Terry McNelley, Sarath Menon
Friction stir processing (FSP) of as-cast NiAl bronzes converts the as-cast microstructure to a wrought condition in the volume of materials subjected to the process. This results in improved properties in the absence of component shape change. With the development of portable systems, friction stir processing may enable in situ repair of defective components such as propellers and thus avoid expensive procedures such as dry docking for such repairs.
Microstructure – mechanical property relationships associated with multi-pass FSP are now of primary interest in work on the cast NiAl bronze materials and have also been examined in Al alloys [8,9]. Factors that adversely affect ductility, especially in orientations transverse to the local direction of tool advance, have been identified. Recent results demonstrate that FSP tools and processing conditions must lead to high and uniform temperatures and strains in order to attain high ductility throughout the stir zone volume.
Applications of FSW in joining of similar and dissimilar metals are now growing rapidly. This process and FSP share many attributes although FSP is less well developed because it is a newer concept. Nevertheless, FSP will also experience rapid growth as the benefits of this unique metallurgical tool become fully apparent. Thus far, development of FSP/W has been largely empirical. The goal of sustained growth in utilization will depend significantly on improved understanding of the fundamental science involved in these technologies. This will also strengthen the underlying foundation in the development of standards for implementation in industry. Both applied and fundamental research activities in support of this goal are needed. Efforts in support of the transition of FSP technology to NiAl bronze propeller materials will be described first, followed by activities that will exploit new NPS capabilities. Finally, an approach to broader, fundamental problems of microstructure control during FSP/W will be outlined.
The thermomechanical cycle of the initial FSP pass on cast NiAl bronze induces characteristic phase transformations that are accompanied by distortion of the resulting microstructure constituents (i.e., the α and β phases). This is often reflected in the development of distinct, elongated primary α and β transformation products in as-processed microstructures. Recent studies involving Gleeble thermomechanical simulations as well as isothermal hot rolling have shown that local von Mises strains varying from 0 to ~3.0 may also be estimated from such distortions of stir zone microstructure constituents. Preliminary comparisons to typical stir zone microstructures suggest that one-third or more of a typical stir volume has experienced local equivalent strains ≤3.0. In contrast, modeling has indicated that equivalent strains are >10.
FSP inherently involves multiple, overlapping passes and the step-over distance between successive passes is an important parameter in addition to the tool rpm/ipm combination. Uniform, equiaxed and refined microstructures, and the absence of texture within stir zones were produced by FSP with small step over distances and a step-spiral tool. Thus, multi-pass FSP results in recrystallization and grain refinement, homogenization, and redistribution of both the primary α and β transformation products. In contrast, excessive step over distance results in regions of less deformed stir zone material that, in turn, lead to strain localization and low apparent ductility.
For more information, contact Terry McNelley at email@example.com
Terry McNelley, Sarath Menon
This program will determine the feasibility of performing crack repair on submarine control surfaces by underwater FSW of HY-80 steel. Microstructures and microstructure – mechanical property relationships will be established for the HY-80 material after FSW.
Submarines exhibit fatigue cracking in the alloy steel (HY-80) plating on the control surfaces due to cyclic stresses caused by depth changes over the life of the ship. Fusion welding is not a practical method of underwater repair of these cracks. High cooling rates will promote the formation of the brittle martensite phase in the weld metal and heat affected zone, resulting in reduced toughness and increased susceptibility to hydrogen assisted cracking. The latter problem will be particularly acute because the welding arc will liberate hydrogen from the water, thereby facilitating hydrogen assisted cracking of the steel when this hydrogen is absorbed into the weld metal and surrounding heat affected zone. For these reasons the method currently available to the navy to repair these cracks is to dry dock the ship.
Control surface inspections and crack repairs are typically scheduled inside CNO docking availabilities, but because of the nature of the work these repairs can cause increased risk to the dry docking schedule and result in increased costs for the availability by delaying the undocking of the submarine. In order to repair cracks on the underside of the control surfaces, trapped water must first be drained or it can affect the quality of the weld repair. In order to drain the trapped water, holes often have to be drilled in the plating and the draining process can take days or even weeks to complete. A delay in undocking of a submarine can result in an increase in cost of $100K per day just in support services alone. Due to tide restrictions on some of the Navy’s drydocks, delaying the undocking by a single day may require shifting the undocking date by 2-3 weeks, with a resulting increase in cost on the order of $1M. Additionally, the holes that were drilled need to be plug welded and often become starting points for future cracks.
