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Compaction, sintering and mechanical properties of elemental 6061 Al powder with and without sintering aids
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Materials & Design
Materials and Design 29 (2008) 752–762 www.elsevier.com/locate/matdes
Compaction, sintering and mechanical properties of elemental 6061 Al powder with and without sintering aids N. Showaiter *, M. Youseffi School of Engineering, Design and Technology, Engineering Materials Unit, University of Bradford, Bradford BD7 1DP, UK Available online 11 February 2007
Abstract Compaction, sintering, microstructural and mechanical properties of the premixed (elemental) 6061 Al powder with and without sintering aids were investigated. The highest green densities (2.6 g/cm3) were obtained at a compaction pressure of 510 MPa. The optimum sintering condition was at a temperature of 620 C for 1 h under pure nitrogen for compaction pressures of 340 or 510 MPa. It was found that additions of 0.12 wt% Pb was the most effective and enhanced the sintering response by obtaining almost a full sintered density (2.7 g/cm3) with a corresponding increase in the hardness and tensile strength values as compared to the base 6061 Al alloy powder mixture without Pb addition. Additions of 0.1 and 0.4 wt% Sn or Ag, respectively, also improved the sinterability of the elemental base powder with sintered densities of 96–98% theoretical (2.65 g/cm3). The highest UTS value of 322 MPa with yield strength of 250 MPa and 9% elongation to failure were obtained for the nitrogen sintered and fully heat-treated (T6) 6061 Al alloy with addition of 0.6 wt% paraffin wax as lubricant and 0.12 wt% Pb as sintering aid. 2007 Elsevier Ltd. All rights reserved. Keywords: Aluminium 6061 alloy; Vacuum and nitrogen sintering; Sintering aids; Mechanical properties
1. Introduction Improvement in sintering properties for P/M aluminium products will allow the use of conventional powder metallurgy as a viable processing alternative to those of casting and wrought operations. Attempts have therefore been made to enhance the compressibility and sinterability in various ways and for different series of Al alloys, both prealloyed or premixed elementally. It is well known that these Al alloy powders are difficult to sinter because of the stable aluminium oxide film covering the powder particles and thus reducing sinterability. It has been found [1,2] that addition of 0.1–1 wt% Mg helps to break up the oxide layers on the surface of the Al particles through the formation of a spinel phase MgAl2O4 [1] and that the relative density of the sintered material increases by 9% by increasing the Mg content to 1 wt% [2]. It was also reported that the tensile *
Corresponding author. Address: Mechanical Engineering, University of Bahrain, P.O. Box 32038, Kingdom of Bahrain. E-mail address: [email protected] (N. Showaiter). 0261-3069/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2007.01.027
properties of the sintered material increased from 60 to 140 MPa for the UTS and from 0% to 12% for elongation as Mg content increased [2]. Mg has also been added in various quantities to 7xxx series and it was found [3] that up to a total Mg content of 3.5 wt% the mechanical strengths were better than those of the present commercial alloy Alumix 431 (Al–5.5Zn–2.5Mg–1.6Cu). However, at higher Mg levels, the secondary phase could not be totally taken into solution and consequently remaining as a network at the grain boundaries causing embrittlement of the alloy. More recently, significant improvements in the sinterability of aluminium alloys have been reported [4–8] by liquid phase sintering leading to improved densification of the alloys. The attributes of liquid formation to assist densification have been described in the literature [6]. The effect of trace addition of selected elements such as Pb, Sn, Se, Bi, and Sb on the sinterability of aluminium alloys was investigated [9–12]. It was found that microalloying with 100 ppm of Pb improved sintering response of an Al–Zn–Mg–Cu alloy produced from premixed elemental powders [9] and that elemental addition of 0.12 wt% Pb increased the UTS
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by 36% from 314 to 427 MPa for the same alloy. Trace addition of Sn also showed some improvement in sintering as well as mechanical strength, but less effective than Pb, and Se had no effect on the strength of the alloy, while Bi and Sb had a large negative effect. It was reported that since these additives, i.e. Sn and Pb had low solubility in the base Al matrix they remained segregated at the liquid–vapour interface, where they reduced the surface tension of the liquid, decreased the wetting angle and hence spreading properly over the matrix powder particles allowing a proper (enhanced) liquid phase sintering process. Recent work [13,14] has described a novel P/M approach for a ternary Al–4Cu–0.5Mg with Sn and Ag addition rather than using conventional ingot aluminium approach. It was found that Ag addition to P/M AA2014 resulted in a minor improvement in mechanical properties as compared to the ternary Al–4Cu–0.5Mg [14]. So far, researchers have largely concentrated on studying the sintering behaviour of the elemental 2xxx and 7xxx series Al alloys whereas the 6xxx series has received little attention [15]. Commercial grade 6xxx series are widely used as medium strength structural Al alloys having good ductility, excellent corrosion resistance and higher response to age hardening. The aim of this work was therefore to investigate the compaction, sintering, microstructure and mechanical properties of the commercial grade 6061 aluminium alloy powder prepared elementally with and without elemental additions of sintering aids such as lead (Pb), tin (Sn) or silver (Ag). These results are also compared to those of prealloyed 6061 Al alloy powder with and without elemental additions of the same sintering aids reported previously [23,24] in more detail.
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compositions are in weight %) as elemental were made to the lubricated base powder and mixed for 15 min each, after which a composition similar to that of wrought 6061Al alloy was gained. The sintering aids of 0.12 Pb, 0.1 Sn or 0.4 Ag as elemental powders were added to the quaternary elemental powder mixture and also mixed for 15 min each.
2.3. Compaction and sintering Squared specimens of size 15 · 15 · 3 mm3 (3.5 g of powder) were pressed at pressures in the range of 250–770 MPa to study the compaction characteristics and to establish the optimum compaction pressure for sintering. Tensile bars (dumbbell shaped) of size 2.5 mm thickness · 4.5 mm width · 15 mm gauge length were machined out of sintered rectangular bars of size 40 · 10 · 4.5 mm3. These bars were pressed (4.5 g of powder) at 250, 340 and 510 MPa for mechanical testing. Zinc stearate was used on the die wall and punches for lubrication before compaction to reduce diewall frictional effects. A single acting hydraulic press (20 Ton-Chirchill) was used for compaction. Sintering characteristics were investigated initially under vacuum (10 4 mbar) in a laboratory ceramic tube furnace (controlled by an Eurotherm 818 P controller) in the range 580–640 C and for 30– 120 min. All compacts were heated to the sintering temperature at a heating rate of 5 C/min, held for 30–120 min and then furnace cooled to room temperature. For comparison, sintering was also carried out under pure nitrogen atmosphere ( 40 dew point) in a standard laboratory Carbolite gas atmosphere recrystallized alumina tube furnace. The pressure change against temperature rise was monitored during the whole sintering cycle in order to study the formation of any liquid phase, appearance of any other phase or any phase transformation via exothermic or endothermic reactions. Green densities were determined from weight and volume measurements. Sintered densities were measured by the Archimedes principle (water displacement technique) using lacquer-coated samples.
2.4. Heat treatment
2. Experimental procedure
Sintered tensile specimens of size 2.5 mm thickness · 4.5 mm width ·15 mm gauge length were fully heat-treated using T6 Temper, i.e. solution heat treated at 520 C for 30 min, water quenched and then aged at 160 C for 18 h.
2.1. Raw materials
2.5. Metallography and mechanical testing
The raw materials used included the following powders and lubricants: Air atomised aluminium (Al) powder, with 99.7% purity and average particle size of 60 lm supplied by Ronald Britton and Co. Magnesium, Mg, with 99.8% purity and maximum particles size of 50 lm. Silicon, Si, with mean particle size of 5 lm and 97.5% purity. Copper, Cu, with maximum particle size of 50 lm and 99.0% purity. Lead, Pb, with maximum particle size of 150 lm as-received, and then sieved below 45 lm with 99.5% purity. Tin, Sn, with maximum particle size of 45 lm and 99.9% purity. Silver, Ag, with maximum particle size of 100 lm as-received, and then sieved below 45 lm with 99.99% purity.Good Fellow Cambridge limited supplied Mg, Si, Cu, Sn, Ag and Pb. Lithium stearate and paraffin wax were used as solid lubricants. The chemical composition of the 6061 Al powder prepared by elemental mixing is as follows: Al–1.0Mg–0.6Si–0.25Cu (All in weight%).
2.2. Mixing Lithium stearate and/or paraffin wax of 0.6% by weight was added to the base Al powder and mixed separately for 20 min using a 3-D Turbula mixer with glass Jar containers. Addition of 0.6 Si, 0.25 Cu and 1 Mg (all
Microstructural examination of the alloys was conducted using an MeF3 optical light microscope and JEOL- JSM 6400 scanning electron microscope (SEM). The chemical analyses of the as-sintered and fully heat-treated specimens were preformed using the SEM equipped with an Energy Dispersive X-ray (EDX) microanalyser. Metallographic samples were prepared conventionally, i.e. grinded and then polished using 6 lm and finished with 1 lm polish. No etching was carried out. Mechanical properties such as apparent macro- and micro-hardness, ultimate tensile strength, and % elongation to failure were measured in the as-sintered and fully heat treated conditions. For the Vickers microhardness a 20 g load were applied using the standard Vickers hardness-testing machines. The ultimate tensile strength (UTS) and % elongation to fracture were obtained using an Instron tensile testing machine with a 10 KN load cell and a crosshead speed of 0.5 mm/min.
