Powder Metallurgy Ti 6Al 4V Research Articles

Crystallographic orientation and slip behavior of powder metallurgy Ti–6Al–4V via multi-directional forging

In this study, we present a microstructural and mechanical analysis of Ti–6Al–4V alloys processed by low-temperature multi-directional forging at 900 °C, achieving near-theoretical density (99.9%), with a tensile strength of 1070 MPa and elongation of 14.9%. The microstructure is defined by both continuous and discontinuous dynamic recrystallization occurring simultaneously, related to an increase in dislocation density. Significantly, this study reveals the interaction between crystallographic orientation and spheroidization during the forging process. Specifically, changes in crystallographic orientation include the asynchronous formation of new subgrains/grains, the disruption of the Burgers orientation relationship, and rotation around the <11 2‾ 0>axis, which not only increases the orientation differences between α lamellae but also enhances the spheroidization process. Moreover, our findings demonstrate that during deformation, the activation of basal and pyramidal slip systems is favored over prismatic slip, indicating that the forging process has enhanced slip activity. The average misorientation angle and the fraction of high-angle grain boundaries are quantified, providing a detailed characterization of the evolved microstructure. This study adopts a new perspective to examine the microstructural evolution during the multidirectional forging process in powder metallurgy, presenting findings that elucidate how multidirectional forging effectively controls.

Crafting high-strength and ductile powder metallurgy Ti6Al4V alloy with a multi-scale microstructure

In this work, a novel multi-scale microstructure of Ti6Al4V duplex alloy with nano-sized β particles was designed based on powder metallurgy. The common lamellar structure was carefully adjusted to reveal the complex effects of microstructure on mechanical properties. The ideal microstructure transforms into a composition dominated by sparse equiaxed primary α, lots of β-nanoprecipitates, and traces of secondary acicular α′ by heat treatment. The uniform nucleation of nano-sized β particles, driven by intragranular element concentration difference and local distortions, is a pivotal reason for high yield strength by dispersion strengthening. More phase boundaries also act as sustainable sources for geometrically necessary dislocations. An uncoordinated phase interface relationship from variant selection improves strain-hardening ability. The intragranular misorientation causes the value inhomogeneity of the Schmid factor in primary α, resulting in an auxiliary slip effect that benefits ductility. Therefore, the fabricated Ti6Al4V alloy displays an ultrahigh ultimate tensile strength of 1329 MPa, yield strength of 1223 MPa, and reasonably large elongation of 8.5 %, respectively. This work underscores the intricate interplay of microstructure evolution and deformation mechanisms in powder metallurgy duplex titanium alloys, offering valuable insights into enhancing the mechanical properties.

Strategy and mechanism to improve the fatigue properties of Ti6Al4V ELI alloy by microstructure modulation combined with surface strengthening process

The hot isostatic pressing (HIP) technique allows obtaining powder metallurgy Ti6Al4V parts with high densities, but its microstructure is not ideal, resulting in a lower fatigue life than expected. In this work, the ideal microstructure was obtained by heat treatment, but this process produces thermally induced porosity (TIP). After heat treatment, the fatigue strength of the bimodal Ti6Al4V alloy reached 680 MPa at 107 cycles, whereas the HIP Ti6Al4V alloy with a fully equiaxed microstructure was lower than 590 MPa. This is because the bimodal Ti6Al4V alloy has better resistance to fatigue crack initiation. This result confirmed that TIPs do not have a significant negative impact on fatigue properties. To further improve the fatigue performance, this study induced gradient nanostructured (GNS) in Ti6Al4V alloy by ultrasonic surface rolling process (USRP), and the fatigue strength of the USRP-treated specimen reached 750 MPa at 107 cycles. Combining microstructure observations and theoretical analysis, the surface hardening mechanism of the USRP-treated Ti6Al4V alloy was quantitatively described. The results showed that the plastic deformation layer is approximately 130 μm, but grain boundary strengthening maintained its effect to a depth of about 200 μm whilst dislocation strengthening maintained its effect to a depth of approximately 350 μm. Under the comprehensive effect of surface hardening and compressive residual stress field, the fatigue crack initiation period of the USRP specimen was significantly extended. This may be the primary reason behind the enhanced fatigue performance obtained through USRP treatment.

