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The Superior Graphite Co. recently completed a successful program to overcome this challenge. The development effort was supported by a $2 million, 3-year grant from the U.S. Department of Commerce through the Advanced Technology Program (ATP). Other participants in the program were the Argonne National Laboratory; Northwestern University; Bio-Imaging Research, Inc.; and BIZTEK Consulting.
What is Electroconsolidation?Electroconsolidation is a method for rapid pressure-assisted densification of preformed powdered materials by the simultaneous application of heat and pressure. As illustrated in Figure 1, the part or parts to be densified are immersed in a bed of free-flowing, electrically conductive granular material contained in a cylindrical vessel or die. Pressure is applied to the parts by rams acting on the surrounding granular material, which serves as a pressure transmitting medium. An electrical current is then passed directly through the compacted granular material, causing it to be electrothermally heated. Heat transfers to the parts by a combination of conduction and radiation.
Electroconsolidation is one of several densification methods referred to as “soft-tooling,” or psuedo-fluid processes, that use a bed of particulate solids as a pressure transmitting medium. The particulate medium used in Electroconsolidation is a porous spheroidal form of graphite. Electroconsolidation differs from other processes of this type by the use of direct resistive heating of the medium. This novel means of “inside-out” heating allows extremely rapid heating with temperature capability to well in excess of 2500°C.
Applications and RequirementsElectroconsolidation is a candidate for use in applications that require pressure to achieve high density, as is generally the case with reinforced composites and products made by Self-propagating High-temperature Synthesis (SHS) reaction technology. Examples include silicon carbide reinforced ceramic oxide cutting tool inserts and high temperature nickel aluminide fasteners. The process has the capability to densify preforms of complex shapes, which distinguishes Electroconsolidation from hot pressing, and, because pressure is applied mechanically through the medium, it is unnecessary to clad the parts as is required with hot isostatic pressing. The rapid heating rate leads to low cycle times, along with the simplification of processing complex shaped parts without cladding. These features result in lower-cost production of components made from many of the advanced materials that require pressure and high temperature for consolidation, such as various carbides and nitrides. The rapid heating to high and ultra-high temperatures by resistively heating granular graphite requires high current densities, which limits the die sizes to less than about 10 in. (~ 25 cm). Therefore, Electroconsolidation is mainly of interest for making large numbers of smaller parts.
The use of Electroconsolidation in industrial applications requires the means to effectively control the densification process. Rapid resistive heating leads to temperature gradients in the die cell. And because granular materials do not transmit pressure isostatically, pressure differences can also occur within the die cell, which affects the temperature distribution. The insertion of thermocouples or other temperature or pressure sensors into the die is not practical in production, and other means to monitor and control the process are needed to meet the increasingly rigorous quality control standards being imposed throughout industry.
Improving the ProcessAt a new Electroconsolidation pilot facility located at Argonne, researchers took several approaches to develop an improved means for process control. One was to develop a mathematical model of the process that could predict the temperature and pressure profiles within the Electroconsolidation die during operation based on a set of equipment design features and the operating conditions to be imposed. Another part of the program was to develop non-intrusive means to measure the temperature within the die cell during actual densification.
Non-Intrusive Temperature Measurement Using Ultra-Sound Pitch/Catch. The insertion of sensors within the Electroconsolidation die is done for experimental work and in initial prototyping, but is not practical for an automated rapid-cycle industrial application. Temperatures can approach 3000°C within several minutes. The measurement environment also involves rapid compaction to pressures up to about 10,000 psi (~700 kg/cm2) and electric currents exceeding 1000 amps/in.2 (~ 150 amps/cm2).
Several alternative methods were considered, but the actual development focused on using the change in the velocity of sound with temperature as sound was transmitted through the Electroconsolidation die. The velocity of sound varies with temperature in a predictable manner—it increases with temperature in gases but decreases with temperature in solids and liquids. However, in the case of graphite, the velocity of sound actually increases with increasing temperature.
No information was available on the characteristics of sound transmission through beds of granular graphite, and this had to be determined. The determination was done by measuring the time-of-flight (TOF) of ultrasonic pulses sent from an ultrasonic transducer located in the upper water-cooled contact plate to a receiving transducer located in the lower water-cooled copper contact plate. The sound waves must propagate through a series of different materials and encounter a number of interfaces between these materials—which include copper, water coolant and the graphite ram—before entering the bed of granular graphite medium and then proceeding through a similar series in traveling to the receiving transducer. To be able to do this and have an adequate signal-to-noise ratio requires a low frequency transmission and the application of pressure.
To measure the TOF, a 500 kHz tone burst signal from a function generator is amplified by a 440 dB-RF power amplifier and applied to a 500 kHz, 1-in. diameter transmitting transducer. The receiving signal is also amplified, filtered and displayed on a digital oscilloscope. The total transit time of the transmitted signal is recorded. The portion of the total TOF corresponding to the copper and the rams is measured independently and subtracted to obtain the TOF through the granular graphite medium material. With the distance through the bed known from the position of the rams during the operation, the velocity of sound in the bed can be readily calculated.
V= Vo – 0.0008 T + 0.0001 T2
V = the velocity of sound in m/sec,
Vo = the velocity of sound at pressure and ambient temperature and
T = the temperature in degrees C.
In an actual industrial operation, the temperature at various fixed positions outside of the die would be monitored by conventional means, such as thermocouples and optical pyrometry. These temperature measurements would be used as a reference for controlling the process from cycle to cycle when the set point conditions are established. Such conventional temperature measurements can be compared with ultrasonic determination for a high temperature sintering operation. The temperature difference between a fixed outside location and the temperature within the die as the temperature is increased can be used to calibrate the ultrasound measurement when the temperature is beyond the temperature limits of conventional sensors.
Process Control with X-Ray ImagingThe material used for the Electroconsolidation die cell is graphite, which is readily penetrated by X-rays. This provides the opportunity to use X-ray imaging to observe dimensional changes of the parts as they undergo densification. Additionally, if small melting point indicators are placed at different positions within the die when the preforms are being loaded, these can also be observed in the X-ray image and provide a modern equivalent to monitoring kiln operations using pyrometric cones.
The source of the X-ray imaging system* used with the Electroconsolidation pilot facility at Argonne is a 160 kV, 2ma X-ray generator (tube). An image intensifier is located on the far side of the die chamber, and both components are attached to the frame of the hydraulic press and positioned to allow a 4 in. (10.2 cm) field of view. At full power, the X-rays can travel through 11 in. (28 cm) of graphite and still have sufficient signal to give a clear image on the display monitor.
A computer is used to program the desired sequence of the images that enable the densification to be observed as it occurs. The images are recorded in the computer’s memory, allowing the history of the densification cycle to be reviewed in detail.
Developing New ApplicationsThe Electroconsolidation pilot facility at Argonne to develop and demonstrate the commercial application of the process comprises a 200-ton hydraulic press fitted with a pair of opposed rams, each independently controlled by computer. The power supply provides up to 10,000 amps DC or pulsed DC current. Various die and ram sets are used for processing parts up to 4 in. (10.2 cm) in size.
Although the recent development work done during the ATP program has focused on equipment design and process control, a variety of materials have been processed. Work is currently being done with companies that are evaluating the technology for specific applications, including whisker and fiber reinforced oxide and non-oxide ceramic matrix composites for cutting tool inserts, ballistic armor and aerospace components. The process is well suited for making dense parts of complex shape directly using SHS reaction synthesis under pressure in the Electroconsolidation die.
With the new methods for process control, numerous opportunities to expand this exciting new forming process will undoubtedly emerge.