Global and Local Mechanical Properties of Autogenously Laser Welded Ti-6Al-4V

Ti-6Al-4V sheets, 3.2-mm in thickness, were butt welded using a continuous wave 4 kW Nd:YAG laser welding system. The effect of two main process parameters, laser power and welding speed, on the joint integrity was characterized in terms of the joint geometry, defects, microstructure, hardness, and tensile properties. In particular, a digital image correlation technique was used to determine the local tensile properties of the welds. It was determined that a wide range of heat inputs can be used to fully penetrate the Ti-6Al-4V butt joints during laser welding. At high laser power levels, however, significant defects such as underfill and porosity, can occur and cause marked degradation in the joint integrity and performance. At low welding speeds, however, significant porosity occurs due to its growth and the potential collapse of instable keyholes. Intermediate to relatively high levels of heat input allow maximization of the joint integrity and performance by limiting the underfill and porosity defects. In considering the effect of the two main defects on the joint integrity, the underfill defect was found to be more damaging to the mechanical performance of the weldment than the porosity. Specifically, it was determined that the maximum tolerable underfill depth for Ti-6Al-4V is approximately 6 pct of the workpiece thickness, which is slightly stricter than the value of 7 pct specified in AWS D17.1 for fusion welding in aerospace applications. Hence, employing optimized laser process parameters allows the underfill depth to be maintained within the tolerable limit (6 pct), which in turn prevents degradation in both the weld strength and ductility. To this end, the ability to maintain weld ductility in Ti-6Al-4V by means of applying a high energy density laser welding process presents a significant advantage over conventional arc welding for the assembly of aerospace components.     

Achieving Superior Strength and Ductility in Ti-6Al-4V Recycled from Machining Chips by Equal Channel Angular Pressing

Ti-6Al-4V machining chips were recycled using equal channel angular pressing. The as-recycled material was fully dense and well bonded, but contained chip boundaries decorated by entrapped surface oxide, giving rise to brittleness in tensile loading. Annealing at high temperatures was effective in removing the oxide. The times required for completely dissolving the oxide layers were calculated using models based on oxygen diffusion in α- and β-Ti, respectively. It is shown that the oxide dissolution is rapid, taking from several minutes to less than one second at temperatures between 973 K and 1323 K (700 °C and 1050 °C) for thicknesses of up to 1 μm. In addition, bands of grains finer than those in the matrix occurred in the vicinity of the prior chip boundaries, caused by the enhanced level of oxygen diffusing away from the dissolving oxide which hindered local grain growth. It would take hours of annealing to homogenize the grain size and composition. The as-recycled material was subjected to conventional mill-annealing, leading to a finer microstructure with superior yield strength (~1150 MPa) and equivalent tensile ductility (~25 pct), compared to a commercial mill-annealed Ti-6Al-4V.     

Microwave Heating, Isothermal Sintering, and Mechanical Properties of Powder Metallurgy Titanium and Titanium Alloys

This article presents a detailed assessment of microwave (MW) heating, isothermal sintering, and the resulting tensile properties of commercially pure Ti (CP-Ti), Ti-6Al-4V, and Ti-10V-2Fe-3Al (wt pct), by comparison with those fabricated by conventional vacuum sintering. The potential of MW sintering for titanium fabrication is evaluated accordingly. Pure MW radiation is capable of heating titanium powder to ≥1573 K (1300 °C), but the heating response is erratic and difficult to reproduce. In contrast, the use of SiC MW susceptors ensures rapid, consistent, and controllable MW heating of titanium powder. MW sintering can consolidate CP-Ti and Ti alloys compacted from ?100 mesh hydride-dehydride (HDH) Ti powder to ~95.0 pct theoretical density (TD) at 1573 K (1300 °C), but no accelerated isothermal sintering has been observed over conventional practice. Significant interstitial contamination occurred from the Al₂O₃-SiC insulation–susceptor package, despite the high vacuum used (≤4.0 × 10^?3 Pa). This leads to erratic mechanical properties including poor tensile ductility. The use of Ti sponge as impurity (O, N, C, and Si) absorbers can effectively eliminate this problem and ensure good-to-excellent tensile properties for MW-sintered CP-Ti, Ti-10V-2Fe-3Al, and Ti-6Al-4V. The mechanisms behind various observations are discussed. The prime benefit of MW sintering of Ti powder is rapid heating. MW sintering of Ti powder is suitable for the fabrication of small titanium parts or titanium preforms for subsequent thermomechanical processing.     

