Novel uses of electric fields and electric currents in powder metal (P/M) processing

This paper describes a series of experiments conducted on the common Fe powder metal alloy 4401 in which the material was compacted during the application of DC current and sintered under electric field strengths up to 7.7 kV cm−1. In the electric field sintering experiments, reduction of the surface porosity of cylindrical 4401 powder specimens by as much as 44% in a 0.2–0.4 mm band below the surface was observed. This effect was confirmed by nitro-carburizing experiments in which carbon diffusion was limited to a depth of only 0.2 mm below the surface. This is comparable to the carburizing depths observed in wrought steel alloys. The specimens not sintered in an electric field showed carbon diffusion depths over ten times (3 mm) that observed in electric field sintered specimens. It is proposed that this decrease in surface porosity is attributable to a decrease in chemical potential of vacancies at or just below the charged external surface. Vacancy flux equations employed to calculate the porosity as a function of distance below the external surface predict that the porosity becomes approximately zero at the distance X=0.4–0.5 mm below the surface, which is in reasonable agreement with metallographic image analysis and the carbon diffusion results. We also conducted experiments using DC current processing for in-die compaction of 4401 Fe iron powders. For these powders, we were able to achieve green state densities of 7.6 g cc−1 (97.6% theoretical density). In conventional powder metal processing, green densities of only 7.1–7.3 g cc−1 can be achieved with combinations of compaction pressure and temperature. The observed increase in powder compressibility is due to a combination of reduced mechanical strength from thermal softening at the particle contact spots and improved particle plasticity caused by electro-plastic effects in the material. Mechanical testing of the green specimens has shown that samples consolidated using in-die current processing have compressive strengths 20 times that of a non-current processed sample compacted at the same pressure.