Subrata Chatterjee

Influence of thermo-mechanical processing and different post-cooling techniques on structure and properties of an ultra low carbon Cu bearing HSLA forging

An ultra low carbon Cu bearing HSLA steel, micro alloyed with Nb and Ti was forged in two stages and subsequently cooled at different cooling rates. Variation in microstructures and the mechanical properties at different cooling rates have been studied. Volume fraction of bainitic phase has been found to vary with different cooling rates. Maximum strength was achieved in this steel at an intermediate cooling due to precipitation hardening of very fine microalloying carbides, carbonitrides and e-Cu precipitates. Finer polygonal ferrite and granular bainite structure obtained at the slowest cooling rate resulted in high impact toughness values at ambient and sub ambient temperatures.

Effect of bonding temperature on interface microstructure and properties of titanium-304 stainless steel diffusion bonded joints with Ni interlayer

Abstract The diffusion bonding was carried out between commercially pure titanium and 304 stainless steel (SS) using nickel as an intermediate material in the temperature range of 800–950°C for 1·8 ks under a 3 MPa uniaxial load in vacuum. The microstructures of the transition joints were revealed in optical and scanning electron microscopy (SEM). TiNi3, TiNi and Ti2 Ni are formed at the nickel/titanium interface whereas, SS–Ni diffusion zones is free from intermetallic phases for all bonding temperatures. At 950°C, Ni/Ti interface exhibits the presence of α-β Ti discrete islands in the matrix of Ti2Ni intermetallic. The bond strength was evaluated and maximum tensile strength of ∼281 MPa along with 7·2% ductility and shear strength of ∼202 MPa were obtained for the diffusion couple processed at 900°C owing to the better coalescence of the mating surfaces. With increasing joining temperature to 950°C, the bond strength drops owing to an increase in the width of the nickel–titanium based reaction products. At lower joining temperatures, bond strengths are also lower owing to incomplete coalescence of the mating surfaces. The activation energy and growth constant were calculated in the temperature range of 800–950°C for the reaction layers and it was found that, the values were maximum for Ti2Ni. Observation of fracture surfaces in SEM using energy dispersion spectroscopy (EDS) demonstrates that, failure takes place mainly through nickel/titanium interface.

Microstructure and mechanical properties of diffusion bonded joints between titanium and stainless steel with copper interlayer

Abstract The solid state joining of titanium to stainless steel with copper interlayer was carried out in the temperature range of 850–950°C for 7·2 ks in vacuum. The interface microstructures and reaction products of the transition joints were investigated with an optical microscope and a scanning electron microscope. The elemental concentration of reaction products at the diffusion interfaces was evaluated by electron probe microanalysis. The occurrence of difference in intermetallics at both interfaces (SS/Cu and Cu/Ti) such as CuTi2, CuTi, Cu4Ti3, χ, FeTi, Fe2Ti, Cr2Ti, α-Fe, α-Ti, β-Ti, T2(Ti40Cu60−xFex; 5<x<17), T4(Ti37Cu63−xFex; 5<x<7) and T5(Ti45Cu55−xFex; 4<x<5) has been predicted from the ternary phase diagrams of Fe–Cu–Ti and Fe–Cr–Ti. These reaction products were detected by X-ray diffraction technique. The maximum tensile strength of ∼91% of Ti strength and shear strength of ∼74% of Ti strength along with ∼ 7·2% ductility were obtained for the joint bonded at 900°C due to better coalescence of mating surfaces. At a lower joining temperature of 850° C, bond strength is poor due to incomplete coalescence of the mating surfaces. With an increase in the joining temperature to 950°C, a decrease in bond strength occurred due to an increase in the volume fraction of brittle Fe–Ti base intermetallics.

Effects of temperature on interface microstructure and strength properties of titanium–niobium stainless steel diffusion bonded joints

Abstract The effects of temperature on interface microstructure and strength properties of Ti/stainless diffusion bonded joint using Nb interlayer, processed in the temperature range 800–950°C for 1·5 h in vacuum were investigated. The stainless steel/Nb interface is free from intermetallic phase up to 900°C; however, Fe2Nb+Fe7Nb6 phase mixture has been observed at 950°C processing temperature. The Nb/Ti interface is free from intermetallic for all processing temperatures. The maximum tensile strength of ∼287 MPa (∼90% of Ti) and shear strength ∼222 MPa (∼75% of Ti) along with 6·9% ductility have been achieved in the diffusion bonded joints, when processed at 900°C. The bonded samples failure takes place through the stainless steel/Nb interface for all processing temperatures during the loading.

Scope for improved properties of dissimilar joints of ferrous and non-ferrous metals

Abstract Dissimilar joints (DSJs) of ferrous and non-ferrous metals have huge technological importance in the frontiers of new designs in new machineries and improved design of conventional systems. This investigation was undertaken to improve mechanical properties of joints of two dissimilar metals: one is Ti-based and the other is Fe-based. DSJs were processed using bonding pressure from 1 to 9 MPa in step of 2 MPa at 750 °C for 60 min. Properties of the DSJs of these two metals using different mechanisms and methods were compared with the present research for verification. Experimental results from the diffusion bonding mechanism for joining the dissimilar metals validated the improvement in properties. Superior mechanical properties of dissimilar-metals joints were achieved mainly due to the third non-ferrous metallic foil, Ni of ~ 200-μm thickness, which avoided the formation of brittle Fe–Ti-based intermetallics in the diffusion zone. DSJs processed are able to achieve maximum strength of ~560 MPa along with substantial ductility of ~11.9%, which is the best ever reported in the literatures so far. Work hardening effect was detected in the DSJs when the bonding was processed at 5 MPa and above. Bulging ratio of the non-ferrous metal (Ti-based) was much higher than that of the ferrous metal (SS) of the DSJs processed. SEM analysis was carried out to know the details of reaction zone, while XRD was carried out to support the SEM results. Reasons for change in mechanical, physical, and fracture properties of the DSJs with the process parameter variations were clarified.

Metallurgists’ Insights on Diffusion-Assisted Dissimilar-Joints of Light- and Heavy-Alloys

In metallurgy and materials engineering, a number of phase transformation in solids like precipitation, oxidation, creep, annealing, homogenization, etc. are brought about by the process of diffusion. Many industrial manufacturing processes utilize solid-state diffusion principle, to name a few: 1. Rotating or sliding parts of steel have a hard outside case for wear resistance and a tough inner core for fracture resistance by gas carburizing procedure; 2. Integrated circuits were produced by diffusing impurity into silicon wafers; and 3. Joints between similar and dissimilar metals, alloys, and non-metals, were made using diffusion bonding (DB) technique. Day by day, the science of solid-state diffusion phenomenon is spreading inevitably into new areas of engineering and technology. Diffusion-Assisted-Joints (DAJs) meet the requirements for most critical structures in terms of strength, toughness, tightness, and resistance to heat and corrosion. DAJs can be made out of 730 pairs of dissimilar metals. Hence, DB is considered as an engineering marvel among all the physical welding metallurgists. Herein, experiments were performed to exactly map the quantum influence of the bonding temperature variation on the dissimilar joints of a popular light alloy, Ti-6Al-4V (TiA), and a heavily used heavy alloy, stainless steel (SS), using diffusion mechanism in high-vacuum environment. Cu foil (~200 μm) was used as an interlayer. Necessary characterization tools for metallurgical investigations were used to understand the extent of diffusion along the TiA/Cu and Cu/SS interfaces, room-temperature mechanical properties, fracture morphologies, and fracture path of the TiA/Cu/SS DAJs. This paper discussed rational reasons backing the results of the characterizations.