New model of microstructural evolution during isothermal forging of Ti—6Al—4V alloy

Abstract

An artificial neural network model of microstructural evolution, in particular grain size and volume fraction of α phase, has been established and trained with the help of experimental results obtained using Ti—6Al—4V alloy subjected to homogeneous deformation under high temperature. The measurement of microstructural variables was carried out using a Leica microscope. By comparison of the calculated results with the experimental data from training specimens and testing specimens, it has been verified that the proposed model can be applied to compute the microstructural evolution of formed Ti—6Al—4V alloy.     

Superplasticity and production of mechanically milled Ti–6Al–4V–0.5B

Abstract

Superplastic forming of conventional titanium alloy sheet is limited commercially by the relatively long cycle times imposed by the high temperatures and slow strain rates required. In order to minimise cycle times material with a fine grain size is required to allow either, an increase in the forming rate or a reduction in the deformation temperature. This study details the manufacture of Ti–6Al–4V–0.5B powder with a nanocrystalline grain size, which was produced by mechanical milling. The material was consolidated by hot isostatic pressing at a range of temperatures during which ～2.5 vol.-%TiB was formed by an in situ reaction between the titanium and boron. The TiB particles limited the growth of the grain size in the titanium from the nanocrystalline structure in the powder to sizes in the range 600 nm–4 µm after consolidation. The consolidated material was hot tensile tested at a range of temperatures and strain rates. A superplastic elongation of 310%was achieved when testing at 900°C at a strain rate of 6×10^-2 s^-1 compared with 220% for conventional Ti–6Al–4V sheet. However, extensive cavitation, induced by the presence of argon, occurred during high temperature deformation and limited the superplastic extensions achieved.     

Phenomenological and crystal plasticity approaches to describe the mechanical behaviour of Ti6Al4V titanium alloy

This paper presents a multiscale study of the quasi-static behaviour of a Ti6Al4V titanium alloy sheet. Tensile and compressive tests were carried out on specimens along several orientations from the rolling direction in order to characterise the material anisotropy. In parallel, X-Ray diffraction texture measurements were performed before and after deformation in tension. A phenomenological model (CPB06exn) and a multiscale crystal plasticity model (Multisite) were investigated to describe the mechanical behaviour of the tested material. The identification of the material parameters provides good predictions of the plastic anisotropy using both tensile and compressive data. The crystal plasticity model is in good agreement with the experiments in tension but it was observed that some improvements should be done to take into account the tension-compression asymmetry displayed by the material. Moreover both models lead to a good prediction of the Lankford’s coefficients and yield strength.     

Experimental and numerical investigation of anisotropic yield criteria for warm deep drawing of Ti–6Al–4V alloy

Accuracy of the finite element simulation of sheet metal forming is significantly dependent on the correctness of input properties and appropriate selection of material models. In this work, anisotropic yield criteria namely, Hill 1948, Barlat 1989, Barlat 1996, Barlat 2000 and Cazacu Barlat have been implemented for Ti–6Al–4V alloy at 400 °C. Material constants required for the yield criteria have been determined and deformation behavior in deep drawing process has been analyzed in finite element software. Also, deep drawing experiments on Ti–6Al–4V alloy have been performed at 400 °C to validate finite element simulation results. Further, comparison of yield criteria based on thickness distribution, earing profile, complexity in material parameter identification and computational time has shown Cazacu-Barlat to be well suited for deep drawing of Ti–6Al–4V alloy.     

Mechanical anisotropy and stress–strain rate characteristics in superplastic deformation of titanium alloys

Abstract

Uniaxial tension and compression tests have been carried out on two titanium based alloys, Ti–6Al–4V in the form of extruded tubes and forged plates and Ti–3Al–10V–2Fe sheets, to study anisotropic behaviour during superplastic deformation. The following were observed: (i) originally round cross-section became elliptical after deformation; (ii) the flow stresses and strain rates were dependent on the orientation of the specimens; and (iii) the strain anisotropy became less severe as the strain rate increased. These characteristics of anisotropy were related to the original microstructure (e.g. the mechanical fibring of the α grains) and the microstructural evolution during superplastic deformation. New constitutive equations for describing anisotropic superplastic deformation have been proposed to explain the effect of strain rate or stress on anisotropy.     

