Medium-frequency electrical resistance sintering of oxidized C.P. iron powder
Abstract
Commercially pure (C.P.) iron powders with a deliberate high degree of oxidation were consolidated by medium-frequency electrical resistance sintering (MF-ERS). This is a consolidation technique where pressure, and heat coming from a low-voltage and high-intensity electrical current, are simultaneously applied to a powder mass. In this work, the achieved densification rate is interpreted according to a qualitative microscopic model, based on the compacts global porosity and electrical resistance evolution. The effect of current intensity and sintering time on compacts was studied on the basis of micrographs revealing the porosity distribution inside the sintered compact. The microstructural characteristics of compacts consolidated by the traditional cold-press and furnace-sinter powder metallurgy route are compared with results of MF-ERS consolidation. The goodness of MF-ERS versus the problems of conventional sintering when working with oxidized powders is analyzed. The electrical consolidation can obtain higher densifications than the traditional route under non-reducing atmospheres.
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metals Article Medium-Frequency Electrical Resistance Sintering of Oxidized C.P. Iron Powder Juan Manuel Montes 1ID , Francisco Gómez Cuevas 2ID , Fátima Ternero 1,*ID , Raquel Astacio 1, Eduardo Sánchez Caballero 1ID and Jesús Cintas 1ID 1Metallurgy and Materials Engineering Group Escuela Técnica Superior de Ingeniería, Universidad de Sevilla Camino de los Descubrimientos s/n, 41092 Sevilla, Spain; [email protected] (J.M.M.); [email protected] (R.A.); [email protected] (E.S.C.); [email protected] (J.C.) 2Department of Chemical Engineering, Physical Chemistry and Materials Science Escuela Técnica Superior de Ingeniería, Universidad de Huelva Campus El Carmen, Avda. Tres de Marzo s/n, 21071 Huelva, Spain; [email protected] *Correspondence: fterner[email protected]; Tel.: +34-954487305 Received: 10 May 2018; Accepted: 4 June 2018; Published: 6 June 2018 Abstract: Commercially pure (C.P.) iron powders with a deliberate high degree of oxidation were consolidated by medium-frequency electrical resistance sintering (MF-ERS). This is a consolidation technique where pressure, and heat coming from a low-voltage and high-intensity electrical current, are simultaneously applied to a powder mass. In this work, the achieved densification rate is interpreted according to a qualitative microscopic model, based on the compacts global porosity and electrical resistance evolution. The effect of current intensity and sintering time on compacts was studied on the basis of micrographs revealing the porosity distribution inside the sintered compact. The microstructural characteristics of compacts consolidated by the traditional cold-press and furnace-sinter powder metallurgy route are compared with results of MF-ERS consolidation. The goodness of MF-ERS versus the problems of conventional sintering when working with oxidized powders is analyzed. The electrical consolidation can obtain higher densifications than the traditional route under non-reducing atmospheres. Keywords: electrical resistance sintering; MF-ERS; FAST; ECAS; iron; hot pressing; sintering; powder metallurgy 1. Introduction The electricity can be used to sinter a conductive powder mass that is passed through by the current, as has been suggested numerous times during the last century, and still remaining as an issue of extraordinary interest at present. The progress of the different modalities of electrical sintering, usually known under the common name of FAST (field assisted sintering techniques), has been reviewed in detail by Grasso et al. [1]. Although with a common name, there are important differences among these techniques, mainly due to: (a) the characteristics of the power source, (b) the electrical features of the die containing the powders, and (c) the time that process lasts (from microseconds to minutes). This strongly influences the physical phenomena taking place in the different processes. Most probably, the FAST modality most widely studied is the so called spark plasma sintering (SPS), where a pulsed DC current directly passes through a graphite die, as well as through the powder compact, in case of conductive samples. This facilitates a relatively high heating rate (up to 1000 K/min), hence the sintering process being generally quite fast (within a few minutes). The main advantages of the technique are the possibility of sintering both conductive and non-conductive Metals 2018,8, 426; doi:10.3390/met8060426 www.mdpi.com/journal/metals