The rapid densification behavior of powder metallurgy Ti alloys by induction heating sintering

Micropores are decisive to mechanical properties and thermal deformation capabilities of powder metallurgy (P/M) Ti alloys sintered compacts. As a result, achieving express densification is of prime importance and has attracted increasing attention recently. Induction heating owns the merits of high efficiency, short process, and low cost, and thus has huge potential to be used as a sintering approach for the fabrication of P/M Ti alloys. Nevertheless, the facilitated densification behavior associated with induction heating sintering remains unclear so far. To address it, powder metallurgy Ti6Al4V is manufactured via induction heating sintering with which the underlying sintering mechanism is investigated in-depth. It is found that induction heating could generate a fully densified compact in a remarkably shortened time, demonstrating its superior sintering efficiency as compared with conventional resistance furnace heating. COMSOL finite element analysis reveals that the maximum current density during induction heating can reach 106 A m–2 though the magnetic field strength is solely 0.02 T, leading to a slight temperature difference of approximately 30 °C between the interior and exterior of the billet. Furthermore, the rapid heating essentially starts at sharp corners of particles due to the potent current concentration effect, which facilitates the cracking of the particle surface oxide film and thus enhances the direct contact between them. Moreover, the electromigration effect caused by induction current promotes the diffusion capability of elements, giving rise to expedited densification, alloying, and chemical homogenization. This work provides not only critical insight into the sintering mechanism of induction heating sintering but also significant guidance for low-cost powder metallurgy materials preparation.

Forging of PM Ti–6Al–4V alloy at the temperature above β-transus and high strain rate: modeling and trials in industrial conditions

The results of the forging process in open dies of the powder metallurgy (PM) Ti–6Al–4V alloy, carried out at the temperature above β-transus and at a high strain rate were presented. As an initial material for the research relatively cheap elemental powders were used. This approach gives a real chance for the implementation of the developed technologies. As the range of phase transition temperature in titanium alloys is influenced also by the technology of their production, the β-transus temperature was estimated for the PM Ti–6Al–4V alloy. Finite element method (FEM) numerical analysis of the forging process at the temperature of 1000 °C and high strain rate was performed. The results obtained by the FEM modeling were verified under industrial conditions. The forging trials were made at the temperature of 1000 °C on a screw press operating at a speed of 250 mm s−1. For comparison, the alloy was also studied in as-cast and hot-rolled conditions, which is widely used as a feedstock. The influence of the method of manufacturing feedstock on the microstructure and selected properties of the forgings was determined. This approach allowed for a qualitative assessment of the PM material. The forging process in open dies of two different feedstocks led to the production of forgings with a uniform and similar lamellar microstructure. Thus, it was shown that the heating conditions, the parameters of the forging process, and the method of cooling the product after forging have a decisive influence on the microstructure condition of the forgings shaped in the temperature range of the β phase.

Wear Behavior of Ti6Al4V Based Hybrid Composites Reinforced with TiB2(7.5%) + TiC (7.5%)

Titanium matrix composites (TMCs) have a greater capacity to realize needs and wishes in aviation, modern firearm systems, and the locomotive sector due to the outstanding specific strength, higher temperature stability, particular stiffness, and resistance to corrosion. However, due to weak tribological characteristics and low hardness, titanium and its alloys' applicability in harsh friction and wear environments are severely constrained. The goal of this research is to improve the wear properties of Ti6Al4V-based hybrid composites reinforced with TiB2(7.5%) + TiC (7.5%). The dry sliding characteristic of powder metallurgy Ti6Al4V alloy and Ti6Al4V/ TiB2(7.5%) + TiC (7.5%) powder metallurgy composites was examined. The wear and friction behaviour of TMCs are explored utilising a pin-on-disk wear tester in a variety of circumstances. The results show that when the sliding speed increases, the particular wear rate increases as well. With the assistance of a Field Emission-Scanning Electron Microscope (FE-SEM) and Energy Dispersive Spectroscopy(EDS), the impact of TiB2 and TiC enhancement on titanium alloy microstructure as well as wear parameters were investigated. The wear resistance of the Ti6Al4V matrix was found to be increased by adding TiB2 and TiC to the sterilised Ti6Al4V matrix.

