AFRL researchers developed a super-high-strength aluminum alloy that engineers can use to improve the capability and performance of aerospace components—cryogenic rocket engine components, in particular. They created an aluminum alloy with specific strength and ductility characteristics surpassing those of the alpha titanium alloy currently used in rocket engine turbopumps. The aluminum alloy also demonstrates less sensitivity to hydrogen embrittlement, is lighter weight, and is potentially less costly to manufacture than the titanium alloy.

Engineers expect the new alloy, developed under the guidelines of the Integrated High-Payoff Rocket Propulsion Technology (IHPRPT) program, to reduce the weight and significantly improve the performance of vital spacecraft propulsion components. The IHPRPT program is a coordinated effort between the Department of Defense (DoD), National Aeronautics and Space Administration, and industry to develop—by the year 2010— revolutionary and innovative technologies that will double rocket propulsion capabilities with respect to 1993 state-ofthe- art technology. The program will improve the nation's capability to move into full-scale development of rocket propulsion systems offering improved performance, affordability, operability, reliability, and maintainability.

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Figure 1. LH2 turbopump (photo courtesy of Rocketdyne)

Liquid rocket engines use liquid hydrogen (LH2) as fuel and liquid oxygen (LOX) as an oxidizer; therefore, engine components must function in cryogenic temperatures as low as 20 K (-253°C). The rocket engine's turbopumps, including the Integrated Powerhead Demonstration (IPD) turbopump, move LH2 and LOX at high speeds through pipes from cryogenic storage tanks to the rocket's combustion chamber. The IPD three-stage turbopump employs three titanium alloy impellers (see Figure 1). Its progressively higher impeller tip speeds provide higher fuel pressure and, correspondingly, higher engine thrust. However, because these current impellers operate near the strength limit of the titanium alloy, engineers cannot further increase impeller tip speeds. In addition, the titanium alloy impellers are expensive to manufacture and maintain and are prone to hydrogen embrittlement.

AFRL researchers teamed with scientists from Universal Energy Systems (UES), Inc., on a Small Business Innovation Research (SBIR)-funded effort designed to meet requirements of the IHPRPT program. Their goal was to improve the thrust-to-weight ratio in rocket engines by identifying an aluminum alloy with (1) specific strength values equal to or exceeding those of the high-strength titanium alloy Ti-5Al-2.5Sn ELI (extra low interstitial), (2) significantly reduced weight, and (3) ductility of no less than 7%. Currently, metallurgists produce high-strength aluminum alloys through an expensive nanophase aluminum process that includes production of the alloy powder; mechanical milling of the powder in liquid nitrogen to produce a nanophase structure; and powder compaction by hot isostatic pressing, extrusion, and forging. Unfortunately, the resulting alloys do not possess the combination of strength and ductility required for use in cryogenic rocket engine applications. During this SBIR effort, AFRL and UES scientists, collaborating with scientists from Rocketdyne, selected an alternative processing approach: they produced alloys using conventional casting technology, which is less expensive than powder metallurgy processes. The properties of new alloys are determined by alloy composition and thermomechanical treatment; the combined effect in the new alloys produces very fine, nonsoluble dispersoids and grain refinement.

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Figure 2. The microstructure of hot-extruded and aged alloy specimens as seen with a transmission electron microscope

During Phase I of the SBIR effort, the research team investigated the microstructure, hardness, and tensile properties of nine alloys. Two different foundries cast 3 in. diameter billets, and the researchers examined them in as-cast and hot-extruded (see Figure 2) conditions using AFRL's Materials Processing Laboratory. They heat-treated the extruded samples to maximum hardness and then determined the tensile properties of the heat-treated samples at standard and cryogenic temperatures. The researchers also analyzed the microstructures of both heat-treated and deformed samples to determine fracture modes.

The research team subsequently selected three of the original nine alloys for further investigation during Phase II of the SBIR effort. The scientists optimized the heat treatment of as-cast alloys to achieve maximum strength and reasonable ductility. They determined the transverse tensile properties of the hot-extruded alloys to verify that their properties would meet design requirements. To ensure that the processing parameters would be acceptable for forging, they also performed compression tests. The team produced several forging pancakes from the alloys with different processing histories and determined tensile properties in different cross sections after heat treatment. This testing allowed the scientists to optimize their forging parameters and determine each alloy's microstructure at different processing steps.

The project team achieved the desired properties with half-sized, subscale preforms. The aluminum alloy is 38% lighter weight, significantly less expensive, and more resistant to hydrogen embrittlement than the titanium alloys currently used in liquid-fueled rocket engine turbopumps. The team has succeeded in efforts to cast the aluminum alloy in bars 76 mm in diameter and 6 m in length. Once the researchers are able to demonstrate fullscale castings with no appreciable material property changes, the alloy will be appropriate for additional applications, and the Air Force, DoD, and aerospace industry will begin to benefit from its accompanying cost reductions and streamlined manufacturing processes.

The project team successfully developed a high-strength aluminum alloy in both cast and wrought forms, both of which exhibit higher specific strength and the same ductility as the titanium alloy used in cryogenic turbopumps. During follow-on testing, scientists will produce larger cast billets and forgings in order to verify the results. They will also complete certification of the forging preforms to determine tensile properties, notch sensitivity, fracture toughness, and high-cycle fatigue response. Finally, they will conduct additional fluidity and tensile property testing to prove the casting ability of the selected alloy. Scientists plan to conduct follow-on IHPRPT efforts to determine whether the super-high-strength aluminum alloy may have a broader impact on aerospace applications.

Dr. Oleg N. Senkov (UES, Inc.), Dr. Daniel B. Miracle, and Mr. Timothy R. Anderl (Anteon Corporation), of the Air Force Research Laboratory's Materials and Manufacturing Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn_index.asp . Reference document ML-H-05-41.