Characterisation of diffusion bonds formed between Ti–6Al–4V and titanium aluminide Super Alpha-2

Abstract

Isostatic diffusion bonds were produced between Ti–6Al–4V and Super Alpha-2 sheet materials at temperatures of 920–940°C and pressures of 6–10 MPa. The diffusion bonds were evaluated using optical microscopy, lap shear testing, and scanning electron microscopy of the lap shear fracture faces. The results of X-ray microanalysis carried out on the bond interface region showed that substantial diffusion of niobium and molybdenum had occurred from the Super Alpha-2 to the Ti–6Al–4V. This caused a local suppression of the β transus in the Ti–6Al–4V alloy and on subsequent cooling the β phase decomposed to give a microstructure of fine acicular α phase and retained β phase in the bond region.

MST/3421     

Identification of Johnson–Cook's Viscoplastic Model Parameters Using the Virtual Fields Method: Application to Titanium Alloy Ti6Al4V

Abstract: The identification of viscoplastic material parameters is addressed using a new powerful method: the virtual fields method (VFM). Contrary to classical procedures that are statically determined, the VFM is applied to heterogeneous mechanical fields. Without any hypotheses of homogeneity required, the exploitation of tests with the VFM is not limited to small levels of strains anymore and it can be taken advantage of the large amount of information available thanks to full‐field measurements. In the case of viscoplastic models, the characterisation of strain‐rate sensitivity with the VFM is attempted in this paper using only one test under high‐speed loading conditions, whereas several tests performed at different constant strain‐rates are required for the classical procedures. This article focuses on the development of the VFM for the characterisation of Johnson–Cook's (JC) viscoplastic model. To his aim a return‐mapping algorithm was developed according to the JC's model with an implicit Euler scheme implemented to integrate the constitutive relations. The whole viscoplastic behaviour of a Titanium alloy (Ti6Al4V) is successfully characterised by the VFM using only two tensile tests on notched flat specimens, with full‐field strain measurements by digital image correlation.     

Post-forming tensile properties of superplastic Ti–6Al–4V alloy

Abstract

A study has been made of the influence of uniaxial superplastic deformation on the ambient temperature tensile properties of Ti–6Al–4V sheet. Material was deformed to various strains up to 200% at temperatures from 850 to 970°C at strain rates in the range 1·1?18 × 10^{;amp;#x2212;4}s^?1 (0·7?11% min^?1). Tests were also performed on statically annealed material to separate the effects of high temperature exposure and superplastic deformation. Mechanical property changes were complex and depended on the relative contributions from the strengthening and softening mechanisms occurring during either superplastic deformation or heat cycling. Structural features influencing mechanical properties were phase size and morphology, dislocation density, and crystallographic texture. The strength after superplastic deformation was always less than that of as-received material but a significant reduction in strength was attributable to heat cycling. In some cases, the strength of the superplastically deformed material was greater than that after heat cycling.

MST/593     

Investigation on high temperature compression behaviour of hydrogenated Ti6Al4V alloy

Abstract

Isothermal compression of hydrogenated Ti6Al4V alloy was carried out on a Gleeble-1500D simulation tester at the strain rate 3×10^?3 s^?1 and high temperatures. Before the isothermal compression, a simplified thermohydrogen processing (THP) was used for Ti6Al4V. Attention was paid to the effect of THP on subsequent compression behaviour. The results show that hydrogen can effectively lower the flow stress and deformation temperature and enhance the strain rate sensitivity index (m value) for isothermal compression. The increasing amount of β phase and the ultrafine and equiaxial microstructure precipitated between the original α or β laths are the main reasons for the simplified THP to improve the formability of Ti6Al4V.     

