TiAl Components Research Articles

Improving the formability and mechanical properties of TiAl alloy by direct forging of uncondensed powder

The limited plasticity and poor formability of TiAl alloys constitute primary constraints on their widespread applicability. In this regard, an innovative forming method is proposed, characterized by the direct forging of unconsolidated pre-alloyed Ti–48Al–2Cr–2Nb powders. In this procedure, spherical powders with a broad particle size distribution are pre-encapsulated and subjected to vacuumed treatment, followed by direct high-temperature forging to attain the final desired shape. Throughout the process, the powders undergo pre-heating to various forming temperatures and are forged at different speeds, enabling a comparative analysis of the effect of forging temperature and speed on densification and the microstructural characteristics of the forged billet. Experimental results indicate that the powders can be nearly fully densified when direct forging is conducted within the α-phase region. By combining the finite element modeling and microstructural characterization, three mechanisms dominating the consolidation process of powders are revealed. Initial particle clustering and spatial rearrangement predominantly occur during the early deformation stage, enhancing densification. Subsequent fragmentation of particles, induced by severe plastic deformation, governs the entire deformation process, leading to the elimination of the prior particle boundaries and substantial densification improvement. The closure of the micrometer-scale voids is controlled by high-temperature diffusion and creep, which plays a pivotal role in weakening the detrimental effect of micro defects on mechanical properties. The above mechanisms act alternately or simultaneously, resulting in a homogeneous microstructure composed of α2+γ lamellae and bulk γ with small sizes, and a good synergy of strength and ductility. Therefore, this novel approach holds considerable potential as a cost-effective alternative strategy for producing TiAl components.

Grindability study of intermetallic titanium aluminide using various abrasive wheels

Nickel (Ni) and Titanium (Ti) based superalloys satisfy the properties of a material suitable for high-performance applications. Titanium aluminide (γTiAl) is a classic example that finds wide application in aerospace and automobile industries. Even though γTiAl is having attractive mechanical and thermal properties, a widespread use of this material is restricted due their poor machinability and high procuring and processing cost. γTiAl during machining often have adverse impact on tool life and surface quality. At room temperature the material exhibits high brittleness and hardness, which along with a low ductility, lower thermal conductivity and fracture toughness, high temperature dependent strength and chemical reactivity with many tool materials. This often makes its machining a challenging task and the material is often considered under the category of difficult to machine material. Majority of the present TiAl components used in aerospace industry undergoes grinding operation. A wider investigation is necessary to understand the grindability of this material, as the variation of composition of the grinding wheel have significant influence on the machining. A consistent and economical machining is essential for the wide recognition of this promising material for wider industrial application. The paper investigates the grindability of intermetallic γTiAl using various abrasive wheels.

In-situ synchrotron high energy X-ray diffraction study on the internal strain evolution of D019-α2 phase during high-temperature compression and subsequent annealing in a TiAl alloy

The residual stress in the D019-α2 phase is known to be significantly higher than that in the L10-γ phase in TiAl alloys after deformation due to the poor plasticity and strong mechanical anisotropy of the α2 phase. However, the internal stress accumulation and relaxation in the α2 phase during high-temperature deformation and annealing are scarcely investigated. In this study, for the first time, the internal strain evolution and load partitioning between the α2 and γ phases at high temperatures are characterized by in-situ synchrotron high energy X-ray diffraction (HEXRD) technique. The plastic deformation is at least initiated at a stress of roughly 200 MPa in the γ phase and 775 MPa in the α2 phase. The intergranular strains in the α2 phase are generated by the onset of dislocation glide in the γ phase, and accentuated with the accumulated dislocations and the ensuing twinning activity. After unloading, great intergranular strains are preserved in the α2 phase constrained by the heavily plastically deformed γ phase. During subsequent heating from 400 to 1000 °C, the internal strains in the α2 phase are almost fully relaxed by substantial dislocation annihilation and rearrangement in the γ phase. During annealing at 800 °C, the internal strain relaxation is rapid in the initial 10 min, whereas considerably retarded subsequently. The extent of relaxation after holding at 800 °C for 1 h is equivalent to that of heating in an effective temperature range of 680–880 °C for 10 min. The in-situ lattice strain measurements with various thermal relaxation schemes provide guidance for the stress relief annealing of TiAl components.

