On an effective approach for reducing TLP bonding time in single crystal superalloys

Excerpt

The ever-increasing demand for higher operating temperatures in aero- and land-based gas turbine engines has resulted in the use of advanced processing techniques and alloy design to produce two major microstructural features in the Ni-based superalloys: (i) a fewer number of grain boundaries and (ii) a higher stability and larger volume fraction of γ′ phase. The number of grain boundaries is reduced or altogether eliminated by resorting to directional solidification casting technique to produce directionally solidified polycrystalline and single crystal (SC) nickel-base superalloys. The aerospace industry has experienced a more extensive use of the higher performing SC superalloys in the past decade. Higher operating temperatures designed for better efficiencies also result in a faster degradation of hot section components through higher levels of creep, fatigue and oxidation. It is usually economically more attractive to repair damaged parts in lieu of a complete replacement. Traditional repair techniques, such as welding are commonly used to repair many superalloy components. However, SC nickel-base superalloys containing high volume fraction of the strengthening phase, γ′ precipitates, are extremely difficult to weld due to their high susceptibility to weld cracking [1]. Transient liquid-phase (TLP) bonding, also known as diffusion brazing, has evolved as an attractive alternate technique for joining difficult-to-weld superalloys due to its technological advantage in producing crack-free joint [2‐5]. The TLP bonding process involves the formation of a joint between two solid substrates through the diffusion-induced isothermal solidification of the melting-point depressant (MPD) solute-rich liquid phase that temporarily exists between the substrates at the bonding temperature [6‐9]. The technique also has a major advantage over conventional brazing in that it can be used to produce joints that are free of deleterious eutectic microconstituents that degrade mechanical properties of brazement. This is possible provided adequate holding time is used during bonding to achieve complete isothermal solidification of the liquid insert during bonding. Therefore, an important parameter in the consideration of TLP bonding for commercial applications is the holding time (t _f) required to complete the diffusional isothermal solidification during holding at the bonding temperature. Unfortunately, the processing time t _f during TLP bonding of SC superalloys is typically very long, which often limits the commercial appeal of the joining technique for industrial applications. One factor that has been suggested as a likely cause of the long t _f in SC alloys is the absence of grain boundaries in these materials, since intergranular regions are generally known to exhibit higher atomic diffusivity compared to bulk lattice diffusion. Nevertheless, a careful numerical modeling simulation and experimental study has showed that long t _f in SC superalloys can not be attributed to the absence of grain boundaries in these materials relative to conventionally cast polycrystalline alloys [6]. It has been, however, found that a fundamental factor that causes long t _f during TLP bonding is the deviation of diffusion-controlled liquid–solid interface migration from its parabolic relationship with holding time [7]. This deviation from parabolic behavior occurs due to continual diffusion-induced reduction of solute concentration gradient (∂C/∂x) in the base material below a critical value (∂C/∂x)_c. …