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Materials Chemistry and Physics 130 (2011) 449–457 Contents lists available at ScienceDirect Materials Chemistry and Physics j ourna l ho me pag e: www.elsevier.com/locate/matchemphys Effect of microwave and conventional heating on sintering behavior and properties of Al–Mg–Si–Cu alloy C. Padmavathi a,, A. Upadhyaya a , D. Agrawal b a Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur, UP, India b Materials Research Institute, Pennsylvania State University, University Park, USA a r t i c l e i n f o Article history: Received 11 February 2011 Received in revised form 28 May 2011 Accepted 2 July 2011 Keywords: Alloys (A) Sintering (B) Microstructure (D) Mechanical Properties (D) a b s t r a c t Effect of heating mode and sintering temperature on sinterability and properties of the 6711 (Al–1Mg–0.8Si–0.25Cu) alloy was investigated. Alloy compacts were consolidated in conventional and microwave furnace at 570, 590, 610 and 630 C under vacuum (10 6 torr). Microwaves coupled with the compacts and resulted in 58% reduction in processing time along with higher heating rates when com- pared to conventional sintering. However rapid heating rate resulted in inhomogeneous microstructure with larger melt fraction at grain boundaries. XRD analysis showed absence of intermetallics due to insuf- ficient time for diffusion. With increasing sintering temperature, all compacts did undergo supersolidus liquid phase sintering (SLPS), accordingly higher densification and shrinkage was observed. The electrical conductivity and hardness followed similar trend as sintered density in both modes. Alloy compact sin- tered at 630 C resulted in significant improvement of mechanical properties (TRS: 57%; UTS: 27%and ductility: 41%) in conventional mode. Age hardening treatment under T6 temper enhanced the tensile strength by 136% with drastic reduction in ductility as compared with sintered compacts. © 2011 Elsevier B.V. All rights reserved. 1. Introduction In recent years, increasing interest in high precision and cost effective processing of lightweight materials has created an opportunity for powder metallurgical (P M 1 ) processing of alu- minum alloys [1]. Due to their lightweight, high compressibility, exceptional specific strength and economical processing, P M 1 aluminum alloys have potential to replace components fabricated by cast and wrought route in automotive industry [2–4]. The for- mation of surface oxide layer during compaction often restricts the densification during sintering of aluminum alloys [5] and conse- quently results in inferior mechanical properties. It was reported by researchers [1,6–10] that successful removal of oxide and sin- tering of aluminum alloys can happen only in the presence of liquid phases during sintering (LPS). The presence of magnesium (0.1–1 wt.%) improves sinterability by breaking the oxide layer through formation of Al–Mg eutectic at 450 C and by forming MgAl 2 O 4 (spinel) structure with surface oxide [5,7]. Savitskii et al. reported the successful liquid phase sintering of Al–Mg (3–32 wt.%) between 460 C and 610 C [11]. Recently, Youseffi and his co-workers [12,13] studied sintering of 6061 prealloyed powders and reported that sinterability was based Corresponding author. E-mail address: [email protected] (C. Padmavathi). on formation of persistent liquid phase above solidus tempera- ture by supersolidus liquid phase sintering (SLPS) mechanism. Ziani and Pelletier [14,15] studied the sintering behavior and tensile properties of vacuum degassed 6061 prealloyed powders during SLPS. Very little research work [12,14,16,17] has been done on Al–Mg–Si–Cu alloys. It was found that unlike other Al alloys, expan- sion in Al–Mg–Si–Cu alloys mainly occurred due to formation of different eutectics but not due to the Kirkendall effect (unbalanced diffusion rates) [16]. The sintering temperature is considered as an important param- eter for varying solid-melt ratio, provided there is flexibility to achieve higher densification in the presence of persistent liquid phase [18]. Another important approach to improve the sinter- ability of Al–Mg based alloys can be through compression of overall processing time. The reduction in sintering time can be achieved through novel techniques such as spark plasma sintering, laser sintering, microwave sintering and hot-isostatic processing [19]. Microwave sintering has been widely recognized due to its higher heating rate, shorter processing time, finer microstructures, improved mechanical properties and environmentally friendliness [20]. Microwave sintering of various particulate metallic systems (Fe, W, and Cu–Sn) has been investigated and produced materi- als with enhanced densities and mechanical properties [20–25]. Microwave sintering of Al based material (Al–SiC composite) was first reported by Leparoux et al. [26,27]. Subsequently, a very few researchers [28–31] have studied the microwave sintering of vari- ous Al based composites in detail. Vauchera et al. [32] investigated 0254-0584/$ see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.07.008

