Diffusion Bonding of CMSX-4 to Udimet 720 Using PVD-Coated Interfaces and HIP

Abstract

There is an increasing interest in development of manufacturing methods for Dual Property BLISKs, consisting of creep resistant airfoils and fatigue resistant disks bonded together by a durable joint. Optimum heat treatments are, however, very different for creep resistant single crystal CMSX-4 and fatigue resistant poly-crystalline Udimet 720 selected in this study, but fortunately the first ageing treatment for CMSX-4 (1140°C, 2–6h, AC) is similar to the partial solution treatment of U 720 HS2 (1115°C, 4h, OQ). Based on this, diffusion bonding was performed by HIP at 1120°C and 200 MPa argon pressure for 4h, followed by cooling to 400°C. Subsequently, a shortened Udimet 720 HS2 two-step ageing treatment was adopted by heating to 650°C for 6h followed by cooling to 400°C, heating to 760°C for 2h, and finally cooling to R.T. under remaining HIP pressure. Plasma etching followed by thin (80 nm) PVD coating with either nickel or titanium were used to clean and protect the polished surfaces before joining. The selection of coatings was governed by the possibility to reduce oxidized nickel by flushing with hydrogen at 330°C during evacuation of the HIP capsules, and by the large solubility of oxygen in titanium. Hot tensile testing was performed at 750°C on both joined and reference materials subjected to the modified heat treatment. Initially solution treated Udimet 720 and CMSX-4 comprised the reference materials. The testing showed that joints with Ni-PVD coatings were almost as strong as Udimet 720 (although with very limited elongation), while the joints with Ti-PVD coatings were weaker.