Alloy steel (4142; 0.43% Carbon) plates (0.25 in thick) were successfully processed by Advanced Metal Products, Provo, UT using a proprietary tool composed of cubic boron nitride (CBN) particles dispersed in a W-Re alloy matrix. One plate was processed dry and the other was immersed in a water tank to a depth of approximately 4 inches. Cooling was supplied to the water box via copper tubing containing refrigerant from the FSW machine. Photos taken during the FSP operation under both dry and wet conditions are presented in Figure 1 to illustrate the underwater setup and also to compare the steel sample behavior under these conditions.
Several RPM/IPM combinations were performed in order to establish a successful processing parameter window where defect-free welds could be produced underwater. Based on the observations here a combination of 400rpm/2ipm was found to be sufficient to produce defect free runs both during dry and wet runs. The progressions in the microstructural development during the dry and wet runs were followed by examining the details of the microstructural constituents and are summarized in Figure 2.
Figure 2(a) illustrates the ferrite + pearlite microstructure in the as- received 4142 steel plate and Figures 2(b) and (c) illustrate the martensitic microstructure observed within the stir zone of the dry and wet runs, respectively. The microstructural constituents in both the cases were quite similar though the thermomechanically affected zone (TMAZ; comparable to the heat affected zone in conventional fusion welding) was much wider in the dry run in comparison to the wet run. It appeared that the pearlitic constituents were always absent within the stir zone suggesting that the austenitization was completed within the stir zone in both the dry as well as the wet FSP conditions. Martensitic microstructure was observed within the SZ and appeared quite uniform throughout this region in all samples
For more information, contact Terry McNelley at firstname.lastname@example.org
Develop novel multi-material dielectrics (e.g. Table, Samples 4-10) with dielectric constants far better than standard (Table, Sample 3) to create capacitors of unprecedented energy density. Initial results show that below 3 volts we have achieved higher energy density than that found in commercial electrolytics. Example: Figure shows MMD producing equivalent to a 10,000 F capacitor in far less volume than a commercial equivalent.
The primary goal of this research is to create novel metal based dielectric material(s) (MMD) with dielectric constants far greater than those of any previous material such that electrostatic and electrolytic capacitors of unprecedented energy density can be created. A second goal of the work is to develop a model of the dielectric constants of the MMD.
For more information, contact Jonathan Phillips at email@example.com
Sebastian Osswald1,2, Dragoslav Grbovic1, Oscar Biblarz2, Marcello Romano2, Darrell Niemann3
We are developing a novel carbon nanotube field ionization engine (CNT-FIE) that will, for the first time, allow for the design of scalable micro-ion thrusters. The unavailability of miniature ion thrusters is the solely remaining factor that inhibits the implementation of novel pico- and nanosatellite technologies.
For more information, contact Sebastian Osswald at firstname.lastname@example.org
|Nanocarbon-reinforced metal armors|
|Organometallic clusters as novel energetic materials|
Joe Hooper, Jim Lightstone, Chad Stoltz
The goal of this work is to explore molecular scale aluminum clusters that are stabilized against oxidation by direct bonds with organic ligand groups. These small clusters crystallize into low-symmetry solid-state structures, and may offer a route to extremely rapid combustion of aluminum that is quite unlike the standard diffusion limited burning of metal particles. This would provide several advantages for solid rocket motor and energetic materials.
The addition of metallic fuels to advanced energetics is now a standard way to increase the overall energy density of warheads. Micron or smaller powders of aluminum result in significant increases in the volumetric heat of combustion of a charge, but the timescales on which aluminum can combust are limited by well-known diffusion and kinetic processes. In this project we are considering a new class of materials that combine the favorable energy density of metallic fuels with the fast reaction rates more characteristic of explosives and propellants.
Aluminum organometallic complexes offer a number of significant benefits for DoD applications, if they can be made air and temperature sensitive. Their high energy density and fast energy release would allow for reductions in warhead size. The potentially high burn rates and high volumetric heats of combustion would allow for new solid rocket motor designs, such as a fast end-burner.
For more information, contact Joe Hooper at jphooper@nps,edu