-scale strengthening strategy, a novel Ni-Fe-Cr alloy reinforced by nano-scale Ni3(Al, Ti)-γ’ and micron-scale TiC phases was designed and prepared by electron beam freeform fabrication with powder-cored wire. In the as-built sample, the long-strip-shaped TiC phases precipitated along the interdendritic regions with an area fraction of 6%, while the precipitation of the γ’ phase was inhibited. Tensile performance exhibited certain anisotropy, with elongation being higher in the vertical direction than in the horizontal direction. To tailor the strengthening phases and enhance mechanical properties, the as-built alloys were subjected to solution and aging treatments with different aging times. After heat treatment, the γ’ phases were adequately precipitated, with their size increasing to over 100 nm, enhancing their strengthening effect. A portion of TiC phases at the grain boundaries were transformed into Cr23C6 carbides through the TiC+γ → Cr23C6 + γ' reaction. Fracture mechanism analysis revealed that the Cr23C6 carbides with a continuous distribution at the grain boundaries significantly compromised the toughness of the alloys. Consequently, after heat treatment, the yield strength of the alloys significantly increased in both the horizontal and vertical directions. Elongation initially increased and then decreased in the horizontal direction, while it continued to decrease in the vertical direction, and the anisotropy in performance was still maintained.

Effects of heat treatments on the microstructure and mechanical properties of Inconel 718 alloy fabricated by electron beam freeform fabrication

The present study investigated the effects of different heat treatments on the microstructure and mechanical properties of Inconel 718 alloy fabricated by electron beam freeform fabrication. The Laves phases, γʹ phases, and (Nb,Ti)C particles were precipitated in the as-deposited specimen. Solution at 960 °C promoted the partial dissolution of Laves phases and the precipitation of δ phases. Further increasing the solution temperature to 1060 °C dissolved the Laves phases effectively, facilitating the adequate precipitation and uniform distribution of γʹ and γʹʹ phases during the subsequent double aging heat treatment. The ultimate tensile strength (UTS) and elongation of the as-deposited specimen were 770 MPa and 12.2 % respectively. The solution heat treatment at 1060 °C increased the elongation to 38.1 % but decreased the UTS to 734 MPa. The microhardness and UTS of specimens that underwent double aging heat treatment were significantly increased, with the maximum average microhardness and UTS reaching 491 HV0.5 and 1285 MPa, respectively. The undissolved Laves phases and the precipitated δ phases at grain boundaries impaired the ductility. The transformation from single solution or double aging heat treatment to solution plus double aging heat treatment led to a shift from a ductile fracture mode to a mixed ductile and brittle fracture mode.

Effects of solution treatments on the microstructure and mechanical properties of Inconel 625 fabricated by electron beam freeform fabrication

The present study investigated the effects of solution treatments on the microstructure and mechanical properties of Inconel 625 fabricated by electron beam freeform fabrication. The results indicated that the microstructure of the as-deposited specimen exhibited typical dendritic characteristics. Meanwhile, the Laves ((Ni,Fe,Cr)2(Nb,Mo,Ti)) phases and needle-shaped δ phases precipitated. The deposition process led to the development of a preferred orientation feature along the building direction, and the typical {001}<100> plate texture was observed. Solution treatments at 900 °C and 1000 °C resulted in the precipitation of more δ phases, and the volume fraction was approximately 5.29 % and 7.96 %, respectively. The precipitated phases fully dissolved at solution temperatures of 1100 °C and 1200 °C, and equiaxed grains with a twin substructure were formed. The recrystallization weakened the preferred orientation, leading to a decrease in grain size and the formation of {111}<1–10> texture. The as-deposited specimen exhibited an ultimate tensile strength (UTS) of 682 MPa, a yield strength (YS) of 410 MPa, and an elongation of 56.4 %. Solution treatment had minimal impact on the UTS, and the YS was mainly contributed by the solid solution strengthening. However, the elongation was greatly improved at elevated solution temperatures, and the elongation was increased to 72.1 % at most. The primary mode of fracture was toughness fracture, which was attributed to the coalescence of microvoids. The solution treatment at 900 °C led to a fracture pattern that exhibited a combination of toughness and brittleness, and decreased the elongation to 48.5 %.

