Abstract
This work aims to investigate the joining of Ti6Al4V alloy to alumina by diffusion bonding using titanium interlayers: thin films (1 µm) and commercial titanium foils (5 µm). The Ti thin films were deposited by magnetron sputtering onto alumina. The joints were processed at 900, 950, and 1000 °C, dwell time of 10 and 60 min, under contact pressure. Experiments without interlayer were performed for comparison purposes. Microstructural characterization of the interfaces was conducted by optical microscopy (OM), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), and electron backscatter diffraction (EBSD). The mechanical characterization of the joints was performed by nanoindentation to obtain hardness and reduced Young’s modulus distribution maps and shear strength tests. Joints processed without interlayer have only been achieved at 1000 °C. Conversely, joints processed using Ti thin films as interlayer showed promising results at temperatures of 950 °C for 60 min and 1000 °C for 10 and 60 min, under low pressure. The Ti adhesion to the alumina is a critical aspect of the diffusion bonding process and the joints produced with Ti freestanding foils were unsuccessful. The nanoindentation results revealed that the interfaces show hardness and reduced Young modulus, which reflect the observed microstructure. The average shear strength values are similar for all joints tested (52 ± 14 MPa for the joint processed without interlayer and 49 ± 25 MPa for the joint processed with interlayer), which confirms that the use of the Ti thin film improves the diffusion bonding of the Ti6Al4V alloy to alumina, enabling a decrease in the joining temperature and time.
Highlights
Ti6Al4V is the most used commercial titanium alloy due to its excellent performance and attractive properties, such as high-temperature specific strength, low density, excellent creep, and corrosion resistance [1]
Titanium is the most used interlayer; for instance, it reacts with Al2O3 by diffusion bonding resulting in the formation of TiO, TiAl, and Ti3Al [8]
The use of interlayers in diffusion bonding can be one option to reduce the bonding temperature, pressure and time and make the process more industrially attractive [39]. This may be even more important in metal-ceramic joining, when the joint is subjected to thermal cycling or thermal shock, large stress concentrations may be introduced in parts of the ceramic
Summary
Ti6Al4V is the most used commercial titanium alloy due to its excellent performance and attractive properties, such as high-temperature specific strength, low density, excellent creep, and corrosion resistance [1]. Obtaining sound joints with good mechanical properties between titanium alloys and ceramic materials has been challenging due to their different properties, e.g., thermal conductivity, coefficient of thermal expansion (CTE), and chemical properties This mismatch of properties induces the formation of residual stresses at the joint’s interface during cooling. The Ti interlayer was selected to allow an interface with a chemical composition similar to one of the base materials, avoiding the formation of phases that will impair the service temperature and with characteristics that promote the diffusion, enabling a decrease in the diffusion bonding processing conditions. The microstructural characterization of the joints’ interface was carried out by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and electron backscatter diffraction (EBSD), while the mechanical characterization was performed by nanoindentation tests across the joints’ interface and shear strength tests
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