Phase formation in Ti/Ni dissimilar welds

We explore phase formation in Ti/Ni dissimilar welds using a combination of microscopy and composition analysis (TEM, SEM and EDS). Main microstructural features are NiTi dendrites and Ti₂Ni grains in the inter-dendritic space. The high temperature B2 phase of NiTi is found to transform to trigonal ‘R’-phase, B19^′martensite, and rhombohedral Ni₄Ti₃ phase; these different transformation products highlight the composition inhomogeneity in the parent B2 phase and probable non-equilibrium solidification events during rapid cooling of the weld. Solidification sequence of NiTi and Ti₂Ni was found to vary depending on local conditions in the weld. Formation of impurity nitride phases of titanium is observed which signify incomplete shielding during welding.     

Characterization and corrosion behavior of NiTi–Ti2Ni–Ni3Ti multiphase intermetallics produced by vacuum sintering

An in-situ synthesis method was employed to produce NiTi–Ti₂Ni–Ni₃Ti multiphase intermetallics. In this regard, the amorphous/nanocrystalline Ni–Ti powders were sintered at 1300 °C for 2 hrs to obtain Ni–Ti alloys with dense structure. Tafel polarization tests were employed to study corrosion behavior of multiphase materials in 3.5% NaCl and 0.1 M H₂SO₄ corrosive media. The results indicated that the microstructure of sintered samples consists of NiTi(Fe) and Ti₂Ni/Ti₄Ni₂O_x phases embedded in a Ni₃Ti matrix. The synthesized multiphase materials had microhardness up to 873 HV1 kg.Further investigations showed the corrosion performance of multiphase samples in 3.5% NaCl solution was inferior to that of wrought NiTi alloy. In contrast, the corrosion resistance of multiphase samples in 0.1 M H₂SO₄ solution was comparable to that of wrought NiTi alloy.     

Microstructures and mechanical properties of Ti3Al/Ni-based superalloy joints brazed with AuNi filler metal

For the purpose of high-temperature service and the weight reduction in aviation engineering applications, the dissimilar joining of Ti₃Al-based alloy to Ni-based superalloy (GH536) was conducted using Au-17.5Ni (wt%) brazing filler metal. The microstructure and chemical composition at the interfaces were investigated by scanning electron microscope, X-ray diffraction and transmission electron microscope. The diffusion behaviors of elements were analyzed as well. The results indicated that the Ti₃Al/GH536 joint microstructure was characterized by multiple layer structures. Element Ni from Au-Ni filler metal reacted with Ti₃Al base metal, leading to the formation of AlNi₂Ti and NiTi compounds. Element Ni from Ti₃Al base metal reacted with Ni and thus Ni₃Nb phase was detected in the joint central area. Due to the dissolution of Ni-based superalloy, (Ni,Au) solid solution ((Ni,Au)ss) and Ni-rich phase were visible adjacent to the superalloy side. The average tensile strength of all the joints brazed at 1253 K for 5–20 min was above 356 MPa at room-temperature. In particular, the joints brazed at 1253 K/15 min presented the maximum tensile strength of 434 MPa at room-temperature, and the strength of 314 MPa was maintained at 923 K. AlNi₂Ti compound resulted in the highest hardness area and the fracture of the samples subjected to the tensile test mainly occurred in this zone.     

Microstructure and martensitic transformation and mechanical properties of cast Ni rich NiTiCo shape memory alloys

Abstract

The influence of Co additions on the microstructure, second phase precipitates, phase transformation and mechanical properties of cast Ni_51?xTi₄₉Co_x (x?=?0, 0·5, 1·5 and 4 at-%) shape memory alloys was investigated. At the expense of Ni, Co added to NiTi alloy significantly increases the martensitic transformation temperature. The matrix phase in the microstructure of Ni₅₁Ti₄₉Co₀ alloy is the austenite phase (B2) in addition to martensite phase (B19′) and precipitates of NiTi intermetallic compounds. However, the parent phase in the other three alloys, Ni_50·5Ti₄₉Co_0·5, Ni_49·5Ti₄₉Co_1·5 and Ni₄₇Ti₄₉Co₄, is martensite. Ti₂Ni phase was found in the microstructures of the all investigated alloys; however, Ni₃Ti₂ phase precipitated only in the NiTi alloy with 0 at-%Co. The volume fraction of Ti₂Ni phase decreased by the additions of 0·5 and 1·5 at-%Co, while it is slightly increased with 4 at-%Co. The hardness value of NiTi alloy is affected by Co additions.     

