Composite marine impellers: Manufacturing technology development (HIP and RTM processing)

Manufacturing technologies for the production of metallic high precision marine impeller designs have been successfully developed. The high degree of precision required to meet the performance criteria was achieved by 5-Axis machining of Powder Metal Hot Isostatically Pressed (PM HIP) billets into shroud and hub/blade sections, which are subsequently electron beam welded together and finish machined. This technique yields finished tolerances within 0.015 inches of the design. While the approach has been successful, the costs associated with the manufacture of an impeller are high. Alternative manufacturing techniques, such as precision casting and one piece molding of composite have been evaluated and prototype parts manufactured. Based on an assessment of technical requirements at that time these processes were concluded to be inadequate for providing acceptable part tolerances (given subsequently acquired information and recent unrelated developments alternative metal manufacturing techniques may now be acceptable). In order to meet both cost and quality requirements, Electric Boat invented a segmented composite impeller manufacturing approach. Initial trials with this technique showed promise of acceptably replacing the baseline technology. The Office of Naval Research sponsored a program to bring this manufacturing technique to maturity under the ManTech program. This effort developed high precision rapid tooling techniques for Resin Transfer Molded parts that were corrected for mold and part shrinkage/distortion. These high precision parts were then assembled to form an impeller. This paper will describe the basic technique, the efforts made to improve the tolerances, and the manufacturing process.