Ti Al Si Alloys Research Articles

Modeling of temperature distribution and clad geometry of the molten pool during laser cladding of TiAlSi alloys

The temperature varying principal of the molten pool is crucial to coatings’ cross-section sizes during laser cladding process, which can influence the performance of the coatings. In this study, a modified three-dimensional (3D) transient finite element model was presented to simulate the temperature evolution and clad geometry of the molten pool of TiAlSi coatings. The difference of thermal properties in TiAlSi powders and TiAlSi alloys were distinguished to improve the precision of simulation results through theoretical calculation. The Gaussian body heat source model was established. A temperature selection judgment mechanism was utilized to compare the average temperature of the element and the melting point of the material to distinguish powder and alloy elements. An exponential distribution of energy attenuation along penetration direction was employed to simulate the laser beam deflected by the powder particles and the void among the particles to correct Gaussian heat source. The effect of different laser power and scan speeds on the cladding layer morphologies were investigated. The track depth and width calculated by simulation were analyzed by polynomial fitting curves. The results showed that the track depth and width were directly proportional to laser power, whereas inversely to scanning speed. Geometric dimensions of cladding layers at arbitrary laser power (600–1800 W) and scan speed (0.005–0.025 m/s) can be computed by polynomial fitting equation. Track depth was more impressed by scanning speed. The track width and longitudinal size of cross-section geometry from the simulation results and experiment measurement were matched to validate the presented model. The modified 3D transient finite element model is able to describe the geometry of single-track laser cladding.

Development of TiAl-Si Alloys-A Review.

This paper describes the effect of silicon on the manufacturing process, structure, phase composition, and selected properties of titanium aluminide alloys. The experimental generation of TiAl–Si alloys is composed of titanium aluminide (TiAl, Ti3Al or TiAl3) matrix reinforced by hard and heat-resistant titanium silicides (especially Ti5Si3). The alloys are characterized by wear resistance comparable with tool steels, high hardness, and very good resistance to oxidation at high temperatures (up to 1000 °C), but also low room-temperature ductility, as is typical also for other intermetallic materials. These alloys had been successfully prepared by the means of powder metallurgical routes and melting metallurgy methods.

High-temperature oxidation of Ti–Al–Si alloys prepared by powder metallurgy

The Ti–Al–Si alloys are promising materials for high-temperature use in automotive, aerospace or cosmic industry. The main advantages of these alloys are their low density (approximately 4 g/cm3), good oxidation resistance, and mechanical properties at elevated temperatures. Addition of silicon into the Ti–Al alloys improves the high-temperature behaviour and improves compactness and adhesion of the oxide layer. The resistance against oxidation can be effectively improved also by an appropriate technology of preparation. In this work, the high-temperature cyclic and isothermal oxidation resistance of the Ti–Al–Si alloys are described. The effect of powder metallurgy production route (reactive sintering, mechanical alloying, Spark Plasma Sintering) on high-temperature behaviour was compared. Cyclic and isothermal oxidation tests were carried out at 800 °C and 1000 °C, well above the air-operating limit for TiAl. It was confirmed, that the Ti–Al–Si alloys are resistant at temperature 800 °C, where only very thin oxide layer was formed. Mechanical alloying followed by Spark Plasma Sintering improved the high-temperature behaviour of these alloys, the oxide layer was even thinner.

High-Pressure Spark Plasma Sintering (HP SPS): A Promising and Reliable Method for Preparing Ti-Al-Si Alloys.

Ti–Al–Si alloys are prospective material for high-temperature applications. Due to low density, good mechanical properties, and oxidation resistance, these intermetallic alloys can be used in the aerospace and automobile industries. Ti–Al–Si alloys were prepared by powder metallurgy using reactive sintering, milling, and spark plasma sintering. One of the novel SPS techniques is high-pressure spark plasma sintering (HP SPS), which was tested in this work and applied to a Ti–10Al–20Si intermetallic alloy using a pressure of 6 GPa and temperatures ranging from 1318 K (1045 °C) to 1597 K (1324 °C). The low-porosity consolidated samples consist of Ti5Si3 silicides in an aluminide (TiAl) matrix. The hardness varied between 720 and 892 HV 5.

