| Ferrous powder metallurgy (PM) materials have been widelyused as engineering parts inmany industries, in particular for automotive applications, due to its good mechanicalproperties and low cost. As known, sintered materials are inherently porous, which is quitedetrimental to their mechanical properties. In order to meet the increasing working demands,much more advanced ferrous PM materials are needed.Electric current activated sintering (ECAS) technique, also known as spark plasmasintering (SPS), plasma activated sintering (PAS), or electric discharge sintering (EDS), is anovel rapid sintering technology that utilizes direct current along with uniaxial load. Incomparison with the conventional sintering, ECAS can consolidate powders to near-fulldensity without remarkable grain growth at a relatively lower temperature and in a shortersintering time. However, the ECAS processes remain poor understood. In spite of this, it isbelieved that ECAS should be a powerful technology to prepare advanced ferrous PMproducts.In this paper, the electrical behavior in the initial stage of ECAS of carbonyl ironpowders was studied. In addition, the electrical conduction of carbonyl iron powder compactswas examined when a constant voltage/current source is supplied, especially under a typicalvoltage9.7V of ECAS. It also reports the microstructure and mechanical properties ofcarbonyl iron powder materials produced by ECAS. Finally, it investigated the microstructureand mechanical properties of three iron-based materials, including Fe-0.8C,Fe-2Cu-1.5Ni-0.5Mo-0.8C and Fe-2Cu-2Ni-1Mo-1C, fabricated byball milling and ECASAn electrical network model is firstly put forward to estimate the uniformity of electriccurrent in a powder compact subjected to different premolding loads in the initial stage. Theimprovement in current uniformity can be reflected from a simultaneous increase in thenumber N and the mass fraction θ of conductive particle chains in the compact. Both N and θare found to follow a power law with the premolding load F for different exponent values.When θ is equal to1, a critical load is reached, at which point the current flows through allparticles during sintering.Using the results of the model and the electrical contact theory, it is also found that onlyan increased temperature of less than20°C across the particle contacts. The distribution oftemperature is uniform in particles. This is clearly different from the general acceptance thatlocal high temperature is created at contact during electric current activated sintering. Theformation and growth of metallic contact at particle interfaces during ECAS are thought to be mainly due to heat bonding, electromigration and contact force, of which effects arepronouncedly enhanced byincreasing the bulk temperature.A reduction in premolding load may cause an increase in the initial electrical resistanceof the compact. Owing to the unique voltage-current characteristic of electric current activatedsintering, a higher initial resistance of compact means more thermal energy is involved,consequently producing a higher bulk temperature and getting a better qualityof sintering.Under a constant voltage stress of9.7V, the time evolution of the current of the carbonyliron powder compact can be clearly divided into four stages. At the very beginning,microsoldering will occur at some of particles contacts. I-V characteristics measurementsprove that microsoldering is a bulk temperature-independent mechanism. In the second andthird stages, bulk temperature-dependent driving forces is dominant. current growth iscontrolled by a power dissipation-temperature-current (resistance) feedback loop based ona competition mechanism. At the end of the stressing process, a sudden drop in resistance isobserved following a power law. This observation can be explained as a thermal breakdown,which occurs when the temperature rise in the compact reaches a percolation threshold.The bulk carbonyl iron powder materials were produced with various temperatures (650-800°C) and uniaxial pressures (20-50MPa). the results shows that when the sinteringtemperature reaches650°C，the microstructure is ferrite and spheroidized carbide. Increasingthe temperature or changing the pressure has no apparent effect on the sintered microstructure.On the other hand, density, average hardness and bending strength of the sintered samplesincrease with an increase in either temperature or pressure. In a uniaxial compaction, pressuregradients are presented in the compact because of the frictional forces between particlesand/or particles and the die wall. Significant density gradients result from the pressuregradients. A low relative density is found in the zone with low pressure. The evolution ofcurrent and temperature distribution in three adjacent zone with different initial relativedensity during sintering was modelled by fnite element calculations. It is found that there isno temperature gradient among three zones, but lower current density is observed in the zonewith low initial density. The observations prove that the densification of the zones does notfollow a self-adjusting mechanism, which was claimed to be a main character of ECAS.Three PM steels, Fe-0.8C, Fe-2Cu-1.5Ni-0.5Mo-0.8C and Fe-2Cu-2Ni-1Mo-1C, werefabricated by mechanical milling and spark plasma sintering. Dense sintered samples withfine and homogeneous microstructure were obtained. According to the results of XRD, DSCand SEM, it is suggested that the temperature of the sample can be~50°C greater than that recorded. The microstructures of the as-sintered samples are divided into two groups. Oneconsists of both ferritic and martensitic structures, and the others are of a ferritic structure. Aconsiderable amount of martensite exists only in those high alloy Fe-2Cu-2Ni-1Mo-1Csamples. The hardness of the sintered samples mainly depends on microstructure andcomposition. It shows that the hardness enhances with the volume fraction of martensite.However, a lower compressive strength is observed in the samples with higher volumefraction of martensite. The analysis of the deformation behavior demonstrates that the yieldstrength and ultimate strength are solely correlated to the properties of ferritic structure.Discontinuously yielding phenomenon, initial work hardening exponent and decreasing rateof strain hardening exponent with strain are considered to be sensitive to the morphology ofcarbides formed in the ferritic structure. |
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