Transient liquid phase diffusion bonding of continuous SiC fibre reinforced Ti–6AI–4V composite to Ti–6AI–4V alloy

Abstract

A continuous SiC fibre reinforced Ti–6Al–4V composite was diffusion bonded in transient liquid phase to Ti–6Al–4V alloy plate using Ti–Cu–Zr amorphous filler metal. Joint strength increased with bonding time up to 1·8 ks and reached the maximum value of 850 MN m^?2 which corresponded to 90% of the tensile strength of Ti–6Al–4V. The extent of deformation of Ti–6Al–4V in the vicinity of the bonding interface was small compared with that of solid diffusion bonding because of the low bonding pressure. The bonding layer had an acicular microstructure which was composed of Ti₂Cu and α titanium with dissolved zirconium. Brittle products such as (Ti, Zr )₅ Si₃ or (Ti, Zr )₅ Si₄ were formed at the interface between the SiC fibres and the filler metal. These products existed only at the end of fibres, in very small amounts, therefore joint strength was not significantly affected by the products.

MST/1989     

Post-forming tensile properties of superplastically bulge formed high strength α/β Ti–Al–Mo–Sn–Si alloy (IMI 550)

Abstract

The effect of biaxial superplastic deformation at 900°C on ambient temperature tensile properties and texture of high strength α/β titanium alloy sheet of nominal composition Ti–4Al–4Mo–2Sn–0·5Si (IMI 550) has been examined. Superplastic straining led to significant decreases in both proof stress and tensile strength values. Heat cycling studies on as received sheet material showed that the decreases in strength were in part due to the influence of temperature, but this had little effect on elongation. The further losses in strength after superplastic forming were attributable to grain growth. The decreases in elongation after superplastic deformation were believed to be due mainly to changes of tensile specimen geometry, while the more isotropic tensile behaviour which was observed was due to the gradual removal of the relatively low level of texture in the as received material. Application of the standard heat treatment to the as received IMI 550 sheet material led to increases in proof stress and tensile strength values of ~70 and ~170 MN m^?2, respectively, and to a slight decrease in elongation. Heat treatment of heat cycled and superplastically bulge formed specimens increased the proof stress and tensile strength values almost to the levels attained in the as received material.

MST/684     

A histomorphometric comparison of the muscular tissue reaction to stainless steel,pure titanium and titanium alloy implant materials

Thein vivo tissue reaction to titanium and titanium-based alloys using quantitative histomorphometry was investigated. According to the guidelines for biomaterials testing suggested by ISO, 2 mm × 6 mm cylindrical specimens of chemically pure (CP) Ti, TiO₂, electrolytically coated Ti, Ti6Al4V, TiO₂-coated Ti6Al4V, TiN physical vapour deposition-coated Ti6Al4V and Ti5Al2.5Fe were implanted in the paravertebral muscles of rats, for 1–52 weeks, 316L stainless steel being used as a control implant material. After PMMA embedding, electrochemical dissolution of the implants, microtome sectioning and Masson's trichrome staining, the tissue reaction was assayed using a semi-automatic method based on the digitization of both the encapsulating membrane contours and the different cell types located within it. All materials induced a close tissue reaction. There was no statistical difference between the tested materials regarding the time-evolution of the inflammatory cells. However, when comparing CP Ti with 316L, a significant difference was found in the fibrocyte kinetics: in the short term, fibrocyte densities were lower for 316L, while beyond 12 weeks, they exhibited higher values than CP Ti. To a lesser extent, a similar observation was made when comparing CP Ti with Ti5Al12.5Fe. No statistical difference was found in the comparison of CP Ti with Ti6Al4V. The membrane thickness was identical for all tested materials and appeared not to be time-dependent.     