Microstructure formation and interface characteristics of directionally solidified TiAl-Si alloys in alumina crucibles with a new Y2O3 skull-aided technology

The microstructure evolution and interface characteristics of a directionally solidified Ti-43Al-3Si (at.%) alloy in an alumina (Al2O3) crucible with new Y2O3 skull-aided technology were investigated. The Y2O3-skull that is in contact with the TiAl-melt is relatively stable, which results in a more controlled reaction between the skull and the melt than in the case of an Al2O3 crucible is used. A thin reaction layer was formed between the mould and the melt through mutual diffusion. The layer thickness increased with increasing reaction time. The thickness of this layer was less than 80 μm for reaction times up to 5800 s. Y2O3 particles were not found in the specimen because the mould coating was prepared with fine Y2O3 powder without a binder, which prevented the Y2O3 particles splitting from the coating as a consequence of thermal physical erosion. The oxygen content of the TiAl-alloy increased with increasing reaction time. The total oxygen content of the solidified specimen was less than that of the specimen solidified in the Al2O3 crucibles. This new Y2O3 skull-aided technology is expected to improve the surface quality of TiAl-alloys and reduce the reaction between the crucible/mould and molten TiAl alloys during directional solidification processing with longer contact times.

Effect of reactive sintering conditions on microstructure of in situ titanium aluminide and silicide composites

In this work, the dependence of microstructure of in situ titanium aluminide and silicide composites on chemical composition and conditions of reactive sintering was studied. It was found that the rate of reactive sintering process increases with growing silicon content. In addition, heating rate strongly affects the structure of Ti–Al–Si alloys with lower content of silicon represented by TiAl23Si10 alloy. The structure consisting of homogeneously distributed Ti5Si3 particles in TiAl matrix can be obtained by rapid heating over 250 K min−1 in this material. TiAl15Si15 alloy forms the desired two phase TiAl–Ti5Si3 structure when a heating rate of at least 10 K min−1 and temperature over 750°C are applied. Replacement of aluminium and silicon powders by Al–Si alloy positively affects the reactive sintering process. The structure of these materials can be refined by milling of Al–Si master alloy powder to the <100 μm fraction, reducing the size of Ti5Si3 particles and porosity significantly.

High-temperature behaviour of Ti–Al–Si alloys produced by reactive sintering

Alloys formed by Ti–Al intermediary phases have low density and very good high-temperature oxidation resistance. Further improvement of high-temperature behaviour can be achieved by a suitable addition of silicon. In this work, Ti–Al–Si alloys containing 20 wt.% of aluminium and 10–20 wt.% of silicon produced by a reactive sintering technology were tested. Isothermal oxidation tests were carried out at 1000 °C in air. The oxidation rate was determined from weight gains caused by oxide formation. Microstructure and phase composition of the oxide layers were studied. Microstructure and hardness changes after various annealing durations were described.

Microstructural Characterization of Silicon Added Titanium Aluminide

Titanium aluminides intermetallic compounds have received great attention during the past decade, since they have the potential, in aircraft and automotive engines, to replace the high density Ni-base superalloys However, these intermetallics possess poor oxidation properties at high temperatures. Previous studies showed that protective alumina scale formation on γ-TiAl can be obtained by small additions (around 2 at.%) of Ag. In the present study, a number of cast Ti–Al–Si alloys were investigated in relation to transient oxide formation in air at 1300°C. After various oxidation times the oxide composition, microstructure and morphology were studied by combining a number of analysis techniques. The TiAl–Si alloys appear to form Al Ti and Si oxides. However, the formation of silicon oxide at the interface of base metal and scale slows down the oxidation rate significantly.

Preparation of Ti–Al–Si alloys by reactive sintering

In this work, Ti–Al–Si alloys were prepared by reactive sintering. The contents of silicon and aluminium ranged between 0–30 wt.% and 0–36 wt.%, respectively. Powders of titanium, aluminum, silicon and AlSi30 master alloy prepared by mechanical machining and/or milling were used for sintering. Powders were blended and pressed at room temperature. Sintering was carried out at 900 °C for 30 min. It was found that it is possible to produce compact materials with the structure corresponding to the as-cast materials when silicon is in the range of 15–20 wt.% and aluminium content between 8 and 30 wt.%. It was shown that these materials contain two phases—Ti5Si3 and TiAl. The central porosity can be reduced by induction heating.

