Microwave Sintering Of Refractory Metals/alloys: W, Mo,

Avijit Mondal et al., Microwave Sintering of Refractory Metals/alloys: W, Mo, Re, W-Cu, W-Ni-Cu and W-Ni-Fe ialenhancementin reactivity and diffusion kinetics, finehomogeneous microstructures and improvedmechanical properties which lead to betterproduct performance. Microwave-matterinteraction and anisothermal situations oftenproduce better quality and new materials thatare normally not possible with conventionalmethods. These characteristics advantagesof microwave sintering can be exploited toovercome the processing difficulties of therefractory metals and alloys.This review article describes recentresearch findings about how certain refractorymetals and alloys (W, Mo, Re, W-Cu, W-NiCu and W-Ni-Fe) have been successfullyconsolidated using microwave sintering anda comparative study with conventional datahas been made. In most cases, microwavesintering resulted in an overall reduction ofsintering time of up to 80%. This sintering timereduction prevents grain growth providingfiner microstructure and as a result bettermechanical properties have been observed.CONSOLIDATION OF TUNGSTENUsually the consolidation of W powderby conventional heating is difficult andrequires very high temperature (2200 C ormore) in electrical resistance sintering underhydrogen atmosphere. The requirementof excessive high temperature and specialtechnique makes the process more expensiveand imparts a restriction in the sizes andshapes of the sintered products.Low-temperature brittleness is themost crucial aspect in the manufacturing ofpure tungsten metal. Therefore, in the pastmuch effort has been directed at loweringthe ductile to- brittle transition temperature(DBTT) and hence improving the fabricabilityof the metal.The brittleness of polycrystallinetungsten at low temperature is attributed tothe weakness of the grain boundaries, whichleads to initiation of cracking in both wroughtand recrystallized tungsten. Pure, singlecrystalline tungsten (free of grain boundaries)remains ductile down to at least 20K. Anyplastic deformation decreases the transitiontemperature by fining the structure [Lassnerand Schubert, 1999]. The presence of smallamount of interstitial impurities such as O, C,N and H2 has a detrimental effect on DBTT.The solubility for such elements intungsten at room temperature is much lowerwhen compare to high temperature. So duringcooling segregation of such elements at thegrain boundary significantly weakens the grainboundary strength. The ductilizing effectof Re in tungsten have gained outstandingimportance in this regard. Addition of Re cancause the transformation temperature to fallbelow room temperature, even for slightlydeformed products. Many studies have shownthat sintering temperature is related to thepowder size, when the size is in nano-scale,the sintering temperature can be decreasedup to several hundred degrees. The reductionof sintering temperature for nano tungstenhas been reported by several researchers[Sarma and Pabi, 2007; Oda et al., 2006;Bose et al., 2008; Engleman et al., 2008;Johnson, 2008;Wang et al., 2008; Jainet al., 2006; Jain et al., 2006]. The reportedsintering temperature of nano sized tungstenproduced by high energy mechanical millingwas drastically decreased from conventionaltemperature of 2500 C to 1700 C [Malewaret al., 2007]. Researchers have also shownthat pressure assisted process such as sparkplasma sintering [Oda et al., 2006], plasmapressure compaction [Bose et al., 2008],and hot isostatic processing (HIP) [Englemanet al., 2008] etc. helps in further reductionin the processing temperature. Selection ofnano powders over microcrystalline powdershas certain distinct advantages. According tothe literature reported data for both metalsand ceramic nano particles was found to startdensification at temperatures of 0.2–0.4 Tm(Tm - melting temperature) compared to 0.5–0.8 Tm for the conventional powders.It is believed that in the case ofnanostructured powders the grain boundaryJournal of Microwave Power and Electromagnetic Energy, 44 (1), 2010International Microwave Power Institute31

