Transient liquid phase (TLP) bonding of Al7075 to Ti–6Al–4V alloy

TLP diffusion bonding of two dissimilar aerospace alloys, Ti–6Al–4V and Al7075, was carried out at 500 °C using 22 μm thick Cu interlayers for various bonding times. Joint formation was attributed to the solid-state diffusion of Cu into the Ti alloy and Al7075 alloy followed by eutectic formation and isothermal solidification along the Cu/Al7075 interface. Examination of the joint region using SEM, EDS and XPS showed the formation of eutectic phases such as, ?(Al₂Cu), T(Al₂Mg₃Zn₃) and Al₁₃Fe along grain boundaries within the Al7075 matrix. At the Cu/Ti alloy bond interface a solid-state bond formed resulting in a Cu₃Ti₂ phase formation along this interface. The joint region homogenized with increasing bonding time and gave the highest bond strength of 19.5 MPa after a bonding time of 30 min.     

As received Ti–6AI–4V/σ-SiC fibre composite

Abstract

A Ti–6Al–4V/σ (SM 1240) composite prepared by diffusion bonding has been studied in the as received condition, using Auger electron spectroscopy, transmission electron energy loss spectroscopy, and scanning electron microscopy. The SiC based σ fibre has a tungsten core, and a duplex coating of carbon (adjacent to the SiC deposit) and TiB_x. It is shown that boron from the TiB_x layer diffused into the matrix and formed TiB needles. Carbon was detected in the TiB_x layer and was present in elemental free form. A continuous SiO₂+ carbon layer was detected at the SiC/carbon layer interface. Analysis of in situ fracture composite surfaces in an Auger spectrometer has shown that the tensile failure was initiated within the carbon layer or at the TiB_x/matrix interface. An oxide layer detected at the TiB_x/matrix interface influenced the fracture behaviour of the composite.

MST/2027     

Microindentation study of Ti–6Al–4V alloy

In order to study the micromechanical behavior of Ti–6Al–4V alloy, microindentation experiments were performed with five different maximum loads of 100, 150, 200, 250 and 300 mN, and with three loading speeds of 6.4560, 7.7473 and 9.6841 mN/s respectively. The experimental results revealed that loading speed has little influence on microhardness and Young’s modulus. Microindentation hardness experiments showed strong indentation size effects, i.e. increase of indentation hardness with the decrease of indentation load or depth. Then microindentation constitutive equation that described the stress as a function of the strain was proposed through dimensional analysis. And the finite element simulation results showed that the predicted computational indentation data from developed constitutive equation can track the microindentation experimental data of Ti–6Al–4V alloy.     

Transient liquid phase bonding of Sn–Bi solder with added Cu particles

The aim of this study was to apply the transient liquid phase (TLP) bonding technique to low-temperature Sn–Bi-based solders to enable their use in high-temperature applications. The microstructure of the eutectic Sn–Bi solders with and without added Cu particles was investigated with the solders sandwiched between two Cu substrates. The flux of the Cu atoms successfully consumed the Sn phase and resulted in the formation of Sn–Cu intermetallic compounds and a Bi-rich phase in the solder joint. This caused the melting point of the solder joint to increase from 139 to 201 °C. The results of this study show the potential of using low-temperature solders in high-temperature applications. This study also provides new insight into the advantages of using particles in the TLP bonding process.     

Compressive failure of alumina continuous fibre reinforced Al–4·5 wt-%Cu alloy at elevated temperatures

Abstract

Alumina continuous fibre reinforced Al–4·5 wt-%Cu alloy composite specimens were compressed in the axis direction at room temperature, 200°C, 300°C, and 400°C. The compressive stress–end shortening relationships at all test temperatures were similar to the elastic response, but with some non-linearity shortly before the macro failures. Composite compressive stress declined at elevated temperatures. The difference between the failure strength and the onset failure strength decreased with increase in temperature. The dominant failure mode at room temperature and 200°C was that of buckling, but it changed to kinking at elevated temperatures. Composite compressive behaviour at all test temperatures conformed to plastic buckling theory.     