Effects of Si and C additions on the thermal stability of directionally solidified TiAl–Nb alloys

The thermal stability of the lamellar microstructure in TiAl–Nb alloys containing Si and C has been investigated by partial melting experiments. The proper compositions, where the lamellar structure is stable enough to be used as a seed material are found for both TiAl–Nb–Si and TiAl–Nb–Si–C alloy systems. The lamellar microstructure of the Ti–44.5Al–3Nb–0.6Si–0.2C and Ti–45Al–2Nb–0.6Si–0.2C (at.%) alloys is indeed successfully aligned parallel to the growth direction by directional solidification (DS) with the seed material from the Ti–44.5Al–3Nb–0.6Si–0.2C alloy. The DS ingots of these alloys exhibit a good combination of room-temperature ductility (8.5%) and high-temperature yield strength (700 MPa at 800 °C). The proper composition range, where the lamellar structure is thermally stable enough to be used as the seed material for the TiAl–Nb–Si alloy is narrower than the corresponding range for previously investigated TiAl–Mo–Si and TiAl–Si alloys. The composition limits for such a region with the lamellar stability are discussed in terms of the critical volume fraction of the α (α 2) phase for the Al-rich side limit and the α-peritectic composition for the Al-lean side limit. It is concluded that Nb is not an effective element to improve the lamellar stability because upon alloying with Nb, no significant change in the volume fraction of the α phase is expected to occur from the shift of the α+γ/γ phase boundary.

Microstructure of rapidly solidified Ti–Al–V and Ti–Al–Si–V alloys

Abstract The microstructure and oxidation resistance of melt-spun Ti–25Al–(0–7.5)Si at.% ribbons containing up to 15 at.% V were studied. V additions do not change the course of the eutectic reaction in the Ti–Al–Si alloys and lead to two phase B2–(Ti, V)(Al, Si) and ζ-(Ti, V) 5 (Si, Al) 3 eutectic structures. TEM observations of hypoeutectic ingots and ribbons showed large antiphase domains (APDs) in the primary B2 dendrites. Rapid solidification by chill block melt spinning resulted in a shift in the coupled zone for the Ti–Al–Si–V eutectic alloys towards the ζ phase. Addition of 15 at.% V significantly decreases the oxidation resistance of Ti 3 Al–Si alloys at high temperatures.

Preparation of metastable precursors with different compositions of Ti–Al–Si by mechanical alloying

With the aim of synthesizing metastable precursors for in situ composite formation in Ti–Al–Si ternary system, five stoichiometric Ti–Al–Si alloys, i.e. Ti–17Al–18Si (all in at.%), Ti–21Al–14Si, Ti–14Al–14Si, Ti–21Al–21Si, Ti–14Al–21Si, were prepared by mechanical alloying (MA) of elemental powder blends for up to 100 h. The structural evolution in these powders was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that elemental powders were progressively transformed into nanocrystalline solid solution and amorphous phases during MA. However, for different contents of Al–Si elements, the behavior of solid-state transformation for the milled powders was quite different, thus distinct precursors could be obtained in each case.

The influence of hydrogen on tensile properties of V-base alloys developed in China

Several V–Cr–Ti–Al–Si alloys were studied with respect to their mechanical performances after exposure to hydrogen environments. The alloys were developed in China by using a melting process in a magnetic floating furnace. Most of the alloys had a high oxygen concentration from 700 to 1100 wppm since the forging and hot rolling were performed in air at elevated temperature. The results showed a synergistic effect of oxygen and hydrogen. The ductility loss correlated largely with the hydrogen concentration up to 113 wppm. Both V4Ti and V4TiSi showed a relatively higher resistance to hydrogen embrittlement in comparison with other alloys such as V4Cr4Ti and V4Ti3Al.

