Majid Etminanbakhsh

The Kinetics of TiAl3 Formation in Explosively Welded Ti-Al Multilayers During Heat Treatment

Abstract

Metallic-intermetallic laminate (MIL) composites are promising materials for many applications, namely in the aerospace industry. One of the interesting laminate composites is Ti/TiAl3. One method to produce these composites is to put titanium and aluminum close to each other and prepare a situation for intermetallic formation by applying temperature and pressure. For this reason studying the kinetic of the intermetallic formation in Ti-Al system can be interesting. In this work, six alternative layers of Ti and Al were putted together and explosively welded. As welded samples annealed at three different temperatures; 630, 600 and 570 ℃ in ambient atmosphere and intermetallic thickness variation by time was calculated. Microstructural investigations were carried out using optical and scanning electron microscope equipped with Energy Dispersive Spectroscopy (EDS) and X-ray diffraction (XRD) technique. The profile of micro-hardness of the layers was also determined. The thickness and the kind of Al-Ti intermetallics were determined. It was found that at each temperature, two different mechanisms exist; reaction controlled and diffusion controlled. Activation energies for reaction controlled and diffusion controlled were 232.1 kJ and 17.4 kJ, respectively.

Keywords: TiAl3; Annealing; Activation Energy; Explosive Welding

  1. Introduction

Over the past several decades, great demand has been placed on the metallurgical community for the development of high strength/low weight substitutes for conventional nickel and cobalt-based superalloys for gas turbine use, automotive and aerospace industry. This has generated a great deal of interesting the study of intermetallic compounds [1]. Aluminum-rich titanium aluminides are currently being considered as potential materials for intermediate temperature applications (700-900°C) due to their low densities and good oxidation resistances. [1-4]. However, these materials have been traditionally considered as brittle, demonstrating low ductility and poor fracture toughness, which has to a large extent limited their practical application [1, 2, 5]

When two metals which form intermediate alloy phases with each other inter-diffuse, layers of the intermediate phases grow between the two metals. This type of chemical diffusion is of commercial Importance in the coating or cladding of one metal with another and in the diffusion bonding or pressure welding of different metals [6].

Solid-state interactions resulting in the formation of new chemical compounds effectively control properties and performance of a variety of materials. Good examples are metal/metal and metal/ceramic composites, bonded and coated components, thin-film electronic devices [7].

In recent years, there has been considerable interest in the design, fabrication and mechanical behavior of a variety of multilayered or laminated composites, such as ceramic–ceramic, metal–ceramic, metal–metal and metal–intermetallic systems. It has been shown that enhanced strength and toughness in combination with improved damage resistance of these composites can be achieved. In particular, Ti–TiAl3 or Ni–Ni3Al metal–intermetallic laminate (MIL) composite systems have a great potential for aerospace, automotive and other structural applications because of their combination of high strength, toughness and stiffness at a lower density than monolithic titanium or other laminate systems. Furthermore, since Al is relatively inexpensive compared to Ti or Ni, the Ti– TiAl3 or Ni–Ni3Al systems are economically more attractive than monolithic Ti or Ni. The design concept of these two kinds of material systems is schematically illustrated in ‎Fig. 1. The ceramic-like aluminide phases (TiAl3 or Ni3Al) give high hardness and stiffness to the composite, while the unreacted Ti or Ni provides the necessary high strength, toughness and ductility for the system to concurrently be flexible [8, 9]. Vecchio has shown that the specific stiffness of “Ti-Al3Ti” is twice that of the specific stiffness of steel. Beryllium and its alloys are the only metallic materials of higher specific stiffness. These positive results for Ti-Al3Ti composites were obtained in tests for static fracture toughness and fatigue crack resistance [10].

  • 1 A schematic vertical cross-section of the Ti–TiAl3 composites before and after the reactive foil sintering process [8].

So in the majority of metallic systems, the kinetics of growth of an intermetallic compound phase layer at an interface between dissimilar metals is controlled by the diffusional supply of components to the reaction front through the layer of reaction products being formed. In some cases, however, the controlling stage of the intermetallic compound phase formation process is the actual chemical reaction [11].

Of the numerous processing techniques available for the formation of a titanium aluminide based LMC, two processes in particular lend themselves well to the synthesis of a Ti–Ti aluminide composite: diffusion bonding and combustion synthesis. Diffusion bonding of elemental titanium and aluminum foils to produce an alternating Ti – Ti aluminide LMC structure provides a low temperature approach to the formation of a composite, allowing growth of the intermetallic phase(s) to occur while both reactant materials (presumably Ti and Al foils) are in the solid state. Combustion synthesis reactions are yet another feasible technique to form metal – intermetallic composites, which require minimal energy input to the process and offer reduced processing times [1]. One method of diffusional bonding of Ti and Al is reactive foil sintering. This technology involves heating foils of the pure material under conditions of 2-4 MPa pressure and higher. The process can take place in an inert atmosphere or vacuum, as well as in the air [8, 10, 12, 13].

Explosive welding is a unique process for producing bimetallic and multilayer compounds with high mechanical properties. The high strength and fracture toughness of explosively welded materials are due to the particular structural state of the welds formed by the dynamic interaction of work pieces [14]. Explosive welding offers an excellent alternative for joining dissimilar metals and alloys with varying physical and metallurgical properties. It is a solid state metal joining process which produces a weld joint by high velocity oblique impact, aided by controlled detonation with an explosive charge [15-17]. Furthermore, the process can be applied to a broad range of thicknesses and area dimensions due to the ability to distribute the high energy of the explosive over the entire welding areas [18]. There are two different setup for explosive welding, parallel setup and inclined setup (setup with initial angle) [19].

Unlike the pressure-applying methods such reactive foil sintering, explosive welding provides a very good contact between layers before heat treatment of layers, without the need to apply high pressure. Preparing metallic layers by explosively welding and heat treatment of multilayers in ambient atmosphere is a very cheap process for producing MIL composites. Kinetics of intermetallic formation in Ti-Al multilayers have been studied by several investigators [11, 20, 21]. However there is lack of information about kinetics of the formation of intermetallics during the heat treatment of explosively welded Al-Ti multilayers. Also in many research activation energy were measured but the achieved activation energy vary widely among different works [22-25]. Therefore, present work is undertaken to study the kinetics of the formation of intermetallics during annealing of explosively welded Al-Ti multilayers and to calculate activation energy in Ti-Al system.

  1. Experimental Procedures
    • Materials and Explosive Welding

Six alternative sheets of aluminum and cp-titanium were explosively welded together. Initial thickness of aluminum and titanium was 0.5 mm and 483 mm, respectively. The chemical composition of initial materials is presented in ‎Table 1. As shown in ‎Fig. 2, parallel set-up for explosive welding was used. Ammonium nitrate mixed with TNT and gas oil used as explosive. The detonation velocity was 4500 m/s. Explosive thickness was equal to 20 mm. The welding assembly was placed on a wooden plate placed on a sand bed. Cold rolling was performed on the as-welded plates to straighten the product.