3. Results and discussion 3.1. Compaction characteristics The compaction characteristics were investigated for the elemental 6061Al alloy without and with addition of sintering aids such as 0.12 Pb, 0.1 Sn or 0.4 Ag at different
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compaction pressures in the range 250–770 MPa. Note also that 0.6 wt% of either paraffin wax (PW) or lithium stearate (LS) was added as lubricant. The compaction curve for the elemental 6061 Al alloy presented graphically in Fig. 1 shows that the highest green density of 2.6 g/cm3 (96% TD) was obtained at a compaction pressure of 510 MPa above which there was little increase in green density. Also, the compaction curves in Fig. 1 show the effect of adding the sintering aids Pb, Sn or Ag on the compressibility of the elemental 6061 Al alloy powder. It can be seen that the green density of compacts with addition of sintering aids is slightly higher (97% TD) at 510 MPa than that without sintering aids. Similar results have been reported elsewhere [18,19] with Sn and Pb additions. These admixed powders are mostly finer than the base Al powder and thus during compaction will be extruded into the pore space. The increase in green density is therefore due to the fine powder particles of Pb, Sn or Ag
filling the smaller voids between the coarse Al powder particles. 3.2. Sintering characteristics The sintering curves for the premixed elemental (EL) Al alloy sintered under vacuum and pure nitrogen at 620 C for 1 h, along with compaction curve in the range 250– 770 MPa processed with 0.6 wt% lubricant and no sintering aids are shown in Fig. 2. The sintering results show that by increasing the compaction pressure the sintered density increases due to more particle bonding and presence of less void as the compaction pressure increases. Fig. 2 also shows clearly that the sintered densities under vacuum were marginally lower than the green densities even by increasing compaction pressure and sintering temperature. This is because the porosity was not completely eliminated, although it was significantly reduced indicating that the
2.68
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2.64 97
2.62 2.6
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2.58
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2.56
Al-1Mg-0.6Si-0.25Cu Al-1Mg-0.6Si-0.25Cu+0.12Pb Al-1Mg-0.6Si-0.25Cu+0.1Sn Al-1Mg-0.6Si-0.25Cu+0.4Ag
2.54 2.52 2.5 0
100
200
300
400
500
600
700
800
94
% Theoretical Density
Green Density, g/cm3
2.66
93 92 900
Compaction Pressure, MPa Fig. 1. Variation in green density with compaction pressure for the premixed elemental 6061 Al powders with and without addition of sintering aids.
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2.64 2.62
97
Density, g/cm3
96
2.58
95
2.56 2.54
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nitrogen sintered green density vacuum sintered
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% Theoretical Density
2.60
93
2.48
92 0
100
200
300
400
500
600
700
800
900
Compaction Pressure, MPa Fig. 2. Variation in green and sintered densities with compaction pressure for the premixed elemental 6061 Al powder compacts sintered at 620 C for 1 h. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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amount of shrinkage produced did not compensate for swelling (due to alloying effects) produced during the vacuum sintering process. The decrease in sintered density is, therefore, related to the expansion of the specimens due to the high solubility of the alloying elements Mg, Si and Cu that form low melting phases which lead to rapid dimensional change, also due to the inefficient removal of the trapped gas and air by sintering under vacuum causing a volume expansion and thus reducing the sintered density. Sintering under nitrogen provided higher densities than that in vacuum for the elementally premixed powder compacts (pressed at 510 MPa) reaching a sintered density of 2.61 g/cm3 (96.7% TD) and only 2.57 g/cm3 (95% TD) in the case of vacuum sintering. The vacuum sintered parts also showed P2–5% volume expansion which suggests clearly that the sintering atmosphere is an important parameter and will affect the final porosity content. It is reported [5,16,18] that by sintering under pure nitrogen the oxide content can be reduced further in the presence of Mg which exposes fresh metal and facilitates the subsequent formation of AlN on exposed surfaces. The variation in sintered density with sintering temperature and presence of 0.12Pb, pressed at 340 and 510 MPa, and sintered for 1 h at different temperatures (580–640 C) under vacuum is shown in Fig. 3. It can be seen from both sintering curves that sintered densities decrease gradually after the optimum sintering temperature at 620 C, but increase more rapidly before this temperature. The sintering data curves also show that sintered densities increase as the compaction pressure increases from 340 to 510 MPa with a similar trend for the under-sintered, optimum-sintered, and over-sintered conditions. These results, therefore, reveal that the optimum sintering condition is at a temperature of 620 C for 60 min under pure nitrogen (or vacuum), which was chosen for the sintering of mechanical tensile bars.
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The effect of Pb, Sn, or Ag content on the sintered density of pressed and sintered 6061 Al alloy is shown in Fig. 4. It can be seen that amongst the sintering aids used in this work, additions of 0.12 wt% Pb was the most effective which enhanced the sinterability by obtaining almost a full sintered density (2.7 g/cm3) with a corresponding increase in hardness and tensile strength values as compared to those without Pb addition. Additions of 0.1 and 0.4 wt% Sn or Ag, respectively, also improved the sinterability of the elemental 6061 Al alloy with sintered densities of P96–98% theoretical (2.65 g/cm3), but being less effective than lead. It can therefore be concluded that trace additions of 0.1 wt% Pb (and Sn or Ag to a lesser extent) is an effective way of enhancing sinterability of the 6061 Al alloy. 3.3. Shrinkage characteristics Percentage volume change for the premixed elemental 6061 Al powders pressed at 340 MPa and sintered at 620 C for 1 h under vacuum or pure nitrogen, with and without 0.12 Pb and 0.6 wt% paraffin wax (PW) or lithium stearate (LS) as lubricants, are given in Table 1. It can be seen that addition of LS as lubricant along with vacuum sintering has caused volume expansion whereas PW and sintering under nitrogen resulted in volume shrinkage and hence higher sintered densities of P98% TD as compared to the vacuum sintered specimens with only 96% TD. It is therefore clear that the sintering atmosphere and type of lubricant plays an important role for the elimination or reduction of pores and expansion or shrinkage of the sintered specimens. This investigation has found that lithium stearate, in particular, has a deleterious effect on sintered density mainly due to its wide burn off range and hence incomplete removal during sintering leaving some black residue as well as causing adverse reaction with Al which was also reported previously [23,24]. The pore
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96 2.58 95
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2.52 340 MPa
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% Theoretical Density
Sintered Density, g/cm3
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2.50 510 MPa
2.48 595
600
605
610
615
620
625
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635
92 640
Sintering Temperature, ˚C Fig. 3. Variation in sintered density with sintering temperature for the premixed elemental 6061 Al alloy powder pressed at 340 and 510 MPa and sintered for 1 h under vacuum with 0.12Pb content.
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100
2.69
99
2.65 98 2.63 97
2.61 2.59
96
Al-1Mg-0.6Si-0.25Cu Al-1Mg-0.6Si-0.25Cu+0.12Pb Al-1Mg-0.6Si-0.25Cu+0.1Sn Al-1Mg-0.6Si-0.25Cu+0.4Ag
2.57 2.55 2.53 100
% Theoretical Density
Sintered Density, g/cm3
2.67
95
94 150
200
250
300
350
400
450
500
550
600
Compaction Pressure, MPa Fig. 4. Variation in sintered density with compaction pressure for the premixed elemental 6061 Al alloy with and without the addition of sintering aids sintered at 620 C for 1 h under pure nitrogen.
content and microstructure also depend on the solubility of the gases in the compacts, i.e. insoluble gases limit pore shrinkage while soluble gases, like hydrogen, in a closed pore can diffuse out during sintering, and are eventually eliminated or reduced in size or content. 3.4. Analysis of heating and cooling curves during the sintering cycle
tering (SLPS) which results in melt formation preferentially at intergranular regions. The intergranular melt contributes mainly to densification by capillary-induced rearrangement, solution-reprecipitation and grain shape accommodation. The endothermic peaks observed are associated with the formation reaction of intermetallics from molten eutectic. The resulting heat output from these reactions
From the analysis of heating data curves during the sintering cycle of the temperature rise versus pressure change for the premixed elemental (EL) and degassed prealloyed (PA) 6061 Al compacts (see Fig. 5a–c), mainly four endothermic peaks at temperatures of 320, 500, 550 and 590 C can be observed corresponding to formation of transient liquid phase, burning peak for the lubricant lithium stearate (LS), and the formation of persistent liquid phase above the solidus temperature, respectively. Use of the prealloyed powder eliminates transient melt formation as seen in Fig. 5b. The predominant sintering mode for the PA powder is, therefore, by supersolidus liquid phase sinTable 1 % Volume change for the premixed elemental 6061 Al alloy powder with and without 0.12 Pb and lubricants pressed at 340 MPa and sintered under vacuum or pure nitrogen Alloy composition
Volume changea (%) Vacuum
EL EL + 0.12Pb EL+LS EL+PW EL + 0.12Pb+LS a
1.56 1.45 +1.89 2.3 +3.96
Nitrogen 1.8 2.14 1.2 2.81 1.8
Positive values indicate expansion and negative values shrinkage. With 0.6 wt% lithium stearate. +PW With 0.6 wt% paraffin wax. +LS
Fig. 5. Sintering cycle showing the heating and cooling curves by monitoring pressure change and temperature rise for: (a) prealloyed 6061 Al alloy with 0.6 LS; (b) prealloyed 6061 Al alloy and no lubricant or sintering aid; and (c) premixed elemental powder with 0.12Pb and 0.6LS; all sintered at a heating rate of 5 C/min, pressed at 510 MPa, and sintered at 620 C for 1 h under vacuum.
