TiAl Joints Research Articles

Effect of brazing temperature on microstructure and tensile strength of γ-TiAl joint vacuum brazed with micro-nano Ti−Cu−Ni−Nb−Al−Hf filler

A novel micro-nano Ti−10Cu−10Ni−8Al−8Nb−4Zr−1.5Hf filler was used to vacuum braze Ti−47Al− 2Nb−2Cr−0.15B alloy at 1160−1220 °C for 30 min. The interfacial microstructure and formation mechanism of TiAl joints and the relationships among brazing temperature, interfacial microstructure and joint strength were emphatically investigated. Results show that the TiAl joints brazed at 1160 and 1180 °C possess three interfacial layers and mainly consist of α2-Ti3Al, τ3-Al3NiTi2 and Ti2Ni, but the brazing seams are no longer layered and Ti2Ni is completely replaced by the uniformly distributed τ3-Al3NiTi2 at 1200 and 1220 °C due to the destruction of α2-Ti3Al barrier layer. This transformation at 1200 °C obviously improves the tensile strength of the joint and obtains a maximum of 343 MPa. Notably, the outward diffusion of Al atoms from the dissolution of TiAl substrate dominates the microstructure evolution and tensile strength of the TiAl joint at different brazing temperatures.

Microstructures and mechanical properties of TiAl joint brazed with Ti-Mn-Fe-Ni-Zr system medium-entropy filler alloy

Microstructures and mechanical properties of TiAl joint brazed with Ti-Mn-Fe-Ni-Zr system medium-entropy filler alloy

Microstructure characteristics and mechanical properties of TiAl alloy joint brazed with CoCuFeNiTi1.2V0.4 high entropy alloy filler

In this study, a high entropy alloy CoCuFeNiTi1.2V0.4 was used as a novel brazing filler metal to braze the TiAl alloy. The microstructure and formation mechanism of TiAl joints were comprehensively investigated, and the effect of brazing temperature on the interfacial microstructure and mechanical properties of the joints was analyzed. The results showed that the joint interfacial formed three different zones with symmetrical characteristics after brazing. The joints were primarily composed of an intermetallic compound phases and high entropy solid solution phases. Ti-rich BCC phase and Cu-rich FCC phase are formed in the brazing seam center zone, which not only improves the strength and plasticity of the joint but as well as releases part of the residual stress. However, the reaction and diffusion of atoms at the interface would break the high entropy effect, and the compound layers could easily form under the conditions of concentration gradient and high temperature. With the brazing temperature increased, the thickness of B2 layer and τ3 layer in the seam increased and the bulk Cu-rich phase internal the seam center basically disappears. In addition, the amount of τ2 compounds precipitated in the B2 phase significantly increased at higher brazing temperatures. The maximum shear strength of the joint was 143.4 MPa when brazed at 1120 °C for 20 min.

Microstructure and mechanical properties of TiAl/TiAl joints brazed with a newly developed Ti–Ni–Nb–Zr quaternary filler alloy

A novel Ti–Ni–Nb–Zr quaternary filler alloy with the composition of Ti-(19∼25)Ni-(15∼25)(Nb ​+ ​Zr) (wt.%) was designed. The filler alloy was composed of (Ti,Nb)ss, (Ti,Zr,Nb)ss ​+ ​(Ti,Zr)2Ni, α-Ti and Ti2Ni phases. It was fabricated into filler foil with a thickness of about 45 ​μm by a rapid solidification technique. The results indicate that the liquidus temperature of the Ti–Ni–Nb–Zr brazing alloy was about 978 oC, and the brazing alloy presented excellent wettability on TiAl substrate. The TiAl joint mainly consisted of β/B2 phase, Ti2Al, Ti2AlNi and α2-Ti3Al phases. The diffusion of Al atom from base metal to brazed seam led to the formation of Ti2Al and Ti2AlNi. Considering that no previously references on XRD pattern of Ti2AlNi compound can be found, Ti2AlNi cast alloy specimen was specially prepared and XRD peaks were specially labeled. The micro-hardness and bending strength tests of the Ti2AlNi phase were carried out, and the results were 761 HV and 192 ​MPa, respectively. The brazing parameters of 1010 oC/10 ​min offered the joint shear strength of 280 ​MPa at room temperature, and the joints exhibited tensile strength of 372 ​MPa at room temperature and 340 ​MPa at 750 oC, indicating that the newly developed filler alloy could offer a stable high-temperature strength.

