A Scientific Approach to the Process Development Bonded Attachments for High-Speed Rotor Application

The quest for increased work per stage of compression to reduce overall gas turbine engine system cost has placed extreme demands on the high-pressure turbine (HPT) system. As an example, the HPT is required to operate at unprecedented levels of $AN2$ (the product of turbine annulus area and mechanical speed squared) to enable compressor performance goals to be met. The typical approach of mechanically attaching blades via firtree or dovetail configured mechanical attachments, limits rotor speed because of the life limiting broach slots (stress concentrators) in the disk rim. Exacerbating this problem is the fact that the disk lugs, which react the blade loading, impose a dead load. Higher disk speed results in higher blade loading requiring a deeper or wider lug to support the blade. This in turn results in a wider disk bore to support the deeper, dead load lug region. The dilemma is that higher speed results in larger stress concentrations at the rim and a wider disk bore to support the added parasitic rim load. The answer to this dilemma lies in creating an integrally bladed rotor (IBR) in which the blades are integral with the disk. Since typically, for an HPT, the blades are single crystal and the disk equiaxed nickel alloys, the IBR design suggested precludes absolute machining as the fabrication approach. A solution lies in metallurgically bonding the blades to the disk rim. Bonded airfoil attachments have the potential to increase $AN2$ and component life by 9–10 percent by eliminating broach induced stress concentrations as noted. Moreover, bonded attachments can reduce external rim loading by upward of 15 percent with a corresponding reduction in disk weight. The key to the solution is a controlled, economical process to concurrently join a full complement of HPT blades in a repeatable manner. This paper discusses how a scientific approach and creative design practice can lead to such a process. Three alternative tooling concepts, and one universal tool that allows independent use of two of these concepts, were developed. Tool stresses and deflections, tool load paths, and bond pressure profiles were all quantified through ANSYS finite element analyses and closed-form analytical solutions. Prior experience has shown that joint strength is sensitive to the bond pressure level. Therefore, the tool materials and geometry were iterated upon until the pressure applied to the blade bond plane was as uniform as possible. Since absolute uniformity is elusive when deformable bodies are part of the bond load train, accurately determining the maximum and minimum bond plane pressure is absolutely essential for subsequent joint characterization and design allowable determination. This allows localized working stresses in the designed attachment to be compared to specific, bond pressure driven, allowable strengths rather than an average strength. This paper will show how applying a scientific approach to the development of a critical technology process can reduce both the cost and risk of process development.