Effect of primary alpha phase variation on mechanical behaviour of Ti–6Al–4V alloy

Small punch tests (SPTs) have been carried out at room temperature to correlate the microstructural variation of Ti–6Al–4V alloy with that of SPT parameters. Microstructural variation in terms of different volume fractions of primary alpha phase of Ti–6Al–4V alloy has been introduced as a result of solution annealing at different temperatures followed by thermal aging. Small punch test parameters, i.e. total area under the load vs displacement curve, area under the zone of elastic bending, plastic bending and plastic instability have been found to increase from the content of 10% primary alpha phase to 20% primary alpha phase and then these are decreasing from the content of 20% primary alpha phase to 30% primary alpha phase.     

Reaction products and growth kinetics during diffusion bonding of SiC ceramic to Ni–Cr alloy

Abstract

Vacuum diffusion bonding of SiC ceramic to Ni–25 at.-%Cr alloy was carried out at 1223–1323 K for 0.9–3.6 ks under a pressure of 7.2 MPa. The kinds of the reaction products and the interface structures of the joints were investigated by scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and X-ray diffraction (XRD), and the kinetic parameters describing the growth of reaction layers were calculated. The experiment and analysis identify that four kinds of reaction products, namely Ni₂Si, graphite (G), Ni₅Cr₃Si₂ and Cr₃Ni₂SiC, have formed during the diffusion bonding of SiC ceramic to Ni–25 at.-%Cr alloy. The interface structure of the SiC/Ni–Cr joints is SiC/(Ni₂Si+G)/(Cr₃Ni₂SiC + Ni₅Cr₃Si₂)/Ni₅Cr₃Si₂/Ni- Cr, and each reaction layer in this structure grows according to a parabolic law. The activation energies for growth of (Ni₂Si+G), (Cr₃Ni₂SiC+Ni₅Cr₃Si₂), and Ni₅Cr₃Si₂ are 217, 248, and 233 kJ mol ­ 1, respectively.     

Interface characterisation of diffusion bonded Ti–6Al–4V alloy and austenitic stainless steel couple

Abstract

In this study, the Ti–6Al–4V alloy was diffusion bonded to austenitic stainless steel at temperatures of 820, 885, 930 and 980°C, under a pressure of 5 MPa for 30 min. The effect of temperature on interface formations and microstructure was investigated using a scanning electron microscope (SEM), energy dispersive spectrograph (EDS) microanalyses, X-ray diffraction and shear strength of bonded specimens. The results showed that intermetallic phases and σ-phase formed in the interface region.     

Corrosion behavior of Ti–8Al–1Mo–1V alloy compared to Ti–6Al–4V

The corrosion behavior of Ti–8Al–1Mo–1V alloy was investigated in 3.5% NaCl and 5% HCl solutions. Corrosion properties of Ti–6Al–4V alloy were also evaluated under the same conditions for comparison. It was found that both Ti–8Al–1Mo–1V and Ti–6Al–4V alloys exhibited spontaneous passivity and low corrosion current densities in 3.5% NaCl solution. The potentiodynamic polarization curves obtained in 5% HCl solution revealed an active–passive transition behavior and similar corrosion rates for the examined alloys. However, the results of the weight loss experiments under accelerated immersion conditions (5 M HCl at 35 °C) indicated that Ti–8Al–1Mo–1V alloy exhibited inferior corrosion behavior compared to Ti–6Al–4V alloy. These results were confirmed by scanning electron microscopy (SEM) analysis of the samples after immersion tests which revealed that the β phase was corroded preferentially for both alloys, but to a larger extent in the case of Ti–8Al–1Mo–1V alloy.     