Effects of oxygen, hydrogen and neutron irradiation on the mechanical properties of several vanadium alloys

Several V–Cr–Ti–Al–Si alloys were studied or reviewed of their mechanical properties after their exposure to oxygen, hydrogen and neutron irradiation environments. The results showed that the oxidation kinetic of V4Cr4Ti alloy exhibited parabolic behavior in a flowing argon gas with low partial pressure oxygen over the temperatures from 450 to 600 °C. Most of the alloys showed ductility losses with the increasing oxygen concentration with an exception of V4Ti3Al. Strong sensitivity to hydrogen embrittlement was found for the alloys with the combination of high oxygen concentration and high mechanical strength. It was found that the effect of hydrogen and neutron irradiation on the uniform deformation capability caused by their hardening are equivalent with that of the strain hardening. Fine grain was recommended for the alloys to improve their properties against hydrogen embrittlement and neutron irradiation induced ductility loss.

Submicron-grained multiphase TiAlSi alloys: Processing, characterization, and microstructural design

Prealloyed powders of the intermetallic γ–TiAl phase and the ceramic Ξ–Ti5Si3phase were high-energy milled and hot-isostatically pressed (HIP) to produce silicide dispersed composite materials with grain sizes in the submicron and nanometer range. The amorphous state of the as-milled powders crystallizes via a multistep decomposition reaction during degassing at 440 °C and HIP. At a pressure of 200 Mpa HIP-temperatures as low as 750 °C are sufficient for a complete densification of the milled powder. The microstructure of the compacts is very homogeneous and consists of equiaxed γ–TiAl crystals and Ξ–Ti5(Si,Al)3particles. Depending on the silicon content, these particles are interspersed within the grain boundary network of the γ–TiAl phase or dispersed inside the γ grains. With respect to technical applications, submicron-grained composites are regarded as promising precursor materials that should allow for easy hot working in the as-prepared state as well as for high-temperature structural applications after a suitable transformation of the microstructure.

Creep deformation of TiAl-Si alloys with aligned γ/α 2 lamellar microstructures

Creep tests were conducted on directionally solidified TiAl-Si alloys to discern the effect of lamellar spacing and lamellar orientation on the high-temperature strength. Directional solidification of a Ti-43Al-3Si alloy was used to produce a model microstructure consisting of a single γ/α2 lamellar colony of controlled orientation for creep testing. Different heat treatments were used to vary the lamellar spacing and tensile creep tests were performed at 750 °C using applied stresses of 180, 210, and 240 MPa. In addition, ingots with large columnar grains of various lamellar orientation were also tested. The results clearly show the beneficial effect of microstructural control with the aligned microstructure having a much superior creep resistance. Furthermore, for specimens with aligned lamellar microstructure, decreasing the lamellar spacing greatly improved creep resistance, as both the primary creep strain and secondary creep rate are much smaller for materials with the fine lamellar spacing.

Phase formation during ball milling and subsequent thermal decomposition of Ti–Al–Si powder blends

With the aim of producing intermetallic composites for high-temperature structural applications, Ti–Al–Si alloys were prepared by mechanical alloying of elemental powder blends. The compositions were chosen to form intermetallic composites consisting of α 2-Ti 3Al, γ-TiAl and Ti 5Si 3 intermetallic compounds in the equilibrium state. The alloys were characterized by X-ray diffraction and differential scanning calorimetry. The metastable phases prepared after milling decompose by a two-step process upon annealing. Thereby four intermetallic metastable phases are formed in the composition range investigated. From the results of the phase analysis, partial Ti–Al–Si phase diagrams for the as-milled state, the intermediate states and the final state are derived. The stability of the composite microstructure is investigated by X-ray diffraction and transmission electron microscopy.

Directional solidification and microstructural control of the [formula omitted] lamellar microstructure in TiAlSi alloys

Composite microstructures were grown from TiAlSi alloys with the gamma/alpha2 lamellar microstructure aligned parallel to the growth direction by directional solidification techniques using a seed material. Within the composition range of 40–50 at% Al, the addition of Si to TiAl shifts the primary alpha region towards a much lower Al content. At a composition of Ti43Al3Si, either alpha or alpha2 can be stable, from melting temperature to room temperature and the orientation of the lamellar microstructure can be controlled using a seed material. In addition to the gamma/alpha2 lamellae, large Ti 5Si 3 particles which formed from the liquid, and much smaller Ti 5Si 3 particles which formed from an eutectoid reaction were observed. The room temperature mechanical properties were determined by tensile and three-point bend tests. From bend specimens oriented with the notch parallel to the lamellar microstructure, the Ti43Al3Si alloy was found to have a greater fracture toughness than a TiAl-PST crystal of the same orientation.