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cause formation of liquid phases (both transient and persistent) which take place depending on the type of powder. Pore volume growth via diffusion of trapped gasses during the sintering process and a corresponding volume expansion also take place depending on the lubricant content and type and the sintering atmosphere. It is to be noted that Mg is aiding liquid formation in the temperature range 400–595 C [21]. The liquid, if present to a sufficient extent, further enhances the densification of the alloy, but the broken oxide film covering the Al powder will hinder sintering to such an extent that the amount of liquid phase may not be sufficient to achieve a good densification. Also, the presence of adsorbed and absorbed gases by the Al particles, as well as water vapour present during vacuum sintering would increase the size of the compacts and therefore reducing their sintered density due to volume expansion. The volume expansion in the premixed EL powder may therefore be attributed to the homogenisation reactions occurring between alloying elements and Al base powder, the use of unsuitable lubricants such as LS having a wide range of burning characteristics, and the formation of transient liquid phases accompanied with the formation of porosity which may not be removed completely by the subsequent shrinkage. The sinterability was, however, improved by addition of appropriate lubricants such as paraffin wax (PW) and sintering aids such as lead. In summary, the heating curves for the elemental 6061 Al alloy show the appearance of mainly four endothermic peaks at temperatures of 320, 500, 550 and 590 C corresponding, respectively to: (1) Diffusion of Mg into solution causing expansion (the Kirkendall effect) as well as the burning of the lubricant (lithium stearate). It has been reported [22] that diffusivity of Mg at 482 C is 100 times higher than that of the other elements such as Si, Cu, and Fe in Al. (2) Formation of an eutectic liquid phase and intermetallics in the range 470–530 C due to temperature rise causing several possible eutectic reactions and further interaction between Mg, Si, Cu and Al particles. (3) As temperature rises, further liquid phase formation occurs at 550 and 590 C due to mainly further diffusion and incorporation of the alloying elements (Mg, Si and Cu) and the liquid reaching its equilibrium composition with the Al at the sintering temperature. The sintering results including the heating data curves (and the microstructural studies) suggest that there are large amount of transient liquid phase formations either as binary, ternary, etc., within the 6061 Al alloy with or without sintering aids. This is because of the formation of one eutectic liquid phase at 500 C followed by another at 550 C (both causing homogenisation) and final reappearance at 590 C which is between the solidus (582 C) and liquidus (652 C) temperatures [17,20] and, therefore, suggesting the occurrence of a supersolidus liquid phase sintering process. This altogether suggests the sinterability of these alloys is based upon the initial formation of transient liquid phases due to the incorporation
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of alloying additions and finally the presence of supersolidus liquid phase sintering. 3.5. Microstructural studies and EDX analysis Backscattered SEM images of both vacuum and nitrogen sintered and fully heat-treated 6061 Al alloy with and without sintering aids are shown in Figs. 6 and 7. Generally, presence of grain boundary eutectic phase in white/ grey contrast along with pores (macro- and micro-porosity) can be seen within the microstructure. In particular, Fig. 6a shows the presence of both macro- and micro-porosity and elongated residue along the grain boundaries most likely due to insufficient delubrication of lithium stearate proving that this lubricant is harmful and should not be used for processing of P/M Al alloys. On the other hand, Fig. 6b shows the presence of only microporosity for the alloy with 0.6 PW and 0.12 Pb additions sintered under pure nitrogen suggesting that both PW as lubricant and Pb as sintering aid are useful and suitable for the processing and sintering of 6061 Al alloy. Fig. 6c and d show similar microstructures with additions of 0.1 Sn or 0.4 Ag and 0.6 PW, with the exception that the alloy with 0.1 Sn has densified better with less porosity as compared to that of 0.4 Ag addition, both sintered under pure nitrogen and fully heat-treated. Figs. 6 and 7 also show show the presence of some white needle-like intermetallics segregated along the grain boundaries in the as sintered and heat treated samples which remained undissolved after solution treatment. The elemental EDX microanalysis showed the presence of mainly Fe and Si along with Al. Presence of iron as an impurity is unavoidable and that the solid solubility of iron in Al is very small resulting in the presence of Al–Fe or Al–Fe–Si within the microstructures. EDX analysis of various phases also proved that the dark/grey phase is rich in Mg and Si and from its morphology it is evidently the solidified liquid phase having an approximate chemical composition of: Al–Mg2Al3–Mg2Si (450 C) and Al–Si–Mg2Si (555 C) [20]. The (near spherical) precipitates revealed presence of Mg and Si along with Al as its primary constituents, thereby indicating a predominantly Mg2Si phase precipitate as a second phase in the matrix formed during the cooling cycle. Generally, it can be said that as temperature increases so does the diffusion of Mg, Si, Cu and Fe into Al resulting in some phase transformation mostly by solid-state diffusion of Mg2Si and eventually formation of eutectic liquid practically at grain boundaries. Associated with this phase are the Cu, Fe, Pb, Sn and/or Ag. The microstructure also contains typical P/M pores of dark colour having various size and shapes. Both optical and SEM microstructural examinations of the vacuum sintered specimens without sintering aids revealed the presence of pores of various size with nonuniform distribution which was expected from a typical P/M Al alloy structure with poor sinterability. The decrease in sintered density for the vacuum sintered
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Fig. 6. SEM micrographs (backscattered image (BSI)) of the fully heat-treated premixed elemental 6061 Al alloy without and with sintering aids, pressed at 510 MPa and sintered under vacuum (a) or pure nitrogen (b, c and d) at 620 C for 1 h: (a) with 0.6 LS; (b) 0.6 PW + 0.12 Pb; (c) 0.6 PW + 0.1 Sn; and (d) 0.6 PW + 0.4 Ag additions.
specimens is related to the presence of two forms of porosity, bulk (within grains) and interparticle grain boundary porosity. It appears that both types are formed and expanded further by the trapped gas and air during sintering under vacuum causing a volume expansion. In particular, the grain boundary porosity is formed by diffusion of the trapped gas/air between particles during sintering which involves delubrication at the same time. It is known that the bulk porosity (due to trapped air, gas or moisture) is present initially in the powder, and the amount of which depend on the atomisation process. It is to be noted that these pores collapse during compaction, but they are not totally eliminated even at higher compaction pressure. The increase in expansion or compact growth at higher compaction pressures and sintering temperatures is believed to be due to trapped air/gas within pores and also due to further pore formation during supersolidus liquid phase sintering which takes place at >580 C and further increase in temperature results
in the formation of more liquid phase mainly along particle boundaries by incipient melting of the 6061 Al powder. There were also some large pores within the assintered specimens specially those without sintering aids most likely produced by agglomeration of the admixed alloying elements which were dissolved in the matrix or by agglomeration of the lubricant which was burned out during sintering and therefore leaving behind large pores. Another reason may be due to the formation of the transient liquid phase (TLP) followed by homogenisation of the alloy causing swelling (expansion) and hence creating some large pores (the Kirkendall effect) [10] which could not be removed during shrinkage. This is believed to be associated with the elemental additions and the prealloyed powder may be more useful in this respect. It is therefore believed that to minimise swelling and formation of porosity in the case of adding no sintering aids, the use of a compressible prealloyed (6061 Al) powder may be necessary and more beneficial as also reported previously
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Fig. 7. SEM micrographs (BSI) of the vacuum and nitrogen sintered and fully heat-treated premixed elemental 6061 Al alloys: (a) with 0.6 PW, sintered under vacuum; and (b) 0.6 PW + 0.12 Pb, sintered under nitrogen. The porosity content is reduced significantly for the nitrogen sintered alloy (b) and the prior particle boundaries are difficult to identify as compared to the vacuum sintered alloy (a).
[23,24]. However, the elemental addition of Pb, Sn or Ag to elemental mixture of Mg, Si, Cu and the base Al powder, is found to be useful for liquid phase sintering (LPS) and the creation of liquid phase by contact melting causing rapid homogenisation by inter-particle diffusion. The incorporation of elements such as Cu will take place above 510 C which is followed by the formation of the equilibrium liquid phase during the sintering temperature (620 C) leading to the formation of the equilibrium alloy with some shrinkage and densification as obtained during nitrogen sintering in this work.
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Generally, there was presence of a light grey phase rich in Mg, Si and Al almost in all the sintered microstructures. This, in fact, is the solidified liquid phase with presence of Pb, Sn or Ag at grain boundaries suggesting the segregation of these elements within the grain boundary liquid phase during sintering. The segregation of the trace elements Pb or Sn is due to their insolubility in Al with Pb having a lower surface tension than Sn and therefore being more effective in activating liquid phase sintering. It follows therefore that 0.1 wt% Pb in 6061 Al alloy was sufficient to activate the sintering process to achieve near full density. It is also clear that due to negligible solubility of Pb in Al, all the Pb will segregate to the liquid phase and hence any solidified liquid phase will be rich in Pb, Mg and Si surrounded by Al as observed within the as-sintered microstructures mainly at the grain boundaries. There were also appearance of a few dark phases, which looked like pores, but in fact they were not pores and happened to be Mg–Al oxide, i.e. a by-product, due to the reaction between the oxide layer on the Al surface and the elemental Mg taking place during the early stages of sintering. This work, therefore, has shown that it is possible to create a proper LPS process within the 6xxx series Al alloys by addition of sintering aids (Pb, Sn or Ag) as trace elements (0.1–0.5 wt%) in order to form adequate liquid phase, which is able to disrupt the highly stable aluminium oxide film covering the Al particles. This liquid phase is also able to penetrate the oxide film through the discontinuities created during pressing allowing neck formation by LPS. It is to be noted that alloying elements such as Mg, Si, Cu and Zn are all soluble in Al, and liquids may form by contact melting, melting of the elements themselves, or by a eutectic reaction between the Al and the admixed elements. The improved sinterability may be explained (Schaffer et al.) by the following mechanism: Firstly, it is to be noted that elemental additions of Pb, Sn or Ag is effective for achieving high densities only by the presence (addition) of 1 wt% elemental Mg. In this way, the Mg addition facilitates oxide disruption by reacting with the Al sesquioxide resulting spinel formation and disruption of the oxide layer and thus allowing wetting by the liquid formed during sintering, while Pb, Sn or Ag additions segregate to the liquid phase and reduce its surface tension and therefore improving sinterability of the Al alloy. It is therefore believed that any excess Mg will diffuse into solution causing swelling (and pore formation) via the Kirkendall effect and that Pb or Sn will segregate to the liquid phase due to their low solubility in the Al matrix and reduce the surface tension of the supersolidus liquid as well as reducing the wetting angle and therefore enhancing wetting of the Al matrix particles and sinterability. It is known that the solubility of Sn in Al is 0.1 wt% at 610 C and decrease to 0% at 660 C. As compared to sintering (under pure nitrogen) without sintering aids, elemental addition of sintering aids (Pb, Sn or Ag) resulted in higher sintered densities with more uniform pore size and distribution. In particular, 0.12% Pb addition resulted in an almost pore free structure
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with presence of some discontinuous grain boundary second phases (intermetallics and eutectic phase). By microanalysis, it was found that the solidified grain boundary liquid phase consisted of various intermetallics, which are known to be brittle, and therefore the as-sintered mechanical properties are expected to be weak due to the presence of grain boundary phases causing premature failure and low tensile strength as reported in the following section. However, the mechanical strength was improved significantly by a full heat treatment using standard T6. 3.6. Mechanical properties Vicker’s microhardness values for the as sintered and fully heat-treated (T6) alloys with and without sintering aids are given in Table 2. The microhardness values increased from 43 to 83 HV0.02 and from 44 to 92 HV0.02 for the vacuum and nitrogen sintered plus fully heat-treated specimens, respectively, and the 0.12Pb addition increased the microhardness values from 53 to 134 HV0.02 for the as-sintered and heat-treated (T6) Table 2 Vickers microhardness for the premixed elemental 6061 Al alloy with and without 0.12Pb and lubricant pressed at 340 and sintered at 620 C for 1 h under vacuum or pure nitrogen and fully heat-treated (T6) condition Alloy composition
Microhadrness HV 0.02 As-sintered
Ela El + 0.12Pba Elb El + 0.12Pbb a b
Fully heat-treated
Vacuum
Nitrogen
Vacuum
Nitrogen
43 44
44 45 50 53
83 130
92 130 110 134
With 0.6 wt% lithium stearate (LS) as lubricant. With 0.6 wt% paraffin wax (PW) as lubricant.
