CERAMIC DEFENSE: Pressing with Controlled Combustion
Forming high-density ceramic parts has traditionally presented a number of challenges. Conventional powder pressing methods often include quite large pieces of equipment and have many moving mechanical parts and/or complex hydraulics that make the systems difficult to maintain. Compaction densities are often lower than desired, and additional multi-step post-processing, machining or finishing are usually needed to achieve the final part specifications.1-5
Recently, a new process has been developed to overcome these challenges. Called combustion-driven compaction (CDC), the technology uses high pressures (>50 to 150 tons per square inch [tsi]) combined with natural gas combustion to provide higher pressed green and sintered part densities, a gentler dynamic loading cycle on the pressed parts, net-shaped or near-net-shaped forming, faster processing times, improved density and manufacturing flexibility, reduced part shrinkage, the potential for nano/composite/multi-layered/functional gradient materials (FGM) fabrication, improved performance, few or no post-machining/grinding requirements, and scalability to higher-capacity CDC press sizes with the potential for automation/rapid fabrication, all in a compact piece of equipment. 6-8
With these benefits, the CDC technology presents a new way to achieve high-density parts with improved properties for applications in defense, energy and other commercial markets.
BackgroundThe CDC process uses a controlled release of energy from the combustion of natural gas and air to compact a variety of ceramic, metallic and composite powders (see Figure 1). The chamber is first filled to a high pressure with a mixture of natural gas and air. As the chamber is being filled, the piston or ram is allowed to move down to pre-compress and remove entrapped air from the powder. The gas supply is then closed, and an ignition stimulus is applied. The ignition causes the pressure in the chamber to rise dramatically, further compressing the powder into its final net shape.
Compaction Loading CycleAs the compaction load to a powdered ceramic or metal is raised, the part density and properties improve. If the powder is compressed too rapidly, shock propagation in some materials can cause internal cracks and separations (over-pressing). Unlike other forging or explosive forming techniques, the CDC loading rate is much gentler and is highly controlled, which provides advantages when compacting crack-sensitive materials, such as brittle ceramic powders, difficult-to-press alloy powders or composites.
The CDC press uses a two-stage fast (but gentle) loading cycle to achieve a high density while avoiding shock propagation and defects in the compacted part. In addition, the load sequence of the CDC technology allows large tonnage loads to be applied without damage to the press or die components. The initial gas-fill sequence aligns the ram and die components while applying a sufficient load to pre-compress the powder and remove entrapped air. When the fill gas is ignited, the ram rapidly presses down without slamming into the tooling or powder. In other words, although the process is fast and powerful, it is smooth and continuous.
The CDC process routinely operates at compaction loads of 2069 MPa (150 tsi). This is in sharp contrast to conventional compaction processes, which generally are limited to 690 MPa (50 tsi). 1-4 One 300-ton CDC press has operated for six years, indicating that the press has a proprietary tooling/die life of more than a few thousand cycles.
Press ScalingSince the CDC press directly converts chemical energy into compaction energy, it is highly energetic and capable of producing enormous compaction pressures in a moderate-sized piece of equipment. To date, 10-, 30- and 300-ton presses have been constructed and operated, and a scaled-up 1000-ton CDC press is in the final stages of design/assembly. A compact version of a 1000-ton CDC press for large-scale part manufacturing (>1 in. diameter with loading pressures up to 150 tsi) can be further scaled up to 3000 tons if needed. Scaling from one size to the next has been relatively straightforward. Since the process works more or less like a piston in an automobile (albeit at much higher pressures), the loads that can be produced are a direct function of the combustion pressure and the area of the ram (piston) rather than the overall size of the press. It is therefore possible to scale a CDC press to very high tonnages without dramatically increasing the size of the press.
The relatively small size of the CDC press compared to traditional hydraulic or mechanical presses can allow ceramic parts to be made in almost any industrial or commercial building that has access to bottled or piped natural gas, including "machining centers." Additionally, the ability to upgrade cost-effectively to higher CDC tonnages (e.g., 1000 tons or higher) provides the potential to rapidly scale up a manufacturing process to meet increasing demand for a new product.
Processed Geometries and Materials BehaviorIn general, CDC compacted parts possess significantly higher green and sintered densities at CDC pressures of above 50 tsi, which is usually the threshold limit offered by traditional powder pressing methods.1-5 Materials that have been processed successfully using the new compaction technology include pure metals (Cu, Al, W, Mo, Re, Ta and Fe), alloys (316 SS, 410SS, FL-4400, FLN2-4405, 737SH and Re/Mo alloys), layered and mixed materials (Al/Ti, SS/Mo, Cu/Ta, Al/Alumina, SS/Ta, FL-4400/Cu, FL-4400/Al, Fl-4400/Ti and FL-4400/Ta), aluminum nitride, nano-silicon carbide, nano-boron carbide, a ferrous alloy matrix with ceramic nanocomposites in various shapes and geometries, multi-layered parts made of steel/copper, copper/stainless steel/niobium, AlN, and nanocomposite magnetic materials such as FeNi-NanoSiO2 (see figures 4-6).
The CDC process routinely operates at compaction loads in the 150-tsi range and above, which significantly improves the final quality of the compacted part both in the green and sintered states. For example, the FeNi-nanoSiO2 composite material compacted with the CDC technology had a relatively higher magnetic permeability and lower eddy current losses compared to a traditionally processed magnet at the tested frequencies of MHz levels. Most materials processed with the CDC technology have exhibited a superior surface finish, reduced shrinkage, and improved mechanical and high-temperature durability properties compared to parts pressed through conventional means.
Applications and Further AdvancesCDC processing is an emerging manufacturing technology that can be used to develop high-density and high-performance net- or near-net-shaped parts for a variety of markets. Anticipated applications of CDC-processed components include X-ray targets, laser optical mirrors, projectiles, lighter and stronger armor tiles, cryogenic parts, accelerator/RF microwave components, fuel cell/battery electrodes, computer hard disk drive accessories, high-performance engine parts, high-temperature nozzle liner parts, heat sinks, tooling inserts, nanostructured composite magnets, gears, bearings, biomedical components, welding/water jet machining nozzles/electrodes, and wear/corrosion resistant tribological components.
The preliminary results are encouraging, and significant potential exists for further evaluating and exploring this new fabrication method.
For more information about the CDC process, contact Karthik Nagarathnam at UTRON Inc., 8506 Wellington Rd., Suite 200, Manassas, VA 20109; (703) 369-5552, ext. 111; fax (703) 369-5298; e-mail firstname.lastname@example.org ; or visit http://www.utroninc.com .