Materials Chemistry and Physics 67 (2001) 85–91

Liquid phase sintering of aluminium alloys

G.B. Schaffer∗, T.B. Sercombe1, R.N. Lumley2

Department of Mining, Minerals and Materials Engineering, The University of Queensland, Brisbane, Qld 4072, Australia

Abstract

The principle that alloys are designed to accommodate the manufacture of goods made from them as much as the properties requiredof them in service has not been widely applied to pressed and sintered P/M aluminium alloys. Most commercial alloys made from mixedelemental blends are identical to standard wrought alloys. Alternatively, alloys can be designed systematically using the phase diagramcharacteristics of ideal liquid phase sintering systems. This requires consideration of the solubilities of the alloying elements in aluminium,the melting points of the elements, the eutectics they form with aluminium and the nature of the liquid phase. The relative diffusivities arealso important. Here we show that Al–Sn, which closely follows these ideal characteristics, has a much stronger sintering response thaneither Al–Cu or Al–Zn, both of which have at least one non-ideal characteristic. © 2001 Elsevier Science B.V. All rights reserved.

Keywords:A-metals; B-sintering

1. Introduction

Powder metallurgy (P/M) can be used to make highstrength and high stiffness aluminium alloys [1–10]. Indeed,room temperature tensile strengths in excess of 800 MPahave been reported [11], which is approaching the theoreti-cal limit for aluminium [12]. However, these alloys are notproduced to near net shape and are therefore expensive tofabricate, which limits their use to niche applications in theaerospace industry. Conventional press-and-sinter P/M is anexemplary net shape process and therefore offers inexpen-sive manufacturing. The commercially available alloys arebased on research that was done in the late 1960s to early1970s [13–16] and there has only been sporadic activity inthe field since then [17–23]. This early work concentratedon wrought alloy compositions and the alloys were notdesigned to be sintered. Because sintering is the step in theP/M process that is most responsible for the development ofstrength and other properties, it is not surprising that currentcommercial alloys do not meet the requirements of manyload bearing applications for which they may otherwise besuitable. Pressed-and-sintered alloys therefore require sub-stantial improvement before widespread use is likely. This

∗ Corresponding author. Tel.:+61-7-3365-4500; fax:+61-7-3365-3888.E-mail address:g.schaffer@minmet.uq.edu.au (G.B. Schaffer).

1 Present address: IRC in Materials, The University of Birmingham,Edgbaston B15 2TT, UK.

2 Present address: CSIRO Manufacturing Science and Technology,Private Bag 33, Clayton, South MDC, Vic. 3169, Australia.

paper reviews recent work at The University of Queens-land in which the traditional compositional restraints havebeen relaxed. We begin, however, with a discussion of theubiquitous oxide film.

2. The surface oxide

Aluminium is always covered by an oxide. The thicknessof the oxide is dependant on the temperature at which itformed and the atmosphere in which it is stored, particularlythe humidity. Fresh oxide on bulk aluminium at room tem-perature is widely reported as being 10–20 Å thick [24–27].The thickness on atomised powder can vary from 50 to 150 Å[28–31]. The oxide on aluminium is usually amorphous[28,31,32] and hydrated [27,31,33,34] with an adsorbed wa-ter layer [35,36]. The oxide crystallises tog-Al2O3 on pro-longed annealing at temperatures above 350◦C [32,37,38].Similar transformations occur in bulk alumina [39].

The oxide prevents solid state sintering in low meltingpoint metals [40], including aluminium [41], but not in allmetals [42–44]. This has been explained in terms of the rel-ative diffusion rates through the oxide and the metal, formetals with stable oxides [45–47]. The use of liquid phasesis an alternative to solid state sintering. An essential require-ment for effective liquid phase sintering is a wetting liquid[48]. The wettability of a solid by a liquid is determined bythe work of adhesion,Wa, [49,50]:

Wa = γ lv(1 + cosθ) = γsv + γ lv − γsl (1)

0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0254-0584(00)00424-7

86 G.B. Schaffer et al. / Materials Chemistry and Physics 67 (2001) 85–91

whereγ lv is the surface tension of the liquid–vapour inter-face,γ sv the surface tension of the solid–vapour interface,γ sl the solid–liquid interfacial tension andθ the contactangle. A liquid is said to wet a solid when cosθ > 0. Highmelting point metal oxides are generally poorly wetted byliquid metals, except above the wetting threshold, a tem-perature beyond whichWa increases sharply [50]. Liquidaluminium is not therefore expected to wet alumina nearthe melting point of the metal. Indeed, the contact angle isvariously given as∼103◦ at 900◦C [51], ∼160◦ at 800◦C[52] or ∼162◦ at 950◦C [53], although this is dependant onthe partial pressure of oxygen and the presence of an oxidefilm on the molten metal [54]. It has been suggested that theAl–CuAl2 eutectic can wet Al2O3 at 600◦C [19]. However,neither Mg, Ce nor Ca additions to molten Al reduce thecontact angle sufficiently to produce wetting [51,52]. Sincethe work of adhesion of liquid metals on oxide surfacesincreases with the free energy of formation of the metaloxide, it is unlikely that Cu will be efficacious. It is there-fore apparent that the oxide on aluminium is a barrier tosintering and needs to be disrupted or otherwise removed.