Microstructure evolution and mechanical property of powder metallurgy Ti6Al4V alloy with high oxygen concentrations

ABSTRACT High oxygen Ti6Al4V alloys are fabricated by vacuum sintering and hot rolling using hydrogenation-dehydrogenation Ti6Al4V powder and ZrO2 powder as raw materials. The complete dissolution of oxygen and zirconium atoms results in an increase in lattice constant and strength. As-rolled Ti64-ZrO2 sample exhibits a bimodal structure with a fine acicular α phase of around 1 μm. Ti64-0.75 ZrO2 alloy with equivalent oxygen content close to 0.6 wt.-% presents a good combination of the high tensile strength (1180.2 MPa) and sufficient elongation (7.3%). Therefore, the critical oxygen content for the ductile-to-brittle transition of powder metallurgy Ti6Al4V alloy can be improved by suitable preparation methods. The formation of a fine β transformation structure may be one of the reasons for maintaining sufficient ductility.

High-temperature plastic deformation behaviour of powder metallurgy Ti6Al4V alloy prepared from HDH powder

ABSTRACT Powder metallurgy (PM) Ti6Al4V alloy was prepared from HDH alloy powder by vacuum pressureless sintering. To study its high-temperature deformation behaviour, isothermal compression testing was carried out with temperature range of 900–1050°C, strain rate range of 0.01–10 s−1, and compression ratio of 60%. Based on the hot deformation data, the corresponding constitutive equation and hot processing map were clarified. The deformation activation energy for PM Ti6Al4V alloy was 606.5 kJ mol−1. Its deformation instability zone was very small, locating in the range of 960∼990°C with strain rate >1 s−1. PM Ti6Al4V alloy had large hot working window and good hot workability. The ideal hot processing domain was at 970–990°C and 0.05–0.01 s−1. Besides, the domain at 1000–1040°C and 1–10 s−1 was also suitable for hot processing. This study proposed an effective guidance for the hot deformation (e.g. forging, extrusion, rolling) of PM Ti6Al4V alloy.

Electrochemical Corrosion Behavior of Powder Metallurgy Ti6Al4V Alloy

The corrosion behavior of powder metallurgy (PM) Ti6Al4V alloys with different relative densities ranged from 95.2 to 99.5% is investigated in 3.5 wt.% NaCl solution at room temperature by electrochemical methods, including potentiodynamic polarization curves and electrochemical impedance spectroscopy. For comparison, traditional wrought Ti6Al4V alloy with full densification is also investigated. The results show that the corrosion resistance of PM Ti6Al4V alloys increases with the increasing density. All the studied samples exhibit low Ip values (in the order of 10−6 A cm−2), indicating good corrosion resistance under current experimental conditions. Therefore, density effect on the corrosion resistance of PM Ti6Al4V alloys is less significant. It is worth noting that the corrosion resistance and mechanical performance of PM Ti6Al4V alloys with nearly full densification are comparable to those of wrought alloys. Therefore, PM Ti alloys are promising candidates in the marine application.