Integration of Hot Isostatic Pressing and Heat Treatment for Advanced Modified γ-TiAl TNM Alloys.

The conventional processing route of TNM (Ti-Nb-Mo) alloys combines casting and Hot Isostatic Pressing (HIP) followed by forging and multiple heat treatments to establish optimum properties. This is a time-consuming and costly process. In this study we present an advanced alternative TNM alloy processing route combining HIP and heat treatments into a single process, which we refer to as IHT (integrated HIP heat treatment), applied to a modified TNM alloy with 1.5B. A Quintus HIP lab unit with a quenching module was used, achieving fast and controlled cooling, which differs from the slow cooling rates of conventional HIP units. A Ti-42.5Al-3.5Nb-1Mo-1.5B (at.%) was subjected to an integrated two HIP steps at 200 MPa, one at 1250 °C for 3 h and another at 1260 °C for 1 h, both under a protective Ar atmosphere and followed by cooling at 30 K/min down to room temperature. The results were compared against the Ti-43.5Al-3.5Nb-1Mo-0.8B (at.%) thermomechanically processed in a conventional way. Applying IHT processing to the 1.5B alloy does indeed achieve good creep strength, and the secondary creep rate of the IHT processed materials is similar to that of conventionally forged TNM alloys. Thus, the proposed advanced IHT processing route could manufacture more cost-effective TiAl components.

Role of Ti3AlC2 MAX phase on characteristics of in-situ synthesized TiAl intermetallics. Part III: microstructure

In this paper, the 3rd part of a series of publications on the sinterability and characteristics of TiAl–Ti3AlC2 composites, the microstructure development during the synthesis and sintering processes was studied by scanning electron microscopy (SEM). Chemical evaluation of various phases in the developed microstructures was performed using energy-dispersive X-ray spectroscopy (EDS) in different ways such as point, line scan and two-dimensional elemental map analyses. For this purpose, five samples were fabricated with different percentages of Ti3AlC2 MAX phase additive (10, 15, 20, 25 and 30 wt%). Ball-milling and spark plasma sintering (SPS: 900 °C/7 min/40 MPa) of as-purchased Al and Ti powders with already-synthesized Ti3AlC2 additive were selected as composite making methodology. SEM/EDS analyses verified the in-situ manufacturing of TiAl/Ti3Al intermetallics as the matrix during the SPS process and the presence of Ti3AlC2 as the ex-situ added secondary phase. Moreover, the in-situ synthesis of Ti2AlC, another member of MAX phases in Ti-Al-C system, was also detected in titanium aluminide grain boundaries and attributed to a chemical reaction between TiC (an impurity in the initial Ti3AlC2 additive) and TiAl components.

Elastic constants and internal friction of forged β-solidifying TiAl alloys at room temperature and high temperature

Forged β-solidifying TiAl alloys can exhibit nine independent elastic constants in accordance with the orthorhombic symmetry of forging texture. In this study, we measured the elastic constants and high-frequency internal friction of two forged Ti–43Al–5V–4Nb (in at.%) alloys with nearly lamellar and triplex microstructures using resonant ultrasound spectroscopy at temperatures between room temperature and 760 °C. We demonstrated that a two-step heat treatment after the forging process produces nearly isotropic elasticity in the TiAl alloys, and the elastic anisotropy is virtually constant with temperature. The Young's modulus of the TiAl alloy with a triplex microstructure was comparable to that of the alloy with a nearly lamellar microstructure. In addition, the triplex-microstructure TiAl alloy showed a significant energy loss for shear vibration modes compared with the nearly lamellar alloy. These findings are supported by the effect of the hexagonal ω-phase present in the βo-phase on mechanical stiffness and loss. Internal friction measurements at high frequencies exhibited an internal-friction peak at approximately 200 °C, which may be related to the isothermal ω-phase. This research may contribute to the development of sophisticated forged TiAl components.