Obtaining of combined titanium-steel structures by electron beam freeform fabrication using niobium and copper interlayers

Obtaining of combined titanium-steel structures by electron beam freeform fabrication using niobium and copper interlayers

Solution heat treatment of electron beam freeform fabricated in-situ TiC-γ'/ Ni–Fe–Cr composite: Strength-toughness synergy by regulating TiC and γ′ precipitates

Nickel matrix composites reinforced by ceramic particles typically exhibit elevated strength but diminished toughness. In this paper, a novel in-situ micron-size TiC and nano-size γ′ phases reinforced Ni–Fe–Cr matrix composite was successfully prepared using the electron beam freeform fabrication (EBF3). Subsequently, the as-built composite was subjected to solution heat treatment at different temperatures. At a solution temperature of 1150 °C, the composite demonstrates superior strength-toughness synergy (ultimate tensile strength = 1109.4 ± 25.4 MPa, elongation = 12.08 ± 0.62%), exhibiting an increase of 17.1% and 104.4%, respectively, compared to the as-built composite. A systematic analysis of the strengthening and toughening mechanisms revealed that the γ′ phase played a predominant role in enhancing strength, while the size, morphology, and content of TiC phase significantly influence fracture toughness. After being solution heat treated at 1150 °C for 2 h, the γ′ phase precipitated uniformly with its size coarsening to 93.85 nm, demonstrating an optimal strengthening effect by shearing mechanism. Meanwhile, the TiC phase partially dissolved into the matrix and transformed from a long-strip shape to a more stable spherical shape. This transformation significantly reduced the formation of microvoids along the TiC/matrix interface during tensile loading, enhancing the fracture toughness. The γ′ and TiC phases were regulated by solution heat treatment to reach a balance of optimising properties, achieving synergistic enhancements in strength and toughness.

Multiscale microstructure containing nanometer-scale precipitations and stacking faults yields a high-strength Al-5Cu alloy by electron beam freeform fabrication

Electron beam freeform fabrication (EBF3) additive manufacturing technique has been successfully employed to print the non-weldable Al-5Cu thin-walled parts through an optimized set of process parameters. Post-heat treatment is conducted to engineer the desirable microstructure containing multiple nanostructures, i.e., the nanoscale θ" and θ' precipitates, stacking faulted zones and Al-Cu-Cd clusters, leading to a high-strength Al-5Cu alloy with good ductility. The grain formation mechanisms are revealed via a combinatorial simulation and experimentation. It is shown that EBF3 process can produce three different types of grain microstructures: fully equiaxed, fully columnar, and the mixture. In the layerwise melt pool solidification process, wide partially melted zone is observed to favor the formation of equiaxed grains due to its inheritance characteristic. After solution and aging heat treatment, Al-Cu-Cd nanoclusters (∼ 2 nm in Dia.) are generated, which facilitates the formation of nanoscale stacking faulted zones (∼11 nm). Meanwhile, as heterogeneous nucleation sites, Al-Cu-Cd nanoclusters effectively promote θ'-Al2Cu phase (∼ 19 - 114 nm) precipitation to enable the coexistence with fine θ"-Al3Cu precipitates (∼ 7 - 37 nm). The multiscale microstructure, especially incorporating the newly identified nanoscale stacking faulted zones, activates multiple strengthening mechanisms. Consequently, its tensile properties at room temperature reach an ultimate tensile stress of ∼ 496.5 MPa, yield strength of ∼ 435.7 MPa, and elongation of ∼ 9.6%, which are superior to other as-cast and additively manufactured Al-Cu alloys. This work offers a promising processing route to directly fabricate high-strength near-net-shape Al-Cu alloy parts for the aerospace and automotive industries.

The Enhancement Effect of Carbides on the Printability and Mechanical Properties of a Ni–Fe–Cr–Al–Ti Alloy Processed by Electron Beam Freeform Fabrication

The Enhancement Effect of Carbides on the Printability and Mechanical Properties of a Ni–Fe–Cr–Al–Ti Alloy Processed by Electron Beam Freeform Fabrication

Temperature and strain-rate dependence of the superelastic response in EBF3-fabricated and post-heat-treated NiTi shape memory alloy

In the present work, the temperature and strain-rate dependence of the superelastic response behavior of NiTi shape memory alloys prepared via electron beam freeform fabrication (EBF3) technique and post-heat-treated is systematically studied. It is demonstrated that the response hysteresis of the EBF3-fabricated NiTi alloys during superelastic cycling decreases with increasing strain-rate at the same test temperature. However, the superelastic response of material gradually alters to a nearly linear mode with the increase of test temperature under the same strain-rate. The corresponding stable superelasticity recovery ratio decreases from 62.6 % to 47.6 % when the test temperature improves from 100 °C to 150 °C. This is mainly due to the activation of dislocations in the {011}<001> slip system of B2 austenite, in which they become plugged and entangled during plastic deformation, deteriorating the material superelastic recovery ability. These findings provide a selection basis for the service conditions of the rapidly prepared NiTi alloys under EBF3-fabrication and post-heat-treatment technology routes.