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    Materials Chemistry and Physics 130 (2011) 449 457

    Contents lists available at ScienceDirect

    Materials Chemistry and Physics

    j ourna l ho me pag e: www.elsev ier .com/ locate /matchemphys

    ffect of microwave and conventional heating on sintering behavior androperties of AlMgSiCu alloy

    . Padmavathia,, A. Upadhyayaa, D. Agrawalb

    Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur, UP, IndiaMaterials Research Institute, Pennsylvania State University, University Park, USA

    r t i c l e i n f o

    rticle history:eceived 11 February 2011eceived in revised form 28 May 2011ccepted 2 July 2011

    eywords:lloys (A)

    a b s t r a c t

    Effect of heating mode and sintering temperature on sinterability and properties of the 6711(Al1Mg0.8Si0.25Cu) alloy was investigated. Alloy compacts were consolidated in conventional andmicrowave furnace at 570, 590, 610 and 630 C under vacuum (106 torr). Microwaves coupled with thecompacts and resulted in 58% reduction in processing time along with higher heating rates when com-pared to conventional sintering. However rapid heating rate resulted in inhomogeneous microstructurewith larger melt fraction at grain boundaries. XRD analysis showed absence of intermetallics due to insuf-

    intering (B)icrostructure (D)echanical Properties (D)

    ficient time for diffusion. With increasing sintering temperature, all compacts did undergo supersolidusliquid phase sintering (SLPS), accordingly higher densification and shrinkage was observed. The electricalconductivity and hardness followed similar trend as sintered density in both modes. Alloy compact sin-tered at 630 C resulted in significant improvement of mechanical properties (TRS: 57%; UTS: 27%andductility: 41%) in conventional mode. Age hardening treatment under T6 temper enhanced the tensilestrength by 136% with drastic reduction in ductility as compared with sintered compacts.

    . Introduction

    In recent years, increasing interest in high precision andost effective processing of lightweight materials has created anpportunity for powder metallurgical (P M1) processing of alu-inum alloys [1]. Due to their lightweight, high compressibility,

    xceptional specific strength and economical processing, P M1

    luminum alloys have potential to replace components fabricatedy cast and wrought route in automotive industry [24]. The for-ation of surface oxide layer during compaction often restricts the

    ensification during sintering of aluminum alloys [5] and conse-uently results in inferior mechanical properties. It was reportedy researchers [1,610] that successful removal of oxide and sin-ering of aluminum alloys can happen only in the presence of liquidhases during sintering (LPS).

    The presence of magnesium (0.11 wt.%) improves sinterabilityy breaking the oxide layer through formation of AlMg eutectict 450 C and by forming MgAl2O4 (spinel) structure with surfacexide [5,7]. Savitskii et al. reported the successful liquid phase

    intering of AlMg (332 wt.%) between 460 C and 610 C [11].ecently, Youseffi and his co-workers [12,13] studied sintering of061 prealloyed powders and reported that sinterability was based

    Corresponding author.E-mail address: [email protected] (C. Padmavathi).

    254-0584/$ see front matter 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.07.008

    2011 Elsevier B.V. All rights reserved.

    on formation of persistent liquid phase above solidus tempera-ture by supersolidus liquid phase sintering (SLPS) mechanism. Zianiand Pelletier [14,15] studied the sintering behavior and tensileproperties of vacuum degassed 6061 prealloyed powders duringSLPS. Very little research work [12,14,16,17] has been done onAlMgSiCu alloys. It was found that unlike other Al alloys, expan-sion in AlMgSiCu alloys mainly occurred due to formation ofdifferent eutectics but not due to the Kirkendall effect (unbalanceddiffusion rates) [16].

    The sintering temperature is considered as an important param-eter for varying solid-melt ratio, provided there is flexibility toachieve higher densification in the presence of persistent liquidphase [18]. Another important approach to improve the sinter-ability of AlMg based alloys can be through compression ofoverall processing time. The reduction in sintering time can beachieved through novel techniques such as spark plasma sintering,laser sintering, microwave sintering and hot-isostatic processing[19]. Microwave sintering has been widely recognized due to itshigher heating rate, shorter processing time, finer microstructures,improved mechanical properties and environmentally friendliness[20]. Microwave sintering of various particulate metallic systems(Fe, W, and CuSn) has been investigated and produced materi-als with enhanced densities and mechanical properties [2025].

