A composite which combines good thermal resistance and high mechanical properties of titanium alloys with good plasticity and low density of aluminium alloys seems to be a promising materials for airspace applications. However, welding of titanium and aluminium is difficult because they are extremely chemically reactive with oxygen and nitrogen at high temperatures. In this concept, the explosive welding (EXW) is one of most promising solid-state welding methods because it gives an opportunity to form the bonding on over the entire junction surface. In order to understand the phenomena taking place at joined surfaces, the detailed microstructural analysis is required, which was a prime goal of the present work.

Ti6Al4V and AA2519 plates 5 mm in thickness were joined via EXW with a detonation speed of 2000 m/s. The samples for scanning electron microscopy (SEM) observations were prepared by using HITACHI IM-4000 ion milling system, which is a damage-less polishing process and allows to see the structure on  SEM through the channelling contrast. SEM observation was carried out in a HITACHI SU-70 field emission Schottky scanning electron microscope at 10 kV accelerating voltage with the in-lens back-scatter electron (BSE) mode at 7mm working distance to get as much as possible surface contrast and resolution. Careful observations of selected areas were carried out on CS-corrected dedicated scanning transmission electron microscope (STEM) Hitachi HD-2700 operating at the accelerating voltage of 200 kV. The STEM thin foilswere prepared via in-situ lift-out in Hitachi NB 5000 focused ion beam. Precise EDX analysis were carried out on FEI Tecnai Osiris  200 kV microscope using Super-X EDX detectors.

Figure 1a shows an overview of a Ti-Al interface. EXW caused a high grain refinement in aluminium plate. The interface zone can be divided into two sections. The first one is  5 µm thickness zone consisted of nano-sized grains of three intermetallic phases: TiAl₃, TiAl₂ and TiAl which were identified by X-ray analysis and electron diffraction. The second zone consist of a small aluminium grains and a white (BSE) Cu rich net around them (Figure 1b). These intermetallic inclusions were often accompanied by nano-cracks (Figure 1b). Figure 2a  shows the magnified structure of this interfacial area in High-Angle Annular Dark-Field (HAADF)-STEM mode. The high resolution imaging (Figure 2b) shows that the copper net region have a crystalline structure. EDX mapping presented on Figure 3 shows the distribution of Ti, Al and Cu in the matrix and a gradient distribution of these alloying compounds in the structure. Copper net phenomenon was explained by the breaking or dissolving of big rich in copper equilibrium particles during explosive welding. Supersaturated solid solution of aluminium cannot contain all dissolved copper, so its excess was secondarily separated in the grain boundaries. 

Figures:

Figure 1. SEM BSE image of (a) the joint structure, (b) the copper net showing nano-cracks

Figure 2. a) STEM cross-section (montage) micrograph of the joint structure, (b) HRSTEM of the grain boundary rich in copper

Figure 3. EDX maps showing the distribution of Ti, Al and Cu in the joint structure

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EMC Abstracts - https://emc-proceedings.com/abstract/characterization-of-the-microstructure-of-composite-ti6al4v-aa2519-obtained-by-explosive-welding/