Stress-based fatigue behavior of Ti–6Al–4V alloy with a discontinuous lamellar microstructure fabricated by thermomechanical powder consolidation

A powder metallurgy (PM) Ti–6Al–4V alloy with a novel discontinuous lamellar microstructure and a high relative density was fabricated by thermomechanical powder consolidation (TMC) of TiH2–Al60V40 powder and subsequent heat treatment. It was found that the PM Ti–6Al–4V alloy exhibited a high fatigue performance relative to that of the Ti–6Al–4V alloy fabricated using the traditional powder metallurgy process of pressing and sintering. The fatigue cracks initiated at grain boundary α phase along the near-basal planes instead of pores due to their small sizes. The TMC defects enlarged and induced secondary cracks under low-stress cyclic loading, leading to stress redistribution at the crack tip. The discontinuous lamellar microstructure caused an increase of fatigue crack deflection frequency and easier crack closure relative to the fully-lamellar microstructure, owing to the increase of the number density of the α/β interfaces and the random distribution of the crystallographic orientations of the fine domains of α/β lamellar structure. The high plasticity of the crack tip makes the crack blunt due to the strain mismatch between the α and β phases of the discontinuous lamellar microstructure. Consequently, the factors associated with microstructures and defects for limiting the fatigue performance of powder metallurgy titanium alloys were identified, and a meaningful route to enhance the fatigue properties was proposed for fabrication of low cost and high performance PM titanium alloys.

A Novel Al‐Rich/Ti–6Al–4V Layered Composite with Superior Wear and Electrochemical Behavior

By adding a certain amount of Al to the surface of Ti–6Al–4V (TC4) alloy, a novel layered composite composed of TC4 alloy matrix and Al‐rich alloy is designed and successfully fabricated by a blended elemental powder metallurgy (BE PM) method. The results demonstrated that after the addition of 1 wt% Al, the layered composite exhibits superior wear and electrochemical behavior over powder metallurgy Ti–6Al–4V (PM TC4) alloy, which indicates that a wear‐resistant and corrosion‐resistant layer can be in situ synthesized on the sample surface. Moreover, the volume fraction of α phase, the thickness of α colonies, porosity, Rockwell hardness, and wear resistance of the layered composites increase significantly with Al addition, resulting in the decrease in area, width, and depth of wear tracks, and the reduction of average friction coefficients (COFs). Furthermore, with the addition of Al, the galvanic effect at the interface of α/β phase reduces, whereas the porosity of the layered composites, the chemical activity, and the corrosion rate of α phase increase significantly. Therefore, the corrosion resistance of the layered composites increases first and then decreases with Al addition, and the optimum value can be achieved when the added amount of Al is 1 wt%.

In-situ tensile deformation behavior of powder metallurgy Ti6Al4V alloys

In this study, in-situ tensile deformation behavior of powder metallurgy (PM) Ti6Al4V alloys was investigated to analyze the crack initiation and propagation. Accordingly, the fracture mechanisms of the as-sintered and forged PM alloys were summarized. At the initial stage of plastic deformation, cracks appeared in the stress concentration area of pores in the as-sintered Ti6Al4V alloy, and the crack propagation direction was along the phase boundary. Due to the existence of pores, early fracture was obtained, resulting in low elongation of 6.3%. After forging, the crack initiation occurred between α lamellar structure, and the propagation direction was along the lamellar direction. The fine lamellar structure in different directions in the forged PM Ti6Al4V alloy can hinder the crack propagation, thus improving the plasticity. As a result, better comprehensive mechanical performance was obtained in the forged sample, with UTS of 960 MPa, YS of 850 MPa, and EL of 16%.

Deformation Behavior and Microstructure Evolution of Powder Metallurgy Ti6Al4V Alloy During Hot Compression

In order to determine appropriate workability parameters for cross-wedge rolling products of powder metallurgy (PM) Ti6Al4V alloys, thermal compression experiments were carried out within the temperature range of 850-1050 °C, the strain rate range of 0.1-10 s−1, and with a height decrease of 60%. The compression deformation behavior was investigated. A strain-compensated constitutive model was established based on the flow stress curves corrected by the friction, which can predict the flow behaviors under the entirety of the experimental deformation conditions. Moreover, the processing map was developed, and stable and unstable deformation regimes were established. The microstructure analysis illustrated that globularization and dynamic recrystallization were the main deformation mechanisms. The porosity of the compressed samples was greatly reduced and influenced by the temperature and strain rate. Considering the material flow stress, hot processing map, microstructure and porosity, the appropriate hot deformation parameters for PM Ti6Al4V alloy preforms are suggested.