Metals 2018,8, 426 2 of 14 powders, achieving a reasonably uniform temperature distribution, whereas its disadvantages are the need of a vacuum system and complicated electronics capable of controlling the high-power pulsating current to be generated. The pulsed current, besides the Joule heating, is supposed to favor the mobility of the atoms on the particles surface, which would cause plasma and discharges between particles. Whether plasma is generated has not yet been confirmed, especially when non-conductive ceramic powders are compacted. However, in case of sintering conductive powders, the equipment needed to undertake electrical consolidation can be greatly simplified. Regarding the many conceived modalities, maybe the simplest is one of the firstly proposed, consisting in a high intensity and low voltage electrical current passing through a powder mass of conductive powders placed inside an insulating die, and being simultaneously under pressure. This is therefore a hot consolidation technique, where the heat is generated in the powder itself by Joule effect. Such technique was described by Taylor in 1933 [ 2 ], and systematically studied later by Lenel [ 3 ], who coined the term ‘electrical resistance sintering’ (ERS) under pressure. Among the advantages of this technique, as compared to the traditional route of cold pressing followed by furnace sintering [ 4 , 5 ], it should be considered: (1) the use of relatively low pressures (around 100 MPa) to reach high densification rates; (2) the need of very short dwelling times (around 1 s); and (3) the possible option of sintering in air, without protective atmospheres. The main drawbacks come from the difficulty of getting a homogeneous temperature distribution inside the powder mass, and from finding the proper die material with a good durability. Other drawbacks arise from the insufficient theoretical knowledge that still exists of the various mechanisms involved [ 6 ]. In spite of this, the interest towards the electrical sintering techniques has been growing rapidly in the last 30 years, mainly in applications involving hard-to-sinter materials, such as amorphous or nanocrystalline structures, nanocomposites, hard metals, and refractory materials [7,8]. On the other hand, it is a well-known fact [ 9 ] the problems arising during conventional sintering of highly oxidized particles when reducing atmospheres are not used. The oxide layer act to impede atomic diffusion and metal-metal contacts between particles, therefore resulting in a deficient sintering. In this paper, the novel medium frequency electrical resistance sintering (MF-ERS) technology will be used with oxidized iron particles. Medium frequency technology is a loan from the field of resistance welding. This technology has been extensively tested and its benefits in improving electronic current control and reducing the size of the required transformers are well known. The adaptation of a welding machine for the new consolidation task is therefore simple, economical, and free of prototyping steps. The compacts densification and electrical resistance evolution during the MF-ERS process will be registered and related to the observed microstructure. Also, metallographic observations of diametrical sections of the MF-ERS compacts will be compared with those obtained by the conventional powder metallurgy (PM) route. 2. Experimental Equipment and Materials 2.1. MF-ERS Equipment There is not a commercial equipment exactly designed to implement the MF-ERS process. However, both the mechanical and electrical requirements are effectively fulfilled by a resistance welding equipment. Welding machines have, up to recently, worked with monophasic and low frequency electric supply (about 50 Hz), but the irruption of the medium frequency technology (about 1000 Hz) has brought about a remarkable change. It has made possible the reduction of core size and weight of the welding transformer without decreasing its power, and also the use of direct current. Direct current eliminates the effect of the inductive resistance (inductance) in the secondary circuit, making the use of longer welding arms possible, but also avoids the current discontinuities, generating more heat for the same welding time. Additionally, the three-phase current of the MF technology is symmetrical and equilibrated, reducing the primary currents and letting lower cable sections and