HCF strength estimation of notched Ti–6Al–4V specimens considering the critical distance size effect

Generally not only the material but also the notch geometry affects the HCF strength. So, when evaluating the fatigue limit of notched Ti–6Al–4V specimens using the conventional theory of critical distance (TCD), a possible critical distance size effect was identified. In this paper, in order to overcome the critical distance size effect, Kt was introduced. And based on an assumption that the product of critical distance and Kt was constant, the TCD was modified to evaluate the HCF strength of notched Ti–6Al–4V specimens. The prediction results fell into an error interval of about ±15% using the modified TCD.     

Constitutive modeling and the effects of strain-rate and temperature on the formability of Ti–6Al–4V alloy sheet

The constitutive model considering the strain-rate and temperature effects was presented by fitting the true stress–strain curves of Ti–6Al–4V alloy over a wide range of strain-rates (0.0005–0.05 s⁻¹) and temperatures (923–1023 K). The Forming Limit Curve (FLC) of Ti–6Al–4V alloy at 973 K was measured by conducting the hemispherical dome test with specimens of different widths. The forming limit prediction model of Ti–6Al–4V alloy, which takes strain-rate and temperature sensitivity into account, was predicted based on Marciniak and Kuczynski (M–K) theory along with Von Mises yield criterion. The comparison shows that the limit strain decreases with temperature lowering but strain-rate increasing. The comparison between theoretical analysis and experiment of FLC verifies the accuracy and reliability of the proposed methodology, which considers the strain-rate and temperature effects, to predict limit strains in the positive minor strain region of Forming Limit Diagram (FLD).     

Model for CO2 laser welding of stainless steel,titanium, and nickel: parametric study

Abstract

The present work has been carried out experimentally and theoretically with the aim of studying the effect of the operative pressure on the weld beads of different materials (AISI 304, Ti–6Al–4V, and Ni), welded using a CO₂ laser beam. At the same time, the reliability of a properly developed analytical model of the laser beam welding process has been confirmed. Such a model, taking into account the dynamics of the process itself, describes the laser induced thermal fields in terms of two heat sources, the first one representing the keyhole effect and the second one, the role played by the plume. A comparison between the experimental data and the beads predicted using the model gives satisfactory results, with average errors less than 5% for Ti–6Al–4V and ~10% for AISI 304 and Ni. The model allows the quantitative evaluation of the power distribution between the keyhole and the plume and a deeper understanding of the entire process.

MST/1591     

Structure‐property investigations on a laser beam welded dissimilar joint of aluminium AA6056 and titanium Ti6Al4V for aeronautical applications. Part II: Resistance to fatigue crack propagation and fracture

Investigations were continued on the dissimilar laser beam welds of AA6056 and Ti6Al4V, fabricated by inserting Ti‐sheet into the profiled Al‐sheet and melting AA6056 alone. By using microstructure, hardness and strength as the criteria, sites exhibiting non‐uniform microstructure and localized plastic deformation due to strength mismatch were investigated in two orientations: ? crack parallel to the weld and ? crack perpendicular to the weld for fatigue crack propagation and fracture toughness at room temperature. Effect of temper of AA6056 on these properties was studied for two conditions; welding in T4 followed by post weld heat treatment T6, and welding in T6 and naturally aged for a defined period. The orientation “crack parallel to the weld” was investigated in 3 locations on the side of AA6056: the interface and the two changeovers on the Al‐side. Firstly, between the fusion zone and the heat affected zone (3 mm from the interface) and secondly, between (primary) heat affected zone and towards the base material (7 mm from the interface). Although brittle intermetallic TiAl₃ had been formed at the interface, uncontrolled separation or debonding at the interface was not observed. Insofar the bond quality of the weld was good. However, the ranking of interface was the lowest since fatigue crack propagation was relatively faster than that in the fusion zone and heat affected zone, and fracture toughness was low. Therefore, unstable fatigue crack propagation is observed when the crack propagates perpendicular to the weld from AA6056 towards Ti6Al4V. The results have shown that the dissimilar joints exhibit improved performance when laser beam welded in the T6 condition.     