Heat-Treatment Effects on the Microstructure and Tensile Properties of Powder Metallurgy Ti-6Al-4V Alloys Modified with Boron

The Ti-6Al-4V (Ti-64) alloys modified with two levels of boron (1B and 1.7B (wt pct)) representing hypoeutectic and hypereutectic compositions, produced via a prealloyed powder metallurgy approach, were subjected to various standard heat treatments of Ti-64 to study the microstructural evolution and its influence on tensile properties. Boron-modified Ti-64 (Ti-64B) alloys exhibited differences in microstructural response to heat treatment compared to that of Ti-64 due to variations in constituent phase fractions and the influence of TiB on the beta-to-alpha phase transformation kinetics. The tensile elastic modulus of Ti-64B alloys increased nearly linearly with the boron content (or TiB volume fraction) and the increase could be satisfactorily predicted with an isostrain rule of mixtures (ROMs) and the Halpin–Tsai model. The Ti-64-1B possessed a good combination of tensile strength (1200 to1370 MPa) and ductility (10 to 13 pct), while Ti-64-1.7B exhibited high strength (1300 to 1695 MPa) and modest ductility (2 to 3.5 pct). Coarse primary TiB particles present in Ti-64-1.7B were found to initiate premature failure. Strength modeling revealed that load sharing by the micron-sized TiB whiskers provides the major contribution for the increase in yield strength.     

Optimization of Blended-Elemental Powder-Based Titanium Alloy Extrusions for Aerospace Applications

The process of canless extrusion in ambient environment, using cold isostatic pressed, and vacuum-sintered, direct-consolidated blended-elemental hydrided ADMA titanium powder, mixed with master alloy powder for the Ti-6Al-4V composition, has been successfully demonstrated. However, these initially processed unoptimized powder-based extrusions also exhibited oxygen content of about 3000 ppm, within the ASTM B817 Standard, but exceeding the AMS Specification 4935 maximum limit of 2000 ppm, and with pre-extrusion residual hydrogen within 300–500 ppm resulting in post-extrusion void nucleation aligned with the extrusion direction. Additional optimization of extrusion billets during the CIP-and-sintering steps has been successfully demonstrated reducing both oxygen and hydrogen contents to levels at or below the AMS Specification limits for Ti-6Al-4V composition (oxygen content of 2000 ppm maximum, and hydrogen content of 125 ppm maximum). Processing-microstructure-property correlations of the optimized, AMS-4935-Specification-conformant, Ti-6Al-4V blended-elemental powder-based product form exhibited an overall mechanical property balance matching that of double-arc-remelted ingot-based extrusions. Property matching was not only in terms of static mechanical properties (room-temperature tensile properties, and monotonic fracture toughness K _IC (K _Q) values), but also in terms of dynamic fatigue properties (combined S/N plus da/dN properties), as well as stress-corrosion resistance, as measured in terms of K _ISCC threshold values.     

The effect of hydrogen as a temporary alloying element on the microstructure and tensile properties of Ti-6Al-4V

Ti-6Al-4V alloy, to which 0.6 wt pct to 1.0 wt pct (22 to 33 at. pct) hydrogen has been added, can undergo a phase transformation which produces unique, fine microstructures. Specimens of the alloy were heated to 870°C, transformed at temperatures between 540°C and 700°C, and the microstructures were determined as a function of hydrogen content and transformation temperature. Microstructures and tensile properties of sheet specimens were determined after such transformation followed by dehydrogenation at temperatures between 650°C and 760°C. The highest yield strength (1130 MPa) and good ductility (9 pct El) were associated with a fine equiaxed microstructure obtained in material charged with approximately 1.0 wt pct hydrogen, transformed at 565°C and dehydrogenated at 675°C. Lower strengths and ductilities were associated with acicular microstructures produced by transformation at higher temperatures or coarser structures producted at higher dehydrogenation temperatures.     

