Additive Manufactured Titanium Alloy Research Articles

Enhanced comprehensive mechanical properties of laser additive manufactured TC17 by design of new heat treatment based on continuous cooling transition

Heat treatment is critical for enhancing the mechanical properties of high strength titanium alloys, especially for exploiting the potential of laser additive manufactured titanium alloys. In this work, the influence of the cooling rate of continuous cooling transition on microstructure evolution and mechanical behavior was investigated in TC17 titanium alloy fabricated by laser directed energy deposition (LDED) technology. It was found that the number density and orientation characteristics of grain boundary α phases (αGB) are jointly influenced by the cooling rate and the structure of β/β grain boundaries (GBs). The average number density (λavg) of αGB has a consistent trend with the degree of variant selection (DVS) for precipitated α clusters subsequently, which is attributed to the autocatalytic effect of the pre-existing α on the post-precipitated α phases. The largest λavg of αGB and the highest DVS of α clusters could be simultaneously obtained at a suitable cooling rate (4 °C/min). In that case, plenty of α/β phase interfaces and dominant variant type ensure high strength, meanwhile, the combinations of activated multi-slip systems and varied crack propagation paths extend work-hardening to maintain greater plastic deformation. This paper provides a novel thought for designing customized heat treatments of LDEDed high strength titanium alloys, and more importantly, promotes the engineering applications of large and complex components prepared by additive manufacturing technology.

Effect of heat treatment on the anisotropic machinability of additive manufactured titanium alloys in micro-milling

For titanium alloy parts prepared by selective laser melting (SLM), post-treatment operation is usually utilized to meet the needed surface accuracy and dimensional tolerances. However, the heat treatment process and anisotropy of different surfaces of SLMed titanium alloy can directly affect its machinability. In this study, the anisotropic machinability of SLMed TC4 before and after heat treatment was comprehensively compared by the milling force, surface quality, top-burr and work hardening. The results indicate that the milling force and work hardening degree of top surface of SLMed TC4 material are larger than those of side surface, and top surface the presents the better surface quality and smaller top-burr. It exhibits obvious anisotropic machinability in micro-milling. After heat treatment, the anisotropic machinability between different surfaces of SLMed TC4 materials were significantly reduced. The average surface roughness difference rate of top and side surfaces decreased from 10.08% to 2.79%, and the total average top-burr size difference rate decreased from 19.53% to 9.99%. The difference rate of milling force and work hardening degree of the top and side surfaces both showed a decreasing trend. This work demonstrates that the anisotropic machinability of SLMed TC4 material in micro-milling can be effectively mitigated by the heat treatment process.

Improving the product of strength and ductility and corrosion resistance by adding He shielding gas in the CMT additive manufacturing process of Ti6Al4V

The cold metal transfer additive manufacturing (CMTAM) of titanium alloys presents a promising solution for the efficient fabrication of large titanium alloy armor components. In this study, different proportions of He gas were added to the welding shielding gas used in the CMTAM of Ti6Al4V, aiming to improve forming quality, the product of strength and ductility and corrosion resistance. Simultaneously, the influence of different He contents on the microstructure, mechanical properties, and corrosion behavior of CMTAM of Ti6Al4V was explored. The research revealed that during the process of CMTAM of Ti6Al4V, as the proportion of He as the shielding gas increased, the arc voltage increases, and the arc becomes more stable. Meanwhile, the wettability of the weld pool improves, resulting in a more spread out weld pool. The fabrication precision of the built material also improved. Microstructural observations indicate that the prior-β grain size gradually increased with the increase in He proportion, the heat input for additive manufacturing also increased，and the martensitic α’ transformed into a basketweave α+β structure. The length and width of the α phase increased, the texture intensity decreased, and the Schmid factor increased. Tensile tests were conducted on samples with different He proportions showed that as the He proportion increased, within the component resulted in a noteworthy enhancement in the product of strength and ductility increase exceeding 50 %. This suggests that He as the shielding gas addition can effectively optimize the strength-plasticity balance in additive manufactured titanium alloys without the need for post-processing heat treatment. Electrochemical tests were conducted in a 3.5 wt% NaCl solution revealed that the addition of He gas significantly improved the corrosion resistance of Ti6Al4V. Specifically, the corrosion resistance was optimal when the shielding gas composition was 50 %He+50 %Ar, far surpassing that of cast Ti6Al4V.