Metals 2018,8, 426 3 of 14 protection devices, therefore reducing costs. These benefits in the welding field are also expected to be beneficial for the MF-ERS process. In this work, a Serra Soldadura S.A. (Spain) press-type resistance welding machine was adapted. The current intensity is electronically controlled, having a 1000 Hz medium frequency 100 kVA three-phase transformer, and servodrived upper head with a force capacity of 15 kN. The head displacement, applied load, and voltage and intensity are monitored during the process by appropriate sensors. The powder containing die consisted in a 12 mm diameter alumina tube reinforced by a steel hoop, in a similar way to the arrangement used by Lenel [ 3 ]. The ceramic die is closed with upper and lower temperature-resistant Cu electrodes (98.9% Cu, 1% Cr, 0.1% Zr), which are in contact with electroerosion resistant and nonsticking heavy metal wafers 75.3% W–24.6% Cu. These wafers, with low thermal conductivity, have the additional objective of damping the heat evacuation from the powder to the water-refrigerated electrodes. The powder to be sintered is placed between both wafers (Figure 1). Metals2018,8,x 3of14 Inthiswork,aSerraSoldaduraS.A.(Spain)press‐typeresistanceweldingmachinewasadapted. Thecurrentintensityiselectronicallycontrolled,havinga1000Hzmediumfrequency100kVAthree‐ phasetransformer,andservodrivedupperheadwithaforcecapacityof15kN.Thehead displacement,appliedload,andvoltageandintensityaremonitoredduringtheprocessby appropriatesensors.Thepowdercontainingdieconsistedina12mmdiameteraluminatube reinforcedbyasteelhoop,inasimilarwaytothearrangementusedbyLenel[3].Theceramicdieis closedwithupperandlowertemperature‐resistantCuelectrodes(98.9%Cu,1%Cr,0.1%Zr),which areincontactwithelectroerosionresistantandnonstickingheavymetalwafers75.3%W–24.6%Cu. Thesewafers,withlowthermalconductivity,havetheadditionalobjectiveofdampingtheheat evacuationfromthepowdertothewater‐refrigeratedelectrodes.Thepowdertobesinteredisplaced betweenbothwafers(Figure1). Figure1.SketchofpunchesanddieusedinMF‐ERSexperiences. 2.2.MaterialandExperimentalProcedure Ac.p.ironpowder(FeWPL200,QMP,Mönchengladbach,Germany)waschosenasstarting material.Powdersweresubjectedtoaheattreatmentat300°Cfor10mininairtopromotetheoxide formingonthepowderssurface.Themainimpuritiesare0.01wt%C,0.2wt%Mn,and0.2wt%O (thelatterfigureistoohighandithinderstheconventionalsinteringprocess,becauseoxygenis mainlyfoundonthesurfaceoftheparticles,intheformofoxides,preventingatomicdiffusion).The apparentdensityofthepowder[10]is2.65g/cm3(a33.7%oftheabsolutedensity)andthemean particlesize78μm(Figure2).Thepowdermassintheexperimentswas3.5g,enoughtoreachafter consolidationaheight/diameteraspectrationear1/2.Initially,powdersarevibratedinsidethedie uptoreachtheirtapdensity[11],of4.45g/cm3. Figure2.SEMmicrographoftheWPL200FepowdertobeconsolidatedbyMF‐ERS. Figure 1. Sketch of punches and die used in MF-ERS experiences. 2.2. Material and Experimental Procedure A c.p. iron powder (Fe WPL200, QMP, Mönchengladbach, Germany) was chosen as starting material. Powders were subjected to a heat treatment at 300 ◦ C for 10 min in air to promote the oxide forming on the powders surface. The main impurities are 0.01 wt % C, 0.2 wt % Mn, and 0.2 wt % O (the latter figure is too high and it hinders the conventional sintering process, because oxygen is mainly found on the surface of the particles, in the form of oxides, preventing atomic diffusion). The apparent density of the powder [ 10 ] is 2.65 g/cm 3 (a 33.7% of the absolute density) and the mean particle size 78 µ m (Figure 2). The powder mass in the experiments was 3.5 g, enough to reach after consolidation a height/diameter aspect ratio near 1/2. Initially, powders are vibrated inside the die up to reach their tap density [11], of 4.45 g/cm3. The sequence of a typical MF-ERS experiment (Figure 3) begins with a cold-pressing period (with a fixed duration of 1000 ms), where the pressure is constant and no current is applied. Then, a heating and pressing period takes place, with electrical current being applied. The sequence finishes with a cooling/forging period (fixed at 300 ms) when only pressure acts.
Metals 2018,8, 426 4 of 14 Metals2018,8,x 3of14 Inthiswork,aSerraSoldaduraS.A.(Spain)press‐typeresistanceweldingmachinewasadapted. Thecurrentintensityiselectronicallycontrolled,havinga1000Hzmediumfrequency100kVAthree‐ phasetransformer,andservodrivedupperheadwithaforcecapacityof15kN.Thehead displacement,appliedload,andvoltageandintensityaremonitoredduringtheprocessby appropriatesensors.Thepowdercontainingdieconsistedina12mmdiameteraluminatube reinforcedbyasteelhoop,inasimilarwaytothearrangementusedbyLenel[3].Theceramicdieis closedwithupperandlowertemperature‐resistantCuelectrodes(98.9%Cu,1%Cr,0.1%Zr),which areincontactwithelectroerosionresistantandnonstickingheavymetalwafers75.3%W–24.6%Cu. Thesewafers,withlowthermalconductivity,havetheadditionalobjectiveofdampingtheheat evacuationfromthepowdertothewater‐refrigeratedelectrodes.Thepowdertobesinteredisplaced betweenbothwafers(Figure1). Figure1.SketchofpunchesanddieusedinMF‐ERSexperiences. 