Microstructures and mechanical properties of dispersion-strengthened high-temperature Al-8.5Fe-1.2V-1.7Si alloys produced by atomized melt deposition process

Dispersion-strengthened high-temperature Al-8.5 pct Fe-pct Si-pct V alloys were produced by atomized melt deposition (AMD) process. The effects of process parameters on the evolution of microstructures were determined using optical metallography and scanning and transmission electron microscopy. The extent of undercooling and the rate of droplet solidification were correlated with process parameters, such as melt superheat, metal/gas flow rates, and melt stream diameter. The size distribution and morphology of silicide dispersoids were used to estimate the degree of undercooling and the cooling rate as functions of process parameters. The tensile properties at 25 °C to 425 °C and fracture toughness at 25 °C of these alloys produced with wide variations in dispersoids size and grain size were determined. Under optimum conditions, the alloy has ultimate tensile strength of 281 MPa and 9.5 pct ductility in the as-deposited condition. Upon hot-isostatic pressing and extrusion, the ultimate tensile strength increased to 313 MPa and ductility increased to 18 pct.     

An investigation of the fracture and fatigue crack growth behavior of forged damage-tolerant niobium aluminide intermetallics

The results of a recent study of the effects of ternary alloying with Ti on the fatigue and fracture behavior of a new class of forged damage-tolerant niobium aluminide (Nb₃Al-xTi) intermetallics are presented in this article. The alloys studied have the following nominal compositions: Nb-15Al-10Ti (10Ti alloy), Nb-15Al-25Ti (25Ti alloy), and Nb-15Al-40Ti (40Ti alloy). All compositions are quoted in atomic percentages unless stated otherwise. The 10Ti and 25Ti alloys exhibit fracture toughness levels between 10 and 20 MPa√m at room temperature. Fracture in these alloys occurs by brittle cleavage fracture modes. In contrast, a ductile dimpled fracture mode is observed at room-temperature for the alloy containing 40 at. pct Ti. The 40Ti alloy also exhibits exceptional combinations of room-temperature strength (695 to 904 MPa), ductility (4 to 30 pct), fracture toughness (40 to 100 MPa√m), and fatigue crack growth resistance (comparable to Ti-6Al-4V, monolithic Nb, and inconnel 718). The implications of the results are discussed for potential structural applications of the 40Ti alloy in the intermediate-temperature (∼700 °C to 750 °C) regime.     

Improvement in mechanical properties of dental cast Ti-6Al-7Nb by thermochemical processing

Hydrogenation and dehydrogenation, that is, thermochemical processing (THP) and its variation with a post-heat treatment (THPH), are investigated in order to improve the balance of strength, elongation, and fatigue strength of cast Ti-6Al-7Nb and Ti-6Al-4V for dental applications. Microstructures of both cast alloys change from coarse Widmanst?tten α structure to super fine α structure with an average diameter of 3 μm by conducting THP or THPH. Tensile strength and fatigue limit of cast Ti-6Al-7Nb and Ti-6Al-4V increase by around 10 and 40 pct, respectively, as compared with those of both as-cast alloys. The balance of strength and ductility of cast Ti-6Al-7Nb is improved by conducting THPH as compared with the case where THP is conducted. This improvement is due to the plastic deformability of unstable β phase because the lattice constant of β phase in each alloy conducted with THPH is much greater than that of each as-cast alloy.     

