A novel sintering technology based on high density electric currents and sintering cycles shorter than a second is presented. Data will be shown that compare it to Hot Pressing and Spark Plasma Sintering and to conventional Press and Sinter. Considerations will be made on the materials, the physical and mechanical characteristics of the objects produced, on the range of present and future products, on the tooling and on the productivity.
Introduction
Using an electric current to heat and sinter or aid a sintering process is not really a novelty. The first pioneers of such methods began at the start and continued throughout the last century when the manipulation of electric currents became increasingly popular for processing. Research in this field has been dense in the last sixty years and there have been a number of reviews that clearly illustrate the main differences of each procedure and method. The electric current assisted technologies that have found direct industrial application, however, are few: Hot Pressing (HP) and, in a more limited way, Spark Plasma Sintering (SPS). Both these very similar methods employ graphite dies and plungers placed inside a controlled atmosphere and heated through a current flow, retrofitted in temperature either through a thermocouple or a pyrometer. Typical control parameters include maximum temperature, heating ramp (°C/s), holding time (in minutes) and pressure/force to apply on the plungers. The main difference between the two methods are in that HP uses alternate currents directly converted from the electrical distribution lines with transformers whereas SPS has a conversion of the current pulses with elaborate power electronics to direct currents and trains of waves of 350 Hz. SPS machines usually also have the possibility to choose between the on and off time of the current pulses and the time of direct current versus alternate current. Besides these sophistications though, the results, as far as sintering time and densification rates or values of maximum density, are quite similar. A typical cycle made of heating and holding the temperature in pressure lasts between 10 and 25 min in both methods and different studies have reported no differences in terms of mechanical properties between them.
This article appeared in the March–April 2018 issue of Metal Powder Report.