2.2.MaterialandExperimentalProcedure Ac.p.ironpowder(FeWPL200,QMP,Mönchengladbach,Germany)waschosenasstarting material.Powdersweresubjectedtoaheattreatmentat300°Cfor10mininairtopromotetheoxide formingonthepowderssurface.Themainimpuritiesare0.01wt%C,0.2wt%Mn,and0.2wt%O (thelatterfigureistoohighandithinderstheconventionalsinteringprocess,becauseoxygenis mainlyfoundonthesurfaceoftheparticles,intheformofoxides,preventingatomicdiffusion).The apparentdensityofthepowder[10]is2.65g/cm3(a33.7%oftheabsolutedensity)andthemean particlesize78μm(Figure2).Thepowdermassintheexperimentswas3.5g,enoughtoreachafter consolidationaheight/diameteraspectrationear1/2.Initially,powdersarevibratedinsidethedie uptoreachtheirtapdensity[11],of4.45g/cm3. Figure2.SEMmicrographoftheWPL200FepowdertobeconsolidatedbyMF‐ERS. Figure 2. SEM micrograph of the WPL200 Fe powder to be consolidated by MF-ERS. Metals2018,8,x 4of14 ThesequenceofatypicalMF‐ERSexperiment(Figure3)beginswithacold‐pressingperiod (withafixeddurationof1000ms),wherethepressureisconstantandnocurrentisapplied.Then,a heatingandpressingperiodtakesplace,withelectricalcurrentbeingapplied.Thesequencefinishes withacooling/forgingperiod(fixedat300ms)whenonlypressureacts. Figure3.TypicalsequenceinanMF‐ERSexperiment,showingthepressure,andcurrentintensityin thedifferentstages.Inalltheexperiments,thecold‐compactionandcoolingperiodslasted1000and 300ms,respectively. MF‐ERSexperimentswerecarriedoutwithdifferentcurrentintensitiesandheatingtimes. Testedintensitieswere6,8,and10kA,andheatingtimesof400,700,and1000mswerechosen.These intensities,normalizedwiththecross‐sectionofthecompacts,representcurrentdensitiesbetween 5.3and8.8kA/cm2.Intensitieslowerthan6kAdidnotsuccessfullyconsolidatethepowdermassfor timesaslowas400ms.Intensitieshigherthan10kAcombinedwithtimesintheorderorhigherthan 1000msresultedincompactsweldedtothewafers. Alltheexperimentswerecarriedoutwithanappliedpressureof100MPa,whichprovokesa porosityreductionfroma55%toapproximately43%.Aquitelowerpressuredoesnotguaranteethe currentpassing,andaquitehigheronecouldreducethedielife‐time,butcouldalsoprovokea deficientsintering.Thisisbecauseahighlydensifiedpowderduringthecoldcompactionstage resultsinaverylowelectricalresistivitypowder,andthethermalpowerdissipatedbytheJouleeffect decreases,eventothelimitofbeinginsufficienttoproduceahigherdensificationandaproper consolidation.Itwouldthenbenecessaryahigherintensity,resultingaprocessmoredifficultto control. PreviouslytoeachMF‐ERSexperiment,theinnerdiewallwaslubricatedwithathingraphite layerthroughagraphite‐acetonesuspension,alsoresultinginanunstickingeffect. Theelectricalresistanceevolutionduringtheprocesswasdeterminedfromthemeasurements ofvoltageandcirculatingcurrentbetweentheelectrodes.Theporosityvariationinthepowdermass wasdeterminedfromtheregisteredupperelectrodedisplacementandthemeasurementofthe compactfinalporosity.Thisfinalporosityisdeterminedbymeasuringthecompactdimensionsand weight.Heightanddiameterweredeterminedfromeightdifferentvalues,angularlyequispaced. AlthoughthemeasurementbytheArchimedesmethodguarantiesaloweruncertaintyforverylow porosities,thisisnotthecaseforhighandopenporosity. Inordertocomparetheporositydistributionincompactsobtainedbytheconventionaland electricalsintering,3.5g–12mmdiametercompactswerecoldcompactedat500MPa(diewall lubrication)andvacuumfurnacesinteredat1175°Cfor30min.Theseconditionswereselectedafter testingseveralconditions,includingtemperaturesof1175°Cand1300°C),pressuresof500,700,and 750MPa,andatmospheresofvacuum,nitrogen,andargon.Withthebestresult,thefinalporosityof thecompactscouldnotbebelow15%.Althoughlowerporositiesofabout8–9%orlesscanbereached withcertainFepowdersandtheuseofreducingatmospheresduringparticularpressandsinter cycles[12,13],itisinterestingforcomparisonpurposesnottousereducingatmospheres,sincethe MF‐ERSprocessiscarriedoutinair,withoutreducingatmosphere. MacrographsofcompactsweretakenwithaD90Canoncamera.Formicroscopicstudies,an optical(EPIPHOT200,Nikon,Tokyo,Japan)andelectronic(SEM,XL30,Phillips,Andover,MA, USA)microscopeswereemployed.Themacroandmicrographicstudieswerecarriedouton diametricalsectionsofthecompacts,embeddedinresinstofacilitatepreparation.Theprocedure Figure 3. Typical sequence in an MF-ERS experiment, showing the pressure, and current intensity in the different stages. In all the experiments, the cold-compaction and cooling periods lasted 1000 and 300 ms, respectively. MF-ERS experiments were carried out with different current intensities and heating times. Tested intensities were 6, 8, and 10 kA, and heating times of 400, 700, and 1000 ms were chosen. These intensities, normalized with the cross-section of the compacts, represent current densities between 5.3 and 8.8 kA/cm 2 . Intensities lower than 6 kA did not successfully consolidate the powder mass for times as low as 400 ms. Intensities higher than 10 kA combined with times in the order or higher than 1000 ms resulted in compacts welded to the wafers. All the experiments were carried out with an applied pressure of 100 MPa, which provokes a porosity reduction from a 55% to approximately 43%. A quite lower pressure does not guarantee the current passing, and a quite higher one could reduce the die life-time, but could also provoke a deficient sintering. This is because a