    Microwave sintering of Al based material (AlSiC composite) wasfirst reported by Leparoux et al. [26,27]. Subsequently, a very fewresearchers [2831] have studied the microwave sintering of vari-ous Al based composites in detail. Vauchera et al. [32] investigated

    dx.doi.org/10.1016/j.matchemphys.2011.07.008http://www.sciencedirect.com/science/journal/02540584http://www.elsevier.com/locate/matchemphysmailto:[email protected]/10.1016/j.matchemphys.2011.07.008

  • 450 C. Padmavathi et al. / Materials Chemistry and Physics 130 (2011) 449 457

    Table 1Chemical composition and physical characteristics of as-received 6711 aluminumalloy powders used in the present study.

    Composition (wt.%) Al0.25Cu1Mg0.8SiProcessing route Gas atomizationParticle size (m)

    D10 24.7D50 83.6D90 170

    Apparent density (g cm3) 1.19

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    Flow rate (s/50 g) 120Specific surface area (m2 g1) 0.129Theoretical density (g cm3) 2.69

    n situ phase transformations occurring during microwave heatingf AlFeCu alloy powders. To the best of our knowledge, there haveeen no reports on microwave sintering of AlMgSiCu alloys.

    The present study investigates the sintering response ofl1Mg0.8Si0.25Cu (6711) alloy that has been consolidated

    hrough microwaves and compares its densification, microstruc-ure, phase changes and mechanical properties with conventionallyintered compacts. Effect of heating mode has been investigated as

    function of sintering temperature. Differential scanning calorime-ry (DSC) and dilatometric studies have been conducted to studyhe phase transformations and in situ dimensional changes also as

    function of temperature. Age hardening of conventionally sin-ered compacts in T4 and T6 ageing conditions was conducted tonhance the mechanical properties.

    . Experimental procedure

    For the present study, the Al1Mg0.8Si0.25Cu (6711) alloy powder contain-ng about 1.5 wt.% acrawax was supplied by AMPAL Inc., NJ, USA. The detailedowder characteristics are provided in Table 1. As-received alloy powder was uni-xially pressed at 400 MPa in a uniaxial hydraulic press (supplier: FIE Pvt. Ltd.,chalkaranji, India) to a green density of 95.3% theoretical density. The sinter-ng response on densification, microstructure and phase evolution, hardness wasvaluated on cylindrical pellets (16 mm diameter and 6 mm height). For mea-uring the transverse rupture strength (TRS) and tensile properties, TRS samples31.7 mm 12.7 mm 5 mm) and flat dog-bone shaped tensile samples were com-acted at 400 MPa as per MPIF specification 41 [33] and 10 [34], respectively.

    A small amount of sample (2570 mg) in alloy powder form was used for differ-ntial scanning calorimetry (DSC) analysis (model: 2960; supplier: TA Instruments,SA). The analysis was performed upto 700 C at a heating rate of 10 C min1 under2 atmosphere. The in situ dimensional changes during sintering was measuredsing a vertical pushrod dilatometry (model: 1161 V; supplier: Anter Instrument

    nc., Pittsburg, USA) by heating the green compact upto 630 C at a heating rate ofC min1 under high purity N2.

    Prior to conventional sintering, debinding of green compacts was carried outt 350 C for 1 h under high purity N2 in a separate SiC tubular furnace (supplier:ahendra Instruments, India). The debinded compacts were sintered in a SiC tubular

    urnace (supplier: Mahendra Instruments, India) coupled with high vacuum pump-ng system at constant heating rate of 5 C min1. The compacts were liquid phaseintered at temperatures ranging from 570 C to 630 C for 1 h under vacuum levelf 106 torr. The details of the experimental set-up are provided elsewhere [35].imilarly, prior to microwave sintering, green compacts were debinded in conven-ional vacuum furnace with roughing vacuum of 102 torr with isothermal held at50 C for 6 h. Microwave sintering of the debinded compacts was done in a multi-ode 6 kW, 2.45 GHz microwave vacuum (106 torr) furnace (model: 56-F industrialicrowave generator, supplier: Cober electronics Inc., USA). Further details of exper-

    mental arrangements and microwave furnace used in present study are describedlsewhere [35].

