In recent years there has been a growing interest in the use of diffusion bonding as a method for the joining of titanium and its alloys, particularly when accompanied by superplastic forming.
Liquid Phase Diffusion Bonding
Liquid-phase bonding relies on the formation of a transient liquid film at the interface which reduces the requirement for accurate fit-up and also accelerates the diffusion process. With solid state diffusion bonding there is more need to have good fit-up before bonding takes place although the combined effect of pressure and temperature means that surfaces tend to be squeezed together. Titanium and its alloys are good materials for diffusion bonding since their surface oxide films are readily dissolved at the temperatures encountered in the process.
With liquid-phase diffusion bonding a layer of another metal is placed along the interface between the titanium surfaces. This metal is normally one which forms a low melting point eutectic with titanium. When heat is applied at this stage a brazing operation takes place and liquid metal flows to accommodate any irregularities in the fit-up. During the later stages the liquid film diffuses into the base material so that by the end of the thermal cycle the bond line is completely solid. A range of materials including aluminium, copper, silver, manganese, iron and nickel have been used as accelerators, of these copper and nickel give the best results.
Solid State Diffusion Bonding
With solid-state diffusion bonding, the surfaces to be joined must be flat before joining, the temperature must be restricted to about 950°C, and the time taken must be less than 2 hours. Bonding pressures of 3.3 to 7 MPa are required and another difficulty is that the process must be carried out in argon or under a vacuum. However, these conditions can be met in the superplastic forming process which is why the two techniques are compatible.
Since high interfacial pressure and plastic deformation at elevated temperatures are beneficial in promoting solid state bonding, the joining operation can sometimes be combined with hot working processes used to reduce the section of the material. Roll diffusion bonding is an example of such a process. Here, titanium alloy components are supported in a steel envelope which is evacuated and sealed prior to hot working. A total of between 75 and 90% reduction is effected at temperatures around 900°C and the steel can subsequently be removed by pickling. The welds are characterised by good strength, ductility, and corrosion resistance but there are difficulties associated with surface quality and control of processing variables which influence the dimensional accuracy of the product.