Dynamic Softening Mechanisms and Microstructure Evolution of TB18 Titanium Alloy during Uniaxial Hot Deformation

In this study, isothermal compression tests of TB18 titanium alloy were conducted using a Gleeble 3800 thermomechanical simulator for temperatures ranging from 650 to 880 °C and strain rates ranging from 0.001 to 10 s−1, with a constant height reduction of 60%, to investigate the dynamic softening mechanisms and hot workability of TB18 alloy. The results showed that the flow stress significantly decreased with an increasing deformation temperature and decreasing strain rate, which was affected by the competition between work hardening and dynamic softening. The hyperbolic sine Arrhenius-type constitutive equation was established, and the deformation activation energy was calculated to be 303.91 kJ·mol−1 in the (α + β) phase zone and 212.813 kJ·mol−1 in the β phase zone. The processing map constructed at a true strain of 0.9 exhibited stability and instability regions under the tested deformation conditions. The microstructure characteristics demonstrated that in the stability region, the dominant restoration and flow-softening mechanisms were the dynamic recovery of β phase and dynamic globularization of α grains below transus temperature, as well as the dynamic recovery and continuous dynamic recrystallization of β grains above transus temperature. In the instability region, the dynamic softening mechanism was flow localization in the form of a shear band and a deformation band caused by adiabatic heating.

Microstructures and Mechanical Properties of Hybrid, Additively Manufactured Ti6Al4V after Thermomechanical Processing

In the present study, we propose a hybrid manufacturing route to produce high-quality Ti6Al4V parts, combining additive powder laser directed energy deposition (L-DED) for manufacturing of preforms, with subsequent hot forging as a thermomechanical processing (TMP) step. After L-DED, the material was hot formed at two different temperatures (930 °C and 1070 °C) and subsequently heat-treated for stress relief annealing. Tensile tests were performed on small sub-samples, taking into account different sample orientations with respect to the L-DED build direction and resulting in very good tensile strengths and ductility properties, similar or superior to the forged material. The resulting microstructure consists of very fine grained, partially globularized alpha grains, with a mean diameter ~0.8–2.3 µm, within a beta phase matrix, constituting between 2 and 9% of the sample. After forging in the sub-beta transus temperature range, the typical L-DED microstructure was no longer discernible and the anisotropy in tensile properties, common in additive manufacturing (AM), was significantly reduced. However, forging in the super-beta transus temperature range resulted in remaining anisotropies in the mechanical properties as well as an inferior tensile strength and ductility of the material. It was shown, that by combining L-DED with thermomechanical processing in the sub-beta transus temperature range of Ti6Al4V, a suitable microstructure and desirable mechanical properties for many applications can be obtained, with the advantage of reducing the material waste.

Analysis of Deformation Behavior and Microstructure Changes for α/β Titanium Alloy at Elevated Temperature

In this paper, the isothermal compressive behavior of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium alloy was investigated on a Gleeble-3500 simulator in the temperature range from 1073 to 1373 K at an interval of 50 K (while the phase transus temperature is approximately 1273 K) and the strain rate range of 0.001–10 s−1. Microstructure evolution and deformation behavior were investigated. The typical flow softening behavior during deformation is observed, which can be explained by the deformation heating effect and microstructure changes. The deformation heating effect is influenced by strain rate and deformation temperature, and it increases with the increasing strain rate and decreasing deformation temperature. In the α + β phase field, the fractions of the primary α phase decrease with the increase of deformation temperature and strain rate. In this case, dynamic recovery may be the main mechanism for microstructure evolution based on the electron back-scatter diffraction (EBSD) analysis. The fully phase transformation occurs above the β transus temperature, which is governed by Burgers orientation relations. The Zener–Hollomon parameter with an exponent-type equation was used to intuitively describe the effects of the deformation temperatures and strain rates on the flow stress behaviors. Furthermore, the influence of strain was incorporated in the constitutive analysis. A fourth-order polynomial was ideally matched to represent the influence of strain. In consequence, the constitutive equation of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium alloy including the phase transus and compensation of the strain was developed based on the experimental results throughout the deformation process. The results indicated that the correlation coefficient (R), root mean square error (RMSE), and the average absolute relative error (AARE) were calculated to be 0.987, 3.585 MPa, and 9.62% in the single-phase region and 0.979, 18.78 MPa, and 9.16% in the duplex-phase region, respectively. Hence, the constitutive model proposed in this research can provide accurate and precise theoretical prediction for the flow stress behavior of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium alloy.