Avijit Mondal et al., Microwave Sintering of Refractory Metals/alloys: W, Mo, Re, W-Cu, W-Ni-Cu and W-Ni-Fe .sliding, dislocation motion, grain rotation,and viscous flow can significantly contributeto the enhanced sintering kinetics [Hahn,1993; Averbach et al., 1996; Bourell andGroza, 1998; Groza, 1999; Groza, 2002].Generally sintering theories hold that thesintering temperature is often correlatedwith melting point of the material. It is longbeen know that the melting temperatureof very fine particles decreases with thesize of the particles [Couchman and Jesser,1977]. Therefore in addition to the fastersintering kinetics, the faster densification innano structured material could be attributedto the lower melting temperature of nanoparticles.In addition to all the above mentionedadvantages, nano tungsten has two distinctadvantages over conventionally sized tungstenpowder. First, the thermal conductivityof nano tungsten is much lower than thatof conventionally sized tungsten powder.Secondly, as in nano dimension grain size areclose to dislocation length, no pinning or pileup of dislocation occurs [Andrey et al., 2004].The contribution of these two phenomenonresults in a material that exhibits a higherpropensity of adiabatic shear, which is veryimportant aspects of this material as a kineticenergy penetrator application is concerned.Wang [Wang et al., 2008] has studied the sizedependent sintering behavior of tungstenpowder.They observed that the startingparticle size before milling plays an importantrole in sinterability of this material, lowerthe size higher will be the sinterability.Although, the final crystallite size aftermilling is same in both the cases. The reasonfor this difference could be attributed withthe difference in particle size after milling.Mohit et al. [Jain et al., 2006] studiedthe application of microwave energy inconsolidating pure tungsten powder. Theyfound 10 to 12% higher sintered density inmicrowave sintering as compare to theirconventional counter part. In another studythey have reported the role of HfO2 and32Y2O3 as successful grain growth inhibitors.They also observed that the introduction ofa secondary oxide (HfO2 and/or Y2O3) had asignificant effect on the powder morphologyand in reducing the primary particle size ofthe as synthesized tungsten powders. Theparticle size was reduced from 350 nm to 80100nm, and the crystallite size was reducedfrom 48 nm to 25 nm with the addition ofdopents [Jain et al., 2006].Prabhu [Prabhu et al., 2008] has alsoinvestigated microwave sintering of puretungsten powder of as received grade andtungsten powder activated by high energymilling (HEM). Their study shows bettersinterability of activated tungsten powder incompare to as received powder.Mondal [Mondal et al., 2009] has alsoreported the similar kind of accelerateddensification in microwave sintered tungstensample. They reported that more than 95%Th density of microwave sintered samplesat 1600 C for 30 min holding in hydrogenatmosphere.Figure 1 describes typical thermalprofile used for their experiments in bothconventional as well as microwave heatingmode.Figure 2 are the SEM micrographof both conventionally and microwaveFigure. 1. Typical thermal profiles of sintered W usingconventional and microwave furnaces [Mondal et al., 2009].Journal of Microwave Power and Electromagnetic Energy, 44 (1), 2010International Microwave Power Institute

Avijit Mondal et al., Microwave Sintering of Refractory Metals/alloys: W, Mo, Re, W-Cu, W-Ni-Cu and W-Ni-Fe .Figure. 2. SEM micrographs of (left) conventional and (rigtj) microwave sintered W at 1600 C for 30 min in H2atmosphere [Mondal et al., 2009].Figure.3. Microwave sintering of complex shaped tungsten samples.sintered samples. Figure 3 show some ofthe commercial pure tungsten products thathave been successfully sintered in microwavefurnace at Penn State’s Microwave ProcessingCenter.CONSOLIDATION OF MOLYBDENUMConventionally the sintering ofmolybdenum powder is conducted using aresistance or induction sintering furnace inan inert atmosphere (argon) or in a reducingatmosphere (hydrogen) [Patrician et al.,1985]. High temperatures in the range of2000 C are employed, resulting in densitiesof 90–95% of theoretical, depending upon thesintering time. Huang [Huang and Hwang,2002] reported the sintering of molybdenumusing vacuum furnaces and obtained densitiesof 97 to 98.5% at a sintering temperature of1750 C with times ranging from 10 to 40 h.Journal of Microwave Power and Electromagnetic Energy, 44 (1), 2010International Microwave Power Institute33