    Sintered densities were obtained through dimensional measurements as wells by Archimedes principle. Three samples were investigated for each set of condi-ions and an average value is reported herein. The variation in initial green densityas taken into account and compact sinterability was also expressed in terms ofimensionless parameter called densification parameter (DP) as per equation 1 [18]:

    ensification parameter = sintered density green densitytheoretical density green density (1)

    ensification parameter was calculated to quantify the amount of densification that

    ccurred during sintering. The positive densification parameter indicates shrinkagend negative indicates swelling during sintering [18].

    The sintered samples were subjected to metallographic preparation and etchedsing Kellers reagent for 30 s. The microstructural analysis was carried out using anptical microscope (supplier: Zeiss, Germany) and scanning electron microscope

    Fig. 1. Comparison of the thermal profiles of 6711 alloy compact sintered in a radi-avtively heated (conventional) and microwave furnace.

    (model: QUANTA 200, Supplier: FEI, The Netherlands) in secondary electron (SE)mode. Phase transformation was studied using X-ray powder diffractometer(supplier: Rich Seifert & Co., Germany) based on Cu-K radiation at a scan rate of3 deg min1. Electrical conductivity measurements were performed on sinteredcompacts with digital conductivity meter (supplier: Technofour, India).

    The bulk hardness of sintered alloy was measured using Vickers microhard-ness testing (Model: SHP 150, Supplier: Barieiss, Germany) at an applied load of20 g for 10 s. The TRS measurements were done using three point bend set upin a universal testing machine at a cross-head speed of 0.5 mm min1 and spanlength of 25 mm. The tensile testing was also performed using an 100 kN capac-ity universal testing machine (model: 1195, supplier: INSTRON, UK) at a crossheadspeed of 0.5 mm min1 and load of 10 kN. From the load-displacement curves, ten-sile strength (UTS), 0.2% offset yield strength (YS) and ductility (% elongation) werecalculated. The fractured surfaces were observed through SEM in secondary mode.

    The age hardening of conventionally sintered 6711 alloy comprised of solutiontreatment (530 C for 1 h) followed by water quenching. Later, samples were sub-jected to natural ageing (T4) at room temperature (35 C) for maximum of 90 daysand artificial ageing (T6) at 160 C in a re-circulating oven (supplier: MahendraInstruments, Kanpur, India) for 18 h. The details of solution treatment and age hard-ening experimental set up and procedure for obtaining the peak hardness has beendescribed elsewhere in detail [35]. Hardness measurement of heat treated sampleswere performed using Rockwell hardness tester (supplier: Indentec) in B scale under100 kg load. Transverse rupture strength (TRS) and tensile properties of heat treatedsamples were obtained as explained above.

    3. Results and discussion

    3.1. Heating profiles and densification response

    The thermal heating profiles of Al1Mg0.8Si0.25Cu alloycompacts in conventional and microwave furnaces are shown inFig. 1. It was observed that the 6711 alloy couples well withmicrowaves and also susceptor (SiC rods) aided the heating whichresulted in higher heating rates without any intermittent temper-ature hold. This behavior is similar to those reported for otherpowdered metallic systems [20,22,25]. The overall heating rateobtained in microwave furnace was 22 C min1 at 590 C for60 min. Considering the heating cycle, 58% reduction in the process-ing time was obtained during microwave sintering as compared toslower heating rate (5 C min1) in a conventional furnace. In caseof conductive metals, the depth of microwave penetration is knownas skin depth [36]. The skin depth is dependent on microwavefrequency (2.45 GHz), magnetic permeability and electrical con-ductivity [35,36]. The equation for skin depth is mathematicallydescribed in detail elsewhere [36]. Fig. 2 shows the effect of tem-perature on skin depth for aluminum in comparison with otherconductive metals as well upon microwave interaction. The val-ues of resistivity as a function of temperature were taken from the

    literature [36,37]. It is clearly seen that metals having higher elec-trical conductivity show lower skin depth values. Accordingly, skindepth of aluminum is 1.7 m [35,36]. But since each particle isabsorbing microwave from all sides to the extent of the skin depth,

  • C. Padmavathi et al. / Materials Chemistry and Physics 130 (2011) 449 457 451

    Fig. 2. The variation in the skin depth of aluminum subjected to 2.45 GHzmicrowaves. For comparison, the skin depth for some common metals have beenplotted for various temperatures [36]. The skin depth for other metals are given inl

    ie

    ditpittmstidsscais

    matl

    of the AlMgSiCu alloy, the phase evolution and dimensionalchanges during sintering were recorded using DSC and dilatom-

    Fs

    ighter shade.

    t gets heated further due to thermal conductivity and as result thentire compact is rapidly heated.