Additive Manufacturing of Ti-Based Intermetallic Alloys: A Review and Conceptualization of a Next-Generation Machine.

TiAl-based intermetallic alloys have come to the fore as the preferred alloys for high-temperature applications. Conventional methods (casting, forging, sheet forming, extrusion, etc.) have been applied to produce TiAl intermetallic alloys. However, the inherent limitations of conventional methods do not permit the production of the TiAl alloys with intricate geometries. Additive manufacturing technologies such as electron beam melting (EBM) and laser powder bed fusion (LPBF), were used to produce TiAl alloys with complex geometries. EBM technology can produce crack-free TiAl components but lacks geometrical accuracy. LPBF technology has great geometrical precision that could be used to produce TiAl alloys with tailored complex geometries, but cannot produce crack-free TiAl components. To satisfy the current industrial requirement of producing crack-free TiAl alloys with tailored geometries, the paper proposes a new heating model for the LPBF manufacturing process. The model could maintain even temperature between the solidified and subsequent layers, reducing temperature gradients (residual stress), which could eliminate crack formation. The new conceptualized model also opens a window for in situ heat treatment of the built samples to obtain the desired TiAl (γ-phase) and Ti3Al (α2-phase) intermetallic phases for high-temperature operations. In situ heat treatment would also improve the homogeneity of the microstructure of LPBF manufactured samples.

Mechanical properties of TiAl fabricated by electron beam melting — A review

As a typical intermetallic material, TiAl is inevitably difficult to process by conventional methods. Additive manufacturing (AM) has recently become a new option for making net-shape TiAl components. Among all AM methods, electron beam melting (EBM) shows the potential to make TiAl components with good mechanical properties and is used for low pressure turbine blades. The mechanical properties, including tensile and compression properties, fracture toughness, fatigue and creep properties of EBM TiAl are reviewed and compared to the conventionally fabricated alloys. Results show that the tensile strength of EBM alloys is higher than cast alloys, and other properties are comparable to the cast/forged alloys. The sensitivity of mechanical properties and microstructure to EBM processing parameters is presented. Issues including layered microstructure, anisotropy in mechanical properties, and fatigue failure from defects are also reviewed. Finally, some opportunities and challenges of EBM TiAl are identified.

Effects of laser polishing on surface morphology and mechanical properties of additive manufactured TiAl components

Abstract The laser deposition manufacturing (LDM) is a free-form metal deposition process, which allows generating a prototype or small series of near net-shape structures. Despite numerous advantages, one of the most critical issues of the technique is that produced pieces have a deleterious surface quality which requires post machining. Traditional machining methods such as milling and grinding have been used to improve the surface quality of the LDM components. However, these methods have some drawbacks such as tool wearing and narrow-area difficult machining. In this paper, the CW laser and the pulsed laser was applied to polish the surface of LDM TiAl alloy samples. The difference between these two polishing processes on the surface morphology, microstructures, corrosion resistance, micro-hardness and wear resistance were investigated. Results revealed that the surface roughness of CW laser polished TiAl can be reduced to about 1.76 μm, which is comparable to a precise machining surface. It was also found that a hardened layer about 600 μm was produced on the TiAl alloy surface after laser polishing, which can improve the mechanical properties of the component. The micro-hardness of the laser polished sample was improved about 8% compared with the initial TiAl alloy, the wear resistance and corrosion resistance were also improved compared with the as-received TiAl surface.