Pre-melted electron beam freeform fabrication additive manufacturing: modeling and numerical simulation

Pre-melted electron beam freeform fabrication additive manufacturing: modeling and numerical simulation

Effect of deposition path of electron beam freeform fabrication on residual stress and deformation of deposited parts

AbstractThe present work focused on the effect of deposition path on the transient temperature distribution, heat transfer, residual deformation and residual stress of a rectangular solid deposited by electron beam freeform fabrication technology. To achieve this aim, three representative deposition paths were involved to produce rectangular solid by electron beam freeform fabrication technology. Thermal‐mechanical decoupling simulations were carried out with respect to three different deposition paths. Both of the experimental and simulation results showed that a shorter deposition track can remarkably reduce the residual stress and warping. After separating the deposition area into small segments, remarkable local bulges were formed near the conjunction lines, and overheating was prone to occur at the turning points of the conjunction lines. The present work is helpful to the selection of deposition path for additive manufacturing process.

Multi-materials additive manufacturing of Ti64/Cu/316L by electron beam freeform fabrication

Titanium/steel multi-material, given full play to their superiorities, has tremendous value for parts exposed to complexity service environments, yet invariably, its common joining techniques are lack of design freedom. To this end, we employ electron beam freeform fabrication (EBF3) to successfully prepare this multi-material system for the first time, whose interfacial microstructure and property are systematically investigated. Applying Cu as an interlayer, instead of causing extensive cracking and delamination at the interface of Ti64/316L, can effectively suppress the formation of continuous Fe–Ti intermetallic compounds (IMCs), and thus improve the strength of Ti64/Cu/316L interfaces. For the Cu/316L interface, besides the characteristic spherical Cu-rich and Fe-rich solid solutions, a few Fe–Ti IMCs appear within the Fe-rich solid solutions owing to the long-range diffusion of minor Ti atoms. Correspondingly, the dendritic Cu-rich solid solutions can also be found at the Ti64/Cu interface, which is regarded as a critical interface due to the concomitant of complex Cu–Ti IMCs. These Cu–Ti IMCs have less negative effect on the Ti64/Cu interface property relative to Fe–Ti IMCs, and the local strain produced by the deformed α-Ti near the interface and Cu-rich solid solutions can effectively relieve the concentration of residual stress. Consequently, the Ti64/Cu interface exhibits maximum micro-hardness to 490 HV and superior shear strength to 196.5 ± 2.2 MPa, which was attributed to the reinforcement for the tip of keyhole molten pool, as well as the synergy between Cu-rich solid solution and interdendritic Cu–Ti IMCs.

Insights into the gradient microstructure and mechanical properties of Ti-6.5Al–2Zr–Mo–V alloy manufactured by electron beam freeform fabrication

Herein, the Ti-6.5Al–2Zr–Mo–V (TA15) alloy with gradient characteristics is fabricated via electron beam freeform fabrication (EBF3) technology. The gradient microstructure and mechanical properties are investigated in detail. Specifically, the microstructural evolution containing the morphology and size along the building direction and its impact on microhardness and tensile properties are systematically illustrated. Besides, the numerical simulation is developed for the establishment of the EBF3 temperature field thermal cycling profile to further elaborate the effect of temperature and thermal history on the microstructural evolution. The results reveal that the columnar to equiaxed transition and coarsening of the prior β grains, as well as successive increase and decrease of thickness of α lamellae along the building direction. The formation of fine lamellar α colony within the layer band is associated with the survived α phase when the peak temperature is just below Tβ. The gradient microstructure endows the TA15 deposit with different mechanical properties. Specifically, the microstructure with fine α lamellae manifests marginally higher microhardness in the layer-band-free region. The elemental dilution, loss and partitioning result in relatively low microhardness in the region with fairly fine α lamellae immediately adjacent to the substrate. The top region of the TA15 deposit, characterized by fine basket-weave microstructure, is imparted with better comprehensive properties relative to the middle and bottom regions, i.e., 1007.4 ± 30.8 MPa, 915.31 ± 28.7 MPa, and 12.08 ± 1.96% for tensile strength, yield strength and elongation, respectively. The integrated effect of coarsened prior β grains and refined α lamellae allow the middle and bottom regions to exhibit similar tensile properties.