highly densified powder during the cold compaction stage results in a very low electrical resistivity powder, and the thermal power dissipated by the Joule effect decreases, even to the limit of being insufficient to produce a higher densification and a proper consolidation. It would then be necessary a higher intensity, resulting a process more difficult to control. Previously to each MF-ERS experiment, the inner die wall was lubricated with a thin graphite layer through a graphite-acetone suspension, also resulting in an unsticking effect. The electrical resistance evolution during the process was determined from the measurements of voltage and circulating current between the electrodes. The porosity variation in the powder mass was determined from the registered upper electrode displacement and the measurement of the compact final porosity. This final porosity is determined by measuring the compact dimensions and weight. Height and diameter were determined from eight different values, angularly equispaced. Although the measurement by the Archimedes method guaranties a lower uncertainty for very low porosities, this is not the case for high and open porosity. In order to compare the porosity distribution in compacts obtained by the conventional and electrical sintering, 3.5 g–12 mm diameter compacts were cold compacted at 500 MPa (die wall lubrication) and vacuum furnace sintered at 1175 ◦C for 30 min. These conditions were selected after
Metals 2018,8, 426 5 of 14 testing several conditions, including temperatures of 1175 ◦ C and 1300 ◦ C), pressures of 500, 700, and 750 MPa, and atmospheres of vacuum, nitrogen, and argon. With the best result, the final porosity of the compacts could not be below 15%. Although lower porosities of about 8–9% or less can be reached with certain Fe powders and the use of reducing atmospheres during particular press and sinter cycles [ 12 , 13 ], it is interesting for comparison purposes not to use reducing atmospheres, since the MF-ERS process is carried out in air, without reducing atmosphere. Macrographs of compacts were taken with a D90 Canon camera. For microscopic studies, an optical (EPIPHOT 200, Nikon, Tokyo, Japan) and electronic (SEM, XL30, Phillips, Andover, MA, USA) microscopes were employed. The macro and micrographic studies were carried out on diametrical sections of the compacts, embedded in resins to facilitate preparation. The procedure consisted of wet mechanical grinding down to 2500 grit silicon carbide paper, and then cloth polishing with 1 µ m alumina powder suspension. 3. Results and Discussion 3.1. A Qualitative Model for Densification The variation of the global porosity, Θ , (i.e., the volumetric fraction of pores) and the electrical resistance, R, is shown in Figure 4for a typical experience of MF-ERS. As shown, the curves do not evolve similarly, with the resistance curve decreasing more abruptly than the porosity one. The vertical lines in Figure 4separate regions where the porosity curve shows remarkable changes. The porosity suffers no changes during the first moments of current passing (zone 1 in the graph), following, still in the heating period, an abrupt decrease in the zone 2, and finally tending to a stabilized residual value in the zone 3 (coincident with the cooling period). Metals2018,8,x 5of14 consistedofwetmechanicalgrindingdownto2500gritsiliconcarbidepaper,andthencloth polishingwith1μmaluminapowdersuspension. 3.ResultsandDiscussion 3.1.AQualitativeModelforDensification Thevariationoftheglobalporosity,Θ,(i.e.,thevolumetricfractionofpores)andtheelectrical resistance,R,isshowninFigure4foratypicalexperienceofMF‐ERS.Asshown,thecurvesdonot evolvesimilarly,withtheresistancecurvedecreasingmoreabruptlythantheporosityone.The verticallinesinFigure4separateregionswheretheporositycurveshowsremarkablechanges.The porositysuffersnochangesduringthefirstmomentsofcurrentpassing(zone1inthegraph), following,stillintheheatingperiod,anabruptdecreaseinthezone2,andfinallytendingtoa stabilizedresidualvalueinthezone3(coincidentwiththecoolingperiod). Figure4.PorosityandelectricalresistanceevolutioninanarbitrarilyselectedMF‐ERSexperiment. Theverticallineat1000msmarksthebeginningofthecurrentpassing.Theotherverticallines separatetheprocessstagesdiscussedinthetext.Theserratedshapeoftheporositycurveisa consequenceofthenoisereductionfunctionofthedisplacementsensor. TheexistenceoftheaforementionedzonesintheMF‐ERSprocesscanbeunderstood,atleast qualitatively,withthehelpofeasyconsiderations,mainlyrelatedtothefactthatmetallicpowder particlesaresurroundedbyoxidelayers.Theelectricalresistivityoftheseoxides(withdielectricor semiconductorcharacter),ingeneral,decaysinanexponentialwaywithtemperature.Thiscan supposeanelectricalresistivitydecreaseofseveralorders(forexample,theFe2O3hasaresistivityof 1016Ωmatroomtemperature,decreasingto10−2Ωmat1000K)[14,15].Regardlessofwhetherornot