Designing advanced intermetallic titanium aluminide alloys for additive manufacturing

Lightweight intermetallic γ-TiAl based alloys are innovative high-temperature structural materials. So far, these alloys are in use as turbine blades or turbocharger turbine wheels in advanced aerospace and automotive engines, where they are produced by means of investment casting as well as wrought processing, e.g. hot-forging. Through the development of powder-based additive manufacturing processes within the last decade, a real paradigm shift for future component production as well as their design and materials properties was created. While so-called proven alloy systems are presently used worldwide for additive manufacturing, the approach of this work is the development of novel process-adapted γ-TiAl based alloys, which on the one hand fulfill the specific requirements of additive manufacturing and on the other hand provide excellent high temperature properties after a suitable heat treatment. Based on the concept of an engineering γ-TiAl based alloy, i.e. the so-called TNM alloy, two alloys are presented. Due to the chemical reactivity of titanium aluminide alloys, electron beam melting processes come into consideration as production methods using optimized manufacturing parameters, providing dense components with only small variations in the Al content between the individual powder layers, which is a decisive factor for the subsequent heat treatment above the γ solvus temperature. The additively produced samples show a fine equiaxed microstructure, whereas the heat-treated samples exhibit a fully lamellar α₂/γ microstructure with an excellent creep resistance. In summary, the adaptation of the additive manufacturing parameters in combination with innovative alloys and subsequent heat treatments are the basis for producing reliable high-performance TiAl components in the near future.

A study on hot-working as alternative post-processing method for titanium aluminides built by laser powder bed fusion and electron beam melting

Intermetallic Titanium Aluminides (TiAl) have been designed for high-temperature lightweight applications. Powder-bed additive manufacturing (AM) processes such as laser powder bed fusion (LPBF) or electron beam melting (EBM) allow near-net-shape production of TiAl components. However, TiAl parts produced by LPBF or EBM do not reach the structural integrity and mechanical properties of forged parts. The main post-processing step of TiAl parts made by AM is hot-isostatic pressing (HIP), which has several disadvantages such as a long process time and an undesired coarsening of the microstructure. High-performance TiAl components such as turbine blades are thus still produced by isothermal forging of preforms made by investment casting and hot isostatic pressing. Due to the low workability of TiAl alloys, often more than one deformation step is required, and the tooling costs and the low material yield make this process chain very expensive. This paper explores the feasibility of combining AM with isothermal forming in order to replace HIP by thermomechanical post-processing. The hot forming behavior of the Ti-43.5Al-4Nb-1Mo-0.1B (at. %) TNM-B1 alloy, manufactured by EBM and LPBF, is analyzed concerning the evolution of the voids and grain sizes and compared to hot isostatic pressing. It is shown that the fine microstructure produced by AM yields a much lower flow stress and faster globularization kinetics in comparison to conventional cast and HIPed material. While HIP experiments are shown to significantly coarsen the microstructure, isothermal hot working is shown to convert the AM microstructure to a dense and refined state. Moderate hot working with total strains of ∼1 of AM pre-forms may thus serve as an alternative process chain to conventional large strains forging of cast pre-forms and to AM + HIP in the series production of high-performance TiAl components.

Additive manufacturing of TiAl-based alloys

The ever-increasing demand for developing lightweight, high-temperature materials that can operate at elevated temperatures is still a subject of worldwide research and TiAl-based alloys have come to the fore. The conventional methods of manufacturing have been used successfully to manufacture the TiAl-based alloy. However, due to the inherent limitations of the conventional methods to produce large TiAl components with intricate near-net shapes has limit the widespread application and efficiency of the TiAl components produced using conventional methods. Metal additive manufacturing such as Electron Beam Melting technology could manufacture the TiAl alloys with intricate shapes but lack geometrical accuracy. Laser powder bed fusion (LPBF) technology could manufacture the TiAl-based alloys with intricate shapes with geometrical accuracy. However, the inherent high rate of heating and cooling mechanisms of the LPBF process failed to produce crack-free TiAl components. Various preheating techniques have been experimented, to reduce the high thermal gradient and residual stress during the LPBF process that causes the cracking of the TiAl components. Although these techniques have not reached industrial readiness up to now, encouraging results have been achieved.

