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.