The oxidation of a metal, M, may be represented as

M + O2 ↔ MO2 (2)

The free energy of formation,1G, of the oxide is given by

1G = −RT ln K1 (3)

whereR is the gas constant,T the temperature in kelvin andK1 the equilibrium constant given by

K1 = (PO2)−1 (4)

wherePO2 is the partial pressure of oxygen when reaction(1) is at equilibrium. For aluminium at 600◦C, a PO2 <

10−50 atm is required to reduce the oxide [55]. Atmospherescontaining hydrogen are often used in powder metallurgy.Hydrogen can reduce a metal oxide by the reaction:

MO + H2 ↔ M + H2O (5)

The equilibrium constant for this reaction,K5, is given by

K5 = PH2O

PH2

(6)

wherePH2 andPH2O are the partial pressure of hydrogen andwater vapour, respectively. The ratio of partial pressures canbe converted to the dew point, effectively the water vapourcontent. A dew point of≤−140◦C at 600◦C is requiredto reduce Al2O3 [56]. Neither a dew point of−140◦C nora PO2 of 10−50 atm is physically attainable and thereforealuminium cannot be sintered in conventional atmospheres.

Magnesium is highly reactive and the free energy of for-mation of its oxide is more negative than that of the oxidesof aluminium. Magnesium therefore has the potential to actas a solid reducing agent in this system. A possible reactionis

3Mg + 4Al2O3 ↔ 3MgAl2O4 + 2Al (7)

which is a partial reduction reaction. This reaction is ob-served in studies of the oxidation behaviour of Al–Mgalloys [57,58] and at bonding interfaces in metal matrixcomposites [59–63]. Detailed analytical transmission elec-tron microscopy (Fig. 1) indicates that spinel crystallitesare indeed present in a sintered Al–Mg alloy. The reac-tion may be facilitated during sintering by diffusion of themagnesium through the aluminium matrix and will be ac-companied by a change in volume, creating shear stressesin the film, ultimately leading to its break up. This willpropitiate diffusion, wetting and therefore sintering.

It has been shown that the sintering of aluminium isenhanced in the presence of magnesium [23,64,65]. Morerecently, X-ray photoelectron spectroscopy indicated thatthe surface oxide can be reduced in the presence of magne-sium, which exposes fresh metal and facilitates the subse-quent formation of AlN on exposed surfaces in a nitrogenatmosphere [66]. The effect that magnesium has on sinter-ing can be shown by dilatometry (Fig. 2). An addition of>0.15% Mg causes shrinkage. The microstructures of theAl–Sn system show that liquid tin only wets aluminium inthe presence of magnesium, when the dihedral angle is verysharp. Without magnesium, the dihedral angle is obtuse andthe liquid is exuded during sintering (Fig. 3). By promotingsintering, magnesium also affects the mechanical proper-ties (Fig. 4). The large increase in strength and ductilityat 0.15% Mg is a direct consequence of improved inter-particle bonding and densification following oxide rupture.The excess magnesium at concentrations >0.15% remainsin solution in the aluminium, causing expansion by theKirkendall effect and solid solution hardening.

It is apparent that the oxide is not the barrier to the sin-tering of aluminium that it is traditionally considered to be.Other factors must therefore be the cause of the poor sinter-ing response.

3. Alloy design

Alloys are generally designed to accommodate the manu-facture of goods made from them as much as the propertiesrequired of them in service. It is for this reason that sinteredsteels, for example, often contain copper or phosphorous inaddition to carbon and nickel. Similarly, cast aluminium al-loys are different to forging alloys which are different againto extrusion alloys. However, with one exception [67], thisprinciple does not appear to have been applied to pressedand sintered P/M aluminium, although Savitskii only exami-ned binary alloys, the oxide phase was not reduced and noallowance was made in the thermal cycle for the transientnature of the sintering liquids. The compositions of the cur-rent commercial alloys are compared to standard wroughtmaterial in Table 1. It is noteworthy that the compositionsof the P/M alloys are essentially identical to those of thewrought material. It is therefore not surprising that their sin-tering response is poor.