    Effect of sintering temperature and heating mode on sinteredensity and densification parameter of 6711 alloy compacts is given

    n Fig. 3. It is worth noticing that irrespective of heating mode,he compacts undergo significant densification at elevated tem-erature (630 C). Microwave sintered compacts showed marginal

    mprovement or lower sintered density as compared to conven-ional counterparts. The distortion of 6711 compacts occurs due tohe presence of compounds such as Al2O3/MgAl2O4 which are poor

    icrowaves absorbers at room temperature[38]. The Al powderurface interacts with microwaves and undergoes joules heatinghrough eddy current losses and is expected to undergo rapid heat-ng. This results in excess liquid formation and leads to compactistortion. The densification parameter follows a similar trend asintered density in both heating modes. The highest sintered den-ity (97%) and densification parameter (0.3) were obtained forompacts sintered at 630 C in conventional furnace. Except 630 C,t other temperatures the compacts slightly swelled during sinter-ng. Gaur [39] too reported that 6061 alloys in general swell afterintering.

    In prealloyed condition, there is no second phase available forelt formation, the powder per se partially melts during sintering

    bove the solidus temperature and this mode of sintering charac-

    eristics to prealloyed powder system is referred to as supersolidusiquid phase sintering (SLPS) [18,40,41]. Fig. 4 shows the schematic

    ig. 3. Effect of sintering temperature and heating mode on (a) sintered density and denintered under vacuum (106 torr).

    Fig. 4. Schematic sketch of the densification and accompanying microstructuralchanges that occur in supersolidus liquid phase sintering.

    sketch of the densification and corresponding microstructuralchanges that occur in supersolidus liquid phase sintering.

    During subsequent heating, depending on the alloy com-position, the formation of liquid phases at particular eutectictemperatures occurs. This liquid formation at intergranular regionsresults in particle fragmentation and further SLPS [18]. Highersintering temperature above solidus, leads to more liquid phaseformation in SLPS reulting further compact densification/shrinkage[12,14]. Similar behavior was observed for vacuum degassed pre-alloyed 6061 powders sintered at 620 C under vacuum (104 torr)to a higher sintered density (96.5% theoretical) [12,13].

    3.2. Thermal analysis (dilatometric and DSC) of 6711 alloy

    For better understanding the phenomenology of densification

    etry, respectively. From DSC curve (Fig. 5a), a slight inflection isnoticed at 530 C during heating. The alloy powder melts in a

    sification parameter of 6711 compact. All compacts were pressed at 400 MPa and

  • 452 C. Padmavathi et al. / Materials Chemistry and Physics 130 (2011) 449 457

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    ig. 5. (a) Differential scanning calorimetry (DSC) plot for 6711 alloy powder whilatometry curve for alloy compact pressed at 200 and 400 MPa; compacts were urves for compacts pressed at 200 MPa at a magnified resolution.

    ange of temperature with solidus and liquidus temperature being80 C and 646 C. It was reported that magnified view of the DSCurve shows the peaks at about 452 C, 558 C and 577 C withmaller intensities which corresponds to the formation of AlMg,lMgSi and AlSi eutectics, respectively [12,42].

    The dilation behavior of 6711 alloy compact heated in conven-ional mode is presented in Fig. 5b and c. It is evident that compactradually expands during heating upto 585 C and then shrinksdensification starts). Compact pressed at 400 MPa shows maxi-

    um expansion of 1.5% at 585 C and thereafter shrinks as muchs 1.2% at 630 C. While compacts pressed at 200 MPa exhibit aarginal dilation of 0.015% and maximum shrinkage of only 0.025%

    t 585 C and 630 C, respectively (Fig. 5c). The trend in dilatom-try results is similar to that reported by Schaffer et al. [7]. Therends discerned through DSC and dilatometry plots correlate wellith the possible phase reactions in the present system and can be

    ummarized as the following [1,39,43]:

    l + Al3Mg L (T 450 C) (2)l + Mg2Si + Si L (T 558 C) (3)l + Si L (T 577 C) (4)l + Mg2Si L (T 586 C) (5)he oxide layer covering the powder particle surface shears off dur-ng compaction and leads to direct contact between Al and Mg [1].he initial swelling behavior of compact may be due to diffusion of

    g into Al [11] that eventually leads to formation of AlMg eutectic

    t 450 C. Savitskii and Martsunova [11] have shown that initiallyhe melt has poor wettability over the matrix and consequentlyhows pronounced swelling upto 500 C. Eventually this eutectic

    as heated up to 700 C at a constant heating rate of 10 C min1 in nitrogen, (b)d in nitrogen up to 630 C at a constant heating rate of 3 C min1 and (c) dilation

    melt reacts with Al2O3 to form spinel (MgAl2O4) and results in com-pact shrinkage [5]. In case, if Si is present in the compact, part ofthe Mg reacts with Si to form Mg2Si [42] and which in turn willreact with the other constit1uents to result in expansion (Eq. (3)).In case of AlMgSiCu alloys, there is an additional eutectic meltformation at 548 C [1,17]. With the exception of Eq. (5), all othereutectic melt is transient in nature and therefore at an elevatedtemperature, they diffuse into the Al matrix to form solid solution[44].

    Initially, it was assumed that the expansion of the compact dur-ing heating is due to the unbalanced diffusion of alloying species(Kirkendall effect) [17,18]. However, diffusivity of Al in Mg and Mgin Al are in the same range [11]. Similarly, diffusion coefficients ofAl in Si and Si in Al at expansion event temperature too exist in thesame range [11]. Hence, compact swelling during heating in 6711alloys cannot be attributed to Kirkendall effect and instead is dueto the formation of the transient eutectic melts at the intermediatetemperatures [16]. While the dilation of the 6711 alloy compactspressed at 200 MPa and 400 MPa exhibit a self-similar behavior, themagnitude of dimensional change is markedly different. This can berationalized in terms of the entrapped gas expansion in compactspressed at higher pressures during sintering [41]. As mentionedabove, the subsequent shrinkage in compacts at elevated temper-atures could be due to supersolidus sintering.

    3.3. Microstructural and phase evolution

    Fig. 6 shows the effect of sintering temperature and heat-ing mode on microstructure of the alloy sintered under vacuum.Microstructure with well developed equiaxed grains is evident inconventionally sintered compacts. Amount of porosity is greatly

  • C. Padmavathi et al. / Materials Chemistry and Physics 130 (2011) 449 457 453

    Fig. 6. Optical micrographs of 6711 alloy sintered at 590 C and 630 C in a (a) radiatively-heated (conventional) and (b) microwave furnace. All compacts were pressed at400 MPa prior to sintering under vacuum (106 torr).

    Fig. 7. Scanning electron micrographs of 6711 alloy sintered at 590 C and 630 C in a (a) radiatively-heated (conventional) and (b) microwave furnace. All compacts werepressed at 400 MPa prior to sintering under vacuum (106 torr).

  • 454 C. Padmavathi et al. / Materials Chemistry and Physics 130 (2011) 449 457

    Fig. 8. XRD analysis of 6711 alloy vacuum sintered at different temperatures in (a) conventional and (b) microwave furnace.

    F ctivityu

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    ig. 9. Effect of sintering temperature and heating mode on the (a) electrical condunder vacuum (106 torr).

    educed by increasing sintering temperature from 590 C to 630 C.urprisingly the grain size is not coarser at higher temperaturehich is attributed to the compensating effect by particle fragmen-

    ation in SLPS. In contrast, microwave sintered samples consisted ofelatively inhomogeneous grains with a proportionately larger frac-ion of dark phase at grain boundaries at 630 C as compared with90 C. Inspite of higher heating rates, coarse grains are observed in

    icrowave sintered compacts. The pores are isolated and restricted

    o grain boundaries in both heating modes. To confirm this, grainoundary regions in microwave sintered compacts were observednder scanning electron microscope (SEM) as shown in Fig. 7.

    ig. 10. 6711 alloy sintered at different temperatures and subjected to different ageing turves.

    and (b) hardness of 6711 alloy. All compacts were pressed at 400 MPa and sintered