Effect of hydrogen on structure and slow strain rate embrittlement of mill annealed Ti6AI4V

Abstract

The slow tensile straining of smooth specimens of mill annealed Ti6Al4V with different hydrogen contents revealed hydrogen induced slow strain rate embrittlement when the hydrogen content exceeded about 2000 ppm. The effect of hydrogen on the mechanical behaviour of the Ti6Al4V at lower hydrogen levels was not so pronounced and may be interpreted in terms of the partitioning of hydrogen between the α and β phases and the effect of hydrogen in solution on the lattice parameters of these phases. Hydrogen levels in excess of about 1200 ppm promote an increase in the amount of β phase at the charging temperature employed and the effect of this on mechanical properties is discussed. The effect of hydrogen solubility and hydride precipitation at the α/β interface on hydrogen induced slow strain rate embrittlement is also considered.

MST/3431     

Decomposition of deformed α′(α″) martensitic phase in Ti–6Al–4V alloy

ABSTRACT

The process of martensitic α′(α″) phase decomposition in titanium alloys has not been sufficiently characterised in the literature – especially in terms of plastically deformed martensite. The research results of water-quenched Ti–6Al–4V alloy, subsequently cold deformed in compression test and tempered at the temperature range of 600–900°C for 1 and 2?h were presented in the paper. Light and scanning electron microscopy observations revealed the influence of plastic deformation on tempered martensite laths morphology – particularly at the temperature of 900°C – it favoured their fragmentation and spheroidisation. The effect of plastic deformation on characteristic temperatures of α′(α″)→α?+?β phase transformation, phase composition and alloying elements distribution in phase constituents of Ti–6Al–4V alloy was identified and evaluated too.

This paper is part of a thematic issue on Titanium.     

Design and characterization of bioceramic coating materials for Ti6Al4V

Novel bioceramics used as coating materials for Ti6Al4V were designed and characterized by adjusting the thermal expansion coefficient. The results show that the thermal expansion coefficient (α) of 6PM-B5-F4 coating is 10.1 × 10⁻⁶/°C, which matched that of Ti6Al4V. The bonding strength between the alloy and 6PM-B5-F4 coating was further measured by the longitudinal pull-off test. The in vitro response of the bioceramic was studied by immersing the specimens in simulated body fluid (SBF). The bioceramic morphology and structure were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transformed infrared spectroscopy (FTIR).     

Microstructure and mechanical properties of TiC/Ti–6Al–4V composites processed by in situ casting route

Abstract

TiC/Ti–6Al–4V composites containing various volume fractions of TiC were produced by induction skull melting and common casting utilising in situ reaction between titanium and carbon powder. The microstructure and room tensile properties of as cast and heat treated TiC/Ti–6Al–4V composites were investigated. Bar-like or small globular eutectic TiC were found in 5 vol.-%TiC/Ti–6Al–4V composite, whereas the equiaxed or dendritic primary TiC particles were found to be the main reinforcements in 10 and 15 vol.-%TiC/Ti–6Al–4V composites. The as cast TiC/Ti–6Al–4V composites have shown higher strength but lower ductility than those of monolithic Ti–6Al–4V alloy. The shape and fracture of TiC particles can strongly influence the fracture and failure of the composites, and so the ultimate tensile strengths and elongations of as cast composites reduce with the increase in volume fraction of TiC. TiC particles appear to be spheroidised, and titanium precipitation can be found within large TiC particles after heat treatment at 1050°C for 8 h, which can promote the resistance to fracture of composites. Therefore, the elongations of the composites increase significantly, and the ultimate tensile strengths also have marginal increase especially for the 10 and 15 vol.-%TiC/Ti–6Al–4V composites after heat treatment.