Improving the properties of turbochargers by the application of TiAl alloys

Bakalářská práce obsahuje popis vlastností slitin TiAl a jejich aplikace se zaměřením na zlepšování vlastností turbodmychadel v porovnání s tradičními materiály. Dále je zde také uvedena technologie výroby TiAl slitin a jejich odlitků.This bachelor thesis contains characterization of TiAl alloys, theirs applications and focusing on improving of the turbocharges properties, compared with conventional materials. Furthermore there is also included manufacturing process of TiAl alloys and castings.

Effect of Boron Content and Cooling Rate on the Microstructure and Boride Formation of β-Solidifying γ-TiAl TNM Alloy

Boron is a unique and popular grain refiner element in cast titanium aluminide (TiAl) alloys, as it helps to improve mechanical properties if properly alloyed. However, the formation mechanism of different types of borides in cast TiAl alloys is not yet clearly understood. This study seeks to correlate the chemical composition and cooling rate during solidification of cast TiAl alloys, with the type of boride precipitated and the resulting microstructure. Several β-solidifying γ-TiAl alloys of the TNM family were cast, alloying boron to a starting Ti-44.5Al-4Nb-1Mo-0.1B (at.%) alloy. The alloys were manufactured with an induction skull melting furnace and poured into a stepped 2, 4, 8 and 16 mm thickness mold to achieve different cooling rates. On one hand, the results reveal that boron contents below 0.5 at.% and cooling rates during solidification above 10 K/s promote the formation of detrimental ribbon borides. On the other hand, boron contents above 0.5 at.% and cooling rates during solidification below 10 K/s promote the formation of a refined microstructure with blocky borides. Finally, the formation mechanisms of both ribbon and blocky borides are proposed

Hot Corrosion and Oxidation Behaviour of TiAl Alloys during Fabrication by Laser Powder Bed Additive Manufacturing Process

This research paper summarises the practical relevance of additive manufacturing with particular attention to the latest laser powder bed fusion (L-PBF) technology. L-PBF is a promising processing technique, integrating intelligent and advanced manufacturing systems for aerospace gas turbine components. Some of the added benefits of implementing such technologies compared to traditional processing methods include the freedom to customise high complexity components and rapid prototyping. Titanium aluminide (TiAl) alloys used in harsh environmental settings of turbomachinery, such as low-pressure turbine blades, have gained much interest. TiAl alloys are deemed by researchers as replacement candidates for the heavier Ni-based superalloys due to attractive properties like high strength, creep resistance, excellent resistance to corrosion and wear at elevated temperatures. Several conventional processing technologies such as ingot metallurgy, casting, and solid-state powder sintering can also be utilised to manufacture TiAl alloys employed in high-temperature applications. This chapter focuses on compositional variations, microstructure, and processing of TiAl alloys via L-PBF. Afterward, the hot corrosion aspects of TiAl alloys, including classification, characteristics, mechanisms and preventative measures, are discussed. Oxidation behaviour, kinetics and prevention control measures such as surface and alloy modifications of TiAl alloys at high temperature are assessed. Development trends for improving the hot corrosion and oxidation resistance of TiAl alloys possibly affecting future use of TiAl alloys are identified

Microstructural Evolution of TiAl-Intermetallic Alloys Containing W and B

The TiAl alloys have been considered as promising candidates for structural-materials applications at around 8000C. In this work, new TiAl alloys, containing tungsten (W) and boron (B), have been developed. Using the scanning-electron microscopy (SEM), electron-microprobe, and transmission-electron microscopy (TEM), the effects of W and B on the microstructural evolution of TiAl alloys, including the colony size and lamellar spacing, were analyzed. It is important to point out that fine uniform microstructures (with the colony size smaller than 50 mm) can be conveniently developed after Hot-Isostatic Pressing (HIP) the as-cast alloys at 1,2500C and 150 MPa for 4 h without the deformation process. It was found that tungsten prefers to react with boron to form borides, and disperses mainly along grain boundaries, and occasionally inside grains. With the increase of the tungsten content, the microstructure can be further refined. Heat treatments at temperatures ranging from 9000C to 1,3100C were conducted. The addition of tungsten can restrain the grain coarsening and stabilize the microstructure up to 1,2800C by hindering the migration of grain boundaries at high temperatures. It is also noteworthy that the beta phase, a high-temperature ductile phase, forms when the tungsten content exceeds 0.4 atomic percent (at.%). The α phase transus temperature, Tα, has been determined through differential-thermal analyses (DTA) and further proved by the investigation of the microstructural changes during various heat treatments. Different microstructures meeting desirable needs can be developed through heat treatments beyond and below the α phase transus temperature. Mechanical testing, such as hardness experiments, has been conducted on the alloys. The addition of the alloying element, tungsten, increases the hardness of TiAl alloys by the solution strengthening and refinement of grain sizes