6061 Al alloys, respectively. It is therefore, clear that the T6 heat-treatment increased the hardness due to precipitation hardening as expected. Ultimate tensile strength (UTS) and % elongation to failure for the EL 6061 Al alloy with and without addition of sintering aids and lubricants pressed at 250, 340, and 510 MPa sintered at 620 C for 1 h under vacuum or pure nitrogen and fully heat-treated using T6 condition are given in Table 3. From Table 3, it can be seen that the strength values increase slightly with an increase in compaction pressure, for the premixed elemental Al alloy without and with addition of sintering aids sintered under nitrogen and addition of 0.6 wt% paraffin wax. The UTS values for a compaction pressure of 250 MPa was only 145 MPa for the heat-treated alloy with 0.12 Pb and 0.6 Li stearate additions, respectively. The low strength value is due to the poor bonding between the powder particles and the presence of 5–10% (by volume) of porosity content as a result of the low compaction pressure at 250 MPa and presence of delubrication residue. The strengths should rise as sintered densities increase or as the porosity content decreases. This was the case for the 6061 Al alloy with addition of 0.12 Pb and 0.6 Li stearate compacted at either 340 or 510 MPa, which showed almost full density (2.69 g/cm3) with UTS value of 260–305 MPa and with 5–13% elongation to failure. The UTS value significantly increased from 266 to 322 MPa for the heat-treated elemental alloys with 0.12 Pb and 0.6 paraffin wax addition, respectively. It is to be noted that the 0.12 Pb additions increased both the hardness and the strengths values for both the as-sintered and heat-treated conditions. It is therefore concluded that, the 0.12 Pb addition increased the sintered densities significantly only under nitrogen sintering reaching almost full density (2.69 g/cm3) for the premixed elemental 6061 Al P/M alloy. The highest UTS of 322 MPa
Table 3 UTS and yield strength values, and % elongation to fracture for the premixed elemental 6061 Al alloy with and without sintering aids and lubricant, pressed at 340 or 510 MPa and sintered at 620 C for 1 h under vacuum or pure nitrogen and fully heat-treated (T6) condition Compaction pressure (MPa)
Alloy composition a
Yield strength (MPa)
Ultimate tensile strength (MPa)
Elongation to fracture (%)
250
El El + 0.12Pba
92 99
140 145
8.7 11
340
Ela Elc El + 0.12Pb c El + 0.12Pba El + 0.1Sna El + 0.4Aga
– 115 141 150 160 198
259 210 233 275 305 233
8.6 6 7 9.3 6 5.3
510
Ela Elb El + 0.12Pba El + 0.12Pbb El + 0.1Sna El + 0.4Aga
113 210 170 250 198 185
266 275 303 322 275 248
a b c
Sintered under pure nitrogen with 0.6 wt% lithium stearate. Sintered under pure nitrogen with 0.6 wt% paraffin wax. Sintered under vacuum with 0.6 wt% lithium stearate.
12 10 8 9 10 13
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was, therefore, obtained for the premixed EL alloy with 0.6 wt% paraffin wax as solid lubricant and 0.12 wt% Pb addition giving a UTS value of 322 MPa with yield strength of 250 MPa and 9% elongation to failure for the nitrogen sintered and heat treated alloy compacted at 510 MPa. 4. Conclusions The highest green densities (2.6 g/cm3) were obtained at a compaction pressure of 510 MPa. Compaction pressures >510 MPa produced very small increase in green density. The optimum sintering temperature was found to be at 620 C for 30 or 60 min for compaction pressures of 340 or 510 MPa. The endothermic peaks during sintering cycle for the premixed elemental alloy showed the formation of both transient liquid phase at temperatures between 320 and 550 C and persistent liquid phase above the solidus temperature at 590 C. Therefore, it is believed that the sintering mechanism is via the formation of both transient and persistent liquid phases. 0.12 wt% Pb addition was found to be the most effective sintering aid as compared to 0.1 Sn and 0.4 Ag additions, resulting almost a full sintered density (2.7 g/ cm3). Sintering under vacuum gave rise to the presence of higher pore content, and excessive amounts of residual porosity at grain boundaries, particularly with 0.6 wt% lithium stearate as solid lubricant. On the other hand sintering under pure nitrogen atmosphere provided higher densities of 97–99% TD with sintering aids as compared to that of vacuum sintering (695% TD). Vacuum sintered microstructures revealed the presence of large, medium, and small size pores with non-uniform distribution as compared to sintering under pure nitrogen which showed smaller pores and more uniform distribution. At a low compaction pressure of 250 MPa the nitrogen sintered specimens showed poor particle bonding which resulted in a low UTS value of 145 MPa. Overall, and by comparison, sintering under nitrogen provided better sintered and mechanical properties than those sintered under vacuum for the 6061 Al alloy. Addition of lithium stearate as solid lubricant to the 6061 Al alloy had deleterious effect on sintered density, microstructure, and mechanical properties, particularly for vacuum sintered specimens. This is believed to be due to its wide burn off range leaving 5% residues and also by adverse reaction with Al matrix during sintering. UTS values of 303 MPa were obtained for the nitrogen sintered and fully heat-treated 6061 Al alloy prepared elementally with presence of 0.12 Pb and a compaction pressure of P340–510 MPa. Similar UTS values were also obtained for the 0.1 Sn addition whereas the 0.4
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Ag addition provided lower UTS values of 233 and 248 MPa for compaction pressures of 340 and 510 MPa, respectively. Compaction pressure had a significant effect on the UTS values mainly due to presence of higher porosity levels at lower compaction pressures 6250 MPa. The highest UTS value was obtained with addition of 0.6 paraffin wax and 0.12 Pb reaching a value of 322 MPa with yield strength of 250 MPa and 9% elongation to failure for the nitrogen sintered and heat treated tensile bars. References [1] Lumley RN, Sercombe TB, Schaffer GB. Surface oxide and role of magnesium during the sintering of aluminium. Metall Mater Trans A 1999;30A:457–63. [2] Kondoh K, Kimura A, Watanabe R. Effect of Mg on sintering phenomenon of aluminium alloy powder particle. Powder Metall 2001;44(2):161–4. [3] Martin JM, Gomez-Acebo T, Castro F. Sintering behaviour and mechanical properties of PM Al–Zn–Mg–Cu alloy containing elemental Mg additions. Powder Metall 2002;45(2):173–80. [4] Lumley RN, Schaffer GB. The effect of solubility and particle size on liquid phase sintering. Scripta Mater 1996;35(5):589–95. [5] Schaffer GB, Sercombe TB, Lumley RN. Liquid phase sintering of aluminium. Mater Chem Phys 2001;67(13):85–91. [6] German RM. Supersolidus liquid-phase sintering of prealloyed powders. Metall Mater Trans A-Phys Metall Mater Sci 1997;28A(7): 1553–67. [7] Schaffer GB, Hall JB. The influence of the atmosphere on the sintering of aluminium. Metall Mater Trans A 2002;33A: 3279–84. [8] Lumley RN, Schaffer GB. The effect of additive particle size on the mechanical properties of sintered aluminium–copper alloy. Scripta Mater 1998;39(8):1089–94. [9] Schaffer GB, Huo SH. On the development of sintered 7xxx series aluminium alloys. Powder Metall 1999;42(3):219–26. [10] Sercombe TB, Schaffer GB. On the use of trace addition of Sn to enhance sintered 2xxx series Al powder alloys. Mater Sci Eng A 1999;268(1–2):32–9. [11] Schaffer GB, Huo SH, Drennan J, Auchterlonie GJ. The effect of trace elements on the sintering of an Al–Zn–Mg–Cu alloy. Acta Mater 2001;49(14):2671–8. [12] Sercombe TB. On the sintering of uncompacted prealloyed Al powder alloys. Mater Sci Eng A 2003;341:163–8. [13] Bishop DP, Li XY, Tandon KN, Caley WF. Dry sliding wear behaviour of aluminium alloy 2014 microalloyed with Sn and Ag. Wear 1998;222(2):84–92. [14] Bishop DP, Cahoon JR, Chaturvedi MC, Kipouros GJ, Caley WF. On enhancing the mechanical properties of aluminium P/M alloys. Mater Sci Eng A-Struct Mater Prop Microstruct Process 2000;29(1–2): 16–24. [15] Dudus JH, Dean WA. The production of precision aluminium P/M parts. Int J Powder Metall 1969;5(2):21–36. [16] Kimura A, Shibata M. Reduction mechanism of surface oxide in aluminium alloy powders containing magnesium studied by X-ray photoelectron spectroscopy using synchrotron radiation. Appl Phys Lett 1997;70(26):3615–7. [17] ASM Handbook, Properties of wrought aluminium and aluminium alloys, Formerly 10th ed. Metals handbook, Vol. 2, Properties and selection: nonferrous alloys and special-purpose materials, 1990, ASM International, p. 130. [18] Sercombe TB. Non-conventional sintered aluminium powder alloys, Ph.D. thesis. The University of Queensland; 1998.
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[19] Nath D, Bolla R, Chandra S. Effect of compaction pressure on green properties of Al–4.5% Cu–Pb powder compacts. Powder Metall Sci Tech 1994;5:25–32. [20] Phillips HWL. Annotated equilibrium diagrams of some aluminium alloy systems. London: The Institute of Metals; 1959. p. 69. [21] Lumley RN. The transient liquid phase sintering of aluminium based alloys, Ph.D. thesis. The University of Queensland; 1998. [22] Hawk JA, Angers LM, Wilsdorf HGF. In: Kim YW, Griffith WM, editors. High volume fraction P/M aluminium alloys: stability and
strength at elevated temperatures, dispersion strengthened aluminium alloys. Warrendale, Pennsylvania: The Minerals, Metals and Materials Society; 1988. p. 337. [23] Youseffi M, Showaiter N, Martyn MT. Sintering and mechanical properties of prealloyed 6061 Al powder with and without common lubricants and sintering aids. Powder Metall 2006;49(1):86–95. [24] Youseffi M, Showaiter N. P/M processing of elemental and prealloyed 6061 aluminium alloy with and without common lubricants and sintering aids. Powder Metall 2006;49(3):240–52.