    The electron micrographs of microwave sintered compacts atboth temperatures revealed the presence of a second phase at grainboundaries interspersed with tiny pin hole type pores as shown inFig. 7b. Furthermore, the relative fraction of this second phase issignificantly higher in compacts sintered at 630 C in microwavefurnace than 590 C. This phase can be attributed to the eutecticmelt formation that occurs during sintering. Faster heating rate

    and shorter process time led to incomplete diffusion of alloyingelements [20,45] thereby formation of more liquid at intergranularregions. Due to the insufficient time, regions near grain boundariesare inhomogeneous in chemical composition. Elsewhere, similar

    reatments (T4 and T6) (a) transverse rupture strength and (b) tensile stressstrain

  • C. Padmavathi et al. / Materials Chemistry and Physics 130 (2011) 449 457 455

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    ig. 11. (a) SEM fractographs of 6711 alloy sintered at different temperatures and s

    esults for microstructural inhomogenization were reported onicrowave sintered premixed CuSn alloys by Upadhyaya and

    ethi [46].Fig. 8 compares the effect of sintering temperature and heat-

    ng mode on phase evolution of alloy compacts using XRD studies.n conventionally sintered compacts, besides -Al peaks, peaks of

    g2Si phases are also observed. With increasing sintering temper-ture from 590 C to 630 C, the peak intensity of Mg2Si increased.his is due to the fact that at higher temperature, all the eutec-ic liquid diffused into the matrix and resulted in precipitation of

    g2Si upon cooling [12,13]. In contrast, microwave sintered com-act reveals only single -Al peak and absence of intermetallichases (Mg2Si) at all temperatures. For instance, rather surpris-

    ngly the compact sintered at 590 C does not show the first intenseeak which is supposed to be at 38.4 C. On the other hand, theicrowave sintered compact at 630 C shows an intense -Al (1 1 1)

    eak at 38.4 C and subsequently other peaks are absent. Thismplies that there was insufficient time available for the liquid dif-usion and precipitation of intermetallic phases. This correlates wellith the literature reports on premixed bronze systems [45,46].

    lsewhere, Peelamedu et al. [47] too reported similar results inicrowave sintering of a mixture of BaCO3 and Fe3O4 and they

    ttributed this to the decrystallization of the material upon inter-ction with microwaves.

    .4. Properties of microwave and conventional sintered compacts

    For both the heating modes, sintered alloy compact exhibitedncreased electrical conductivity with increasing sintering tem-erature as shown in Fig. 9a. The electrical conductivity dependsn the surface microstructural characteristics [41]. Higher sinter-ng temperature (630 C) resulted in higher sintered density that

    mplies less porosity and better electrical conductivity. Higheronductivity for conventional sintered compacts attributed tohe well sintered interparticle bonds and less porosity. Whereas,on-uniform distribution of alloying elements in microwave

    ted to heat-treatment under (a) 590 C, (b) 630 C, (c)T4 and (d) T6 condition.

    sintered compacts resulted in marginally lower conductivityvalues.

    Fig. 9b compares the effect of heating mode and sintering tem-perature on hardness of 6711 alloy. The conventionally sinteredcompacts show higher hardness due to the presence of uniformlydistributed Mg2Si precipitates at 630 C as compared at 590 C.Microwave sintered compacts show contrasting behavior to this.Higher hardness at 590 C could be due to the fine grain size andthe presence of alloying elements in Al matrix. Higher temperature(630 C) results in larger amount of eutectic liquid and remains atthe grain boundaries without being diffused into matrix. Differencein hardness trend between microwave and conventional sinteringattributed to the inhomogenity and insufficient time which altersthe liquid formation reactions [46]. Conventionally sintered andheat treated 6711 alloy were evaluated for their strength underflexural and tensile conditions. The microwave sintered compactsunderwent distortion and this limited the evaluation of tensileproperties in these alloys.

    From Fig. 10a, it is obvious that higher temperature results insignificant enhancement in bending strength (276 MPa) which isequivalent to those reported for wrought aluminum alloys [48].Fig. 10b shows the stress-strain curves for vacuum sintered andheat treated 6711 alloy. For above mentioned (experimental sec-tion) tensile testing conditions, the alloys undergo extensive strainhardening and significant plastic deformation prior to failure.Table 2 summarizes the strength and ductility of sintered and heattreated 6711 alloys. The tensile strength, yield strength and duc-tility follow the similar trend as transverse rupture strength (TRS)and increase with sintering temperature. The elevated consolida-tion temperature results in significant improvement in mechanicalproperties (TRS: 57%; UTS: 27%, YS: 59%, ductility: 41%). Thiscan be attributed to the higher efficacy of supersolidus sintering indensification during sintering at elevated temperature. Fig. 11a and

    b compare the fractured surface morphology of 6711 alloy. Theyboth exhibited dimpled morphology, which is indicative of duc-tile failure mode and extent of dimpling was higher for compactssintered at 630 C.