Synergistic effect of oscillating laser-Nb foil on microstructure and mechanical properties of TC4/7075 dissimilar joints

Oscillating laser welding of TC4 titanium (Ti) to 7075 aluminum (Al) alloys with assistance of Nb foil was performed in an overlap Ti-on-Al configuration, and Nb foil was determined to improve the metallurgical reactions based on analysis from thermodynamic calculations. The synergistic effect of oscillating laser-Nb foil on microstructure and mechanical properties of TC4/7075 dissimilar joints was investigated. Results show that the reduction of Ti–Al joint properties is mainly due to the weak regions at Ti/Al interface composed of TiAl and TiAl3 compounds, where lattice distortion inside TiAl3 and the dislocations in Al matrix grains adjacent to TiAl3 are detected. A sound Ti/Al dissimilar joint is obtained under the beam rotation of oscillating laser assisted by Nb foil. The joint connection area is increased and interfacial heat input is reduced with the beam oscillation, which arises from the weakened heat transfer in penetration direction and heat absorption of Nb foil, thus it is beneficial to reducing the Al melting amount and preventing the overmixing and mutual diffusion between Ti and Al. At the optimal welding parameters, the maximum tensile-shear property of joints increases from 1663 N to 2131 N, representing a 28.14% enhancement compared with that of non-oscillation mode. Moreover, oscillating laser produces a significant effect on the metallurgical regulation of Nb foil, under which the phase composition at Ti/Al interface is replaced by Nb-containing Ti–Al compounds with lower brittleness, as well as some new phases like Ti2NbAl and NbAl3. However, when oscillation frequency is high, such as 100Hz, the melting amount of Nb foil is reduced, and the dual-interface joint is formed which is composed of Ti/Nb and Nb/Al interfaces, where the Nb metallurgy regulation is weakened.

A novel zipper shape joint for improving microstructure and mechanical property of electron beam welded TiAl joint

TiAl alloy is sensitive to welding cracks and exhibits brittle nature at room temperatures. The formation of the α2-Ti3Al phase coupled with high welding residual stress has been proved to be the key factor triggering the cracking of the joint. To improve the microstructure of the weld, a zipper shape joint is designed to decrease the cooling rate of the joint. Finite element simulation analysis of the traditional line-shape joint and the zipper shape joint was carried out. The results reveal that the special zipper shape design effectively decreases the cooling rate of the joint than the straight-line shape joint. The zipper shape can play a role in preheating and post-heat treatment during the welding process. Compared with the straight-line shape joint, the high temperature (above 750 °C) dwelling time of the zipper shape joint is greatly prolonged, which promoted the α → γ phase transformation. However, such weld path also causes stress concentration in the corner region of zipper shape weld bead. Though welding cracks still exist, the crack frequency of the zipper shape joint is decreased obviously. The tensile tests also exhibit that the strength of the zipper shape joint reaches 417 MPa, 40.4 % higher than that of the straight-line shape joint of 297 MPa. Hence, joint shape design can be regarded as a promising direction for further investigations referring to crack control of TiAl joints.

Effect of oscillating laser beam on the interface and mechanical properties of Ti/Al fusion welding joint

To solve the problem that cracks and brittle Ti-Al intermetallic compounds restrict the mechanical properties of Ti-Al joint, an effective connection method has been explored for laser welding of TC4 titanium alloy and 7075 aluminum alloy by oscillating laser beam, and the effect of oscillating laser beam on the interface and mechanical properties of Ti/Al fusion welding joint was also investigated. With the application of beam oscillation, some defects like pores and cracks can be avoided in welding joint, the connection width of interface layer is increased, and the diffusion of Al-to-Ti is suppressed due to the reduced interfacial heat input and the stirring effect induced by the keyhole rotating. As a result, Al-rich Ti-Al compound at interface is reduced while Ti-rich compound is formed. Generally, the property of Ti/Al joint can be significantly improved by the reduction of joint defects and the formation of lower brittle TiAl/Ti3Al compounds. As the oscillation frequency increase, the maximum load of the joint increases first but then decreases, and reached the highest value of 1852N when the 150 Hz oscillating frequency is applied, representing 76.38% enhancement compared with that without oscillation. However, under the condition of excessive oscillating frequency, the interface layer is too thin to form an effective connection, which deteriorates the joint properties.