The effect of alloying additions on the superplastic properties of Ti-6 pct Al-4 pct V

The superplastic deformation properties of Ti-6 pct Al-4 pct V and modified alloys containing 1/4 pct, 1/2 pct, 1 pct, and 2 pct of either cobalt or nickel have been investigated in the temperature range 950 to 750 °C. The results show that both cobalt and nickel modified alloys have reduced flow stresses, in comparison with Ti-6 pct Al-4 pct V, the reductions being particularly marked at the lower temperatures and lower strain rates. The results are shown to be consistent with an isostress model for the deformation of (α + β) two-phase alloys in which the varying β volume fractions and differing diffusivities of titanium, cobalt, or nickel in the β phase are taken into account.     

Ti-6Al-4V-2Ni as a matrix material for a SiC-reinforced composite

Ti-6Al-4V-2Ni is being considered as a composite matrix material because of its potential for a lower consolidation temperature and reduced reaction product formation compared with conventional Ti-6A1-4V. Stress/strain-rate measurements of Ti-6Al-4V-2Ni in sheet form provided data for calculation of diffusion bonding parameters required for efficient consolidation. These data were used as consolidation parameters for fabrication of SiC (SCS-6) reinforced Ti-6Al-4V-2Ni. The composite with 10.5 vol pct SiC exhibits room temperature tensile strength approximately 80 pct of that observed for conventional Ti-6Al-4V/SiC having 35 to 40 vol pct SiC. Scanning and transmission electron microscopy revealed that the fiber-matrix reaction zone is roughly one-half the thickness of that found in SiC-reinforced Ti -6A1-4V, and that it consists of TiC and Ti₅Si₃. Nickel does not enter into the reaction zone products, but rather promotes the formation of Ti₂Ni in the matrix.     

Diffusion Bonding of Ti-6Al-4V Sheet with Ti-6Al-4V Foam for Biomedical Implant Applications

Advanced metallic bone implants are designed to have a porous surface to improve osseointegration and reduce risks of loosening. An alternative approach to existing surface treatments to create a porous surface is to bond separately produced metallic foams onto the implant. To assess the feasibility of this approach, a Ti-6Al-4V foam was diffusion bonded onto bulk Ti-6Al-4V in an argon atmosphere at temperatures between 1173 K and 1223 K (900 °C and 950 °C) for times between 45 and 75 minutes. These specimens were tested in tension to determine bond quality: failures occurred in the foam, indicating a strong diffusion-bonded interface. The quality of the bond was confirmed by metallographic studies, indicating that this approach, which can also be applied to creating of sandwich with porous cores, is successful.     

Effect of Bonding Time on Interfacial Reaction and Mechanical Properties of Diffusion-Bonded Joint Between Ti-6Al-4V and 304 Stainless Steel Using Nickel as an Intermediate Material

In the current study, solid-state diffusion bonding between Ti-6Al-4V (TiA) and 304 stainless steel (SS) using pure nickel (Ni) of 200-μm thickness as an intermediate material was carried out in vacuum. Uniaxial compressive pressure and temperature were kept at 4 MPa and 1023 K (750 °C), respectively, and the bonding time was varied from 30 to 120 minutes in steps of 15 minutes. Scanning electron microscopy images, in backscattered electron mode, revealed the layerwise Ti-Ni-based intermetallics like either Ni₃Ti or both Ni₃Ti and NiTi at titanium alloy-nickel (TiA/Ni) interface, whereas nickel-stainless steel (Ni/SS) interface was free from intermetallic phases for all the joints. Chemical composition of the reaction layers was determined by energy dispersive spectroscopy (SEM–EDS) and confirmed by X-ray diffraction study. Maximum tensile strength of ~382 MPa along with ~3.7 pct ductility was observed for the joints processed for 60 minutes. It was found that the extent of diffusion zone at Ni/SS interface was greater than that of TiA/Ni interface. From the microhardness profile, fractured surfaces, and fracture path, it was demonstrated that the failure of the joints was initiated and propagated apparently at TiA/Ni interface near Ni₃Ti intermetallic for bonding time less than 90 minutes, and through Ni for bonding time 90 minutes and greater.     