Recent progress in the high-cycle fatigue behaviour of γ-TiAl alloys

The high cycle fatigue (HCF) properties of γ-TiAl alloys are reviewed, particularly with regards to the deformation mechanisms active in the near-threshold cyclic loading regime. By examining the influence of lamellar orientation and thickness on the HCF threshold, in addition to more conventional microstructural considerations such as the grain size or the volume fraction of lamellar colonies, factors to improve the γ-TiAl microstructure for HCF are assessed. Finally, experimental methods and loading strategies are surveyed to identify techniques for improving HCF testing of γ-TiAl alloys. In this, we consider both the conservativism of differing approaches, and the possibility to measure with suitable resolution the local mechanical behaviour under HCF of the lamellar γ-TiAl microstructure

Microstructural Control and Alloy Design of the Ti-Al-Nb-W-B Alloys

The TiAl alloys have been considered as promising candidates for structural-materials applications at around 8000C. The major concern for the structural use of the TiAl alloys is their low ductility and poor fracture resistance at ambient temperature. Refining the grain size of the TiAl alloys can be an effective way to improve the mechanical properties of the alloys. In this work, new TiAl alloys, containing tungsten (W) and boron (B), have been developed. Using the scanning-electron microscopy (SEM), electron-microprobe, and transmission-electron microscopy (TEM), the effects of W and B on the microstructural evolution of TiAl alloys, including the grain size and lamellar spacing, were analyzed. It is important to point out that fine uniform microstructures (with the grain size smaller than 50 μm) can be conveniently developed after Hot-Isostatic Pressing (HIP) the as-cast alloys at 1,2500C and 130 MPa for 5 h, produced through arc-melting. With the increase of the tungsten content, the microstructure of the TiAl-based alloy can be refined. The addition of tungsten can restrain the grain coarsening and stabilize the microstructure up to 1,2800C by hindering the migration of grain boundaries at high temperatures. It is also noteworthy that the beta phase, a high-temperature residual phase, forms when the tungsten content exceeds 0.4 atomic percent (at.%). The α-phase transus temperature, Tα, has been determined through differential-thermal analyses (DTA) and further proved by the investigation of the microstructural changes during various heat treatments. Different microstructures meeting desirable needs can be developed through heat treatments beyond and below the α-phase transus temperature. The mass production of the TiAl-based alloy, with the optimal composition developed through arc-melting, has been made through a magnetic-floatation-melting method. A comparison in the microstructures of the mass production and arc-melting small production has been made. A larger grain size, with a significant amount of the β phase, has been observed in the large ingot. Heat treatments have been conducted in order to obtain desirable microstructures and to minimize the amount of the β phase in the alloy. Hot forging is another effective method chosen to refine the grain size and eliminate the β phase in the alloy. Hot simulation has been conducted to the alloy in order to obtain the optimal parameters for the hot deformation of the TiAl-based alloy. Mechanical testing, such as hardness measurements and tensile tests, have been performed on the alloys. The addition of the alloying element, such as tungsten, increases the hardness of TiAl alloys by the solution strengthening and refinement of grain sizes. The room-temperature ductility and yield strength of the alloy have been enhanced through the alloy development and heat treatments. A ductility as high as 1.9% has been obtained in the newly-developed TiAl-based alloy at ambient temperature in the heat-treated cast samples. This result demonstrates the potential use of the cast material for structural applications of TiAl-based alloys with controlled compositions and optimal heat treatments

Optimizing 3d printed metallic object’s postprocessing : a case of gamma‐tial alloys

Gamma‐TiAl (γ‐TiAl) alloys can be used in high‐end products relevant to the aerospace, defense, biomedical, and marine industries. Fabricating objects made of γ‐TiAl alloys needs an additive manufacturing process called Electron Beam Melting (EBM) or other similar processes because these alloys are difficult‐to‐cut materials. An object fabricated by EBM exhibits poor surface finish and must undergo postprocessing. In this study, cylindrical specimens were fabricated by EBM and post‐processed by turning at different cutting conditions (cutting speed, depth of cut, feed rate, insert radius, and coolant flowrate). The EBM conditions were as follows: average powder size 110 μm, acceleration voltage 60 kV, beam current 10 mA, beam scanning speed 2200 mm/s, and beam focus offset 0.20 mm. The surface roughness and cutting force were recorded for each set of cutting conditions. The values of the cutting conditions were set by the L36 Design of Experiment approach. The effects of the cutting conditions on surface roughness and cutting force are elucidated by constructing the possibility distributions (triangular fuzzy numbers) from the experimental data. Finally, the optimal cutting conditions to improve the surface finish of specimens made of γ‐TiAl alloys are determined using the possibility distributions. Thus, this study’s outcomes can be used to develop intelligent systems for optimizing additive manufacturing processes. © 2021 by the authors. Licensee MDPI, Basel, Switzerland