Material Characterization and Electrochemical Properties of Titanium Alloy 5553 Prepared by Selective Laser Melting as Processed and after Abrading and Polishing.

The microstructure, Vickers microhardness, and electrochemical properties of an additive manufactured titanium alloy, Ti-5553 (Ti-5Al-5Mo-5V-3Cr wt %), are reported on. The alloy specimens were fabricated by selective laser melt processing. The surface morphology and electrochemical properties of the as-processed and surface-pretreated (abraded and polished) Ti-5553 specimens were investigated. The as-processed specimens had a nominal density of 4.62 ± 0.04 g/cm3. Based on comparison with the reported density for the die-cast alloy, the specimens were 99-100% dense with ca. 1% porosity. Optical microscopy and scanning electron microscopy revealed some micropores, balling features, and fusion pore defects across the surface of the alloy (XZ plane-orthogonal to the build direction). The nominal Vickers microhardness was 292 ± 2 HV. Detailed electrochemical characterization of the as-processed and surface-pretreated alloys revealed reproducible open-circuit potentials (OCPs), linear polarization resistances (R p), and potentiodynamic polarization curves for both specimen types in naturally aerated 3.5 wt % NaCl at room temperature. For the surface-pretreated alloys, the OCP was 225 mV more noble, the anodic current in the potentiodynamic polarization curves was 72× lower, the cathodic current was 8× lower, and R p was larger by 426× than the values for the as-processed specimens. The collective electrochemical data revealed that the surface-pretreated alloys exhibit greater corrosion resistance than the as-processed alloys due to a reduction of the real area and the formation of a more passivating oxide layer.

The robot grinding and polishing of additive aviation titanium alloy blades: a review

PurposeThe purpose of this review is to comprehensively consider the material properties and processing of additive titanium alloy and provide a new perspective for the robotic grinding and polishing of additive titanium alloy blades to ensure the surface integrity and machining accuracy of the blades.Design/methodology/approachAt present, robot grinding and polishing are mainstream processing methods in blade automatic processing. This review systematically summarizes the processing characteristics and processing methods of additive manufacturing (AM) titanium alloy blades. On the one hand, the unique manufacturing process and thermal effect of AM have created the unique processing characteristics of additive titanium alloy blades. On the other hand, the robot grinding and polishing process needs to incorporate the material removal model into the traditional processing flow according to the processing characteristics of the additive titanium alloy.FindingsRobot belt grinding can solve the processing problem of additive titanium alloy blades. The complex surface of the blade generates a robot grinding trajectory through trajectory planning. The trajectory planning of the robot profoundly affects the machining accuracy and surface quality of the blade. Subsequent research is needed to solve the problems of high machining accuracy of blade profiles, complex surface material removal models and uneven distribution of blade machining allowance. In the process parameters of the robot, the grinding parameters, trajectory planning and error compensation affect the surface quality of the blade through the material removal method, grinding force and grinding temperature. The machining accuracy of the blade surface is affected by robot vibration and stiffness.Originality/valueThis review systematically summarizes the processing characteristics and processing methods of aviation titanium alloy blades manufactured by AM. Combined with the material properties of additive titanium alloy, it provides a new idea for robot grinding and polishing of aviation titanium alloy blades manufactured by AM.

Enhancement of the Microstructure and Fatigue Crack Growth Performance of Additive Manufactured Titanium Alloy Parts by Laser-Assisted Ultrasonic Vibration Processing

Post-processing techniques can efficiently improve the surface quality, address microstructural defects, and optimize mechanical properties in additively manufactured parts. Surface severe plastic deformation processes such as ultrasonic nanocrystal surface modification (UNSM) integrated with localized laser heating were explored to enhance the surface properties, microstructure as well as the fatigue crack growth properties (FCG) in both directions of built. We successfully induced greater plasticity flow and achieved beneficial refinement of the surface grain structure by precisely controlling the heat and impact energies during surface treatment process. The LA-UNSM process, with the parameters utilized in this study considerably decreased the FCG rates of treated samples, when compared to samples without surface treatment. Improved fatigue crack growth properties along vertical and horizontal orientations after the post-process treatment were attributed to the induced-microstructural changes, improved surface quality, induced compressive residual stresses through gradient structure deformation layer that was prepared on the surface of the material. The fractographic analysis revealed that the cracks mostly originated from the pores in the as-produced state. This observation shows a clear correlation between the surface treatment performed and a substantial improvement in fatigue crack growth resistance.