Mechanochemical processing of nanocrystalline Ti-6Al-4V alloy

Synthesis of nanocrystalline Ti-6Al-4V was explored using mechanochemical processing. The reaction mixture was comprised of CaH₂, Mg powder, anhydrous AlCl₃, anhydrous VCl₃, and TiCl₄. The milled powder (reaction product) primarily consisted of nanocrystalline alloy hydride having a composition (Ti-6Al-4V)H_1.942, along with MgCl₂ and CaCl₂ as by-products. Aqueous solutions of nitric acid, sulfuric acid, and 1 pct sodium sulfite were found to be very effective in leaching of the chlorides from the milled powder. The (Ti-6Al-4V)H_1.942 on dehydrogenation at 375°C resulted in nanocrystalline Ti-6Al-4V alloy powder.     

Microstructure and Mechanical Properties of Wire and Arc Additive Manufactured Ti-6Al-4V

Wire and arc additive manufacturing (WAAM) is a novel manufacturing technique in which large metal components can be fabricated layer by layer. In this study, the macrostructure, microstructure, and mechanical properties of a Ti-6Al-4V alloy after WAAM deposition have been investigated. The macrostructure of the arc-deposited Ti-6Al-4V was characterized by epitaxial growth of large columnar prior-β grains up through the deposited layers, while the microstructure consisted of fine Widmanstätten α in the upper deposited layers and a banded coarsened Widmanstätten lamella α in the lower layers. This structure developed due to the repeated rapid heating and cooling thermal cycling that occurs during the WAAM process. The average yield and ultimate tensile strengths of the as-deposited material were found to be slightly lower than those for a forged Ti-6Al-4V bar (MIL-T 9047); however, the ductility was similar and, importantly, the mean fatigue life was significantly higher. A small number of WAAM specimens exhibited early fatigue failure, which can be attributed to the rare occurrence of gas pores formed during deposition.     

Effects of oxygen and heat treatment on the mechanical properties of alpha and beta titanium alloys

Two alloys, Ti-6Al-2V and Ti-2Al-16V, simulating the alpha and beta phases of Ti-6A1-4V, respectively, were prepared with oxygen concentrations from 0.07 to 0.65 wt pct (0.20 to 1.83 at. pct). Their microstructure, deformation behavior, and strength were investigated with X-ray diffraction, microscopy, and mechanical tests to determine the effects of oxygen concentration and heat treatment. In both alloys the hardness increases in identical fashion with the square root of oxygen concentration. The alloys' strengths also depend on heat treatment, but in different ways. Whereas the alpha alloy is non-age-hardenable, the beta alloy's strength can be doubled by aging. The hardening effect of oxygen is generally unaffected by heat treatment, except for the alloys with the highest oxygen concentrations. During aging of the alpha a small amount of Ti₃Al can form, and slight age-hardening occurs. The ductility of the alpha alloy is little affected by aging. On the other hand, oxygen causes a change from good ductility at low oxygen concentration (0.07 wt pct) to total brittleness at 0.65 wt pct oxygen, independent of heat treatment. In the beta alloy there are complex phase transformations depending on heat treatment. Its deformation behavior varies from very ductile in solutiontreated and quenched (STQ) condition to totally brittle in aged conditions. The aging embrittlement appears to be caused by alpha and some omega precipitation. Decoration of the beta grain boundaries with precipitates accounts for the intergranular brittle fracture. Oxygen, on the other hand, is not an embrittler, although it reduces the ductility of the beta alloy.     