Effect of cellular structure on the mechanical properties of 316L stainless steel fabricated by EBF3

The cellular structure formed in additive manufactured 316L stainless steel (316L SS) plays a crucial role in achieving strength–ductility synergy. The cellular structure was also found in the 316L SS fabricated by electron beam freeform fabrication process (EBF3), and its microstructural characterization was analyzed. Interrupted tensile tests were carried out to demonstrate the effect of the cellular structure on the mechanical properties. The results showed that the microstructure of the as-deposited 316L SS consisted of the single austenite phase, and the enrichment of Cr and Mo elements was found in the cellular boundary. The orientation of the cellular structure and boundary was similar, and the obvious {100} preferred orientation was parallel to the building direction. The yield strength and fracture elongation of the as-deposited 316L SS parts were 316 MPa and 37.2%, respectively. Its ultimate tensile strength with a value of 632 MPa, was higher than that manufactured by conventional methods. The hardness and elastic modulus in the cellular interior were 2.88 ± 0.11 GPa and 227.45 ± 10.66 GPa, respectively. The cellular boundary had a higher hardness and a lower elastic modulus than the cellular interior. At the initial stage of tensile deformation, the cellular boundary deforms first due to its lower elastic modulus, causing the dislocation pileup and orientation change. The high dislocation density improved the slip resistance leading to the appearance of deformation twins after necking. The micron-dimples and nano-dimples formed in the fracture surface with the deformation of the cellular boundary and interior. Different from 316L SS fabricated by other additive manufacturing processes, the cellular structure improved the strain hardening capacity of the 316L SS fabricated by the EBF3 process.

The electron beam freeform fabrication of NiTi shape memory alloys. Part II: Influence of the heat treatment on the superelastic behaviour

In a previous study (part I), the deposition of functional single-track and multi-layer Ni-rich NiTi using the electron beam freeform fabrication (EBF3) technique was demonstrated. The processed microstructure consists of columnar millimetre range grains that grow parallel to the build direction. Based on the same parameters of part I, this work successfully deposited a stable multi-track and multi-layer NiTi block. Different heat treatments were applied to modify the superelastic properties of the deposited material. Initially, the solution treatment reduces the chemical inhomogeneity and the thermal stresses that originated during the deposition and cooling. Subsequently, due to ageing, Ni4Ti3 particles precipitate at 350 °C and 450 °C for 1, 6 and 12 h, modified the superelastic behaviour of the deposited NiTi. The differential scanning calorimetry results did not reveal any difference in the martensitic starting temperature between as-built and solution-treated samples. However, a remarkable influence of the ageing temperature was detected: while ageing at 350 °C impacted R-phase starting temperatures, 450 °C altered the martensitic starting one. The superelastic behaviour was evaluated by cycling compression. As-built and solution heat-treated samples presented poor mechanical performance, with about 40% of strain recovered after 10 cycles. The ageing at 350 °C for 1 h led to a maximum measured strain recovery of 72.5% after 10 uniaxial compression cycles up to 860 MPa. This condition also resulted in 3.3% permanent deformation out of 11.3% of the total. This study addressed for the first time the fabrication by the EBF3 technique of a stable multi-track and multi-layer NiTi block besides the importance of heat treating it, demonstrating that this post-treatment improves the superelastic performance of EBF3 fabricated parts.

Microstructure and mechanical properties of in-situ TiC hybrid reinforced TC4 matrix composites by a new additive manufacturing method

As a new method to fabricate TiC-reinforced Ti matrix composites, pre-melted electron beam freeform fabrication was proposed based on in situ metallurgy using a graphite diversion nozzle. The feasibility of the method was investigated. Initially, the liquid TC4, an α+β Ti alloy, flowed into the graphite diversion nozzle, corresponding to the reaction between TC4 and graphite. Then, the fabrication of TiC occurred, achieving Ti matrix composites. The liquid TC4 was subsequently deposited on the TC4 substrate, indicating consolidated metallurgical bonding and the optimized hardness and abrasive resistance presented by the surface of the TC4 substrate. The deposited layer's microstructure and the TiC strengthening phase morphology in different areas were varied, considering the difference in the cooling rate and the flow mode. The distribution and morphology of the TiC strengthening phase in different regions were analyzed to reveal the underlying reasons for the property optimization of the TiC-TC4 deposited layer.

