Diffusion bonding of CMSX-4 to UDIMET 720 using PVD-coated interfaces and HIP

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