ap-4-12.dvi Acta Polytechnica Vol. 52 No. 4/2012 Influence of Various Process Parameters on the Density of Sintered Aluminium Alloys Mateusz Laska1, Jan Kazior1 1 Cracow Univeristy of Technology, ul. Warszawska 24, 31-155 Krakow Correspondence to: mgrmlaska@gmail.com Abstract This paper presents the results of density measurements carried out on Alumix sintered parts. ECKA Alumix aluminium powders were used because of their wide application in the powder metallurgy industry. The compacts were produced using a wide range of compaction pressures for three different chemical compositions. The compacts were then sintered under a pure dry nitrogen atmosphere at three different temperatures. The heating and cooling rates were the same throughout the entire test. The results showed that the green density increases with compaction pressure, but that sintered density is independent of green density (compaction pressure) for each sintering temperature. Keywords: Alumix, sintering, powder metallurgy, density. 1 Introduction PM aluminium alloys have growing potential in the automotive industry, because desired properties of structural components can be achieved at relatively low cost. In recent times, considerable efforts have been made to develop AlZnMgCu sintered alloys. The alloying elements were introduced as elemental powders, or in the form of master alloys, since pre- alloyed powders are incompressible and cause some technological problems during sintering. However, developments in aluminium-silicon alloys remain an area of intense interest. In principle, silicon additions are made to aluminium casting alloys in order to in- crease the fluidity of the molten alloy. The addition of silicon to aluminum alloys in PM technology offers some advantages in the production cycle over cast- ing, in particularly in its ability to produce hyper- eutectic alloys with a relatively fine silicon particle, which can for example provide wear-resistant prod- ucts. With the use of proper sintering parameters, densities of almost 99 % of the theoretical density can be achieved [1–5]. The powders used in this paper were produced by ECKA Granules, under the desig- nation ECKA Alumix. EA231, EA 321 and EA 431D were used. The purpose of this research work was to study the influence of various process parameters on the densification behavior of various aluminium alloy powders. 2 Experimental procedure The powders used in this study were EA231, EA 321 and EA 431D, supplied by ECKA. The characteris- tics of the powders are summarized in Table 1. The powder mixtures were uniaxially pressed in steel dies at 450, 500, 550 and 600 MPa compaction pressure to obtain cylindrical samples 20 mm in diameter and 5 mm in height. The sintering process was carried out at temperatures of 580, 590 and 600 ◦C for 30 min- utes, under pure dry nitrogen. The heating and cool- ing rates were set at a constant level of 5 ◦C/min. The densities of the green compacts were determined from the mass and the dimensions of the compacts, while the densities of the sintered compacts were de- termined using the Archimedes principle. The theo- retical density (TD ) was calculated using the sim- plified additive function, applying the formula: T D = 100 P 1 D1 + P 2 D2 + . . . + P x Dx Where the T D is Theoretical Density, Px is mass percentage of respective elements, Dx is density of the respective ingredients in elementary form. A Netzsch 402C dilatometer was used for deter- mining the dimensional changes during sintering, and 15 × 5 × 5 prismatic specimens were sintered under the same conditions as the cylindrical specimens. Table 1: Chemical compositions and theoretical densities Material Cu (wt. %) Mg (wt. %) Si (wt. %) Zn (wt. %) Wax (wt. %) Al (wt. %) T D (g/cm 3 ) EA231 2.5 0.5 14 – 1.5 Balance 2.68 EA321 0.2 1 0.5 – 1.5 Balance 2.69 EA431 1.5 2.5 – 5.5 1 Balance 2.79 93 Acta Polytechnica Vol. 52 No. 4/2012 3 Results and discussion Figure 1 presents density measurements as a function of compaction pressure. It is evident that green den- sity increases with compaction pressure. However, for the same compaction pressure the green density is different, due to differences in chemical compositions, in particular in the amount of silicon and copper in the powders under study here. Figure 2 presents the relationship between sintered density and com- paction pressure for different sintering temperatures. It was observed that while the green density increases with compaction pressure, the sintered density is in- dependent of the green density (compaction pressure) for each sintering temperature. Additionally, the densification factor for all sin- tered specimens was defined, using the formula: DF = Sd − Gd T d − Gd (1) where DF is densification factor, Sd is sintered den- sity, Gd is green density, and T d is theoretical density. A negative densification coefficient indicates expan- sion, while a positive value represents shrinkage. The relationship between densification factor and com- paction pressure is presented in Figure 3. The fact that both powders containing silicon (EA231 and EA321) have a densification factor significantly lower (samples expanded) than the EA431 powder contain- ing zinc (shrinkage after sintering) leads to the con- clusion that the two different groups of powders be- have in different ways during sintering. Figure 1: Influence of compaction pressure on the green densities of compacts It is interesting to note that, in principle, the trend of the densification factor is lower for higher compaction pressures. This does not apply, however, for the sample of EA431 compacted at 600 MPa and sintered at 590 ◦C, and this phenomenon will be a topic for further study. Figure 4 presents a typi- cal dilatometric curve for EA431. Dilatometry in- dicates that samples undergo significant dimensional changes during different parts of the sintering cycle. The specimens expand rapidly just before isothermal sintering and then shrink during isothermal sintering and cooling. The peak of expansion could be related to penetration of the liquid phase by the interparti- cle capillaries, which forces the sample apart. The shrinkage during sintering is a result of densification mechanisms, and during cooling it is a result of ther- mal contraction. Figure 2: Sintered density to theoretical density ratio as a function of compaction pressure for: (A) EA231, (B) EA321, (C) EA431 The negative values of the densification factor curves of the EA231 and EA321 powders therefore correspond with the observed changes in the geom- etry of the samples and the swelling of the sintered parts. General observations of the surface showed that the faces of samples EA231 and EA321 were rough, unlike the smooth surfaces of the EA431 spec- imens. Further investigations are necessary for a better understanding of all the dimensional changes and the whole densification mechanism as a function of the chemical composition of aluminium alloys. 94 Acta Polytechnica Vol. 52 No. 4/2012 Figure 3: Densification factor as a function of compaction pressure for: (A) EA231, (B) EA321, (C) EA431 4 Conclusion An evaluation of the influence of compaction pressure and sintering temperature on the density of different sintered aluminium alloys shows that these parame- ters do not affect the densification process of the alloy powders under study here. The EA231 and EA321 powders have significantly lower densification rates than EA431. In all cases, the EA231 and EA321 parts had lower density than their green compacts. Figure 4: Dilatometric curve as a function of process time and temperature for EA431, compaction pres- sure 600 MPa This can be partially explained by the swelling of the samples. The EA431 powder showed not only a positive densification factor, but also the smoothest surface of all the investigated specimens. Acknowledgement This research was supported by project Euro- pean Funds Portal — Innovative Economy POIG No. 01.01.02-00-015/09-00 References [1] Martin, J. M., Castro, F.: Liquid phase sinter- ing of P/M aluminium alloys: effect of process- ing conditions. In Journal of Materials Process- ing Technology, 2003, Vol. 143–144, p. 814–821. ISSN 0924-0136. [2] Mondolfo, L.F.: Aluminium alloys: Structure and properties. London : Butterworths, 1976. [3] ASM Specialty Handbook, Aluminium and Alu- minium Alloys, Materials Park, OH, USA, 1993. [4] Greasley, A., Shi, H. Y.: Powd. Metall. 36 (1993) 288. [5] Jatkar, A. D., Sawtell, R. R.: Proceedings of the International Conference on P/M Aerospace Ma- terials, Paper 15, Lausanne, 1991. 95