theoxidelayerissufficientlythin,thewholeparticleisaffectedbythelayerresistivity[16,17]. Therefore,increasingtemperature,evenbeforereachingavaluehighenoughtoprovokematerial softening,makestheparticleresistivitytodecrease,asshowninzone1.Then,thecurrentincreases, andsodoesthetemperature,softeningthepowderandmakingmaterialdensificationpossible becauseofthecontinuouslyappliedpressure(zone2).Oncedensificationstarts,theresistivityofthe remainingoxidedecreasesenoughtonolongeraffectthecompactresistance,whichwillbethen governedbythemetal–metalcontactsandtheporosityreduction,withasubsequentresistance decrease,althoughnotsignificantly.Thisreductionwillbeinpartcounterbythemetalresistivity, whichincreaseswithtemperatureuptoamaximumvalueduringtheprocess.Figure5summarizes theseideas. 0 5 10 15 20 25 30 35 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0 500 1000 1500 2000 2500 Resistance,R[m] Porosity Time,t[ms] Porosity Resistance Figure 4. Porosity and electrical resistance evolution in an arbitrarily selected MF-ERS experiment. The vertical line at 1000 ms marks the beginning of the current passing. The other vertical lines separate the process stages discussed in the text. The serrated shape of the porosity curve is a consequence of the noise reduction function of the displacement sensor. The existence of the aforementioned zones in the MF-ERS process can be understood, at least qualitatively, with the help of easy considerations, mainly related to the fact that metallic powder particles are surrounded by oxide layers. The electrical resistivity of these oxides (with dielectric or semiconductor character), in general, decays in an exponential way with temperature. This can suppose an electrical resistivity decrease of several orders (for example, the Fe 2 O 3 has a resistivity
Metals 2018,8, 426 6 of 14 of 10 16 Ω m at room temperature, decreasing to 10 −2Ω m at 1000 K) [ 14 , 15 ]. Regardless of whether or not the oxide layer is sufficiently thin, the whole particle is affected by the layer resistivity [ 16 , 17 ]. Therefore, increasing temperature, even before reaching a value high enough to provoke material softening, makes the particle resistivity to decrease, as shown in zone 1. Then, the current increases, and so does the temperature, softening the powder and making material densification possible because of the continuously applied pressure (zone 2). Once densification starts, the resistivity of the remaining oxide decreases enough to no longer affect the compact resistance, which will be then governed by the metal–metal contacts and the porosity reduction, with a subsequent resistance decrease, although not significantly. This reduction will be in part counter by the metal resistivity, which increases with temperature up to a maximum value during the process. Figure 5summarizes these ideas. Metals2018,8,x 6of14 Figure5.MicroscopicinterpretationofthedifferentstagesoftheMF‐ERSprocess.(a)Initialstatus withoxidizedpowderparticlesundercompression;(b)withheating,theoxideresistivitydecays, althoughstillnotaffectingtheporosity;(c)afterasufficientlyhightemperatureisreachedinparticle contacts,pressureprovokesaparticlesrearrangementwithmechanicaldescalingoftheoxidelayers, andahighdensificationtakesplace;(d)whenrearrangementisnofurtherpossible,densificationis duetoparticleshotdeformationwithalowvariationofporosityandresistance. 3.2.DensificationKinetics Figure6showstheglobalporosity(Θ)evolutionoftheMF‐ERScompacts,forthedifferent heatingtimesandcurrentintensities. 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0 500 1000 1500 2000 2500 Porosity, Time,t[ms] 400ms 6kA 8kA 10kA (a) Figure 5. Microscopic interpretation of the different stages of the MF-ERS process. ( a ) Initial status with oxidized powder particles under compression; ( b ) with heating, the oxide resistivity decays, although still not affecting the porosity; ( c ) after a sufficiently high temperature is reached in particle contacts, pressure provokes a particles rearrangement with mechanical descaling of the oxide layers, and a high densification takes place; ( d ) when rearrangement is no further possible, densification is due to particles hot deformation with a low variation of porosity and resistance. 3.2. Densification Kinetics Figure 6shows the global porosity ( Θ ) evolution of the MF-ERS compacts, for the different heating times and current intensities. It is clearly observed that increasing the intensity provokes a decrease in the final porosity of the compact, which is reached in a more abrupt way. Intensities of 8 and 10 kA make the zone 1 in Figure 4 practically to disappear. Also, increasing the current dwelling time yields to a lower final porosity, although its influence is lower than that of the current intensity. The dependence of the final porosity ΘFwith the current intensity and the heating time, is shown in Table 1.