Interfacial microstructure and tensile strength of TiAl joint brazed with an improved Ti-Zr-Cu-Ni filler

Ti-16.5Zr-13.5Cu-8.5Ni (wt.%) as a filler and brazing process as 950 °C/60 min were supplied in this study for brazing cast TiAl alloys with fully lamella structure. The interface microstructure and element diffusion were analyzed and measured by SEM, EBSD, and EPAM. And the mechanical properties of joints were evaluated. The sound joints were achieved with strength as 575 MPa at room temperature and remained as 410 MPa and 385 MPa in average at 700 °C and 760 °C, respectively. It was attributed to lots of fine grains with soft orientation as BCC [120] in the joint to the tensile direction. It was found that the fracture happened in the local zones with very large grains and the extreme misorientation between grains in the joint. Even though most grains as Ti3Al phase were found in the joint, there was no sign indicating the formed IMC related to the fracture.

Research on interfacial layer of laser-welded aluminum to titanium

Formation mechanism of intermetallic compounds (IMCs) and its relation with strength reduction for laser-welded aluminum to titanium were investigated in this study. It was found that the interfacial layer is composed of α-Ti, TiAl3, Ti2Al, TiAl, Ti3Al and α-Al. TiAl3 is precipitated from liquid Al and formed in an orientation relationship with α-Al. Strength reduction of TiAl joints with increment of IMCs are due to the weak regions including lattice distortion at interface between TiAl/Ti3Al and stacking fault in TiAl3. The results intend to explore the factors determining joint strength and contributed to the welding process improvement.

Brazeability evaluation of Ti-Zr-Cu-Ni-Co-Mo filler for vacuum brazing TiAl-based alloy

Abstract Ti−47Al−2Nb−2Cr−0.15B (mole fraction, %) alloy was vacuum brazed with amorphous and crystalline Ti−25Zr− 12.5Cu−12.5Ni−3.0Co−2.0Mo (mass fraction, %) filler alloys, and the melting, spreading and gap filling behaviors of the amorphous and crystalline filler alloys as well as the joints brazed with them were investigated in details. Results showed that the amorphous filler alloy possessed narrower melting temperature interval, lower liquidus temperature and melting active energy compared with the crystalline filler alloy, and it also exhibited better brazeability on the surface of the Ti−47Al−2Nb−2Cr−0.15B alloy. The TiAl joints brazed with crystalline and amorphous filler alloys were composed of two interfacial reaction layers and a central brazed layer. Under the same conditions, the tensile strength of the joint brazed with the amorphous filler alloy was always higher than that with the crystalline filler alloy. The maxmium tensile strength of the joint brazed at 1273 K with the amorphous filler alloy reached 254 MPa.

Tungsten inert gas welding of dissimilar metals aluminum to titanium with aluminum based wire

The T40 and 5A06 alloy were welded by tungsten inert gas (TIG) welding-brazing technology. The effects of welding parameters on cross-section macrostructure, microstructure, mechanical properties and fracture features were investigated. During the TIG welding-brazing of Ti–Al, the low melting point Al alloy and filler metal melted and resulted in the formation of fusion welded joint, whereas the molten filler metal spread on the solid Ti plate and resulted in the brazed joint. TiAl3 IMC layer formed at the brazing area and the thickness of IMC increased with the increase of welding current. Besides, the thickness of IMC layer of lower zone is less than that of upper zone. The optimal performance of Ti–Al joint is gained under the current of 80 A and the highest joining strength reaches 235 MPa. However, as the current exceeded 80 A, tensile strength of joint decreased owing to the metallurgical defects at the fusion welding area and brazing area.

Cold Metal Transfer Welding–Brazing of Pure Titanium TA2 to Aluminum Alloy 6061‐T6

Pure titanium TA2 and aluminum alloy 6061‐T6 are joined using AlSi5 filler wire by cold metal transfer (CMT) process in the form of two lap configurations. Results indicate that these two various lap joints are both composed of fusion welded joint and brazing joint. The fusion welded joint consists of α‐Al solid solution and Al–Si eutectic. Brazing joint is formed between local molten Ti base metal and weld metal, which produces a certain thickness of reaction layer. Al–Ti joint only has one joining interface with a thickness of 1–5 μm and the optimum tensile strength can reach up to 305.5 N mm−1, while Ti–Al joint forms three joining interfaces with tensile strength of 168.2 N mm−1. Moreover, Al–Ti joint fractures at the weld metal with plastic fracture mode, while Ti–Al joint fractures at joining interface.