Phase transformation and microstructure and mechanical properties of as cast NiTiRe shape memory alloys

Abstract

The microstructure, martensitic transformation and mechanical properties of as cast Ni₅₂Ti_48?xRe_x shape memory alloys (SMAs) were investigated. The microstructure of these alloys consists of B19′ martensite phase as a matrix and B2 austenite in small percentages in addition to some precipitations of NiTi intermetallic compounds. There are two types of NiTi precipitates: the first one is Ti₂Ni, which can be seen in the all microstructures of the three alloys, and the other is Ni₂Ti, which is found only in the microstructure of Ni₅₂Ti_47·7Re_0·3 alloy. According to differential scanning calorimetry measurements, one stage of transformation reaction B2 to B19′ accompanied with forward and backward martensitic transformation was observed. The martensitic peak and the austenitic peak were increased with the addition of rhenium. Both are increased as the number of valence electron per atom increase and the valence electron concentration decrease. Hardness measurements of Ni₅₂Ti_48?xRe_x SMAs are improved by the Re additions.     

Dissimilar joining of Ti3Al-based alloy to Ni-based superalloy by arc welding technology using gradient filler alloys

In this study, Ti–Al–Nb, Ti–Ni–Nb and Ni–Cr–Nb system alloys were designed and incorporated in order to construct a gradient structure at the surface of the joined Ti₃Al base material. And the Ti₃Al-based alloy and Ni-based superalloy were successfully joined together using gas tungsten arc (GTA) welding technology. The microstructure evolution, mechanical properties and fractured behaviors of the joints were investigated. The gradient structure remarkably decreased the formation tendency of brittle phases within the joints compared with a single filler alloy and thus improved the joint strength effectively. The average room-temperature tensile strength of the Ti₃Al/In718 dissimilar joint reached 353 MPa, and the strength value at 873 K was 245 MPa. At the Ti–Ni–Nb/Ni–Cr–Nb interface, some Ni₃(Nb, Ti) + (Nb, Ti)Cr₂ and TiNi₃ phases were detected in the Ti–Ni–Nb matrix. It was believed that their presence decreased the room-temperature strength of the Ti–Ni–Nb alloy but improved its high-temperature strength.     

Characterization and corrosion behavior of NiTi-Ti2Ni-Ni3Ti multiphase intermetallics produced by vacuum sintering

An in-situ synthesis method was employed to produce NiTi-Ti₂Ni-Ni₃Ti multiphase intermetallics. In this regard, the amorphous/nanocrystalline Ni-Ti powders were sintered at 1300 °C for 2 hrs to obtain Ni-Ti alloys with dense structure. Tafel polarization tests were employed to study corrosion behavior of multiphase materials in 3.5% NaCl and 0.1 M H₂SO₄ corrosive media. The results indicated that the microstructure of sintered samples consists of NiTi(Fe) and Ti₂Ni/Ti₄Ni₂O_x phases embedded in a Ni₃Ti matrix. The synthesized multiphase materials had microhardness up to 873 HV1 kg.Further investigations showed the corrosion performance of multiphase samples in 3.5% NaCl solution was inferior to that of wrought NiTi alloy. In contrast, the corrosion resistance of multiphase samples in 0.1 M H₂SO₄ solution was comparable to that of wrought NiTi alloy.     

Precipitation of second phases in aged Ni rich NiTiRe shape memory alloy

Abstract

This study investigated the effect of aging on the structure and precipitation of second phases of Ni₅₂Ti_47·7Re_0·3 shape memory alloys. The alloy was solutionised at 1000°C for 24 h before aging at various temperatures ranging from 300 to 600°C for 3 h. The matrix phase in both solutionised and aged specimens was martensite. Ti₂Ni phase was also present in the microstructure of both solutionised and aged specimens and its volume fraction decreased as the aging temperature increased. Ni₄Ti₃ phase began in appearance by increasing aging temperature to 400°C. Ni₄Ti₃ precipitates had lenticular and non-geometry shapes. Aging at 600°C led to precipitation of Ni₃Ti phase in the microstructure. This precipitated phase formed in white blocky shapes. Ti/Ni ratio increased and/or Ni content decreased in the matrix with increasing in aging temperature.     