Finite element analysis of the stress distribution in a thermally and transversely loaded Ti–6Al–4V/SiC fibre composite

The load transfer between fibre and matrix in a metal matrix composite (MMC) depends on the properties and conditions of the fibre/matrix interfacial region. The objective of this investigation is to gain a better understanding of the stresses generated within a continuously reinforced MMC, particularly at this interface. Finite element analysis is used to investigate the effect of thermal and transverse mechanical loading on the SiC/Ti–6Al–4V composite system. The effect on the stress field of a carbon coating on the SiC fibres is also investigated. The results indicate that the interfacial region affects the stress distribution, with the presence of the carbon coating significantly altering the stress profiles generated. It is also found that the residual stresses generated as a result of cooling down the composite from processing temperature, has a marked effect on the stress profile and the behaviour of the composite when subsequent mechanical loading is applied.     

Thermal analysis and phase transformation behaviour during additive manufacturing of Ti–6Al–4V alloy

Despite the fact that the additive manufacturing (AM) technique has been established for almost two decades, its optimisation is still performed by trial and error experimentation. In the present work, a finite element modelling approach was used to study both the temperature distribution and heat flux vector characteristics during multi-layer deposition of a Ti–6Al–4V alloy that take place in the AM process. The results revealed the difference between different powder deposition time intervals on thermal cycles, heat flux vectors and the resulting microstructures. Good agreement between the numerical and experimental results was found. The results obtained can be used for process optimisation.     

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

Abstract

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

MST/593     

Diffusion bonding of alumina reinforced 6061 alloy metal matrix composite using Al–Li interlayer

Abstract

This article describes a study of the application of a solid state diffusion welding technique to an aluminium alloy (6061) matrix composite reinforced with alumina particles in two different percentages (10 and 20 vol.-%) using an Al–Li alloy interlayer. The influence of bonding parameters on joint formation and the effect of the reinforcement in the bond line were determined by microstructural study of the joints. Shear tests using single overlap joints were used to evaluate the strength of these bonds.     

Effects of SiC coating on ablation resistance of carbon fibre reinforced BN–Si3N4 matrix composite

Abstract

A SiC coating was prepared on the surface of a carbon fibre reinforced BN–Si₃N₄ composite by chemical vapour deposition. The coating was characterised by SEM and XRD, and the ablation behaviours of the coated and uncoated composites were investigated and compared. The coating is mainly amorphous SiC and quite compact; the ablated area of the composite is reduced considerably by the coating and the coated composite presents a lower linear ablation rate of 21˙4% and a lower mass ablation rate of 51˙6%. The SiC coating covers over the pores on the surface of the ablative composite, which prevents the flame from spreading to other regions and from penetrating the inside of the composite. As a result, both the chemical erosion and the mechanical denudation are restrained and the ablation resistance of the composite is improved.     

Role of Ti in direct active bonding of SiC substrate using Sn–Ag–Ti alloy filler

Journal of Materials Science: Materials in Electronics - Low-temperature active bonding of silicon carbide substrate using Sn3.5Ag4Ti(Ce,Ga) active solder filler was carried out at...     

Interfacial microstructure and strengthening mechanism in Ti–6Al–4V reinforced Al-7075 alloy

In the present study, an Al-7075 alloy reinforced with 15?wt-% TC4 (Ti–6Al–4V) was synthesised by hot pressing. The results of transmission electron microscopy suggest that a well-bonded multilayer interface was created, and that the multilayer has the ability to limit crack initiation at the interface. Sintering at 913?K for 0.5?h proved to be the most effective procedure for limiting the sources of cracking inside the multilayer interface, resulting in an enhanced tensile strength and a compressive strength for the composite of 454 and 873?MPa, respectively. The desired multilayer structure and the strengthening mechanism are discussed in detail. This structure provides guidance for the development of new methods for fabricating composites with excellent mechanical properties.     