Materials & Design
Materials and Design 29 (2008) 752–762 www.elsevier.com/locate/matdes
Compaction, sintering and mechanical properties of elemental 6061 Al powder with and without sintering aids N. Showaiter *, M. Youseffi School of Engineering, Design and Technology, Engineering Materials Unit, University of Bradford, Bradford BD7 1DP, UK Available online 11 February 2007
Abstract Compaction, sintering, microstructural and mechanical properties of the premixed (elemental) 6061 Al powder with and without sintering aids were investigated. The highest green densities (2.6 g/cm3) were obtained at a compaction pressure of 510 MPa. The optimum sintering condition was at a temperature of 620 C for 1 h under pure nitrogen for compaction pressures of 340 or 510 MPa. It was found that additions of 0.12 wt% Pb was the most effective and enhanced the sintering response by obtaining almost a full sintered density (2.7 g/cm3) with a corresponding increase in the hardness and tensile strength values as compared to the base 6061 Al alloy powder mixture without Pb addition. Additions of 0.1 and 0.4 wt% Sn or Ag, respectively, also improved the sinterability of the elemental base powder with sintered densities of 96–98% theoretical (2.65 g/cm3). The highest UTS value of 322 MPa with yield strength of 250 MPa and 9% elongation to failure were obtained for the nitrogen sintered and fully heat-treated (T6) 6061 Al alloy with addition of 0.6 wt% paraffin wax as lubricant and 0.12 wt% Pb as sintering aid. 2007 Elsevier Ltd. All rights reserved. Keywords: Aluminium 6061 alloy; Vacuum and nitrogen sintering; Sintering aids; Mechanical properties
1. Introduction Improvement in sintering properties for P/M aluminium products will allow the use of conventional powder metallurgy as a viable processing alternative to those of casting and wrought operations. Attempts have therefore been made to enhance the compressibility and sinterability in various ways and for different series of Al alloys, both prealloyed or premixed elementally. It is well known that these Al alloy powders are difficult to sinter because of the stable aluminium oxide film covering the powder particles and thus reducing sinterability. It has been found [1,2] that addition of 0.1–1 wt% Mg helps to break up the oxide layers on the surface of the Al particles through the formation of a spinel phase MgAl2O4 [1] and that the relative density of the sintered material increases by 9% by increasing the Mg content to 1 wt% [2]. It was also reported that the tensile *
Corresponding author. Address: Mechanical Engineering, University of Bahrain, P.O. Box 32038, Kingdom of Bahrain. E-mail address: [email protected] (N. Showaiter). 0261-3069/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2007.01.027
properties of the sintered material increased from 60 to 140 MPa for the UTS and from 0% to 12% for elongation as Mg content increased [2]. Mg has also been added in various quantities to 7xxx series and it was found [3] that up to a total Mg content of 3.5 wt% the mechanical strengths were better than those of the present commercial alloy Alumix 431 (Al–5.5Zn–2.5Mg–1.6Cu). However, at higher Mg levels, the secondary phase could not be totally taken into solution and consequently remaining as a network at the grain boundaries causing embrittlement of the alloy. More recently, significant improvements in the sinterability of aluminium alloys have been reported [4–8] by liquid phase sintering leading to improved densification of the alloys. The attributes of liquid formation to assist densification have been described in the literature [6]. The effect of trace addition of selected elements such as Pb, Sn, Se, Bi, and Sb on the sinterability of aluminium alloys was investigated [9–12]. It was found that microalloying with 100 ppm of Pb improved sintering response of an Al–Zn–Mg–Cu alloy produced from premixed elemental powders [9] and that elemental addition of 0.12 wt% Pb increased the UTS
N. Showaiter, M. Youseffi / Materials and Design 29 (2008) 752–762
by 36% from 314 to 427 MPa for the same alloy. Trace addition of Sn also showed some improvement in sintering as well as mechanical strength, but less effective than Pb, and Se had no effect on the strength of the alloy, while Bi and Sb had a large negative effect. It was reported that since these additives, i.e. Sn and Pb had low solubility in the base Al matrix they remained segregated at the liquid–vapour interface, where they reduced the surface tension of the liquid, decreased the wetting angle and hence spreading properly over the matrix powder particles allowing a proper (enhanced) liquid phase sintering process. Recent work [13,14] has described a novel P/M approach for a ternary Al–4Cu–0.5Mg with Sn and Ag addition rather than using conventional ingot aluminium approach. It was found that Ag addition to P/M AA2014 resulted in a minor improvement in mechanical properties as compared to the ternary Al–4Cu–0.5Mg [14]. So far, researchers have largely concentrated on studying the sintering behaviour of the elemental 2xxx and 7xxx series Al alloys whereas the 6xxx series has received little attention [15]. Commercial grade 6xxx series are widely used as medium strength structural Al alloys having good ductility, excellent corrosion resistance and higher response to age hardening. The aim of this work was therefore to investigate the compaction, sintering, microstructure and mechanical properties of the commercial grade 6061 aluminium alloy powder prepared elementally with and without elemental additions of sintering aids such as lead (Pb), tin (Sn) or silver (Ag). These results are also compared to those of prealloyed 6061 Al alloy powder with and without elemental additions of the same sintering aids reported previously [23,24] in more detail.
753
compositions are in weight %) as elemental were made to the lubricated base powder and mixed for 15 min each, after which a composition similar to that of wrought 6061Al alloy was gained. The sintering aids of 0.12 Pb, 0.1 Sn or 0.4 Ag as elemental powders were added to the quaternary elemental powder mixture and also mixed for 15 min each.
2.3. Compaction and sintering Squared specimens of size 15 · 15 · 3 mm3 (3.5 g of powder) were pressed at pressures in the range of 250–770 MPa to study the compaction characteristics and to establish the optimum compaction pressure for sintering. Tensile bars (dumbbell shaped) of size 2.5 mm thickness · 4.5 mm width · 15 mm gauge length were machined out of sintered rectangular bars of size 40 · 10 · 4.5 mm3. These bars were pressed (4.5 g of powder) at 250, 340 and 510 MPa for mechanical testing. Zinc stearate was used on the die wall and punches for lubrication before compaction to reduce diewall frictional effects. A single acting hydraulic press (20 Ton-Chirchill) was used for compaction. Sintering characteristics were investigated initially under vacuum (10 4 mbar) in a laboratory ceramic tube furnace (controlled by an Eurotherm 818 P controller) in the range 580–640 C and for 30– 120 min. All compacts were heated to the sintering temperature at a heating rate of 5 C/min, held for 30–120 min and then furnace cooled to room temperature. For comparison, sintering was also carried out under pure nitrogen atmosphere ( 40 dew point) in a standard laboratory Carbolite gas atmosphere recrystallized alumina tube furnace. The pressure change against temperature rise was monitored during the whole sintering cycle in order to study the formation of any liquid phase, appearance of any other phase or any phase transformation via exothermic or endothermic reactions. Green densities were determined from weight and volume measurements. Sintered densities were measured by the Archimedes principle (water displacement technique) using lacquer-coated samples.
2.4. Heat treatment
2. Experimental procedure
Sintered tensile specimens of size 2.5 mm thickness · 4.5 mm width ·15 mm gauge length were fully heat-treated using T6 Temper, i.e. solution heat treated at 520 C for 30 min, water quenched and then aged at 160 C for 18 h.
2.1. Raw materials
2.5. Metallography and mechanical testing
The raw materials used included the following powders and lubricants: Air atomised aluminium (Al) powder, with 99.7% purity and average particle size of 60 lm supplied by Ronald Britton and Co. Magnesium, Mg, with 99.8% purity and maximum particles size of 50 lm. Silicon, Si, with mean particle size of 5 lm and 97.5% purity. Copper, Cu, with maximum particle size of 50 lm and 99.0% purity. Lead, Pb, with maximum particle size of 150 lm as-received, and then sieved below 45 lm with 99.5% purity. Tin, Sn, with maximum particle size of 45 lm and 99.9% purity. Silver, Ag, with maximum particle size of 100 lm as-received, and then sieved below 45 lm with 99.99% purity.Good Fellow Cambridge limited supplied Mg, Si, Cu, Sn, Ag and Pb. Lithium stearate and paraffin wax were used as solid lubricants. The chemical composition of the 6061 Al powder prepared by elemental mixing is as follows: Al–1.0Mg–0.6Si–0.25Cu (All in weight%).
2.2. Mixing Lithium stearate and/or paraffin wax of 0.6% by weight was added to the base Al powder and mixed separately for 20 min using a 3-D Turbula mixer with glass Jar containers. Addition of 0.6 Si, 0.25 Cu and 1 Mg (all
Microstructural examination of the alloys was conducted using an MeF3 optical light microscope and JEOL- JSM 6400 scanning electron microscope (SEM). The chemical analyses of the as-sintered and fully heat-treated specimens were preformed using the SEM equipped with an Energy Dispersive X-ray (EDX) microanalyser. Metallographic samples were prepared conventionally, i.e. grinded and then polished using 6 lm and finished with 1 lm polish. No etching was carried out. Mechanical properties such as apparent macro- and micro-hardness, ultimate tensile strength, and % elongation to failure were measured in the as-sintered and fully heat treated conditions. For the Vickers microhardness a 20 g load were applied using the standard Vickers hardness-testing machines. The ultimate tensile strength (UTS) and % elongation to fracture were obtained using an Instron tensile testing machine with a 10 KN load cell and a crosshead speed of 0.5 mm/min.