Preparation of NixTiy compound via Ni plating followed by thermal treatment of Ti powder

In this research, Ni_xTi_y compound was prepared by thermal treatment of Ni-plated Ti powder. For this purpose, Ti powder was plated in an electroless Ni bath for various times (120, 225, 300, and 720?min). Hydrazine hydrate was used as a reductant for the deposition of pure Ni on the Ti particles. The plated powder (225?min) was heat treated under argon atmosphere to achieve Ni_xTi_y powder. Finally, the heated/plated powder was pressed by CIP followed by sintering at 980°C for prepare the Ni_xTi_y bulk sample. The plated powders as well as sintered one were characterized using scanning electron microscopy, energy dispersive spectrometer, X-ray fluorescence, X-ray diffraction and differential scanning calorimetric. The NiTi₂, NiTi, and Ni₃Ti phases were detected in the XRD patterns of heated/plated Ti powder. According to DSC data, the heated/plated Ti powder showed reversible martensitic transformation at temperature range of ?38.0°C to +38.1°C, while sintered/heated/plated Ti powder displayed reversible transformation at temperature range of 16.0°C–15.4°C.     

Porous NiTi shape memory alloys produced by SHS: microstructure and biocompatibility in comparison with Ti2Ni and TiNi3

Shape memory alloys based on NiTi have found their main applications in manufacturing of new biomedical devices mainly in surgery tools, stents and orthopedics. Porous NiTi can exhibit an engineering elastic modulus comparable to that of cortical bone (12–17 GPa). This condition, combined with proper pore size, allows good osteointegration. Open cells porous NiTi was produced by self propagating high temperature synthesis (SHS), starting from Ni and Ti mixed powders. The main NiTi phase is formed during SHS together with other Ni–Ti compounds. The biocompatibility of such material was investigated by single culture experiment and ionic release on small specimen. In particular, NiTi and porous NiTi were evaluated together with elemental Ti and Ni reference metals and the two intermetallic TiNi₃, Ti₂Ni phases. This approach permitted to clearly identify the influence of secondary phases in porous NiTi materials and relation with Ni-ion release. The results indicated, apart the well-known high toxicity of Ni, also toxicity of TiNi₃, whilst phases with higher Ti content showed high biocompatibility. A slightly reduced biocompatibility of porous NiTi was ascribed to combined effect of TiNi₃ presence and topography that requires higher effort for the cells to adapt to the surface.     

Study on cold metal transfer welding–brazing of titanium to copper

3 mm Pure titanium TA2 was joined to 3 mm pure copper T2 by Cold Metal Transfer (CMT) welding–brazing process in the form of butt joint with a 1.2 mm diameter ERCuNiAl copper wire. The welding–brazing joint between Ti and Cu base metals is composed of Cu–Cu welding joint and Cu–Ti brazing joint. Cu–Cu welding joint can be formed between the Cu weld metal and the Cu groove surface, and the Cu–Ti brazing interface can be formed between Cu weld metal and Ti groove surface. The microstructure and the intermetallic compounds distribution were observed and analyzed in details. Interfacial reaction layers of brazing joint were composed of Ti₂Cu, TiCu and AlCu₂Ti. Furthermore, crystallization behavior of welding joint and bonding mechanism of brazing interfacial reaction were also discussed. The effects of wire feed speed and groove angle on the joint features and mechanical properties of the joints were investigated. Three different fracture modes were observed: at the Cu interface, the Ti interface, and the Cu heat affected zone (HAZ). The joints fractured at the Cu HAZ had higher tensile load than the others. The lower tensile load fractured at the Cu interface or Ti interface was attributed to the weaker bonding degree at the Cu interface or Ti interface.     

