Field Activated Sintering Technology: Multi-physics Phenomena ModelingDr. Jing Zhang - Indiana University - IUPUI
Initially Dr. Zhang shared the titles of some of his previous studies which represent collaborated efforts between the University and Industry.
Tonight's presentation concerns a method of densification of powdered metals and other materials into useful solid shapes. Traditionally, powders have been compacted by either uniaxial pressing (either cold or hot) or by isostatic pressing followed by heating (sintering) to form interparticle bonds. These methods produce good products but are characterized by being slow and energy intensive and have significant grain growth during the sintering process.
For some time a more efficient and faster process FAST (Field Activated Sinter Technology) has been known and practiced. This process involves pressing the powder (metal or ceramic) uniaxially between graphite-tungsten punches in graphite-tungsten dies while electrical power is applied through the punches and the part. The power is generally applied as a series of rapid pulses. The final effect is that the part comes out densified (at least as much as by traditional methods) with minimal grain growth, in seconds (as compared to much longer times by the two-step "press-sinter" methods). It is also noted that FAST is effective with both metallic and ceramic powders. It is not however fully understood how and why this method is so effective and the purpose of this research is to provide some of these answers to potentially allow the process to be scaled-up to production environments.
The technique has it roots with Taylor who in 1933 proposed resistance sintering using DC, Pulse, Multi-pulse, Plasma, and AC currents. There are claims of sparking between powder particles which creates a cleaning effect. Also proposed are an Electro-plastic effect, and a Joule energy (resistance) heating effect. There has been some ambiguity of measured specimen temperature in studying the process because of limitations to measurement of localized temperatures.
This study uses computer modelling to study both possible electrical transport and thermal analysis mechanisms as related to the densification. The goal is to provide a proper design tool for production applications.
The electrical side of the model involves the concepts of imperfect contact, and change of bulk resistivity during in the compact during the process, as well as vast resistivity differences between the materials being compacted (alumina and graphite for the study). Alumina has a huge voltage gradient, while graphite has relatively much less. On the thermal side, the study considers heat conduction, Joule heating, convective heat transfer, interface resistance as well as heat losses trough the loading tooling (early) and later the die material.
Modeling showed that conductivity as a function of compaction was nearly unity for isostatic compaction, but displays an increase (to about a 1.2 ratio) during early stages of uniaxial compaction, falling back to unity as the densification continues.
The model shows that the primary cause of densification is due to thermal conduction from the upper and lower punch material which undergo resistance heating during the passage of the electrical current. For both the carbon and alumina slugs, the initial electrical path involves the die material and is most resistive in the punches themselves causing them to provide the primary heat source. For the ceramic material, this continues throughout compaction. For the graphite (conductive) material, the compact itself contributes significantly to the electrical path as densification proceeds. Heat losses can be reduced by externally insulating the die during graphite FAST processing. In both cases most heating is the result of thermal conduction from punches to the compact. This would explain the success of the technique for both ceramic and metallic compacts.
A conclusion of this study is that anisotropy is about 120% in the compacting direction of what it is in the other direction during compaction.