Simultaneous enhancements of strength and ductility of additively manufactured Ti-6.9Al-6.8Zr-2.3Mo-2.2V alloy by cyclic heat treatment and solution-aging

In the paper, in terms of the new developed additively manufactured Ti-6.9Al-6.8Zr-2.3Mo-2.2V alloy, different microstructural design strategies were proposed to tune the morphology and content of α phase to optimize the mechanical properties of the alloy and the corresponding deformation mechanism of the alloy were characterized and analyzed. The results show that after cyclic heat treatment and solution-aging, the best comprehensive mechanical properties (yield strength ∼ l009 MPa, and elongation ∼12%) are obtained. The better comprehensive tensile properties are attributable to the microstructure consisting 16% equiaxed αp and βt structure. The cycle heat treatment can effectively decrease the length-width ratio of lamellar α phase and improve spheroidization of primary α phase. The equiaxed primary α phase has higher ability to coordinate local plastic strain, leading to the activation of extensive prismatic <a> slip and improving the ductility of the alloy. After aging-solution, large amount of fine αs precipitate from β phase. The high hetero-deformation induced stress at αs/β interfaces in βt structure promotes the activation of the pyramidal <a> slip systems, thereby increasing the yield strength. The present findings provide significant guidance for additive manufactured titanium alloy having good combination of tensile ductility and strength.

Effect of processing parameters on the defects, microstructure, and property evaluation of Ti-6Al-4V titanium alloy processed by laser powder bed fusion

In this study, we designed the processing windows for laser powder bed fusion (LPBF) of Ti-6Al-4V (Ti-64) alloy by using central composite design and made a detailed investigation into the influence of processing parameters on the defects. The purpose is to investigate the effect of defects on mechanical properties. It was found that insufficient energy density could lead to the formation of lack of fusion (LOF) defects and produce non-melted powders on the surface, while excessive energy density could lead to cracks that were detrimental to mechanical performance. In addition, the microstructural evaluation found that relatively low energy density could lead to shorter columnar prior-β grains, while prior-β grains in the sample processed by the high energy density extended almost the entire height of the cross-section, which could lead to the strong mechanical property anisotropy. The prior β grains are formed by heterogeneous nucleation on the partially melted material powder. As the energy input increases, all the powder powders in the molten pool can be melted so that these particles do not act as nucleation sites and the prior β grain can grow through more layers without forming new grains being able to nucleate. The prior β-grain in as-built Ti-64 samples consisted of acicular α’ martensite with myriads of lattice distortions, as a precursor to a phase transition, which lead to strong tensile strength and poor ductility. Annealing heat treatment promoted the improvement of the ductile performance of LPBF Ti-64. Overall, this study provides comprehensive views on the effects of processing parameters (laser power, scanning speed, and hatch distance) on the internal (pores and LOF) and external (unmelted powder, sintering neck, and crack), defects, microstructure, and tensile property evaluation of LPBF Ti-64, which offer insights for the development of additive manufactured titanium alloys with excellent mechanical property.

Parent Grain Reconstruction in an Additive Manufactured Titanium Alloy

Electron backscatter diffraction (EBSD) is an excellent tool for characterizing the crystallographic orientation aspects of the microstructure of polycrystalline material. In some additively manufactured materials, the material may undergo a phase transformation during the forming process. Although EBSD can only characterize the final microstructure, neighbor information from orientation mapping allows the microstructure before the phase transformation to be reconstructed, provided that the parent–child orientation relationship is known. An investigation of the effectiveness of the reconstruction algorithms for capturing the grain size as well as orientation gradients is undertaken with a focus on additively manufactured Ti-alloy. The EBSD results, coupled with reconstruction algorithms, reveal information on the prior grain size as well as the plastic flow of the material.