3. Results and discussion 3.1. Compaction characteristics The compaction characteristics were investigated for the elemental 6061Al alloy without and with addition of sintering aids such as 0.12 Pb, 0.1 Sn or 0.4 Ag at different
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compaction pressures in the range 250–770 MPa. Note also that 0.6 wt% of either paraffin wax (PW) or lithium stearate (LS) was added as lubricant. The compaction curve for the elemental 6061 Al alloy presented graphically in Fig. 1 shows that the highest green density of 2.6 g/cm3 (96% TD) was obtained at a compaction pressure of 510 MPa above which there was little increase in green density. Also, the compaction curves in Fig. 1 show the effect of adding the sintering aids Pb, Sn or Ag on the compressibility of the elemental 6061 Al alloy powder. It can be seen that the green density of compacts with addition of sintering aids is slightly higher (97% TD) at 510 MPa than that without sintering aids. Similar results have been reported elsewhere [18,19] with Sn and Pb additions. These admixed powders are mostly finer than the base Al powder and thus during compaction will be extruded into the pore space. The increase in green density is therefore due to the fine powder particles of Pb, Sn or Ag
filling the smaller voids between the coarse Al powder particles. 3.2. Sintering characteristics The sintering curves for the premixed elemental (EL) Al alloy sintered under vacuum and pure nitrogen at 620 C for 1 h, along with compaction curve in the range 250– 770 MPa processed with 0.6 wt% lubricant and no sintering aids are shown in Fig. 2. The sintering results show that by increasing the compaction pressure the sintered density increases due to more particle bonding and presence of less void as the compaction pressure increases. Fig. 2 also shows clearly that the sintered densities under vacuum were marginally lower than the green densities even by increasing compaction pressure and sintering temperature. This is because the porosity was not completely eliminated, although it was significantly reduced indicating that the
2.68
99 98
2.64 97
2.62 2.6
96
2.58
95
2.56
Al-1Mg-0.6Si-0.25Cu Al-1Mg-0.6Si-0.25Cu+0.12Pb Al-1Mg-0.6Si-0.25Cu+0.1Sn Al-1Mg-0.6Si-0.25Cu+0.4Ag
2.54 2.52 2.5 0
100
200
300
400
500
600
700
800
94
% Theoretical Density
Green Density, g/cm3
2.66
93 92 900
Compaction Pressure, MPa Fig. 1. Variation in green density with compaction pressure for the premixed elemental 6061 Al powders with and without addition of sintering aids.
98
2.64 2.62
97
Density, g/cm3
96
2.58
95
2.56 2.54
94
nitrogen sintered green density vacuum sintered
2.52 2.50
% Theoretical Density
2.60
93
2.48
92 0
100
200
300
400
500
600
700
800
900
Compaction Pressure, MPa Fig. 2. Variation in green and sintered densities with compaction pressure for the premixed elemental 6061 Al powder compacts sintered at 620 C for 1 h. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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amount of shrinkage produced did not compensate for swelling (due to alloying effects) produced during the vacuum sintering process. The decrease in sintered density is, therefore, related to the expansion of the specimens due to the high solubility of the alloying elements Mg, Si and Cu that form low melting phases which lead to rapid dimensional change, also due to the inefficient removal of the trapped gas and air by sintering under vacuum causing a volume expansion and thus reducing the sintered density. Sintering under nitrogen provided higher densities than that in vacuum for the elementally premixed powder compacts (pressed at 510 MPa) reaching a sintered density of 2.61 g/cm3 (96.7% TD) and only 2.57 g/cm3 (95% TD) in the case of vacuum sintering. The vacuum sintered parts also showed P2–5% volume expansion which suggests clearly that the sintering atmosphere is an important parameter and will affect the final porosity content. It is reported [5,16,18] that by sintering under pure nitrogen the oxide content can be reduced further in the presence of Mg which exposes fresh metal and facilitates the subsequent formation of AlN on exposed surfaces. The variation in sintered density with sintering temperature and presence of 0.12Pb, pressed at 340 and 510 MPa, and sintered for 1 h at different temperatures (580–640 C) under vacuum is shown in Fig. 3. It can be seen from both sintering curves that sintered densities decrease gradually after the optimum sintering temperature at 620 C, but increase more rapidly before this temperature. The sintering data curves also show that sintered densities increase as the compaction pressure increases from 340 to 510 MPa with a similar trend for the under-sintered, optimum-sintered, and over-sintered conditions. These results, therefore, reveal that the optimum sintering condition is at a temperature of 620 C for 60 min under pure nitrogen (or vacuum), which was chosen for the sintering of mechanical tensile bars.
755
The effect of Pb, Sn, or Ag content on the sintered density of pressed and sintered 6061 Al alloy is shown in Fig. 4. It can be seen that amongst the sintering aids used in this work, additions of 0.12 wt% Pb was the most effective which enhanced the sinterability by obtaining almost a full sintered density (2.7 g/cm3) with a corresponding increase in hardness and tensile strength values as compared to those without Pb addition. Additions of 0.1 and 0.4 wt% Sn or Ag, respectively, also improved the sinterability of the elemental 6061 Al alloy with sintered densities of P96–98% theoretical (2.65 g/cm3), but being less effective than lead. It can therefore be concluded that trace additions of 0.1 wt% Pb (and Sn or Ag to a lesser extent) is an effective way of enhancing sinterability of the 6061 Al alloy. 3.3. Shrinkage characteristics Percentage volume change for the premixed elemental 6061 Al powders pressed at 340 MPa and sintered at 620 C for 1 h under vacuum or pure nitrogen, with and without 0.12 Pb and 0.6 wt% paraffin wax (PW) or lithium stearate (LS) as lubricants, are given in Table 1. It can be seen that addition of LS as lubricant along with vacuum sintering has caused volume expansion whereas PW and sintering under nitrogen resulted in volume shrinkage and hence higher sintered densities of P98% TD as compared to the vacuum sintered specimens with only 96% TD. It is therefore clear that the sintering atmosphere and type of lubricant plays an important role for the elimination or reduction of pores and expansion or shrinkage of the sintered specimens. This investigation has found that lithium stearate, in particular, has a deleterious effect on sintered density mainly due to its wide burn off range and hence incomplete removal during sintering leaving some black residue as well as causing adverse reaction with Al which was also reported previously [23,24]. The pore
2.62
97
96 2.58 95
2.56 2.54
94
2.52 340 MPa
93
% Theoretical Density
Sintered Density, g/cm3
2.60
2.50 510 MPa
2.48 595
600
605
610
615
620
625
630
635
92 640
Sintering Temperature, ˚C Fig. 3. Variation in sintered density with sintering temperature for the premixed elemental 6061 Al alloy powder pressed at 340 and 510 MPa and sintered for 1 h under vacuum with 0.12Pb content.
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100
2.69
99
2.65 98 2.63 97
2.61 2.59
96
Al-1Mg-0.6Si-0.25Cu Al-1Mg-0.6Si-0.25Cu+0.12Pb Al-1Mg-0.6Si-0.25Cu+0.1Sn Al-1Mg-0.6Si-0.25Cu+0.4Ag
2.57 2.55 2.53 100
% Theoretical Density
Sintered Density, g/cm3
2.67
95
94 150
200
250
300
350
400
450
500
550
600
Compaction Pressure, MPa Fig. 4. Variation in sintered density with compaction pressure for the premixed elemental 6061 Al alloy with and without the addition of sintering aids sintered at 620 C for 1 h under pure nitrogen.
content and microstructure also depend on the solubility of the gases in the compacts, i.e. insoluble gases limit pore shrinkage while soluble gases, like hydrogen, in a closed pore can diffuse out during sintering, and are eventually eliminated or reduced in size or content. 3.4. Analysis of heating and cooling curves during the sintering cycle
tering (SLPS) which results in melt formation preferentially at intergranular regions. The intergranular melt contributes mainly to densification by capillary-induced rearrangement, solution-reprecipitation and grain shape accommodation. The endothermic peaks observed are associated with the formation reaction of intermetallics from molten eutectic. The resulting heat output from these reactions
From the analysis of heating data curves during the sintering cycle of the temperature rise versus pressure change for the premixed elemental (EL) and degassed prealloyed (PA) 6061 Al compacts (see Fig. 5a–c), mainly four endothermic peaks at temperatures of 320, 500, 550 and 590 C can be observed corresponding to formation of transient liquid phase, burning peak for the lubricant lithium stearate (LS), and the formation of persistent liquid phase above the solidus temperature, respectively. Use of the prealloyed powder eliminates transient melt formation as seen in Fig. 5b. The predominant sintering mode for the PA powder is, therefore, by supersolidus liquid phase sinTable 1 % Volume change for the premixed elemental 6061 Al alloy powder with and without 0.12 Pb and lubricants pressed at 340 MPa and sintered under vacuum or pure nitrogen Alloy composition
Volume changea (%) Vacuum
EL EL + 0.12Pb EL+LS EL+PW EL + 0.12Pb+LS a
1.56 1.45 +1.89 2.3 +3.96
Nitrogen 1.8 2.14 1.2 2.81 1.8
Positive values indicate expansion and negative values shrinkage. With 0.6 wt% lithium stearate. +PW With 0.6 wt% paraffin wax. +LS
Fig. 5. Sintering cycle showing the heating and cooling curves by monitoring pressure change and temperature rise for: (a) prealloyed 6061 Al alloy with 0.6 LS; (b) prealloyed 6061 Al alloy and no lubricant or sintering aid; and (c) premixed elemental powder with 0.12Pb and 0.6LS; all sintered at a heating rate of 5 C/min, pressed at 510 MPa, and sintered at 620 C for 1 h under vacuum.