Effect of Ti–Al content on microstructure and mechanical properties of Cf/Al and TiAl joint by laser ignited self-propagating high-temperature synthesis

Cf/Al composites and TiAl alloys were joined by laser ignited self-propagating high-temperature synthesis (SHS) with Ni–Al–Ti interlayer. The effect of Ti–Al content on interfacial microstructure and mechanical properties of the joints was investigated. Localized melt of the substrates occurred in the joints. γ-Ni0.35Al0.30Ti0.35, NiAl3 and Ni2Al3 reaction layers formed adjacent to the substrates. Joint flaws, such as pores and cracks, made the joint density decrease and worked as the fracture source, which led to the sharp decline of joint strength. Additive Ti–Al increased joint density and strengthened the interlayer adhesion to Cf/Al. The joint flaws could be controlled by changing the Ti–Al content. When the Ti–Al content was 0.1, the joint was free of cracks with high density and reached the maximum shear strength of 24.12 MPa.

Effects of brazing temperature and testing temperature on the microstructure and shear strength of γ-TiAl joints

Ti–47Al–2Nb–2Cr–0.15B (at%) alloy was brazed with an amorphous Ti–25.65Zr–13.3Cu–12.35Ni–3Co–2Mo (wt%) alloy foil in vacuum furnace. The effects of brazing temperature and testing temperature on the interfacial microstructure and mechanical property of the TiAl joints were investigated in detail. Sound TiAl brazed joints are obtained at brazing temperature of 925–1050°C for 5min. The joint is mainly composed of two zones, and the intermetallic phases in the joint are (Ti, Zr)2, (Cu, Ni) and Ti3Al. The brazing temperature has a significant effect on the interfacial microstructure of the TiAl joint. The room temperature shear strength of the joint first increases and then decreases with the brazing temperature. The highest room temperature shear strength of the joint is 211MPa when the TiAl alloys were brazed at 1000°C for 5min. The shear strength decreases with the testing temperature and maintains above 155MPa at a temperature below 700°C. However, solid-state diffusion and serious oxidation in the central brazed layer II lead to the sharp reduction of the joint shear strength when the testing temperature exceeds 700°C. All the failures of the joints occur in the central brazed layer II and the (Ti, Zr)2 (Cu, Ni) intermetallic compounds occupy most of the fracture surfaces.

Microstructures of the TiAl joints brazed with Ti-Zr-based filler metals

TiAl is brazed with Ti-Zr-Fe and Ti-Zr-Cu-Ni-Fe brazing foils at 1,323 and 1,373 K for 5 min. Ti-rich, Ti3Al, and white blocky Ti-Zr-Al-Fe phase are found in the joint brazed with Ti-Zr-Fe filler metal at 1,323 K. Ti2Al phase is formed with the improved brazing temperature. When using Ti-Zr-Cu-Ni-Fe filler metal, the joints are mainly comprised of Ti-rich, Ti3Al, and Ti2Al and white blocky Ti-Zr-Al-(Cu, Ni, Fe) phase. The joint widths and the size and amount of white blocky Ti-Zr-based phases in the center of the joints are increased when the brazing temperature is increased. And under the same brazing condition, the joint width, the size of Ti3Al and Ti-Zr-Al-(Cu, Ni, Fe) phase in the joint brazed with Ti-Zr-Cu-Ni-Fe filler metal, are larger than that of Ti-Zr-Fe filler metal. More TiAl base metal dissolved into the joint induces the wider joint and the higher concentration of element Al in the joint, which lead to form Ti2Al phase. Too large size compounds such as Ti-Zr-based phases and Ti3Al would deteriorate the joint property. A lot of these phases also might have a negative effect on the joint property.

Brazing TiAl intermetallics using TiNi–V eutectic brazing alloy

A novel TiNi–V25 (at.%) eutectic brazing alloy, which was composed of TiNi intermetallics and V, was designed and fabricated. Subsequently, the brazing alloy was used to braze TiAl intermetallics (Ti–42.5Al–9V–0.3Y (at.%)) and reliable joints were obtained. The typical interfacial microstructure of TiAl brazed joints was characterized by employing SEM, EDS and XRD, and the brazing mechanism was analyzed in detail. The brazed joints mainly consisted of B2 phase and τ3-Al3NiTi2 intermetallics. Interfacial morphology of TiAl joints varied with the increase of brazing temperature, while the phase constitution in joints did not change sound TiAl joints with the highest average shear strength of 196MPa were obtained when the specimens were brazed at 1220°C for 10min. The presence of B2 phase and τ3-Al3NiTi2 intermetallics in joints deteriorated the joining properties due to their brittleness, which led to cleavage-dominated fracture after shear test. The crack propagation path and fracture location were mainly determined by the content and distribution of τ3-Al3NiTi2 phase.