Avijit Mondal et al., Microwave Sintering of Refractory Metals/alloys: W, Mo, Re, W-Cu, W-Ni-Cu and W-Ni-Fe .This also results in abnormal grain growth.Microwave sintering of molybdenum metalpowder has been reported for the first timeby Chillar [Chhillar et al., 2008]. In theirwork, the authors reported sintering of nanomolybdenum powder to obtain submicron grainsize microstructure using microwave energy.As received Mo powder was agglomeratedwith a mean agglomerate size of 1.6 um,but equivalent surface area based on N2adsorption suggests an average particle sizeof 200 nm. Sintering was carried out using theas received powder. Samples with densities ashigh as 98% of theoretical density (TD) wereobtained with limited grain growth in 5 minof sintering time in microwaves, compared to10–20 h in a conventional process.The highlight of this research wasachieving 98%TD in 1 min at 1650 C with asubmicron grain size. Microwave sinteringresult showed that near theoreticaldensities can be obtained at much reducedtemperatures, and with much reducedsintering times, as compared to conventionalsintering.Conventionally sintered samples at1400 C for 10 h resulted in 98%TD. However,using microwave energy 99%TD could beobtained at 1400 C in just 30 min. Thisconclusively shows that microwave sinteringis much faster than conventional sintering.CONSOLIDATION OF RHENIUMArc melting of rhenium in an inertatmosphere or vacuum is possible but themetal produced tends to have coarse grainsize and may have segregation of rheniumoxides at the grain boundaries. These issuesare a problem for further fabrication of aproduct and therefore powder metallurgy hasshown to mitigate some of these problems.Rhenium powder is consolidated using pressuretechniques to a density of approximately 60%of the theoretical density.The pressed compacts are then presintered in a hydrogen atmosphere to facilitatehandling before final sintering. Proper choiceof powder sizes, careful blending and adequate34sintering times and temperatures producebars of high yield and small grain size withthe same homogeneity attainable by electronbeam or arc melting. Subsequent fabricationis performed by swaging, rolling, forging anddrawing with intermediate annealing.Some of these operations may have tobe conducted at elevated temperatures. Theoptimum sequence of these operations variesfor rhenium and it alloys and depends on thefinal end form. Microwave sintering of rheniumpallet has been successfully conducted atPenn State’s microwave processing center.Fig. 4: SEM micrographs of Re pallet in (above) aspressed and (below) microwave sintered at 2000 C for10 min [Mondal et al., 2009].Journal of Microwave Power and Electromagnetic Energy, 44 (1), 2010International Microwave Power Institute

Avijit Mondal et al., Microwave Sintering of Refractory Metals/alloys: W, Mo, Re, W-Cu, W-Ni-Cu and W-Ni-Fe .Relatively high sintered density in the orderof 95% of theoretical has been achieved inmicrowave heating at 2000 C, 20 min soakingtime. Figure 4 shows a SEM micrograph ofas-pressed and microwave sintered rheniumcompact.CONSOLIDATION OF W-CuThere are two ways used to formW–Cu composites: copper infiltration of aporous tungsten skeleton and liquid phasesintering (LPS) of mixed phase compacts. Foreconomical reasons, shape, complexity, andlarge scale production consideration, thelatter technique is the only widely adaptedfabrication routes for W-Cu alloys. However,the lack of solubility between tungsten andcopper makes it very difficult to achieve fulldensification through liquid phase sintering.In the LPS approach, high compactionpressures increase the green density, butresult in increased particle-particle contactsthat limit particle rearrangement duringLPS. Other than rearrangement shrinkage,the sintering shrinkage is due to solid statesintering of the tungsten skeleton, even afterliquid formation. Smaller tungsten powdersinduce faster skeletal sintering or allowfor a lower sintering temperature, which isdesirable since copper evaporation occurs athigh temperatures. The mutual insolubility ofthe W–Cu system denies solution precipitationcontrolled LPS densification. On the otherhand, the addition of small quantities oftransition elements (such as Ni, Co and Fe),activate tungsten skeletal sintering and havea positive impact on the sinterability ofthe W–Cu system. However, these additionsdegrade electrical and thermal conductivity.Higher sintering temperature or longerholding time always help to improve thedensification but copper may diffuse out fromthe skeleton which leads to non homogeneousmicrostructure. Slow heating rate, coreduction of oxides or other thermo chemicalprocesses are all efficient method to improvethe densification of this material. Novelsintering technique has also been explored toenhance W-Cu composite densification whichincludes resistance sintering, plasma sprayingand laser sintering. Microwavesinteringof W-Cu has also been successfully reportedby several researchers [Mondal et al., 2008;Mondal et al., 2009; Mondal et al., 2007].Mondal showed that full densification can beachieved by microwave sintering of W-15Cucomposition. In spite of higher heating rate inmicrowave sintering, cracking in the sinteredsamples was not observed. This is attributedto the volumetric heating nature of microwavesintering. They also reported that microwavesintering lowers the sintering temperatureto achieve the optimum densification aswell as optimum mechanical properties.Figure 5. SEM micrographs of (above) conventional and(below) microwave sintered W-30Cu alloys, sintered at1200 C for 30 min in H2 atmosphere.Journal of Microwave Power and Electromagnetic Energy, 44 (1), 2010International Microwave Power Institute35