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cause formation of liquid phases (both transient and persistent) which take place depending on the type of powder. Pore volume growth via diffusion of trapped gasses during the sintering process and a corresponding volume expansion also take place depending on the lubricant content and type and the sintering atmosphere. It is to be noted that Mg is aiding liquid formation in the temperature range 400–595 C [21]. The liquid, if present to a sufficient extent, further enhances the densification of the alloy, but the broken oxide film covering the Al powder will hinder sintering to such an extent that the amount of liquid phase may not be sufficient to achieve a good densification. Also, the presence of adsorbed and absorbed gases by the Al particles, as well as water vapour present during vacuum sintering would increase the size of the compacts and therefore reducing their sintered density due to volume expansion. The volume expansion in the premixed EL powder may therefore be attributed to the homogenisation reactions occurring between alloying elements and Al base powder, the use of unsuitable lubricants such as LS having a wide range of burning characteristics, and the formation of transient liquid phases accompanied with the formation of porosity which may not be removed completely by the subsequent shrinkage. The sinterability was, however, improved by addition of appropriate lubricants such as paraffin wax (PW) and sintering aids such as lead. In summary, the heating curves for the elemental 6061 Al alloy show the appearance of mainly four endothermic peaks at temperatures of 320, 500, 550 and 590 C corresponding, respectively to: (1) Diffusion of Mg into solution causing expansion (the Kirkendall effect) as well as the burning of the lubricant (lithium stearate). It has been reported [22] that diffusivity of Mg at 482 C is 100 times higher than that of the other elements such as Si, Cu, and Fe in Al. (2) Formation of an eutectic liquid phase and intermetallics in the range 470–530 C due to temperature rise causing several possible eutectic reactions and further interaction between Mg, Si, Cu and Al particles. (3) As temperature rises, further liquid phase formation occurs at 550 and 590 C due to mainly further diffusion and incorporation of the alloying elements (Mg, Si and Cu) and the liquid reaching its equilibrium composition with the Al at the sintering temperature. The sintering results including the heating data curves (and the microstructural studies) suggest that there are large amount of transient liquid phase formations either as binary, ternary, etc., within the 6061 Al alloy with or without sintering aids. This is because of the formation of one eutectic liquid phase at 500 C followed by another at 550 C (both causing homogenisation) and final reappearance at 590 C which is between the solidus (582 C) and liquidus (652 C) temperatures [17,20] and, therefore, suggesting the occurrence of a supersolidus liquid phase sintering process. This altogether suggests the sinterability of these alloys is based upon the initial formation of transient liquid phases due to the incorporation
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of alloying additions and finally the presence of supersolidus liquid phase sintering. 3.5. Microstructural studies and EDX analysis Backscattered SEM images of both vacuum and nitrogen sintered and fully heat-treated 6061 Al alloy with and without sintering aids are shown in Figs. 6 and 7. Generally, presence of grain boundary eutectic phase in white/ grey contrast along with pores (macro- and micro-porosity) can be seen within the microstructure. In particular, Fig. 6a shows the presence of both macro- and micro-porosity and elongated residue along the grain boundaries most likely due to insufficient delubrication of lithium stearate proving that this lubricant is harmful and should not be used for processing of P/M Al alloys. On the other hand, Fig. 6b shows the presence of only microporosity for the alloy with 0.6 PW and 0.12 Pb additions sintered under pure nitrogen suggesting that both PW as lubricant and Pb as sintering aid are useful and suitable for the processing and sintering of 6061 Al alloy. Fig. 6c and d show similar microstructures with additions of 0.1 Sn or 0.4 Ag and 0.6 PW, with the exception that the alloy with 0.1 Sn has densified better with less porosity as compared to that of 0.4 Ag addition, both sintered under pure nitrogen and fully heat-treated. Figs. 6 and 7 also show show the presence of some white needle-like intermetallics segregated along the grain boundaries in the as sintered and heat treated samples which remained undissolved after solution treatment. The elemental EDX microanalysis showed the presence of mainly Fe and Si along with Al. Presence of iron as an impurity is unavoidable and that the solid solubility of iron in Al is very small resulting in the presence of Al–Fe or Al–Fe–Si within the microstructures. EDX analysis of various phases also proved that the dark/grey phase is rich in Mg and Si and from its morphology it is evidently the solidified liquid phase having an approximate chemical composition of: Al–Mg2Al3–Mg2Si (450 C) and Al–Si–Mg2Si (555 C) [20]. The (near spherical) precipitates revealed presence of Mg and Si along with Al as its primary constituents, thereby indicating a predominantly Mg2Si phase precipitate as a second phase in the matrix formed during the cooling cycle. Generally, it can be said that as temperature increases so does the diffusion of Mg, Si, Cu and Fe into Al resulting in some phase transformation mostly by solid-state diffusion of Mg2Si and eventually formation of eutectic liquid practically at grain boundaries. Associated with this phase are the Cu, Fe, Pb, Sn and/or Ag. The microstructure also contains typical P/M pores of dark colour having various size and shapes. Both optical and SEM microstructural examinations of the vacuum sintered specimens without sintering aids revealed the presence of pores of various size with nonuniform distribution which was expected from a typical P/M Al alloy structure with poor sinterability. The decrease in sintered density for the vacuum sintered
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Fig. 6. SEM micrographs (backscattered image (BSI)) of the fully heat-treated premixed elemental 6061 Al alloy without and with sintering aids, pressed at 510 MPa and sintered under vacuum (a) or pure nitrogen (b, c and d) at 620 C for 1 h: (a) with 0.6 LS; (b) 0.6 PW + 0.12 Pb; (c) 0.6 PW + 0.1 Sn; and (d) 0.6 PW + 0.4 Ag additions.
specimens is related to the presence of two forms of porosity, bulk (within grains) and interparticle grain boundary porosity. It appears that both types are formed and expanded further by the trapped gas and air during sintering under vacuum causing a volume expansion. In particular, the grain boundary porosity is formed by diffusion of the trapped gas/air between particles during sintering which involves delubrication at the same time. It is known that the bulk porosity (due to trapped air, gas or moisture) is present initially in the powder, and the amount of which depend on the atomisation process. It is to be noted that these pores collapse during compaction, but they are not totally eliminated even at higher compaction pressure. The increase in expansion or compact growth at higher compaction pressures and sintering temperatures is believed to be due to trapped air/gas within pores and also due to further pore formation during supersolidus liquid phase sintering which takes place at >580 C and further increase in temperature results
in the formation of more liquid phase mainly along particle boundaries by incipient melting of the 6061 Al powder. There were also some large pores within the assintered specimens specially those without sintering aids most likely produced by agglomeration of the admixed alloying elements which were dissolved in the matrix or by agglomeration of the lubricant which was burned out during sintering and therefore leaving behind large pores. Another reason may be due to the formation of the transient liquid phase (TLP) followed by homogenisation of the alloy causing swelling (expansion) and hence creating some large pores (the Kirkendall effect) [10] which could not be removed during shrinkage. This is believed to be associated with the elemental additions and the prealloyed powder may be more useful in this respect. It is therefore believed that to minimise swelling and formation of porosity in the case of adding no sintering aids, the use of a compressible prealloyed (6061 Al) powder may be necessary and more beneficial as also reported previously
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Fig. 7. SEM micrographs (BSI) of the vacuum and nitrogen sintered and fully heat-treated premixed elemental 6061 Al alloys: (a) with 0.6 PW, sintered under vacuum; and (b) 0.6 PW + 0.12 Pb, sintered under nitrogen. The porosity content is reduced significantly for the nitrogen sintered alloy (b) and the prior particle boundaries are difficult to identify as compared to the vacuum sintered alloy (a).
[23,24]. However, the elemental addition of Pb, Sn or Ag to elemental mixture of Mg, Si, Cu and the base Al powder, is found to be useful for liquid phase sintering (LPS) and the creation of liquid phase by contact melting causing rapid homogenisation by inter-particle diffusion. The incorporation of elements such as Cu will take place above 510 C which is followed by the formation of the equilibrium liquid phase during the sintering temperature (620 C) leading to the formation of the equilibrium alloy with some shrinkage and densification as obtained during nitrogen sintering in this work.
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Generally, there was presence of a light grey phase rich in Mg, Si and Al almost in all the sintered microstructures. This, in fact, is the solidified liquid phase with presence of Pb, Sn or Ag at grain boundaries suggesting the segregation of these elements within the grain boundary liquid phase during sintering. The segregation of the trace elements Pb or Sn is due to their insolubility in Al with Pb having a lower surface tension than Sn and therefore being more effective in activating liquid phase sintering. It follows therefore that 0.1 wt% Pb in 6061 Al alloy was sufficient to activate the sintering process to achieve near full density. It is also clear that due to negligible solubility of Pb in Al, all the Pb will segregate to the liquid phase and hence any solidified liquid phase will be rich in Pb, Mg and Si surrounded by Al as observed within the as-sintered microstructures mainly at the grain boundaries. There were also appearance of a few dark phases, which looked like pores, but in fact they were not pores and happened to be Mg–Al oxide, i.e. a by-product, due to the reaction between the oxide layer on the Al surface and the elemental Mg taking place during the early stages of sintering. This work, therefore, has shown that it is possible to create a proper LPS process within the 6xxx series Al alloys by addition of sintering aids (Pb, Sn or Ag) as trace elements (0.1–0.5 wt%) in order to form adequate liquid phase, which is able to disrupt the highly stable aluminium oxide film covering the Al particles. This liquid phase is also able to penetrate the oxide film through the discontinuities created during pressing allowing neck formation by LPS. It is to be noted that alloying elements such as Mg, Si, Cu and Zn are all soluble in Al, and liquids may form by contact melting, melting of the elements themselves, or by a eutectic reaction between the Al and the admixed elements. The improved sinterability may be explained (Schaffer et al.) by the following mechanism: Firstly, it is to be noted that elemental additions of Pb, Sn or Ag is effective for achieving high densities only by the presence (addition) of 1 wt% elemental Mg. In this way, the Mg addition facilitates oxide disruption by reacting with the Al sesquioxide resulting spinel formation and disruption of the oxide layer and thus allowing wetting by the liquid formed during sintering, while Pb, Sn or Ag additions segregate to the liquid phase and reduce its surface tension and therefore improving sinterability of the Al alloy. It is therefore believed that any excess Mg will diffuse into solution causing swelling (and pore formation) via the Kirkendall effect and that Pb or Sn will segregate to the liquid phase due to their low solubility in the Al matrix and reduce the surface tension of the supersolidus liquid as well as reducing the wetting angle and therefore enhancing wetting of the Al matrix particles and sinterability. It is known that the solubility of Sn in Al is 0.1 wt% at 610 C and decrease to 0% at 660 C. As compared to sintering (under pure nitrogen) without sintering aids, elemental addition of sintering aids (Pb, Sn or Ag) resulted in higher sintered densities with more uniform pore size and distribution. In particular, 0.12% Pb addition resulted in an almost pore free structure
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with presence of some discontinuous grain boundary second phases (intermetallics and eutectic phase). By microanalysis, it was found that the solidified grain boundary liquid phase consisted of various intermetallics, which are known to be brittle, and therefore the as-sintered mechanical properties are expected to be weak due to the presence of grain boundary phases causing premature failure and low tensile strength as reported in the following section. However, the mechanical strength was improved significantly by a full heat treatment using standard T6. 3.6. Mechanical properties Vicker’s microhardness values for the as sintered and fully heat-treated (T6) alloys with and without sintering aids are given in Table 2. The microhardness values increased from 43 to 83 HV0.02 and from 44 to 92 HV0.02 for the vacuum and nitrogen sintered plus fully heat-treated specimens, respectively, and the 0.12Pb addition increased the microhardness values from 53 to 134 HV0.02 for the as-sintered and heat-treated (T6) Table 2 Vickers microhardness for the premixed elemental 6061 Al alloy with and without 0.12Pb and lubricant pressed at 340 and sintered at 620 C for 1 h under vacuum or pure nitrogen and fully heat-treated (T6) condition Alloy composition
Microhadrness HV 0.02 As-sintered
Ela El + 0.12Pba Elb El + 0.12Pbb a b
Fully heat-treated
Vacuum
Nitrogen
Vacuum
Nitrogen
43 44
44 45 50 53
83 130
92 130 110 134
With 0.6 wt% lithium stearate (LS) as lubricant. With 0.6 wt% paraffin wax (PW) as lubricant.