Brazing high Nb containing TiAl alloy using TiNi–Nb eutectic braze alloy

High Nb containing TiAl alloy (Ti–45Al–5Nb-(W, B, Y) (at.%)) was successfully brazed using a novel TiNi–Nb eutectic braze alloy (Ti40Ni40Nb20 (at.%)), which was fabricated by vacuum arc remelting technique in the lab. The interfacial microstructure and mechanical properties of TiAl brazed joints were characterized by electronic microscopy and shear test, respectively. Microstructure analysis results showed that new intermetallic phases of O–Ti 2AlNb and τ 3-Al 3(Ti,Nb) 2Ni formed in the joint. Moreover, the brazing temperature had a great influence on the interfacial morphology of TiAl joints. With increasing brazing temperature the dissolution style of TiAl substrate into molten braze alloy changed significantly. The shear test results showed that the maximum shear strength at room and high temperature reached 308 MPa and 172 MPa, respectively. Brittle fractures were observed in all of the TiAl brazed specimens and the fracture location varied with brazing temperature and shear test temperature. The interaction of materials and joining mechanism was systemically discussed.

Interfacial structure and fracture behavior of TiB whisker-reinforced C/SiC composite and TiAl joints brazed with Ti–Ni–B brazing alloy

Abstract In situ formed TiB whisker-reinforced TiAl and C/SiC joints, containing 30% of TiB whiskers by volume, were produced by vacuum brazing with (Ti–66Ni) 1− x B x ( x = 3 wt.%) alloy at 1160 °C for 10 min. The brazing alloy prepared by arc-melting was mainly comprised of TiNi 3 and TiNi eutectic microstructure, in which some black TiB 2 blocks formed. TiB 2 was transformed into TiB whiskers during brazing due to the successive dissolution of TiAl substrate. The interfacial structure of the brazed joint consisted of β reaction layer transformed from TiAl, TiB whisker-reinforced central zone and TiC reaction layer adjacent to C/SiC. The in situ formed TiB whiskers were beneficial in reducing the joint residual stresses and reinforcing the joint. The average shear strength reached 90 MPa and 65 MPa at room temperature and 600 °C, respectively. Fracture primarily propagated in the SiC matrix with the carbon fiber being pulled out. Contributing factors to the high strength included the possibility of the pull-out of carbon fibers, the diversion of crack propagation direction and the “nail effect” which formed by the infiltration of the brazing alloy.

Vacuum diffusion bonding of TiB2 cermet to TiAl based alloys

Vacuum diffusion bonding of TiB2 cermet to TiAl based alloys was carried out at 1123 – 1323 K for 0.6 – 3.6 ks under 80 MPa. The microstructural analyses indicate that a compound Ti(Cu, Al)2 is formed in the interface of the TiB2 /TiAl joints, and the width and quantity of the Ti(Cu, Al)2 compound increase with the increase of the bonding temperature and bonding time. The experimental results show that the shear strength of the diffusion bonded TiB2 /TiAl joint is 103 MPa, when TiB2 cermet is bonded to TiAl based alloy at 1223 K for 1.8 ks under 80 MPa.

Infrared joining of TiAl intermetallics using Ti-15Cu-15Ni foil—II. The microstructural evolution at high temperature

The microstructural evolution of TiAl joint during infrared joining at 1150°C under different holding times using Ti-15Cu-15Ni foil as brazing filler metal was investigated. Based on the observed microstructures, a five-step microstructural evolution mechanism at 1150°C joining temperature is proposed in this study. These time-dependent evolution steps including (a) β-Ti layer formation, (b) columnar α + β two-phase zone formation, (c) α2-phase nucleation, (d) high Al% α-phase formation and (e) α2-phase integration, are consistent with the multiphase diffusion theories in solid-state systems. Since different joining temperatures (Tw) have different corresponding ternary isotherms and stable phases, small variations of Tw can result in significant changes of the microstructural morphologies, especially concerning the microstructural evolution of zones of α2- and the high Al% α-phases. The mechanism proposed in this study has predicted such evolutions, which agree well with observed microstructures. All the observed microstructures at ambient temperatures can be clearly elucidated by this proposed mechanism.