The composite materials of carbon fibres have eventually replaced the metals and metal alloys in the aircraft manufacturing industry. However, aluminium alloys are still playing a significant role in making aircraft structures like fuselage and wings due to their high strength-to-weight ratio, corrosion resistance and good machinability properties [1]. Aluminium alloy (AA) 2219 is an effective structural material widely used in aerospace applications, including space launch vehicles. It offers high specific strength, good fracture toughness, and excellent stress corrosion resistance. The composition, with copper as the primary alloying element, gives it superior wear and corrosion resistance compared to other aluminium alloys. Its inherent strength is significantly enhanced during solidification due to the presence of ductile copper. AA2219 is precipitation-hardenable, containing additional elements like manganese, iron, zinc, and titanium. It has favourable mechanical properties and fracture toughness across a wide temperature range. Its exceptional qualities make it a globally significant material for cryogenic applications, especially in constructing cryogenic fuel tanks for storing liquid oxygen and hydrogen [2–6]. On the other hand, Ti-6Al-4V is an alpha-beta alloy with excellent properties like high strength, lightweight, fracture toughness, and high-temperature strength. It is commonly used in airframe structures, engine parts, automobile connecting rods, and engine valves [7–8].
The direct bonding of dissimilar materials can create bonded surfaces that are corrosion-resistant and highly conductive. These bonded interfaces may need the creation of small joints for lightweight and portable technological applications [9]. The joining of aluminium and titanium alloys has gained much attention to produce high-performance and lightweight components for aerospace, marine, and military applications [10]. However, joining dissimilar metals by conventional techniques such as fusion welding is difficult as the intermetallic and cracks form at the weld nugget area of the joints due to the differences in cooling rate between the weld area and parent materials. Moreover, it is not feasible to join aluminium and titanium due to significant differences in their thermal and mechanical properties [11–12]. SSDB is a kind of solid state welding that joins similar and dissimilar metals without using any secondary metal phase and retains the parent metal microstructure and mechanical properties. SSDB requires specific combinations of an elevated temperature in the range of 0.5–0.9 times the melting temperature of the base metal to necessitate the molecular activity and grain expulsion across the interface, adequate pressure to facilitate proper contact of metals to be joined and optimum bonding time to produce strong joints. Furthermore, several process parameter combinations would be experimented with to achieve good metallurgical bonds. However, a high bonding temperature is not always necessary for some metal joints, where favourable mechanical properties can also occur at lower bonding temperatures [13–18]. The strong and stable oxide stability and reasonable oxygen solubility of aluminium at higher temperatures make the diffusion bonding difficult by inhibiting the diffusion of atoms, resulting in a weak bond. Hence, it is essential to chemically cleanse the aluminium surface before it is kept for bonding. Titanium, on the other hand, has high oxygen solubility, and the stable oxide dissolves gradually at higher temperatures [19–22].
Few studies are obtainable on solid state diffusion bonding of aluminium and titanium alloys. Akca et al. [23] examined the SSDB process using Ti-6Al-4V and pure aluminium at bonding temperatures ranging from 520–640°C. Interestingly, this temperature range is quite close to the melting temperature of aluminium. It was found that successful bonding occurred only after a bonding duration of 60 minutes, highlighting the importance of optimal bonding temperature, and holding time for producing good bonding joints. Rajkumar et al. [24] achieved SSDB joints between AA7075 and pure titanium, and obtained maximum shear strength of 87 MPa at a bonding temperature of 510°C, a holding time of 37 minutes, and a bonding pressure of 15 MPa. Notably, this bonding pressure is relatively high for joining aluminium and titanium. Wei et al. [25] expounded the diffusion bonding of pure titanium and aluminium with a bonding temperature range of 500–650°C and observed joint strength near that of pure Al. However, the study utilised a high bonding temperature near the melting point of pure Al and an exceptionally long holding time of 600 minutes. Additionally, the intermetallic compound TiAl3 has been consistently observed at the interface of bonding joints in all these studies. The intermetallic formation is inevitable during the solid-state bonding process of aluminium and titanium. Furthermore, the presence of TiAl3 significantly affects the mechanical properties of the joints.
The direct diffusion bonding studies on joining aluminium and titanium alloys are scanty, and furthermore, no studies are found on the solid state diffusion bonding of AA2219 and Ti-6Al-4V. This research aims to illustrate the SSDB between AA2219 and Ti-6Al-4V. Moreover, the microstructure and mechanical properties at various bonding temperatures are investigated.