Anisotropy of plasticity in Ti–6Al–4V alloy processed by electron beam direct energy deposition

Additive manufactured titanium alloys are generally characterized by columnar grain structures along the vertical direction (VD), thereby causing a relative lower plasticity in the horizontal direction (HD) than that in the VD direction. However, herein we report that such columnar grain structures in the Ti–6Al–4V alloy produced by electron beam direct energy deposition (EB DED) can induce a superior uniform elongation in the HD direction than that in the other loading directions (22.5°,45°, 67.5° and VD directions). Besides the orientation of α laths with a favorable soft and hard match when the loading direction is along the HD, such anisotropic plasticity also can be attributed to the columnar grain structures which can also effectively prevent dislocation slip and suppress the necking of the HD sample, thereby triggering a greater work-hardening rate and strain hardening exponent in the stage of uniform deformation, and increasing the uniform elongation in the HD. The greater total elongation in the VD is mainly reflected in its higher non-uniform elongation, rather than uniform elongation, which is mainly caused by the α laths possessing easily activated basal slip system inducing strain softening. As for the 22.5° and 45° samples, most of the α laths under such loading directions present hard orientation, thereby leading to both lower non-uniform elongation and total elongation. The above information offers a deeper understanding about anisotropic plasticity in additive manufactured titanium alloys, thereby providing a theoretical basis for optimizing the consistency of mechanical properties.

Effect of cyclic heat treatment on microstructure and tensile property of a laser powder-bed fusion-manufactured Ti–6Al–2Zr–1Mo–1V alloy

Laser powder-bed fusion (L-PBF) is increasingly being employed to fabricate titanium components for aerospace applications. However, L-PBF fabricated titanium alloy parts typically exhibit high strength but low ductility because of fine α′ martensite. Post-heat treatment is essential to compose α′ into stable α, thereby achieving improved ductility. In this study, we put forward a novel post-heat treatment, i.e., a cyclic treatment that included multiple cycles of low-to-high and high-to-low temperature annealing, to improve the ductility of an L-PBF fabricated Ti–6Al–2Zr–1Mo–1V (TA15) alloy. Its effect on the microstructures and tensile properties was investigated. The results showed that the as-fabricated TA15 alloy exhibited fine martensite, leading to high strength (i.e., an ultimate tensile strength of 1168 MPa) and low ductility (i.e., an elongation of 7.1 %). Conventional post-heat treatments introduced lamellar α+β structures and resulted in improved ductility of 9–13.9 % and reduced strength of 896–1070 MPa. The cyclic multistep treatment created trimodal structures of α-laths, equiaxed α, and thin film β. Especially, it contributed to the formation of up to 57 % equiaxed α grains compared to less than 5 % in the conventional treatments, leading to the enhanced ductility. This specific structure displayed superior properties, an ultimate tensile strength of ∼900 MPa, and elongation of ∼16 %, which are comparable to the TA15 alloy wrought counterparts. The optimized strength and ductility also demonstrated the potential of cyclic multistep treatment to enhance the performance of other additive manufactured titanium alloys.

Micro-deformation behavior of laser melting deposited multi-gradient TC4/TC11 alloy under tension–tension cyclic loading

The microstructure of additive manufacturing titanium alloy is closely related to its mechanical properties. In this study, a TC4/TC11 titanium alloy with 20 layers was prepared by laser melting deposition (LMD) technology. Microstructure analysis revealed a transformation of the columnar grains into equiaxed grains along the build direction (BD), accompanied by the presence of heat affected zone (HAZ) bands and pore defects. The defect families of specimens in different directions were characterized by X-ray micro-computed tomography (micro-CT). The porosity of BD specimens was 0.0021%, and the porosity of the transverse direction (TD) specimens was 0.0001%, indicating that the material was dense. Specimens with different directions were subjected to tension–tension cyclic loading, and their deformation was analyzed by digital image correlation (DIC). The results show that the presence of noticeable pore defects leads to crack initiation around these regions due to local strain concentration. Moreover, the dispersion and anisotropy of mechanical properties are attributed to the influence of porosity and microstructure deformation, the latter being the dominant factor. In addition, the deformation mechanism and failure mode of the microstructure in LMD TC4/TC11 are proposed in this paper. The deformation of the α laths of TD specimen is hindered by the columnar grain boundary, resulting in improved resistance against crack nucleation and propagation compared to the BD specimen.