Effect of hot working on flow behavior of Ti–6Al–4V alloy in single phase and two phase regions

Hot deformation behavior of the alloy Ti–6Al–4V was investigated via conducting hot compression tests at temperatures of 800–1150 °C and at strain rates, ranging from 0.001 s⁻¹ to 1 s⁻¹, at an interval of an order of magnitude. The apparent differences of flow stress curves obtained in dual phase α + β and single phase β regions were analyzed in term of different dependence of flow stress to temperature and strain rate and different microstructural evolutions. The values of strain rate sensitivity and apparent activation energy were obtained respectively as 0.20 and 530 kJ/mol for two phase microstructure. However, for single phase β microstructure they were approximated as 0.19 and 376 kJ/mol, respectively. It was found that in two phase region the values of strains corresponding to peak point, ε_p, and the highest rate of flow softening, ε^*, are almost independent to Zenner–Hollomon parameter. In single phase region, ε_p and ε^* exhibited a direct relationship to Z parameter and the corresponding empirical equations were proposed.     

Effect of cold rolling on superplasticity of Ti–6AI–4V sheet

Abstract

The tensile and bulging characteristics of Ti–6Al–4V alloy sheet have been investigated under superplastic conditions following various prior treatments. These prior treatments. consisted of cold rolling to different degrees of reduction and recrystallization. It was found that cold rolling markedly improves the superplasticity of the alloy, and even recrystallization, which causes grain coarsening, does not eliminate this effect. Moreover, the ratio of the total tensile elongation to the bulging strain to failure was found to decrease with increased prerolling.

MST/172     

Pretreatment process of SiC particles and fabrication technology of SiC particulate reinforced Zn–Al alloy matrix composite

Abstract

In the present paper, a novel pretreatment process for SiC particulate and a new mechanical–electromagnetic combination stirring process for fabricating Zn–Al(ZA27)/SiC_p composites are described. The optimal pretreatment route and the most appropriate SiC particle parameters were experimentally determined. The pretreated SiC particles were easily incorporated and dispersed in the ZA27 alloy melt and were not agglomerated before addition to the melt. The surface status of the SiC particles before and after pretreatment was observed and analysed by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, and transmission electron microscopy. It was found that gas existing on the SiC particle surfaces by physical and chemical absorption was a significant hindrance to the incorporation and dispersion of SiC particles in the alloy melt. The gas absorption was induced by ultrafine SiC powders, fracture steps, and ions existing on the SiC particle surfaces. The carbon, silicon, and oxygen contents on the SiC surface were varied with different pretreatment techniques. Moreover, a dense layer of amorphous SiO₂, which improves wetting of SiC particles in the ZA27 melt, was formed owing to calcination of SiC particles in air. The new combined stirring process exploits the advantages of both mechanical and electromagnetic stirring of the melt at the different processing stages during fabrication. The microstructural characteristics of the resulting composites are: homogeneously distributed SiC particles, fewer macro gas blows and inclusions, and little shrinkage porosity in comparison to composites fabricated by a mechanical stirring process. Finally, the mechanisms of degassing and reducing the porosity and the number of oxide inclusions are discussed.     

Effect of process parameters on semi-solid TLP bonding of Ti–6Al–4V to Mg–AZ31

The semi-solid transient liquid-phase bonding (Semi-solid TLP bonding) of titanium alloy Ti–6Al–4V to magnesium alloy Mg–AZ31 was performed using a eutectic forming nickel foil. The process parameters were optimized to achieve higher shear strength. The effect of temperature and pressure on microstructure evolution and mechanical characteristics were examined for bonding time between 5 and 60 min. Three reaction layers L1, L2 at Ni/Mg–AZ31 interface and L3 along the Ni/Ti–6Al–4V interface were determined within joint zone at a bonding temperature of 515 °C. The L1 and L2 reaction layers continued to be seen when the bonding temperature increased to 540 °C. When the bonding pressure increases from 0.2 to 0.7 MPa, a new reaction layer L4, at the Ni/Ti–6Al–4V interface was observed. The results showed that as the bonding time increased up to 60 min, the width of the joint decreased due to isothermal solidification. Maximum shear strength of 39 MPa was obtained for 540 °C and 0.2 MPa with a holding time of 20 min. However, further increase in bonding time to 60 min resulted in a decrease in shear strength to 8 MPa, and this decrease in strength was attributed to the increase in intermetallics forming within the joint zone.