Metals 2018,8, 426 7 of 14 Metals2018,8,x 6of14 Figure5.MicroscopicinterpretationofthedifferentstagesoftheMF‐ERSprocess.(a)Initialstatus withoxidizedpowderparticlesundercompression;(b)withheating,theoxideresistivitydecays, althoughstillnotaffectingtheporosity;(c)afterasufficientlyhightemperatureisreachedinparticle contacts,pressureprovokesaparticlesrearrangementwithmechanicaldescalingoftheoxidelayers, andahighdensificationtakesplace;(d)whenrearrangementisnofurtherpossible,densificationis duetoparticleshotdeformationwithalowvariationofporosityandresistance. 3.2.DensificationKinetics Figure6showstheglobalporosity(Θ)evolutionoftheMF‐ERScompacts,forthedifferent heatingtimesandcurrentintensities. 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0 500 1000 1500 2000 2500 Porosity, Time,t[ms] 400ms 6kA 8kA 10kA (a) Metals2018,8,x 7of14 Figure6.EvolutionoftheglobalporosityΘ,asafunctionofthecurrentintensity,fordifferentheating times:(a)400ms,(b)700ms,and(c)1000ms.Theintensitystartspassingthroughthespecimenafter 1000msandstops300msbeforetheendoftheprocess. Itisclearlyobservedthatincreasingtheintensityprovokesadecreaseinthefinalporosityofthe compact,whichisreachedinamoreabruptway.Intensitiesof8and10kAmakethezone1inFigure 4practicallytodisappear.Also,increasingthecurrentdwellingtimeyieldstoalowerfinalporosity, althoughitsinfluenceislowerthanthatofthecurrentintensity.Thedependenceofthefinalporosity ΘFwiththecurrentintensityandtheheatingtime,isshowninTable1. Table1.Valuesofthecompactsfinalporosity(ΘF)infunctionoftheintensity(inkA)and heatingtime(inms)forthedifferentMF‐ERSexperiments HeatingTime 400ms700ms1000ms Intensity 6kA0.300.280.24 8kA0.190.160.13 10kA0.120.080.06 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0 500 1000 1500 2000 2500 Porosity, Time,t[ms] 700ms 6kA 8kA 10kA (b) 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0 500 1000 1500 2000 2500 Porosity, Time,t[ms] 1000ms 6kA 8kA 10kA (c) Figure 6. Evolution of the global porosity Θ , as a function of the current intensity, for different heating times: ( a ) 400 ms, ( b ) 700 ms, and ( c ) 1000 ms. The intensity starts passing through the specimen after 1000 ms and stops 300 ms before the end of the process.
Metals 2018,8, 426 8 of 14 Table 1. Values of the compacts final porosity ( ΘF ) in function of the intensity (in kA) and heating time (in ms) for the different MF-ERS experiments. Heating Time 400 ms 700 ms 1000 ms Intensity 6 kA 0.30 0.28 0.24 8 kA 0.19 0.16 0.13 10 kA 0.12 0.08 0.06 Porosities in Table 1follow a reasonable trend, with the lower value for the bottom right cell. Data in Table 1have been represented in Figure 7for a better analysis. Metals2018,8,x 8of14 PorositiesinTable1followareasonabletrend,withthelowervalueforthebottomrightcell. DatainTable1havebeenrepresentedinFigure7forabetteranalysis. Figure7.Compactsfinalporosity(ΘF)versus(a)intensityand(b)heatingtime,forthedifferentMF‐ ERSexperiments.Thecontinuouslinesinbothgraphsrepresentthetrendofthewholedatacloud. Thediscontinuouslinesin(b)describethetrendsofexperiencescarriedoutwiththesamecurrent intensity. AsindicatedbythecoefficientsofdeterminationR2ofthequadratictrendlinescorresponding toallthedata(continuouslines),thecurrentintensityresultsamuchmoreinfluentialparameterthan theheatingtimetodeterminethevalueofthefinalporosity,atleastinthestudiedrange.Itshould benotedthattheporosityvaluescanbeaffectedbyapproximatelya5%ofexperimentaluncertainty. Theexpectedfinalporosity(ΘF)ofacompact,sinteredwithgivenvaluesofcurrentintensity andheatingtime,couldbeestimatedfromthedatainTable1bytwo‐dimensionalinterpolations. However,itisexpectedthat,accordingtothenatureoftheprocess,thethermalenergygeneratedby theJouleeffectresultsmoresuitablethantheintensityandheatingtimeforthestudyoftheMF‐ERS experiences. R²=0.906 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 4 6 8 10 12 FinalPorosity, F Intensity,I[kA] 6kA/400ms 6kA/700ms 6kA/1000ms 8kA/400ms 8kA/700ms 8kA/1000ms 10kA/400ms 10kA/700ms 10kA/1000ms (a) R²=0.092 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 100 400 700 1000 1300 FinalPorosity, F HeatingTime,t H [ms] 6kA/400ms 6kA/700ms 6kA/1000ms 8kA/400ms 8kA/700ms 8kA/1000ms 10kA/400ms 10kA/700ms 10kA/1000ms (b) Figure 7. Compacts final porosity ( ΘF ) versus ( a ) intensity and ( b ) heating time, for the different MF-ERS experiments. The continuous lines in both graphs represent the trend of the whole data cloud. The discontinuous lines in ( b ) describe the trends of experiences carried out with the same current intensity.