G.B. Schaffer et al. / Materials Chemistry and Physics 67 (2001) 85–91 87

Fig. 1. (a) Transmission electron micrograph of a sintered Al–2.5%Mg alloy, showing a multitude of spinel crystallites. The inset shows the selectedareadiffraction pattern from this region; it can be indexed to spinel. (b) EDS spectra from (a) showing that the fine crystallites contain significantly moremagnesium and oxygen than does the aluminium matrix (c) [80].

Fig. 2. Dilatometry curves for Al–xMg alloys, wherex is 0, 0.15 and1.5 wt.% Mg showing the effect of trace additions of magnesium toaluminium cause shrinkage during sintering [80].

Fig. 3. Exuded liquid on the surface of an Al–8Sn alloy after sinteringat 620◦C.

88 G.B. Schaffer et al. / Materials Chemistry and Physics 67 (2001) 85–91

Fig. 4. As sintered tensile properties for Al–Mg alloys sintered 30 min at 620◦C [80].

Instead of producing sintered alloys which simply mimicexisting wrought alloys, it is preferable to develop alloys thatare specifically designed to be sintered. Based on an under-standing of fundamental liquid phase sintering phenomena,German and co-workers [68,69] recognised that it is possi-ble to define certain ideal phase diagram characteristics. Thekey features of an ideal liquid phase sintering system are asfollows:• The additive should have a lower melting point than the

base. The alternative is a low melting point eutectic whichis less advantageous because liquid formation does notoccur spontaneously on heating.

• The solubility of the additive in the base should be lowbecause this ensures that the additive remains segregatedto particle boundaries and maximises the liquid volume.

• While the base should be soluble in the liquid, it is notnecessary for the base to be soluble in the solid additive.Completely miscible liquids ensures that mass transportis not constrained. In addition, the base should also havea high diffusivity in the liquid. This ensures high rates ofmass transport and therefore rapid sintering.

3.1. The Al–Cu system

Copper is one of the primary alloying elements for alu-minium, based largely on the substantial age hardening re-sponse of Al–Cu alloys. They are arguably the most widelystudied P/M alloys [15,19–22,70,71]; they are certainly themost widely used. The binary phase diagram is shown in

Table 1The composition and properties of sintered aluminium alloys and theequivalent wrought alloys [79]

Alloy Type Composition Density (%) T6 properties

Cu Si Mg UTS (MPa) εf (%)

6061 Wrought 0.3 0.6 1 100 310 12601 P/M 0.25 0.6 1 94 232 2

2014 Wrought 4.4 0.8 0.5 100 483 13201 P/M 4.4 0.8 0.5 93 323 0.5

Fig. 5. It has two of the ideal features: there is a single liquidphase in which aluminium is continuously soluble and themaximum solid solubility of copper in aluminium is 5.65%at 548.2◦C. However, the melting point of copper is almostdouble that of aluminium. The liquid phase forms as a eutec-tic between (Al) and Al2Cu, this is shown in Fig. 6. Becausethere is some solid solubility of copper in aluminium, theliquid is partially transient. The sequence of events duringsintering of Al–Cu is:• interdiffusion takes place on heating from room tempe-

rature and a series of Al–Cu intermetallics form;• the first liquid forms at 548◦C on Al–Al2Cu (θ ) bound-

aries;• Cu is drawn from the liquid into solution in the aluminium

and is replaced by dissolution of the intermetallics, whichare replenished in turn by solid state diffusion fromadjacent Cu particles;

• the intermetallics disappear when all the Cu is completelydissolved;

• all the liquid is absorbed into the Al particles if the Cucontent is low, although most alloys retain some liquidthroughout sintering.

Fig. 5. The Al–Cu phase diagram [81].

G.B. Schaffer et al. / Materials Chemistry and Physics 67 (2001) 85–91 89

Fig. 6. Optical micrograph of an Al–5.5Cu alloy quenched from 575◦Cshowing the eutectic liquid forming between the Al matrix and the Al2Cuphase.

The major problem in this system is that the diffusivity ofcopper in aluminium is almost 5000 times faster than that ofaluminium in copper. The diffusivity,D, of Cu in Al at 600◦Cis 5.01× 10−9 cm2 s−1 whereas the diffusivity of Al in Cuis 1.14× 10−12 cm2 s−1 [72]. While the faster diffusivity ofcopper in aluminium enhances the rate of homogenisation,it causes expansion via the Kirkendall effect. Sintering ofthe Al–Cu system is therefore dependent on the processvariables, particularly the copper particle size and the heatingrate [73,74]. This is non-ideal.

