Effect of phase transformation and intermetallic compounds on the microstructure and tensile strength properties of diffusion-bonded joints between Ti–6Al–4V and AISI 304L

Abstract

The occurrence of a phase transformation and the effect of intermetallic compounds on the microstructure and tensile strength properties of diffusion-bonded (DB) joints between Ti–6Al–4V and AISI 304L were studied in the temperature range of 875–950 °C with an interval of 25 °C, a bonding time of 60 min and pressures of 4 MPa and 8 MPa. A maximum tensile strength of 242.6 MPa, was observed for diffusion-bonded joints that were processed at a temperature of 900 °C, bonded for 60 min at a pressure of 4 MPa and annealed for 2 h at 750 °C. Optical microscopy and scanning electron microscopy (SEM) were used to examine the grain growth and the fine details of the interface structure. Energy dispersive X-ray analysis (EDAX) and X-ray diffraction analysis (XRD) revealed the existence of intermetallic compounds and corroborated the phase transformation.

Introduction

Titanium alloys (Ti-alloys) are light and strong and they possess good corrosion resistance properties that favour their usage in aerospace and corrosion resistance applications [1]. Among titanium alloys, Ti–6A–4V is the most widely used alloy for aerospace applications because of its low density, high strength, and high temperature properties. The cost of Ti-alloy is very high compared to that of stainless steels. The cost and material consumption could be reduced by bonding Ti–6Al–4V with AISI 304L, with or without an interlayer, to achieve the desired properties. Fusion welding is not suitable for joining advanced materials because it leads to the formation of hard and brittle phases at the welded region. Because of the limitations of fusion welding, diffusion bonding (DB) is more widely used to join dissimilar materials of two different species. The DB process is capable of joining all kinds of materials whose chemical and metallurgical properties are suitable for engineering applications.

Several authors have reported the feasibility of DB of dissimilar materials, with and without the use of interlayer materials, to improve their physical and metallurgical properties. Vigraman et al. [2], [3] performed DB of dissimilar ferrous materials duplex stainless steel SAE 2205 with medium-carbon steel AISI 1035 and austenitic stainless steel AISI 304L to low-carbon steel with an interlayer material AISI 304L steel. The combined techniques of superplastic forming and DB of similar titanium alloys were investigated in the literature for the manufacture of advanced aircraft engine blades and heavy parts in an inert gas environment [4], [5]. Xun and Tan [6] reported the application of combined superplastic forming and diffusion bonding to manufacture hollow engine blades. The low temperature superplastic properties were studied, and the production of hollow panel diffusion-bonded structures was reported in Ref. [7]. The effect of alloying elements on the microstructure of dual phase Ti-alloy subjected to superplastic deformation was published in Ref. [8]. Different diffusion bonding techniques were developed to bond pure titanium and Ti-alloys with several other metal combinations. The formation of diffusion-bonded joints between pure ‘Ti’ and pure ‘Al’ was reported by Wei et al. [9]. Titanium aluminide samples were diffusion-bonded with alternate layers of pure ‘Al’ and ‘Ti’ in another published work [10]. In a separate study [11], the use of a composite barrier layer for DB of TiAl to steel was reported. Konga et al. [12] conducted hot-pack roll bonding of Ti–6Al–4V with TiAl and obtained high bond strength for the diffusion-bonded joints. Kundu et al. [13], [14] described the joint properties and microstructures of pure titanium diffusion bonded to austenitic stainless steel with a nickel interlayer. Torun and Celikyurek [15] performed the diffusion bonding of pure titanium with nickel under vacuum conditions and reported good bond formation; in addition, the highly polished DB samples were borided.

Yan et al. [16] performed vacuum hot-roll bonding of Ti-alloy and stainless steel using a nickel interlayer and obtained good tensile strength. The DB of Ti-alloy to stainless steel with an aluminium alloy interlayer was performed below the melting point of the aluminium alloy and reported by He et al. [17]. The interface microstructure and mechanical properties of DB joints between titanium and stainless steel with a niobium interlayer were studied by Kundu et al. [18]. The effect of the impulse pressure on the bonded samples between Ti-alloy and stainless steel was studied and presented by Yuan et al. [19]. Hot isostatic DB of Ti–6Al–4V and stainless steel (304 grade) was successfully performed and the joint strength was presented in the literature [20].

In the current work, direct DB of Ti–6Al–4V to AISI 304L was conducted by maintaining a vacuum at high temperatures and maintaining the surface finish requirements. The phase transformations that occurred at various temperatures and their effect on the tensile strength were studied. High pressures were applied prior to DB to plastically deform the surface asperities of the contact surfaces, finishing the surface and breaking the thin oxide layer formed after mechanical and chemical cleaning. The experimental evidence indicated the presence of intermetallic compounds at the interface and a phase transformation in the solid state. The diffusion-bonded joints exhibited brittleness and deleterious effects on strength of the joints when they were subjected to cyclic heating and cooling. However, the study was conducted to understand the phase transformations taking place at the joint region. This material combination could be used for corrosion resistance applications, but these joints should not be exposed to higher temperatures for longer duration.