6061 Al alloys, respectively. It is therefore, clear that the T6 heat-treatment increased the hardness due to precipitation hardening as expected. Ultimate tensile strength (UTS) and % elongation to failure for the EL 6061 Al alloy with and without addition of sintering aids and lubricants pressed at 250, 340, and 510 MPa sintered at 620 C for 1 h under vacuum or pure nitrogen and fully heat-treated using T6 condition are given in Table 3. From Table 3, it can be seen that the strength values increase slightly with an increase in compaction pressure, for the premixed elemental Al alloy without and with addition of sintering aids sintered under nitrogen and addition of 0.6 wt% paraffin wax. The UTS values for a compaction pressure of 250 MPa was only 145 MPa for the heat-treated alloy with 0.12 Pb and 0.6 Li stearate additions, respectively. The low strength value is due to the poor bonding between the powder particles and the presence of 5–10% (by volume) of porosity content as a result of the low compaction pressure at 250 MPa and presence of delubrication residue. The strengths should rise as sintered densities increase or as the porosity content decreases. This was the case for the 6061 Al alloy with addition of 0.12 Pb and 0.6 Li stearate compacted at either 340 or 510 MPa, which showed almost full density (2.69 g/cm3) with UTS value of 260–305 MPa and with 5–13% elongation to failure. The UTS value significantly increased from 266 to 322 MPa for the heat-treated elemental alloys with 0.12 Pb and 0.6 paraffin wax addition, respectively. It is to be noted that the 0.12 Pb additions increased both the hardness and the strengths values for both the as-sintered and heat-treated conditions. It is therefore concluded that, the 0.12 Pb addition increased the sintered densities significantly only under nitrogen sintering reaching almost full density (2.69 g/cm3) for the premixed elemental 6061 Al P/M alloy. The highest UTS of 322 MPa
Table 3 UTS and yield strength values, and % elongation to fracture for the premixed elemental 6061 Al alloy with and without sintering aids and lubricant, pressed at 340 or 510 MPa and sintered at 620 C for 1 h under vacuum or pure nitrogen and fully heat-treated (T6) condition Compaction pressure (MPa)
Alloy composition a
Yield strength (MPa)
Ultimate tensile strength (MPa)
Elongation to fracture (%)
250
El El + 0.12Pba
92 99
140 145
8.7 11
340
Ela Elc El + 0.12Pb c El + 0.12Pba El + 0.1Sna El + 0.4Aga
– 115 141 150 160 198
259 210 233 275 305 233
8.6 6 7 9.3 6 5.3
510
Ela Elb El + 0.12Pba El + 0.12Pbb El + 0.1Sna El + 0.4Aga
113 210 170 250 198 185
266 275 303 322 275 248
a b c
Sintered under pure nitrogen with 0.6 wt% lithium stearate. Sintered under pure nitrogen with 0.6 wt% paraffin wax. Sintered under vacuum with 0.6 wt% lithium stearate.
12 10 8 9 10 13
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was, therefore, obtained for the premixed EL alloy with 0.6 wt% paraffin wax as solid lubricant and 0.12 wt% Pb addition giving a UTS value of 322 MPa with yield strength of 250 MPa and 9% elongation to failure for the nitrogen sintered and heat treated alloy compacted at 510 MPa. 4. Conclusions The highest green densities (2.6 g/cm3) were obtained at a compaction pressure of 510 MPa. Compaction pressures >510 MPa produced very small increase in green density. The optimum sintering temperature was found to be at 620 C for 30 or 60 min for compaction pressures of 340 or 510 MPa. The endothermic peaks during sintering cycle for the premixed elemental alloy showed the formation of both transient liquid phase at temperatures between 320 and 550 C and persistent liquid phase above the solidus temperature at 590 C. Therefore, it is believed that the sintering mechanism is via the formation of both transient and persistent liquid phases. 0.12 wt% Pb addition was found to be the most effective sintering aid as compared to 0.1 Sn and 0.4 Ag additions, resulting almost a full sintered density (2.7 g/ cm3). Sintering under vacuum gave rise to the presence of higher pore content, and excessive amounts of residual porosity at grain boundaries, particularly with 0.6 wt% lithium stearate as solid lubricant. On the other hand sintering under pure nitrogen atmosphere provided higher densities of 97–99% TD with sintering aids as compared to that of vacuum sintering (695% TD). Vacuum sintered microstructures revealed the presence of large, medium, and small size pores with non-uniform distribution as compared to sintering under pure nitrogen which showed smaller pores and more uniform distribution. At a low compaction pressure of 250 MPa the nitrogen sintered specimens showed poor particle bonding which resulted in a low UTS value of 145 MPa. Overall, and by comparison, sintering under nitrogen provided better sintered and mechanical properties than those sintered under vacuum for the 6061 Al alloy. Addition of lithium stearate as solid lubricant to the 6061 Al alloy had deleterious effect on sintered density, microstructure, and mechanical properties, particularly for vacuum sintered specimens. This is believed to be due to its wide burn off range leaving 5% residues and also by adverse reaction with Al matrix during sintering. UTS values of 303 MPa were obtained for the nitrogen sintered and fully heat-treated 6061 Al alloy prepared elementally with presence of 0.12 Pb and a compaction pressure of P340–510 MPa. Similar UTS values were also obtained for the 0.1 Sn addition whereas the 0.4
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Ag addition provided lower UTS values of 233 and 248 MPa for compaction pressures of 340 and 510 MPa, respectively. Compaction pressure had a significant effect on the UTS values mainly due to presence of higher porosity levels at lower compaction pressures 6250 MPa. The highest UTS value was obtained with addition of 0.6 paraffin wax and 0.12 Pb reaching a value of 322 MPa with yield strength of 250 MPa and 9% elongation to failure for the nitrogen sintered and heat treated tensile bars. References [1] Lumley RN, Sercombe TB, Schaffer GB. Surface oxide and role of magnesium during the sintering of aluminium. Metall Mater Trans A 1999;30A:457–63. [2] Kondoh K, Kimura A, Watanabe R. Effect of Mg on sintering phenomenon of aluminium alloy powder particle. Powder Metall 2001;44(2):161–4. [3] Martin JM, Gomez-Acebo T, Castro F. Sintering behaviour and mechanical properties of PM Al–Zn–Mg–Cu alloy containing elemental Mg additions. Powder Metall 2002;45(2):173–80. [4] Lumley RN, Schaffer GB. The effect of solubility and particle size on liquid phase sintering. Scripta Mater 1996;35(5):589–95. [5] Schaffer GB, Sercombe TB, Lumley RN. Liquid phase sintering of aluminium. Mater Chem Phys 2001;67(13):85–91. [6] German RM. Supersolidus liquid-phase sintering of prealloyed powders. Metall Mater Trans A-Phys Metall Mater Sci 1997;28A(7): 1553–67. [7] Schaffer GB, Hall JB. The influence of the atmosphere on the sintering of aluminium. Metall Mater Trans A 2002;33A: 3279–84. [8] Lumley RN, Schaffer GB. The effect of additive particle size on the mechanical properties of sintered aluminium–copper alloy. Scripta Mater 1998;39(8):1089–94. [9] Schaffer GB, Huo SH. On the development of sintered 7xxx series aluminium alloys. Powder Metall 1999;42(3):219–26. [10] Sercombe TB, Schaffer GB. On the use of trace addition of Sn to enhance sintered 2xxx series Al powder alloys. Mater Sci Eng A 1999;268(1–2):32–9. [11] Schaffer GB, Huo SH, Drennan J, Auchterlonie GJ. The effect of trace elements on the sintering of an Al–Zn–Mg–Cu alloy. Acta Mater 2001;49(14):2671–8. [12] Sercombe TB. On the sintering of uncompacted prealloyed Al powder alloys. Mater Sci Eng A 2003;341:163–8. [13] Bishop DP, Li XY, Tandon KN, Caley WF. Dry sliding wear behaviour of aluminium alloy 2014 microalloyed with Sn and Ag. Wear 1998;222(2):84–92. [14] Bishop DP, Cahoon JR, Chaturvedi MC, Kipouros GJ, Caley WF. On enhancing the mechanical properties of aluminium P/M alloys. Mater Sci Eng A-Struct Mater Prop Microstruct Process 2000;29(1–2): 16–24. [15] Dudus JH, Dean WA. The production of precision aluminium P/M parts. Int J Powder Metall 1969;5(2):21–36. [16] Kimura A, Shibata M. Reduction mechanism of surface oxide in aluminium alloy powders containing magnesium studied by X-ray photoelectron spectroscopy using synchrotron radiation. Appl Phys Lett 1997;70(26):3615–7. [17] ASM Handbook, Properties of wrought aluminium and aluminium alloys, Formerly 10th ed. Metals handbook, Vol. 2, Properties and selection: nonferrous alloys and special-purpose materials, 1990, ASM International, p. 130. [18] Sercombe TB. Non-conventional sintered aluminium powder alloys, Ph.D. thesis. The University of Queensland; 1998.
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[19] Nath D, Bolla R, Chandra S. Effect of compaction pressure on green properties of Al–4.5% Cu–Pb powder compacts. Powder Metall Sci Tech 1994;5:25–32. [20] Phillips HWL. Annotated equilibrium diagrams of some aluminium alloy systems. London: The Institute of Metals; 1959. p. 69. [21] Lumley RN. The transient liquid phase sintering of aluminium based alloys, Ph.D. thesis. The University of Queensland; 1998. [22] Hawk JA, Angers LM, Wilsdorf HGF. In: Kim YW, Griffith WM, editors. High volume fraction P/M aluminium alloys: stability and
strength at elevated temperatures, dispersion strengthened aluminium alloys. Warrendale, Pennsylvania: The Minerals, Metals and Materials Society; 1988. p. 337. [23] Youseffi M, Showaiter N, Martyn MT. Sintering and mechanical properties of prealloyed 6061 Al powder with and without common lubricants and sintering aids. Powder Metall 2006;49(1):86–95. [24] Youseffi M, Showaiter N. P/M processing of elemental and prealloyed 6061 aluminium alloy with and without common lubricants and sintering aids. Powder Metall 2006;49(3):240–52.