Durability Assessment of TiAl Alloys

The durability of TiAl is a prime concern for the implementation of TiAl into aerospace engines. Two durability issues, the effect of high temperature exposure on mechanical properties and impact resistance, have been investigated and the results are summarized in this paper. Exposure to elevated temperatures has been shown to be detrimental to the room temperature ductility of gamma alloys with the most likely mechanisms being the ingress of interstitials from the surface. Fluorine ion implantation has been shown to improve the oxidation resistance of gamma alloys, and ideally it could also improve the environmental embrittlement of high Nb content TiAl alloys. The effect of F ion implantation on the surface oxidation and embrittlement of a third generation, high Nb content TiAl alloy (Ti-45Al-5Nb-B-C) were investigated. Additionally, the ballistic impact resistance of a variety of gamma alloys, including Ti-48Al-2Cr- 2Nb, Ti-47Al-2Cr-2Nb, ABB-2, ABB-23, NCG359E, 95A and Ti-45Al-5Nb-B-C was accessed. Differences in the ballistic impact properties of the various alloys will be discussed, particularly with respect to their manufacturing process, microstructure, and tensile properties

Interdiffusion Behavior of Aluminide Coated Two-Phase α2-Ti3Al/γ-TiAl Alloys at High Temperatures

Lower density materials of TiAl based intermetallic alloys have recently attracted intensive attention for the replacement of nickel-based superalloys used at high temperatures. As aluminium-rich titanium aluminide intermetallic compounds are normally brittle, two-phase α2-Ti3Al/γ-TiAl alloys have been developed. To increase the corrosion resistance of these alloy systems, an aluminide coating of TiAl3 layer is normally applied. During operation at high temperatures, however, interdiffusion between the coating and the alloy substrate can occur and decrease the TiAl3 layer thickness of the coating. The effects of temperature exposure on the growth of the TiAl2 interdiffusion zone layer on two-phase α2-Ti3Al/γ-TiAl alloys with a chemical composition of Ti-47Al-2Nb-2Cr-0.5Y-0.5Zr are presented in this paper. The exponents for kinetics and rate constant of the TiAl2 interdiffusion layer growth of this multi-component system were found under variation of temperature. The results were compared with those from other researchers

Innovative materials for high temperature structural applications: 3rd Generation γ-TiAl fabricated by Electron Beam Melting

In the aeronautics industry, the propulsion systems stand among the most advanced and critical components. Over the last 50 years, gas turbine aeroengines were subjected to intensive research to increase efficiency and reduce weight, noise and harmful emissions. Together with design optimization, breakthrough in materials science for structural applications triggered the development of the most advanced gas turbine engines. For low temperatures, basically ahead of the combustion section, lightweight Ti alloys are preferred for their good mechanical properties. For high temperatures instead, Ni-based superalloys exhibit outstanding properties up to very high temperatures despite a rather high material’s density. Research have focused on enhancing to the maximum the potential of materials in gas turbine engines. According to the application, the components experience various mechanical and environmental constraints. Special designs, manufacturing process, material compositions and protective coatings have been developed to push the limits of advanced materials. Nowadays, the attention is focused on innovative materials to replace the existing Ti and Ni based alloys leading to substantial benefits. Light weight composite materials in particular were found very attractive to replace some components’ Ti alloys. At higher temperatures, it is of great interest to replace Ni-based superalloys by materials with lower density and/or higher temperatures applications, which in turn would lead to substantial weight reduction and increase efficiency. At the highest temperatures range, in particular in the combustion chamber and high pressure turbine sections, ceramic based materials offer promising balance of properties. Research are dedicated to overcome the drawbacks of ceramics for such structural applications, and in particular their brittle fracture behavior, by addition of reinforcing fibers. At lower temperatures range, TiAl based intermetallics emerged as very promising materials at half the density of Ni-based superalloys. Significant weight reduction could be achieved by the introduction of TiAl based alloys for rotating components of the compressor and low pressure turbine. 2nd generation γ-TiAl alloys were lately introduced in GE’s GEnx and CFM’s LEAP engines. The present work concerns the fabrication by the additive manufacturing technique Electron Beam Melting of 3rd generation γ-TiAl alloys for high temperatures application in gas turbine aeroengines. EBM, building parts layer by layer according to CAD, offers many advantages compared to other manufacturing processes like casting and forging. Reported by Avio, 2nd generation γ-TiAl alloys have been successfully fabricated by EBM. To increase the material’s potential, the production of 3rd generation γ-TiAl alloys Ti-(45-46)Al-2Cr-8Nb was therefore studied. The optimization of the EBM parameters led to high homogeneity and very low post-processing residual porosity ≤ 1%. The fine equiaxed microstructure after EBM could be tailored towards the desired mechanical properties by simple heat treatment, from equiaxed to duplex to fully lamellar. In particular, a duplex microstructure composed by about 80 % lamellar grains pinned at grain boundaries by fine equiaxed grains was obtained after heat treatment slightly over the α transus temperature. The study showed that addition of a higher amount of Nb significantly increased the oxidation resistance of the material, thus increasing the application temperature range of these γ-TiAl alloys