Breaking through the strength-ductility trade-off dilemma in powder metallurgy Ti–6Al–4V titanium alloy

A novel approach was developed to fabricate low-oxygen powder metallurgy Ti–6Al–4 V (wt.%) titanium alloy with a highly desirable combination of ultrahigh tensile strength and good ductility. Using clean and unpassivated TiH2 based powder compact, together with cleaning effect of in-situ dehydrogenation during heating the powder compact resulted in a low oxygen content of 0.26% in final extrusion consolidated alloy. The nature of cleaning effect during sintering was proved to be a reduction reaction of TiO2 + 4[H] = Ti + 2H2O (g). Besides, solution & aging treatment produced a unique composite lamellar microstructure, which mainly consists of fine α platelets and β-transformed structures containing high number density of ultrafine acicular α precipitates. Such composite lamellar microstructure gave rise to extra geometrically necessary dislocations during tensile deformation and brought about significant dislocation hardening together with high global strain. Consequently, this Ti–6Al–4 V alloy exhibited a superior ultimate tensile strength of up to 1320 MPa and meanwhile a high elongation to fracture of not less than 12.5%.

Preparation and mechanical properties of powder metallurgy Ti–6Al–4V alloy produced by hot isostatic pressing

Ti–6Al–4V pre-alloyed powder was produced by a gas atomisation process and fully dense powder metallurgy Ti–6Al–4V alloy was prepared by hot isostatic pressing. Tensile properties of powder metallurgy powder compact compared with those of typical cast and wrought alloys under various heat treatments were investigated. The results show that tensile properties of powder metallurgy Ti–6Al–4V alloy are better than those of as-cast alloy, and comparable with those of wrought alloy. The tensile strength of powder metallurgy alloy varies more slightly with the cooling rate compared with cast/wrought alloys. Electron beam welding was used to join the powder metallurgy Ti–6Al–4V plates. It indicated that the welding joint shows high metallurgy quality and excellent mechanical performance.

Enhanced fatigue behavior of powder metallurgy Ti–6Al–4V alloy by applying ultrasonic impact treatment

The effect of severe plastic deformation induced by multiple sliding impacts by the specimen surface produced at ultrasonic impact treatment (UIT) on the stress-controlled fatigue response of the powder metallurgy Ti–6Al–4V alloy is studied in this paper. Specimens of Ti–6Al–4V alloy were produced from Ti hydride precursor powders via the cost-effective blended elemental powder metallurgy technique. Structure investigations were performed by XRD, TEM, LM and SEM techniques. After UIT, fatigue strength was increased by about 60% on the base of 107 cycles, and lifetime was prolonged by two orders of magnitude at applied stress amplitudes of 300–400MPa. The UIT process leads to approx. four times decrease in the surface roughness parameters. Increased by 65 and 20% microhardeness magnitudes are respectively registered on the top surface and on the depth of 100μm. The UIT induced compressive stresses achieve about two thirds of the alloy yield stress. The hardness increase is shown to be coupled with the increased dislocation density, essentially refined of α+β microstructure and with randomization in α-grains orientations. Observations of cross-sections of the UIT processed specimens revealed the pores free near-surface layer of approx. 200μm thick, which is formed thanks to micro-pore closure process promoted by high shear strains produced at the UIT induced sliding impacts. Analysis of fracture surfaces revealed subsurface cracks initiations and numerous fracture steps indicating on the cracks branching and deflection in the surface layers of the UIT processed specimens instead of superficial crack initiation and the grain boundary cleavages mainly observed in the pristine samples. Experimentally registered magnitudes of fatigue limit were successfully predicted by accounting for the effective stress intensity factor range ΔKth and observed by TEM microstructural units responsible for fatigue fracture (α-phase colonies, α-grains or micro-pores) and compared with literature data on PM Ti–6Al–4V alloy. Enhanced fatigue strength and prolonged lifetime of PM Ti–6Al–4V alloy after the UIT process are concluded to be associated with (i) minimized surface roughness; (ii) compressive residual stresses; (iii) UFG and nano-scale α+β microstructure; and (iv) micro-pore healing.