Achieving isotropic microstructure in an additively manufactured Ti-6Al-4V alloy enabled by dual laser processing

With the development of science and technology and the popularization of intelligent manufacturing, laser additive manufacturing of titanium alloys is being attractive in industrial adoption due to its advantages of near-net shape and cost-saving features, and especially laser melting deposition is an effective additive manufacturing technique for large-scale and lightweight components such as TC4 alloy. However, inside the as-prepared alloy, large scale columnar crystals are formed along the forming direction, which will lead to anisotropic macroscopic properties of the alloy parts, further affecting their service life. In view of this, we report an innovative strategy of dual laser processing integrating laser melting deposition and laser shock peening, which can be used to achieve isotropic microstructure along three building directions in Ti-6Al-4V alloy. Interestingly, the isotropic microstructure is basket-weave structure of ultrafine α and nanometer β lamellar colonies together with abundant nanometer β fragments/particles distributed dispersedly inside ultrafine α matrix. Especially, the as-fabricated alloy exhibits approximately isotropy of mechanical properties along three building directions, and circumvents the strength–ductility trade-off of Ti-6Al-4V alloy with different microstructures reported so far. This study validates the feasibility of manufacturing high performance complex structural components by coupling technology of laser melting deposition and laser shock peening, for grain refinement and isotropic microstructure of additive manufactured titanium alloys in a low-cost and effective method. Impact statementThe proposed dual laser processing technology integrating additive manufacturing of laser melting deposition and synchronous laser shock peening, can provides insight into fabricating isotropic metal components with higher performance through additive manufacturing of one-step forming.

Investigation of fatigue behavior of laser powder bed fusion Ti-6Al-4V: Roles of heat treatment and microstructure

High cycle fatigue (HCF) performance of additive manufactured (AM) titanium products is of great significance for its structural and functional applications. However, tying fatigue performance to the complex microstructures in AM titanium alloys is challenging. The work here carried out a thorough investigation into the influence of microstructures (particularly the grain boundary α-phase (GB-α) with varied morphologies) on the fatigue performance of laser powder bed fusion (LPBF) Ti–6Al–4V (Ti-64). Results showed that the improvement in the high-cycle fatigue life of LPBF Ti-64 could be achieved by the formation of low aspect ratio α lath and discontinuous GB-α via optimized post-fabrication heat treatment. Discontinuous GB-α could fully accommodate the deformation, improving the fatigue crack propagation resistance. Moreover, α lath with a low aspect ratio could lead to less strain accumulation on the interface between adjacent α lath, and thereby inhibit the crack initiation at these interfaces. This study enhances the understanding of how LPBF-induced complex microstructures influence fatigue behavior, and provides a pathway for the improvement of fatigue performance of additive manufactured titanium alloy.

Investigation on surface quality in micro milling of additive manufactured Ti6Al4V titanium alloy

The additive manufactured technology is widely applied to produce titanium alloy parts with complex structures. The subsequent machining operations are necessary for additive manufactured parts to achieve the required machining quality. In this paper, the theoretical surface roughness model in micro milling was established. The model indicated the theoretical milled roughness mainly depends on the feed per tooth, tool nose radius and bottom edge inclination angle. Then, micro milling experiments are conducted on additive manufactured titanium alloy using PCD micro end mill. The surface roughness on milled surface was measured and investigated. At the middle position of micro milled surface, the distribution curves of surface roughness Ra and Sa show the same variation trend under different milling parameters. Moreover, the measured surface roughness results on different transverse positions of milled surface showed that the difference value between the actual and theoretical surface roughness of milled surface is mostly induced by the unstable cutting behaviors, which is also mainly responsible for the surface roughness size effect. The transverse distribution of surface roughness on milled surface exhibited a W shape because of the variation of the instantaneous undeformed chip thickness and material removal mode at different transverse positions.

Tool wear mechanisms of PCD micro end mill in machining of additive manufactured titanium alloy

Tool wear mechanisms of PCD micro end mill in machining of additive manufactured titanium alloy

In Situ Tensile Observation of the Effect of Annealing on Fracture Behavior of Laser Additive Manufactured Titanium Alloy.