3.2. The Al–Zn system

The Al–Zn system (Fig. 7) shows some of the characte-ristics of an ideal liquid phase sintering system in that zinchas a lower melting point than aluminium, no intermediatephases form and there is complete miscibility in the liquid.However, the solid solubility ratio is non-ideal. The maxi-mum solid solubility of zinc in aluminium is 83.1%, whilethe maximum solid solubility of aluminium in zinc is 1.2%.The liquid phase during sintering of Al with Zn is thereforehighly transient and Al–Zn alloys are extremely processsensitive. Fast heating rates and coarse zinc particle sizesenhance sintering [73]. Where fine zinc particles are used,the zinc dissolves in the aluminium before substantial quan-tities of liquid phase can form. Where coarse zinc particlesare used, the aluminium becomes locally saturated before

Fig. 7. The Al–Zn phase diagram [81].

homogenisation is achieved. Hence the additive forms aliquid which aids sintering. High heating rates also favourliquid formation because the opportunity for diffusion to oc-cur before melting is minimised and the reaction is delayedto higher temperatures where the equilibrium solubility issmaller and therefore local saturation can occur more easily.

The 7000 alloys have the greatest response to age hard-ening of the conventional aluminium alloys and are there-fore used as high strength forgings in the aerospace industry.Because zinc is a poor sintering aid, however, these alloys,which contain 3–8% Zn, do not have a good sintering re-sponse either. The high vapour pressure of zinc also givesrise to additional porosity in these alloys, particularly whenelemental powders are used [75]. It is therefore necessary touse master alloy powders [76] or microalloying additions inorder to achieve acceptable sintered properties [77].

3.3. The Al–Sn system

An examination of the binary aluminium phase diagramsindicates that Al–Sn is perhaps the only one which exhibitsalmost all of the features of an ideal system (Fig. 8). Themelting point of tin (232◦C) is considerably lower than thatof aluminium (660◦C) and there are no intermetallic phases.Tin is sparingly soluble in solid aluminium: the maximumsolid solubility is<0.15%. Aluminium is completely solublein liquid tin and no immiscible liquids form. In addition, thediffusivity of Al in liquid Sn is faster than the diffusivityof either Cu or Zn in liquid Sn and about five times greaterthan the self diffusivity of liquid Sn [78].

In the presence of magnesium, tin is indeed a very ef-fective sintering aid. This is illustrated in Fig. 9, which isa densification contour map for the Al–Sn–Mg system. Thedensification is a function of the green density, sintered den-sity and theoretical density and is a measure of the sinteringresponse: positive values indicate shrinkage, negative valuesindicate expansion; full density is achieved at a value of 1.

90 G.B. Schaffer et al. / Materials Chemistry and Physics 67 (2001) 85–91

Fig. 8. The Al–Sn phase diagram [81].

The closely spaced, parallel contour lines at low magnesiumconcentrations indicate that small quantities of magnesiumare required to activate the system. The widely spaced, gen-tly sloping contour lines at higher magnesium concentra-tions indicate that the system is relatively insensitive to Snconcentration and Mg levels greater than the critical con-centration. At a tin concentration of 8%, the sintered densityapproaches 99% of theoretical.

The Al–Sn–Mg system, having been designed to besintered, is also effective for uncompacted powder, i.e. alu-minium can be gravity sintered to near full density. Thisfacilitates free form fabrication and rapid prototyping. Bycombining the flexibility of free forming and the easy sin-tering of the Al–Sn–Mg system, functionally graded metalmatrix composites can also be manufactured (Fig. 10).The mechanical properties of the Al–Sn system, however,are low because tin does not provide much strengthening.

Fig. 9. Map showing densification of sintered aluminium as a function ofmagnesium and tin concentration. Each contour represents a densificationof 0.2.

Fig. 10. The macrostructure of a section of one tooth of a functionallygraded, freeform fabricated gear. The centre is the Al–8Sn–4Mg alloy;the surface contains a 10 wt.% loading of alumina [82].

Copper could be incorporated, but sintering of the quater-nary system becomes complicated, partly because of theformation of immiscible liquid phases.

4. Conclusions

Sintering of aluminium has always been considered to beproblematical because of the oxide film present on the sur-face of the powder particles. However, trace additions ofmagnesium react with the oxide to form spinel. This breaksup the oxide, which facilitates sintering. It is therefore ap-parent that in contradiction to the standard paradigm, theproperties of pressed-and-sintered aluminium alloys are notlimited by the “oxide problem”. Aluminium P/M alloys canbe improved without recourse to hot working or master al-loy powders if their design is based on an understandingof the underlying sintering processes and the characteris-tics of an ideal liquid phase sintering system. The Al–Snsystem is close to ideal and Al–Sn–Mg alloys can be sin-tered to 99% of the theoretical density. Once an alloy hasbeen optimised for sintering, a variety of new options canbe realised. Free formed, functionally graded, aluminiummatrix composites and high strength materials are two suchexamples.

Acknowledgements

This work has been supported in part by the AustralianResearch Council, ACL Bearing Company, Comalco Alu-minium Ltd. and Ampal Inc.

G.B. Schaffer et al. / Materials Chemistry and Physics 67 (2001) 85–91 91

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