Electron beam freeform fabrication of Ti6Al4V alloy and the role of post-heat treatment in the microstructure, texture, and mechanical properties

Electron beam freeform fabrication (EBF3) is a promising manufacturing technique to fabricate large-scale and fully-dense metallic components, showing broad potential applications in the aerospace industry. A Ti6Al4V alloy block was produced using the EBF3 process at an ultra-fast wire feeding rate (3.5 m/min), and the role of post-heat treatments in the microstructure, texture, and mechanical properties was investigated. The results revealed that the as-deposited alloy was characterized by the large columnar prior β grains and the inhomogeneous periodic microstructures. Duplex annealing was considered to be an optimal post-heat treatment candidate to tune the as-deposited microstructure, although the columnar prior β grains maintain. Annealing at 950 ℃ for 1 h followed by air cooling created a unique bimodal microstructure consisting of coarse crab-like primary α and fine lamellar secondary α laths, and then secondary annealing at 630 ℃ for 2 h followed by air cooling facilitated the increased volume fraction of secondary α phase, thereby weakening the α texture. Such microstructure and texture evolution are favorable to a superior and isotropic overall tensile property. The ultimate tensile strength (UTS), yield strength (YS) and tensile fracture elongation (EL) were 889 MPa (transverse direction, TD)/875 MPa (verticle direction, VD), 821 MPa (TD)/801 MPa (TD) and 7.6% (TD)/8.1% (TD), respectively. Overall, the results offer a reasonable reference to the optimization in the microstructure and mechanical performance of EBF3-ed Ti6Al4V alloy.

Microstructure evolution and shape memory function mechanism of NiTi alloy by electron beam 4D printing

Compared to the traditional additive manufacturing techniques, the electron beam freeform fabrication (EBF3), is carried out in a vacuum atmosphere, indicating its superiority in the manufacturing of element content-sensitive alloy, e.g., NiTi shape memory alloy (NiTi SMA), thus deriving promising application prospects. In this study, with the TC4 alloy was utilized as the substrate, we used EBF3 to fabricate equiatomic Ni50Ti50 SMA successfully. Microstructure evolution, phase transition temperature and structure, mechanical properties, and most significantly, shape memory functional ability after multiple deformations were systematically studied. The deposition was composed of four typical regions: Ni diffusion layer, Ni-Ti reaction layer, Ti diffusion region, and NiTi stable deposited region. The mechanical properties of each region varied from each other. A single reversible martensitic transition occurred during heating and cooling (B2↔B19′). The microstructure of the stable deposited region was B2 austenite phase (Space group: Pm3m), while a small amount of NiTi2 and Ti4Ni2Ox phases precipitated at the grain boundaries. Dislocations tend to form in B2 phase deriving from more slip systems. Because of the incomplete phase transition, tangling and piling up occurred both inside B19′ phase and phase interface during the martensitic transition, which affected the integrity of the B19′ phase and hindered the transition (B19′→B2). As the consequence, the incomplete martensitic transition was also detrimental to the shape memory effect (SME).

The effect of electron beam surface remelting on the wear behavior of Ti-6Al-4V by EBF3

Ti-6Al-4V alloy is one of the key materials in the aerospace and chemical industries. Additive manufacturing (AM), e.g., electron beam freeform fabrication (EBF3), is increasingly applied to manufacture the titanium part due to its low cost, high flexibility, high efficiency, etc. At the same time, the wear resistance and hardness of the Ti-6Al-4V alloy synthesized by AM can deteriorate during fabrication. In this paper, electron beam surface remelting (EBSR) is used to improve the wear resistance and hardness of the titanium alloy made by EBF3. The phase, microstructure, element composition, and wear track profile of layers remelted at three EBSR-beam currents were analyzed. According to the results, the synthesized alloy consists of a homogeneous α′ martensitic structure with numerous embedded nano-scale particles rather than a dual α + β lamellar structure when a rapid cooling rate is applied during EBSR. Simultaneously, the coarser prior-β grain boundary was eliminated in the process. The wear rate of the as-obtained remelted layers at the EBSR-beam currents of 0 (as-deposited), 3, 6, and 9 mA was determined as 7.7 × 10−10, 5.7 × 10−10, 7.9 × 10−10, and 8.9 × 10−10 m3/Nm, respectively. The evolution of the structure accounts for the high hardness and superior wear resistance. EBSR successfully modified the as-deposited microstructure to achieve favorable wear properties, which widens the application potential and extends service life.

The Inhomogeneous Microstructure and Mechanical Properties of Ti–6Al–4V Additively Manufactured by Electron Beam Freeform Fabrication

The Inhomogeneous Microstructure and Mechanical Properties of Ti–6Al–4V Additively Manufactured by Electron Beam Freeform Fabrication