Mechanical properties of additive laser-welded NiTi alloy

Laser welding would be a suitable joining technique for NiTi shape memory alloy if the mechanical properties of laser weld were improved. With this purpose, effects of additive on mechanical properties of laser-welded NiTi alloy have been experimentally studied. Welding specimens used in this study were 2 mm thick hot-rolled plates with a chemical composition of Ni50.9Ti49.1. (Ni50.9Ti49.1)-Ce2 (at.%) alloy foil or Ni47Ti44Nb9 plate was used as filler metal to add Ce or Nb element into NiTi laser weld metal. Both tensile strength and the toughness of additive-welding specimens were improved significantly compared with non-additive-welding specimen. The mechanical property improvement was attributed to the fine solidification NiTi grains and good grain-linking in weld center. The microstructure control mechanisms of these two additive welds were discussed.     

Microstructure and mechanical behaviors of electron beam welded NiTi shape memory alloys

The vacuum electron beam welding (EBW) technique was employed to weld Ni_50.8Ti_49.2 shape memory alloy sheets, and the microstructure, transformation behaviors and mechanical behaviors of the welding joints were investigated systematically. The microstructure observation showed that the weld seam was composed of coarse columnar crystals at the center and relatively fine columnar crystals near the fusion line. The abnormal high intensity of B2_2 0 0 peak in XRD patterns and preferred orientation in EBSD indicated that the grains in the weld seam have grown preferentially along the 〈1 0 0〉 crystal orientation. Differential scanning calorimetry (DSC) curves exhibited an increase of the martensite start temperature (M_s) of the weld seam, which led to the mixed microstructure of martensite and austenite at room temperature. As a result, the ultimate tensile strength of the welding joint was 85% as high as that of the base metal at room temperature, while it could reach 93% at 223 K when both the weld seam and the base metal were in pure martensitic state.     

Microstructural characteristics of directionally solidified Ni–43Ti–7Al alloys

Abstract

Ni–43Ti–7Al (at-%) alloy was directionally solidified at different withdrawal rates (2, 20 and 100 μm s^?1) and a constant temperature of 1550°C by liquid metal cooling method. Results show that as the withdrawal rate decreases from 100 to 2 μm s^?1, the cellular arm spacing increases from 39·5 to 126 μm, the size of Ti₂Ni and the stability of the liquid/solid interface also increase, while the volume fraction of Ti₂Ni decreases from 3·1 to 0·9%. Moreover, microstructural analysis reveals that a NiTi+Ti₂Ni anomalous eutectic structure is formed in intercellular regions of directionally solidified samples withdrawn at 20 and 100 μm s^?1. However, in the sample withdrawn at 2 μm s^?1, Ti₂Ni phases represent strip and liquid droplet morphologies in the intercellular region. Finally, the possible explanation to the change of microstructure is discussed.     

The study of constitutional phases in a Ni47Ti44Nb9 shape memory alloy

The constitutional phases and microstructure of Ni₄₇Ti₄₄Nb₉ alloy have been studied by means of optical microscopy, electron probe X-ray microanalyses (EPMA) and X-ray diffraction. It has been shown that the microstructure of the experimental alloy consists of three phases:TiNi matrix, niobium-rich phase and Ti₃(Ni,Nb)₂ compound. The Nb-rich phase is determined to be β -Nb with bcc structure containing a small amount of Ni and Ti. The β -Nb is a soft phase which forms a eutectic structure with TiNi phase during solidification. After hot working the soft β -Nb phase is dispersed in TiNi matrix and gives rise to a wide transformation hysteresis in the alloy. The Ti3(Ni,Nb)₂ is a harder and embrittlemental phase.     

The effect of the combustion channels on the compressive strength of porous NiTi shape memory alloy fabricated by SHS as implant material

Porous NiTi shape memory alloy (SMA) with ideal porosity and high compressive strength as an implant material was fabricated by self-propagating high-temperature synthesis (SHS). In this study, a new ignition technique “high voltage electric arc” was used to ignite the green specimens and control the orientation of combustion channels which effect compressive strength. It was determined that the compressive strength of specimens was increased when the combustion channels were parallel along the specimen axis, and the compressive strength was decreased when the combustion channels were perpendicular to specimen axis. The desired phases such as B2(NiTi) and B19′ (NiTi) were dominant while the second phases (Ni₄Ti₃ and NiTi₂) in small amount. The undesired phases (such as pure Ni and Ni₃Ti) for biocompatibility are not found in the structure. The transformation temperatures were higher for medical applications by heat treatment and partly decreased at every next thermal cycle where the heating rate of the specimen was increased.     

