Shear-induced, solid-state joining of Ti-6Al-4V and Ti17 titanium alloys
Introduction
Solid-state joining (SSJ) of dissimilar titanium alloys is an attractive technology for many aerospace applications. In the past, SSJ by inertia-friction, linear-friction, and friction-stir welding [1], [2] has been shown to provide a number of benefits relative to arc welding due to short joining time, direct heat input at the weld interface, and a relatively-thin heat-affected zone [3]. Furthermore, friction-welding techniques are generally melt-free, enabling better control over grain size and structure, although the peak temperature at and near the weld interface can be very high and lead to the formation of brittle intermetallic compounds or other defects/porosity. For SSJ of Ti-6Al-4V and β21S, for example, the complex thermomechanical conditions in friction-stir welding have been observed to result in non-uniformity in the flow pattern, composition, and microstructure [4], all of which can be related to pin-rotation and translation effects. In other work [5], it was shown that rapid heating above the β-transus temperature and insufficient time at high temperature results in an inhomogeneous distribution of alloying elements and non-equilibrium microstructures in the weld zone [5]. The difference in mechanical and physical properties between the two joined materials can also result in the development of substantial residual stresses during cooling following the joining operation per se.
The focus of the present work was on the development of a SSJ process involving shear-induced inter-mixing and inter-diffusion at temperatures below the β-transus. In particular, the joining of sheet panels of dissimilar alloys via asymmetric rolling was investigated. This metal-forming operation induces severe shear as well as dilatational strains and thus enhances inter-mixing. The high strains also lead to nanostructuring of the material within the interface region and thereby accelerated inter-diffusion [6]; i.e., the fine grain size increases the number of short-circuit paths for mass transport via grain-boundary diffusion and results in the formation of ultra-fast diffusion paths [7]. In addition, large shear strains promote the formation of vacancies, vacancy clusters, subgrain structures, and micro-shear bands. These features all contribute to an increase in both the bulk and grain-boundary diffusivities [8]. Therefore, asymmetric rolling of dissimilar titanium alloys might produce high-integrity bonds. The mechanical properties following joining could also be enhanced due to interfacial shear mixing and the formation of intermetallic compounds and/or unique microstructures within the interface zone. In the past, it has been demonstrated that high levels of nanostructuring cannot be achieved in single-phase alloys, however [9], [10]. The importance of high shear deformation was not recognized until recently, but it may be crucial for the co-deformation and bonding of dissimilar alloys.
In the present work, the deformation and bonding of Ti-6Al-4V (Ti64) and Ti17 during symmetric rolling (SR) and asymmetric rolling (AR) were compared. This work provided insight into the mechanisms of bond formation under conditions of large shear and high hydrostatic pressure.
Section snippets
Materials
Ti64 and Ti17 sheets with dimensions 200 mm × 450 mm × 1.5 mm produced by Timet Division of PCC were used in this investigation. The final step in the process used by Timet was a heat treatment comprising 1110 K/4 h/water quench + 1140 K/4 h/water quench + 868 K/8 h/air cool. The nominal compositions of the two materials are summarized in Table 1. Preliminary rolling of the as-received materials to a 50% thickness reduction at RT and 773 K revealed a low ductility for Ti17; continuous cracks
Effect of rolling asymmetry dr on microstructure evolution and joint formation
BSE-SEM observations revealed the effect of dr on microstructure evolution and the nature of interface bonding in fine-ground, no-shift samples. A deformed α+β structure elongated along the RD was observed in both Ti64 and Ti17 for all processing trials (Fig. 5). The area fraction of primary α-particles in both materials is shown in Fig. 6. The results suggest it has not changed as a result of deformation and has not been affected by thick α-platelets. Furthermore, there were no obvious signs
Summary and conclusions
The effect of roll asymmetry and the location of the interface (contact) plane on the solid-state joining of Ti-6Al-4V to Ti17 sheets was established. It was found that bond formation due to high, local shear strain and interface pressure results from a combination of mechanical intermixing and inter-diffusion. No evidence of the formation of intermetallic phases was observed. The thickness and strength of the bonded zone is proportional to the level of shear strain. The development of a
Acknowledgements
This research was supported by the Asian Office of Airspace Research and Development (AOARD) of the US Air Force Office of Scientific Research (AFOSR), grant number FA2386-16-1-4049. Deakin University's Advanced Characterisation Facility is acknowledged for use of the FEI Quanta 3D SEM and JEOL 2100F TEM instruments and assistance from Dr. Mark Nave and Ms. Rosey Squire. The authors also acknowledge use of facilities within the Monash Centre for Electron Microscopy, particular, the use of FIE
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