Avijit Mondal et al., Microwave Sintering of Refractory Metals/alloys: W, Mo, Re, W-Cu, W-Ni-Cu and W-Ni-Fe .Finer particle size and more homogeneousdistribution of tungsten in the microstructurehave been reported in microwave sintered W15Cu composites. Figure 5 exhibits finer grainsize and homogeneous distribution of boththe phases in microwave and conventionallysintered W-30Cu composites.TUNGSTEN HEAVY ALLOYS (WHAs)Tungsten heavy alloy (WHA) is a groupof two-phase composites, based on W-Ni-Cuand W-Ni-Fe. Price [Price et al., 1938] proposedliquid phase sintering as a viable techniquefor consolidating the W-Ni-Cu alloy. The WNi-Cu system provided an important basis forunderstanding the phenomenology of liquidphase sintering. In fact Cannon [Cannon andLenel, 1953] referred the mechanism of liquidphase sintering as ‘heavy alloy mechanism’.WHA possesses unique combination ofproperties such as high density (16-18 g/cm3), high strength (1000-1700 MPa) and highductility (10-30%).WHAs can be classified into two maingroups based on the binder composition: WNi-Cu and W-Ni-Fe. Besides the high densityand unique combination of high strength andductility, there are other attributes, whichmake WHA a versatile product [Lassner andSchubert, 1999]: The high modulus of elasticityExcellent vibration dampingcharacteristicsIts good machinabilityThe high absorption ability forx-rays and γ-raysGood thermal and electricalconductivitiesLow electrical erosion andwelding tendencyGood corrosion resistanceThe combination of density, ductility,strength, thermal conductivity and corrosionresistance makes them unique in manyapplications such as radiation shields,vibration dampers, kinetic energy penetrators36and heavy-duty electrical contacts. Being astrategic material, all the details regardingthe processing are not available in the openliterature. These alloys are very structuresensitive and therefore a firm understandingof the processing and their physical metallurgyare very critical for attaining better productperformance. In the past, substantialwork has been focused on modifying thematrix composition and post-sintering heattreatment to achieve full densification andoptimal mechanical properties. Residualporosity greater than 0.5% drastically reducesthe mechanical properties, especially thetoughness and ductility.CONSOLIDATION OF W-Ni-Cu ALLOYSIn W-Ni-Cu alloys, normally the nickelto-copper ratio ranges from 3:2 to 4:1. Price[Price et al, 1938] were the first to proposeNi-Cu as the binder for tungsten heavy alloys.They have shown that highly dense W-Ni-Cuheavy alloys could only be obtained by liquidphase sintering.Subsequently, researchers lookedat full densified W-Ni-Cu composites bysolid state sintering. Solid state activatedsintering of tungsten powder with various NiCu additions was studied by Brophy [Brophyet al., 1966]. Their study was mainly confinedto densification mechanism. They proposedthat shrinkage occurs during the initial twostages of sintering: (i) rearrangement and (ii)solution-reprecipitation stages. The role ofphase relationships on the activated sinteringof tungsten was studied by Prill [Prill, 1964].They proposed that at lower Ni:Cu content,the acceleration of sintering of tungstendiminishes. Kothari [Kothari, 1967] studiedthe densification and grain growth kineticsof W-Ni-Cu heavy alloys with different Ni:Curatios.The rate constant for the sinteringprocess was evaluated from the volumechange as a function of sintering period. Theactivation energy was found to be independentof binder composition. Grain growth rate wasproportional to the sintering period. MakarovJournal of Microwave Power and Electromagnetic Energy, 44 (1), 2010International Microwave Power Institute

Avijit Mondal et al., Microwave Sintering of Refractory Metals/alloys: W, Mo, Re, W-Cu, W-Ni-Cu and W-Ni-Fe .[Makarov et al., 1965] studied the coalescencephenomenon in W-Ni-Cu alloys during liquidphase sintering. The effect of tungsten andcopper powder size variation on the sinteredproperties of W-Ni-Cu heavy alloys was carriedout by Srikanth [Srikanth et al., 1983; Srikanthand Upadhyaya, 1984]. They found enhancedsintered properties with finer particle size ofthe constituent with increasing nickel contentin the binder. The effect of composition andsintering temperature on the densificationand microstructure of W-Ni-Cu heavy alloyswas studied by Ramakrishanan [Ramakrishnanand Upadhyaya, 1990]. Kuzmic [Kuzmic, 1966

refractory metals and alloys. This review article describes recent research findings about how certain refractory metals and alloys (W, Mo, Re, W-Cu, W-Ni-Cu and W-Ni-Fe) have been successfully consolidated using microwave sintering and a comparative study with