Microstructural evolution and mechanical properties of powder metallurgy Ti–6Al–4V alloy based on heat response

In present work, powder metallurgy (PM) Ti–6Al–4V alloy was produced by hot isostatic pressing (HIPing) from gas atomized powder. Various HIPing conditions and heat treatments were used to investigate the heat response of PM Ti–6Al–4V alloy. The results show that the optimization of HIPing parameters is temperature from 920 to 940°C, pressure over 120MPa and holding for 3h. The microstructure of powder compact changes significantly after different heat treatments, while there was no obvious difference in tensile properties. Temperature induced porosity (TIP) in powder compact occurred after annealing at 930°C for 1h plus aging. The TIP has no obvious effects on tensile, impact, and fracture toughness properties of powder compact, but the TIP has an adverse effect on fatigue property, especially at shorter fatigue lives. In order to eliminate the TIP in powder compact, several probable solutions were suggested for the application of titanium powder components.

Microstructure and intermediate-temperature mechanical properties of powder metallurgy Ti–6Al–4V alloy prepared by the prealloyed approach

The mechanical properties of powder metallurgy (P/M) Ti–6Al–4V alloys, produced from prealloyed powders by hot isostatic pressing, were evaluated over the intermediate temperature range of 300–500°C. A fully equiaxed or lamellar structure was observed depending on the processing temperature. Microstructural changes were attributed to the beta-transus temperature, predicted as 977°C. As the test temperature increased from 300 to 500°C, the strength of the Ti alloys decreased and their elongation increased, regardless of their initial microstructure. Over the entire temperature range tested, the strength and ductility of equiaxed phase morphologies were higher than those of the lamellar structure. In addition, finer equiaxed structures exhibited higher strength and lower ductility than coarser morphologies. These results were attributed to the initial microstructures of the P/M Ti–6Al–4V alloy samples, as confirmed by evaluating their fracture features.

Understanding competing fatigue mechanisms in powder metallurgy Ti–6Al–4V alloy: Role of crack initiation and duality of fatigue response

Multiple mechanisms of fatigue crack initiations from pores leading to short-term and long-term fatigue life have been observed in a powder metallurgy (PM) Ti–6Al–4V alloy made by the hydrogen sintering and phase transformations (HSPT) process. The pores in the sintered alloy could be classified into two groups; type-I being caused by particle interstices that did not fully close during sintering (sinter-pores) and the type-II originating from larger voids that did not close during powder packing (cave-pores). The fatigue life trends can be classified by the type and the location of pore from which the crack initiated. Interestingly, the two types of pores, and the two different locations (surface versus interior) from which the cracks initiated lead to four distinct S–N trends in the fatigue data. When the stress amplitude level and the pore size were accounted for through initial stress intensity factors, the fatigue data reduced to dual S–N fatigue curves. The results offer new insights into how pores, especially their type and size, and location control the fatigue behavior of PM Ti–6Al–4V alloy. Large improvements in fatigue life can be obtained if the large cave-pores are eliminated or their volume density reduced to very low levels.

Oxidation and corrosion protection by halide treatment of powder metallurgy Ti and Ti6Al4V alloy

The aim of this work is to study the effect of halide treatment in powder metallurgy Ti–6Al–4V alloy, in order to improve high temperature oxidation and electrochemical corrosion resistance. Halide treatment was performed by immersing samples in a powder mixture containing 3wt% NH4Cl, at 200°C and 950°C. Oxidation resistance was assessed by exposure to 600°C for 300h. The microstructure and phase constitution of the formed oxide layers was characterized. Corrosion resistance in aircraft simulated environment was evaluated with electrochemical impedance spectroscopy. Notable improvement in oxidation and corrosion resistance was observed for samples treated at 200°C.