Microstructural evolution in AZ91D magnesium alloy during electron beam welding

Vacuum electron beam welding can have a low heat input, which means there is a minimum heat affected zone during welding of AZ91D magnesium alloy. From the observed microstructure, the weld of the AZ91D magnesium alloy can be divided into four regions, which are the weld metal zone, a partially-melted zone adjacent to the fusion boundary, a partially-melted zone adjacent to the base metal and the base metal zone. A sharp transition from the fusion zone to the non-melted zone, especially the characteristic partial melting microstructure and nature of the alloy elements, was observed. It was found that significant partial melting had taken place in the very narrow region around the weld metal of the AZ91D magnesium alloy. The Al content of eutectic β-Mg₁₇Al₁₂ in the partially-melted zone adjacent to the fusion boundary was close to the content in the continuously precipitated eutectic β particles in the fusion zone and much lower than the eutectic β in the base metal. The fully melted eutectic β-phase coexisted with the partially melted eutectic β phase in the partially-melted zone adjacent to the base metal.     

Martensitic transformation behaviors of Ti-rich Ti–Ni alloy fibers fabricated by melt overflow

Ti₅₀Ni₅₀, Ti_50.5Ni_49.5, Ti₅₁Ni₄₉ and Ti_51.5Ni_48.5 fibers were fabricated by melt overflow process. The rapid solidification can increase the solubility above the equilibrium solubility. The effects of the rapid solidification of Ti-rich Ti–Ni alloys on the microstructure, transformation temperatures and shape memory characteristics are investigated. The addition of 0.5 at.% Ti in Ti₅₀Ni₅₀ alloy greatly increases the transformation temperature. However, the transformation temperatures decrease again for Ti content exceeding 51 at.%. Results of thermal cycling tests under various constant stress levels reveal that the recoverable elongation associated with B2–B19 martensitic transformation of Ti_50.5Ni_49.5 fibers is two times larger than that of Ti_51.5Ni_48.5 alloy fiber.     

Microscopic examination of an Alloy 600/182 weld

A microscopic study was carried out to examine the microstructural and compositional features of an Alloy 600/182 weldment. The grain boundaries of the Alloy 600 base metal consisted of low angle boundaries (4.4%), coincidence site lattice boundaries including twins (46.6%), and random high angle grain boundaries (49.0%). The precipitates in the matrix of Alloy 600 were identified as Cr₇C₃, irrespective of the intergranular and intragranular ones. The microstructure of the Alloy 182 weld metal consisted of cellular dendrites in the grains epitaxially solidified from the heat affected zone, and the Mn concentration increased periodically across the dendritic interfaces. Contrary to the Alloy 600 base metal, most of the grain boundaries of the Alloy 182 weld metal were low angle and random high angle grain boundaries, with a negligible fraction of coincidence site lattice boundaries. Tiny Cr-rich M₂₃C₆ and Nb carbides were distributed on the grain boundaries in the weld metal, and a Cr-depletion zone was formed due to the Cr carbide precipitation. Precipitates of (Nb,Ti)C, Al₂O₃ type and TiO₂ type oxides were found in the Alloy 182 weld metal.     

Laser welding of Ti–6Al–4V to Nitinol

Formation of brittle intermetallic phases in addition to different thermal expansion coefficients associated with dissimilar welding leads to the formation of transverse cracks in weld metal and eventually restricts widespread applications of dissimilar joints. Therefore, joining technology should be expanded in field of dissimilar welding in order to solve its difficulties. In the present study, an experimental work with pulsed Nd:YAG laser was performed for dissimilar welding of Ti–6Al–4V and Nitinol. Autogenous welding of these two alloys resulted in joints with poor strength and ductility due to the formation of transverse cracks in the weld metal. Therefore, the chemical composition of the weld metal has to be modified in order to reduce the formation of brittle phases and eliminate subsequent cracking. In this work, this was done by insertion of a copper interlayer with a thickness of 75 μm between the base metals. The results indicated that insertion of copper interlayer has a great influence on the reduction of the amount of Ti₂Ni brittle intermetallic phase, elimination of transverse cracks through the weld metal and eventually improvement of mechanical properties of the joints. Insertion of copper interlayer was very useful since it altered the cracked autogenous joint to a joint which could withstand a tensile stress of 300 MPa.