The use safety of laser additive manufactured (LAM) titanium alloys is closely related to each one's fracture failure mode. In this study, in situ tensile tests were carried out to study the deformation and fracture mechanisms of LAM Ti6Al4V titanium alloy before and after annealing treatment. The results showed that plastic deformation promoted slip bands to occur inside the α phase and shear bands to generate along the α/β interface. In the as-built sample, cracks initiated in the equiaxed grains and propagated along the columnar grain boundary, showing a mixed fracture mode. However, it transformed into a transgranular fracture after annealing treatment. The widmanstatten α phase acted as a barrier for slip movement and improved the crack resistance of grain boundaries.

Role of structural hierarchy on mechanics and electrochemistry of wire arc additive manufactured (WAAM) single phase titanium

The absence of understanding of correlations between structure, mechanics, and electrochemical behavior in single-phase additive-manufactured titanium components impedes the advancement of large-scale processing. The present study addresses this scientific knowledge gap by investigating the evolution of structural hierarchy and its impact on the hardness and corrosion resistance of wire arc additive manufactured (WAAM) commercially pure titanium (cp-Ti) along three mutually orthogonal deposition directions. Integrated X-ray diffraction and optical microscopy revealed that a single-phase α-Ti monolithic structure is devoid of observable deposition tracks and interfaces with traces of titanium oxide. Homogeneous nucleation in the single phase at low undercooling and slow solidification rates during WAAM resulted in coarse grains of size 1 mm. The grains are randomly oriented with slight preferential texture along the hexagonal close-packed basal {0001} planes due to their parallel orientation with the close-packed body-centered cubic {011} plane during the β – α phase transition. The single-phase structure results in a uniform distribution of microhardness along layer-by-layer buildup direction and the bulk with an average of 1.6 GPa, which is about 80 % of conventionally cast cp-Ti. An increased dislocation density at the surface and from thermal strain during WAAM enhances nanohardness to 3.0 GPa. The absence of galvanic potential difference between two adjacent grains and the coarse size imparts high corrosion resistance with a rate of 80 × 10−5 mm per year, less than 50 % of other additive-manufactured titanium alloys. Overall, this study advances the state-of-the-art in homogenous, single-phase WAAM processing as a potential large-sale additive manufacturing alternative to conventionally processed Ti components.

The State of the Art in Machining Additively Manufactured Titanium Alloy Ti-6Al-4V

Titanium alloys are extensively used in various industries due to their excellent corrosion resistance and outstanding mechanical properties. However, titanium alloys are difficult to machine due to their low thermal conductivity and high chemical reactivity with tool materials. In recent years, there has been increasing interest in the use of titanium components produced by additive manufacturing (AM) for a range of high-value applications in aerospace, biomedical, and automotive industries. The machining of additively manufactured titanium alloys presents additional machining challenges as the alloys exhibit unique properties compared to their wrought counterparts, including increased anisotropy, strength, and hardness. The associated higher cutting forces, higher temperatures, accelerated tool wear, and decreased machinability lead to an expensive and unsustainable machining process. The challenges in machining additively manufactured titanium alloys are not comprehensively documented in the literature, and this paper aims to address this limitation. A review is presented on the machining characteristics of titanium alloys produced by different AM techniques, focusing on the effects of anisotropy, porosity, and post-processing treatment of additively manufactured Ti-6Al-4V, the most commonly used AM titanium alloy. The mechanisms resulting in different machining performance and quality are analysed, including the influence of a hybrid manufacturing approach combining AM with conventional methods. Based on the review of the latest developments, a future outlook for machining additively manufactured titanium alloys is presented.

Fatigue behaviour of L-DED processed Ti-6Al-4V with microstructures refined by trace boron addition

Boron is a powerful grain solute that promotes substantial microstructural refinement in titanium alloys produced by additive manufacturing. However, the fatigue performance of boron refined additive manufactured titanium alloys remained unknown. In this study, a thorough investigation into the high-cycle fatigue life and crack propagation rates of laser direct energy deposited Ti-6Al-4V with a trace boron addition was carried out. The high-cycle fatigue life was significantly improved, as the refined microstructure increased the crack initiation resistance. However, the crack propagation resistance estimated from spectrum loading was reduced, due to the refined prior-β grains and TiB particles.