Microstructure and Mechanical Properties of a High-Strength Ti-4Al-2Fe-3Cu Alloy Fabricated by Sintering and Hot Extrusion

A Ti-4Al-2Fe-3Cu (wt pct) alloy containing only low-cost alloying elements was fabricated by vacuum sintering a blend of TiH₂, Al, Fe, and Cu powders at 1200 °C for 1 hour followed by hot extrusion at the same temperature. The as-extruded alloy exhibited a microstructure consisting of mainly α/β lamellar colonies and Ti₂Cu as a minor phase. The average colony size and lamella thickness were 118 and 12 µm, respectively, and Fe and Cu were predominantly distributed in the β lamellae. The as-extruded alloy had a high tensile yield strength (YS) and ultimate tensile strength (UTS) of 1248 and 1270 MPa, respectively, but a limited ductility (elongation to fracture: 2.3 pct). Annealing at 750 °C for 4 hour caused the average colony size and lamella thickness of the alloy to increase to 145 and 17 µm, respectively, and the volume fraction of the β phase decreased with the annealing. These microstructural changes resulted in a slight decrease of the YS and UTS to 1221 and 1253 MPa, but a clear increase of the ductility with the elongation to fracture reaching 4 pct. This work demonstrates that a combination of relatively low-temperature vacuum sintering, hot extrusion, and annealing can be effectively utilized to fabricate a low-cost Ti-4Al-2Fe-3Cu alloy with high strength and appreciable tensile ductility.

     

An investigation of the fatigue and fracture behavior of a Nb-12Al-44Ti-1.5Mo intermetallic alloy

This article presents the results of a study of the fatigue and fracture behavior of a damage-tolerant Nb-12Al-44Ti-1.5Mo alloy. This partially ordered B2 + orthorhombic intermetallic alloy is shown to have attractive combinations of room-temperature ductility (11 to 14 pct), fracture toughness (60 to 92 MPa√m), and comparable fatigue crack growth resistance to IN718, Ti-6Al-4V, and pure Nb at room temperature. The studies show that tensile deformation in the Nb-12Al-44Ti-1.5Mo alloy involves localized plastic deformation (microplasticity via slip-band formation) which initiates at stress levels that are significantly below the uniaxial yield stress (∼9.6 pct of the 0.2 pct offset yield strength (YS)). The onset of bulk yielding is shown to correspond to the spread of microplasticity completely across the gage sections of the tensile specimen. Fatigue crack initiation is also postulated to occur by the accumulation of microplasticity (coarsening of slip bands). Subsequent fatigue crack growth then occurs by the “unzipping” of cracks along slip bands that form ahead of the dominant crack tip. The proposed mechanism of fatigue crack growth is analogous to the unzipping crack growth mechanism that was suggested originally by Neumann for crack growth in single-crystal copper. Slower near-threshold fatigue crack growth rates at 750 °C are attributed to the shielding effects of oxide-induced crack closure. The fatigue and fracture behavior are also compared to those of pure Nb and emerging high-temperature niobium-based intermetallics.     

The Influence of Precipitation of Alpha2 on Properties and Microstructure in TIMETAL 6-4

Samples of Hot Isostatically Pressed (HIPped) powder of TIMETAL 6-4 (Ti-6Al-4V, compositions in wt pct unless indicated), which was HIPped at 1203 K (930 °C), and of forged bar stock, which was slowly cooled from above the beta transus, were both subsequently held at 773 K (500 °C) for times up to 5 weeks and analyzed using scanning and transmission electron microscopy and atom probe analysis. It has been shown that in the samples aged for 5 weeks at 773 K (500 °C), there is a high density of alpha2 (α₂, an ordered phase based on the composition Ti₃Al) precipitates, which are typically 5 nm in size, and a significantly smaller density was present in the slowly cooled samples. The fatigue and tensile properties of samples aged for 5 weeks at 773 K (500 °C) have been compared with those of the HIPped powder and of the forged samples which were slowly cooled from just above the transus, and although no significant difference was found between the fatigue properties, the tensile strength of the aged samples was 5 pct higher than that of the as-HIPped and slowly cooled forged samples. The ductility of the forged samples did not decrease after aging at 773 K (500 °C) despite the strength increase. Transmission electron microscopy has been used to assess the nature of dislocations generated during tensile and fatigue deformation and it has been found that not just is planar slip observed, but dislocation pairs are not uncommon in samples aged at 773 K (500 °C) and some are seen in slowly cooled Ti6Al4V.