Metals 2018,8, 426 9 of 14 As indicated by the coefficients of determination R 2 of the quadratic trend lines corresponding to all the data (continuous lines), the current intensity results a much more influential parameter than the heating time to determine the value of the final porosity, at least in the studied range. It should be noted that the porosity values can be affected by approximately a 5% of experimental uncertainty. The expected final porosity ( ΘF ) of a compact, sintered with given values of current intensity and heating time, could be estimated from the data in Table 1by two-dimensional interpolations. However, it is expected that, according to the nature of the process, the thermal energy generated by the Joule effect results more suitable than the intensity and heating time for the study of the MF-ERS experiences. 3.3. Specific Thermal Energy Released The parameter STE (specific thermal energy) is the thermal energy released per powder unit mass due to the Joule effect. STE values for the different experimental MF-ERS conditions are shown in Table 2. These values of thermal energy come from the integration during the process of the electrical power that is generated in the powder aggregate; that is, STE =1 MZtH 0I2R(τ)dτ where Iis the intensity passing through the powder mass, Ris the electrical resistance, Mis the powder mass, and t H is the heating time. As expected, the higher STE value in Table 2is found in the bottom right cell. Table 2. Values of the STE (in kJ/g) in function of the intensity (in kA) and heating time (in ms) for the different MF-ERS experiments Heating Time 400 ms 700 ms 1000 ms Intensity 6 kA 0.35 0.47 0.59 8 kA 0.43 0.60 0.77 10 kA 0.53 0.72 1.02 The compacts final porosity ( ΘF ) is represented versus the STE in Figure 8. As shown, it can be said that the final porosity decreases by increasing the STE. Metals2018,8,x 9of14 3.3.SpecificThermalEnergyReleased TheparameterSTE(specificthermalenergy)isthethermalenergyreleasedperpowderunit massduetotheJouleeffect.STEvaluesforthedifferentexperimentalMF‐ERSconditionsareshown inTable2.Thesevaluesofthermalenergycomefromtheintegrationduringtheprocessofthe electricalpowerthatisgeneratedinthepowderaggregate;thatis, 2 0 1() H t STE I R d M whereIistheintensitypassingthroughthepowdermass,Ristheelectricalresistance,Misthe powdermass,andtHistheheatingtime.Asexpected,thehigherSTEvalueinTable2isfoundinthe bottomrightcell. Table2.ValuesoftheSTE(inkJ/g)infunctionoftheintensity(inkA)andheatingtime(in ms)forthedifferentMF‐ERSexperiments HeatingTime 400ms700ms1000ms Intensity 6kA0.350.470.59 8kA0.430.600.77 10kA0.530.721.02 Thecompactsfinalporosity(ΘF)isrepresentedversustheSTEinFigure8.Asshown,itcanbe saidthatthefinalporositydecreasesbyincreasingtheSTE. Figure8.Compactsfinalporosity(ΘF)versusreleasedspecificthermalenergy(STE)forthedifferent MF‐ERSexperiments.Thecontinuouslinedescribesthetrendofthewholedatacloud,whereasthe discontinuouslinescorrespondtothetrendsofexperiencescarriedoutwiththesamecurrent intensity. AlthoughtheglobaltrendofthedifferentvaluesrepresentedinFigure8isadecreasingtrend, itisfoundasignificantscattering,withacoefficientofdeterminationR2ofonly0.673.Thisvalueis intermediatebetweenthoseoftherepresentationsversusintensityandheatingtime.Inthesame graph,thetrendinglinescorrespondingtovaluesobtainedinexperienceswiththesamecurrent intensityhavebeenincluded.Thesepartialtrends,moreorlessalignedwiththeglobaltrend,reach coefficientsofdeterminationhigherthan0.94,meaningthattheintensityvaluesaremore R²=0.673 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.2 0.4 0.6 0.8 1.0 1.2 FinalPorosity,F SpecificThermalEnergy,STE [kJ/g] 6kA/400ms 6kA/700ms 6kA/1000ms 8kA/400ms 8kA/700ms 8kA/1000ms 10kA/400ms 10kA/700ms 10kA/1000ms Figure 8. Compacts final porosity ( ΘF ) versus released specific thermal energy (STE) for the different MF-ERS experiments. The continuous line describes the trend of the whole data cloud, whereas the discontinuous lines correspond to the trends of experiences carried out with the same current intensity.