Research progress on refractory composition and deformability of shell molds for TiAl alloy castings

At present, most TiAl components are produced by an investment casting process. Environmental and economic pressures have, however, resulted in a need for the industry to improve the current casting quality, reduce manufacturing costs and explore new markets for the process. Currently, the main problems for investment casting of TiAl alloys are cracks, porosities, and surface defects. To solve these problems, many studies have been conducted around the world, and it is found that casting defects can be reduced by improving composition and properties of the shell molds. It is important to make a summary for the related research progress for quality improvement of TiAl castings. So, the development on refractory composition of shell molds for TiAl alloy investment castings was reviewed, and research progress on deformability of shell mold for TiAl alloy castings both at home and abroad in recent years was introduced. The existing methods for deformability characterization and methods for improving the deformability of shell molds were summarized and discussed. The updated advancement in numerical simulation of TiAl alloy investment casting was presented, showing the necessity for considering the deformability of shell mold during simulation. Finally, possible research points for future studies on deformability of shell mold for TiAl alloy investment casting were proposed.

A Combined Electromagnetic Levitation Melting, Counter‐Gravity Casting, and Mold Preheating Furnace for Producing TiAl Alloy

In this work, the authors describe the development of TiAl castings over a wide range of approaches. To overcome casting defects and cracks that appear in TiAl castings, a novel furnace is designed and constructed. The design combines induction skull melting, counter‐gravity casting, and mold heating, which facilitates both filling and microstructure formation via a controllable process. This aim is to improve shaping capabilities and microstructural control for TiAl castings. Melting and casting experiments on TiAl alloys with a nominal composition corresponding to Ti–48Al–2Cr–2Nb (at%) are carried out and discussed. X‐ray examinations indicate that the shaping of the TiAl components dose not contain macro casting defects, validating the advantages of this technique. The results are of interest to researchers devoted to technical innovations and modifications for TiAl casting at the industrial scale.

Effect of thermal stresses on the microstructure of the continuous cooling TiAl alloys

A series of continuous cooling tests were performed on TiAl alloys using a Gleeble3500 machine to investigate the effect of thermal stresses on the microstructure. The results show that macroscopic thermal stresses promote correlated nucleation of γ lamellae. The trend of the dominance of one twin-group γ variants in local regions is weakened, and the γ/γ interfaces tend to be true twin and pseudotwin boundaries rather than 120° rotational faults under macroscopic thermal stresses. Meanwhile, thermal-induced deformation generated under the effect of both microscopic and macroscopic thermal stresses results in numerous low angle grain boundaries (LAGBs) and dislocations. The LAGBs and dislocations distribute heterogeneously among lamellar colonies and phases. No mechanical twins are observed due to the low strain and low strain rate characteristics of the thermal-induced deformation. These findings could shed light on understanding and preventing the cracking of TiAl components during cooling process.

Titanium aluminide (Ti-48Al) powder synthesis, size refinement and sintering

Titanium aluminide based alloys have shown significant potential in high temperature applications, but the high production cost of TiAl considerably limits its utilisation. Although the use of powder metallurgy processes can reduce the cost by minimising post-machining, an economical powder production route is still required. Therefore, in the present study a pre-alloyed Ti-48Al powder is developed using an elemental Ti and Al powder blend prepared using a simple vacuum heat treatment. A formation model of the intermetallic phases (i.e. TiAl, Ti3Al, TiAl2, TiAl3) during powder synthesis is proposed. In order to improve the sinterability, various milling methods (i.e. ball, attrition and shatterbox milling) are examined to reduce the particle size. The sintered microstructures, particularly the two-phased (α2-Ti3Al γ-TiAl) lamellar structures are also investigated. Improved densification is achieved at 1300°C, held for 2h, using the manufactured powder, compared to the elemental powder blend (∼55%). With higher sintering temperatures or longer hold periods, increased density TiAl components are possible.