  • 456 C. Padmavathi et al. / Materials Chemistr

    Table 2Effect of sintering temperature and ageing conditions on the tensile strength, 0.2%yield strength and ductility of 6711 alloy sintered in conventional furnace undervacuum (106 torr).

    Sinteringtemperature(C)/aged condition

    Ultimate tensilestrength (MPa)

    0.2% Yieldstrength (MPa)

    Ductility (%)

    590 89 67 13630 115 100 19630/T4 189 172 19.1

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    630/T6 271 256 1

    The effect of heat treatment on the peak aged hardness of 6711lloy sintered at 630 C was investigated. As compared to artifi-ial ageing, the peak hardness in natural aged condition attained atuch longer duration [18 h (T6) 66 HRB versus 30 days (T4) 62RB] and also artificial ageing resulted in higher hardness than nat-rally aged compacts. The T4 and T6 treated alloys attained higherardness as compared to sintered compacts. The smaller grain size

    n sintered 6711 alloy provided more grain boundaries for nucle-tion which increased the rate of strain hardening at 160 C thanbserved during natural ageing. It is interesting to note that artifi-ial ageing enhanced the tensile strength by about 136%, however,rastic reduction in the ductility is seen in Table 2. This implieshat T6 treatment would have resulted in preferential precipita-ion at the grain boundary regions attributing to embrittlement49]. Natural ageing enhanced tensile strength significantly com-ared to as-sintered alloy without affecting its ductility. Elsewhere,imilar observation has been reported by Dudas and Dean [50] asell in 601 AB alloys. The SEM of fractured surface for T4 and T6

    reated alloys are shown in Fig. 11c and d. The fractured surface of6 treated alloy shows intergranular decohesion and cleaved sur-ace which indicates a non-ductile mode of failure i.e. brittle failure.he fracture surface of T4 treated alloy contained ductile mode ofracture with larger number of smaller dimples indicating ductile

    ode of fracture.

    . Conclusions

    . Al1Mg0.8Si0.25Cu (6711) aluminum alloy powders weresuccessfully consolidated at different temperatures through con-ventional and microwave sintering.

    . Microwave sintering resulted significant (58%) reduction in theprocessing time as compared to conventional samples.

    . Due to their higher heating rate and reflectivity of 6711 compactsto microwaves in vacuum led to distortion, in spite of energy andtime savings during microwave sintering.

    . However, microwave sintering resulted in inhomogeneousmicrostructure due to rapid heating and insufficient time fordiffusion of alloying elements.

    . Microwave sintered compacts showed absence of intermetallicsphase (Mg2Si) due to lesser time available for the precipitationof intermetallic phases.

    . Conventionally sintered compacts at 630 C resulted in higherdensity and improved mechanical properties which can beattributed to grain refinement in SLPS microstructure and Mg2Siprecipitates.

    . Higher ductility (19%) was observed for 6711 alloy compacts sin-tered in vacuum. The sintered compacts resulted in higher TRSvalues in T4 and T6 condition; it was also interesting to notethat artificial ageing enhanced tensile strength by 136% with

    reduction in ductility.

    . The unique result from the present study is retention of ductility(19%) levels in the naturally aged (T4) samples.

    [[[

    y and Physics 130 (2011) 449 457

    Acknowledgements

    The authors would like to thank Mr. Jessu Joys of AMPAL Inc.,Palmerton, USA for providing the 6711 aluminum alloy powders forthe present study. This study was conducted under the NetworkedCenter for Microwave Processing of Metal-Ceramic Compositesfunded by the Indo-US Science and Technology Forum (IUSSTF),New Delhi, India.

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    Effect of microwave and conventional heating on sintering behavior and properties of AlMgSiCu alloy1 Introduction2 Experimental procedure3 Results and discussion3.1 Heating profiles and densification response3.2 Thermal analysis (dilatometric and DSC) of 6711 alloy3.3 Microstructural and phase evolution3.4 Properties of microwave and conventional sintered compacts

    4 ConclusionsAcknowledgementsReferences