Effect of Cooling Rates during a Double Stage Solution Treatment on the Massive Phase Formation in Ti-6Al-4V Alloy

The effect of cooling rates during a double stage solution treatment (DSST) on the volume fraction of the massive phase (αm) in Ti-6Al-4V alloy was successfully confirmed in the present study. The morphology of Ti-6Al-4V alloy depends on the cooling rates during the cooling from the β region. The αm, which has a transformation characteristic between martensite (α′) and α diffusion, is reported to be a potential method for obtaining a fine lamellar α/β by thermal decomposition. The different fraction of αm was found after DSST with the first stage was conducted above the β-transus temperature at 1050 °C, followed by second annealing at different temperatures in the α+β region. It was found that the formation of αm exists in a specific temperature region. A longer period in this region, which was calculated based on different cooling rates during DSST, will increase the fraction of αm in the specimen. All specimens after DSST contain αm with the α width of approximately 1μm and white-dot particles, which is predicted to be V-enriched precipitates. The DSST can be a potential method for producing a high fraction of αm, which can be thermally decomposed into a fine lamellar α/β, introducing a Ti-6Al-4V alloy with superior mechanical properties.

Development of CALPHAD database for both Ti- and TiAl-based alloys

Phase diagram calculations and phase transformation simulations have been widely employed to materials design and process optimization. Recent development of a 27-element thermodynamic database (TCTI2) together with a compatible mobility database (MOBTI3) for Tiand TiAl-based alloys is reported. The TCTI2 database has been developed in a systematic way in order to cope with the complexity in phase relations and phase transformations in both conventional Ti-based and newly emerging TiAl-based alloys. It can be used with Thermo-Calc and the add-on Diffusion Module (DICTRA) and Precipitation Module (TC-PRISMA) for calculations of multi-component alloys. Feasible calculations are ranging from the traditional stable and metastable phase equilibria (β-transus temperature, evolution of phase fractions vs. temperature, martensitic temperature, etc.) to some thermophysical properties (density, thermal expansion, etc.). Using a combination of TCTI2 and MOBTI3, one can simulate diffusion-controlled phase transformation and precipitation kinetics. The intermetallic TiAl-based alloys are known for their own complexities and the present modeling for α2-Ti3Al and γ-TiAl based alloys will be discussed. Typical calculated examples for various properties in titanium alloys are presented with the emphasis on validation against experimental observations in multi-component commercial alloys. This database is expected to efficiently support further development of Ti- and TiAl-based alloys, as well as to promote process simulations with accurate prerequisites.

Electropulsing-Induced α to β Phase Transformation of Ti–6Al–4V

Electrically assisted forming (EAF) has been increasingly utilized as an effective auxiliary processing technology to improve the formability of hard-to-deform metals. Previous works have revealed that the phase transformation of titanium alloys subjected to electropulsing treatment (EPT) can occur at a lower temperature and in a remarkably shorter time compared with those subjected to the traditional heating treatment (THT). However, an in-depth experimental verification and further analysis is still missing so far. Therefore, to characterize the specific effects of EPT on α → β transformation process, both EPT and THT experiments were conducted on Ti–6Al–4V sheet specimens. After that, a calculation method based on the analysis of optical microscopic (OM) metallographs was developed to characterize the amount of phase transformation in EPT and THT. According to the results, it was found that the pulse current can significantly reduce the phase transus temperature and accelerate the transformation process in EPT compared with that in THT. Furthermore, the specific effects of EPT on transus temperature and phase transformation rate were investigated in detail. Based on that, the transformation kinetics of the electropulsing-induced α → β phase transformation was also analyzed using the Johnson–Mehl–Avrami model. It is revealed that the activation energies of both nucleation and growth of phase transformation are reduced by electric current. Hence, the phase transformation can start at a lower temperature and with a higher rate in EPT. The mechanism behind the effects was also discussed in detail in the present work.