Effect of substrate preheating on the texture, phase and nanohardness of a Ti–45Al–2Cr–5Nb alloy processed by selective laser melting

The crystallographic texture, phase composition and evolution, and nanohardness of a Ti–45Al–2Cr–5Nb alloy processed by selective laser melting (SLM) at various substrate preheating temperatures were investigated. The α2 phase decreases whereas the γ and B2 phases increase with increasing preheating temperature, and the relationship in orientation between B2, α2 and γ phases is described as follows: 112̅0α2//111γ//111B2. The SLM-processed TiAl alloy shows much higher nanohardness than its traditional casting counterpart, which increases with the increase of preheating temperature. The findings would be a valuable reference for fabricating TiAl components with acceptable texture, phase compositions and nanohardness by SLM.

Crystal orientation, crystallographic texture and phase evolution in the Ti–45Al–2Cr–5Nb alloy processed by selective laser melting

A TiAl-based alloy, Ti–45Al–2Cr–5Nb (at.%), has been processed by selective laser melting (SLM) using different energy density inputs. The experimental results show that when the energy density input increased from 250J/mm3 to 350J/mm3, the crystallographic texture varied from a strong (0001) orientation to a combination of (0001), 101̅1 and 112̅1 orientations. The SLM-processed TiAl alloy are dominated by high-angle (>15°) grain boundaries (HAGBs) and α2 (Ti3Al) phase. The contents of HAGBs and α2 are 92.8% and 90% respectively at the maximum density input of 350J/mm3. Moreover, a small amount of γ (TiAl) and B2 phases in a range of several hundred nanometers are uniformly distributed within the α2 matrix. The phase evolution mechanism in the SLM-processed TiAl alloy can be as follows: (210)β transformed to 202̅0α2 and (110)γ, and then the residual B2 and the incompletely transformed γ phase homogeneously distributed in the α2 phase matrix. The orientation relationship between B2, α2 and γ phases observed via HRTEM can be expressed as: 111B2//11̅0γ//112̅0α2. Those observations and discussions provide a deep insight into the microstructure characteristics and phase evolution in the SLM-processed TiAl alloy, and the findings would be a valuable reference for optimizing the energy density input in SLM to fabricate TiAl components with acceptable grain structure and phase compositions.

Development and evaluation of a geographic information retrieval system using fine grained toponyms

Geographic information retrieval (GIR) is concerned with returning information in response to an information need, typically expressed in terms of a thematic and spa- tial component linked by a spatial relationship. However, evaluation initiatives have of- ten failed to show significant differences between simple text baselines and more complex spatially enabled GIR approaches. We explore the effectiveness of three systems (a text baseline, spatial query expansion, and a full GIR system utilizing both text and spatial in- dexes) at retrieving documents from a corpus describing mountaineering expeditions, cen- tred around fine grained toponyms. To allow evaluation, we use user generated content (UGC) in the form of metadata associated with individual articles to build a test collec- tion of queries and judgments. The test collection allowed us to demonstrate that a GIR- based method significantly outperformed a text baseline for all but very specific queries associated with very small query radii. We argue that such approaches to test collection development have much to offer in the evaluation of GIR.

Modeling of TiAl Alloy Grating by Investment Casting

The investment casting of TiAl alloys has become the most promising cost-effective technique for manufacturing TiAl components. This study aimed to investigate a series of problems associated with the investment casting of TiAl alloys. The mold filling and solidification of this casting model were numerically simulated using ProCAST. Shrinkage porosity was quantitatively predicted by a built-in feeding criterion. The results obtained from the numerical simulations were compared with experiments, which were carried out on Vacuum Skull Furnace using an investment block mold. The investment casting of TiAl grating was conducted for verifying the correctness and feasibility of the proposed method. The tensile test results indicated that, at room temperature, the tensile strength and elongation were approximately 675 MPa and 1.7%, respectively. The microstructure and mechanical property of the investment cast TiAl alloy were discussed.