Densification of Al powder and Al–Cu matrix composite (reinforced with 15% Saffil short fibres) during axial cold compaction
Graphical Abstract
The densification behavior of Al and Al–Cu powders with and without 15% Saffil short fibres were analyzed using models of Artz, Konopicky, Panelli–Ambrosio and Kawakita. Konopicky resulted the best fitting for the all cases. Composite showed a clear hardening behavior and its yield stress increased compared with unreinforced powder.
Research Highlights
► Curvature in Konopicky plots agree with the hardening behavior during compaction. ► Kawakita model defines clear straight fitting lines for the three materials. ► 15% Saffil fibres raises the flow strength for pure Alumix 13 powder. ► Microhardness Vickers values could fit the flow strengths obtained by Konopicky.
Introduction
Aluminum-based Metal Matrix Composites (MMC) can be produced [1], [2] by powder metallurgy (PM) mixing Al powder with hard (usually brittle) discontinuous phases like ceramics, intermetallic particles [1] or short fibres. The traditional PM involves the cold consolidation by closed-die or by isostatic compaction followed by sintering. For both PM techniques the porosity (1 − D) is eliminated from the initial density D0 [2]. The fraction of hard phase decreases the obtained density due to the reduction of the effective pressure over soft Al particles [2]. The ductile particles need extra plastic deformation to fill the volume gap between the hard reinforcements. Most PM/MMC works are focused on particles composites and only a few are on short fibres composites. The most widely test used to study this process is the axial die compaction due to its simplicity. However, some geometric parameters like pellet height/diameter ratio must be reduced to minimize friction effects. Several parameters are used such as: powder morphology, particles size, oxide content, friction between particles and mechanical properties which are involved in the densification control. Many phenomenological and micromechanical relations have been proposed to describe the plastic powders densification process. The basic developments relate relative density D with pressure P introducing characteristic powder parameters.
Artz [3] developed a micromechanical model for a random array of monosized spheres of radii R isostatically pressed during the initial stage [4] of densification. The spheres deformed suffering of a progressive flattening, thus bringing them closer and rising the density of the packing. The effective pressure between the particles is a continuous function of contact area A = 3 (D − D0), in units of R2, and coordination number Z = Z0 + 9.5 (D − D0), and it is described below, where σYS is the yield stress
Kawakita [5] model expresses the compaction pressure P and the degree of the volume reduction C to define two parameters characteristic of the powder a and bwhere a is the initial porosity equal to (1 − D0), V and V0 are the actual and initial volumes respectively, and b represents the ability to compact porous materials, sometimes called compressibility parameter, and in certain conditions b is related to 1/σYS.
Panelli and Ambrosio Filho [6] have proposed a phenomenological relation of the logarithm of porosity inverse (1/(1 − D)) as the result of square root function of pressurewhere BPan = ln (1/(1 − D0)) and kPAN is related to the compaction capacity of the powder which is inversely proportional to the yield stress.
Earlier Konopicky and Shapiro [7] developed a grossly similar relationship to Panelli's function, but linearly dependent on the pressure.where BMK is a constant depending on D0. It is interesting to remark that Konopicky's empirical model can be obtained from a micromechanical model developed by Torre [8] taking into account a hollow sphere isostatically pressed. The sphere of relative density D is made of fully plastic material. The work showed that k is proportional to 3/(2 σYS). Later Hewitt and his co-workers [9] extended this model to strain hardening materials.
The aim of this work is to analyze the plastic densification during die compaction in the light of previous described models, for the three different powder compositions. Theoretical and experimental limitations and difficulties found in each model are discussed. Finally, measures of microhardness were made to compare and analyze the mechanical properties obtained with those models.
Section snippets
Materials
The raw materials were two different powders i) aluminum Alfa 99 (Aesar, Johnson–Matthey) and ii) Alumix 13 (Eckart–Werke) which is a commercial mixture of aluminum powder with 4.5 wt.% of Cu, 0.5 Mg and 0.2 Si. Alfa 99 powder (probably made by atomization) is integrated by spherical particles of about 18–20 μm mean diameter, while smaller particles (5–10 μm) were observed under scanning electronic microscopy (SEM, Philips 515) attached to the large ones. According to Aesar, the purity of Al was
Results
Load and displacement data of five compactions were transformed into applied pressure P and relative density D. The compressibility curves P vs. D of every test are plotted in Fig. 3, where “Ref CP” data was inserted for a comparative purpose. The latter test was obtained from an axial compaction, at room temperature, of 45 μm particle size of pure Al atomized powder [11]. In all the cases the main densification was obtained during load rate (ΔP/Δt stage) for all cases, until DRP was reached.
Discussion
The densification rate with pressure, according to the theoretical equation developed by Panelli and Ambrosio, can be expressed as:which means that in zero pressure condition (i.e. at the beginning of pressure ramp) the densification rate would tend toward infinite. This implies the limitation of the model during finite values of applied pressure. Beyond this theoretical consideration, two experimental limitations have been found in this work. The first
Conclusions
Pure Alfa 99 and Alumix 13 powders and mixture of Alumix 13 with 15 wt.% of Saffil were compacted at 80 °C in a single action die up to pressures of 250–385 MPa. High relative densities (D > 0.95) were obtained for pure powders while noticeable reduction of relative density was measured for MMC pellet.
Closed porosity defined by flattened particle faces and massive or significant plastic deformation were detected for both unreinforced powders, when external and internal pellet surfaces were observed.
Acknowledgements
CONICET and ANPCyT, both of Argentina, are thanked for providing funds for student grants and projects. The groups of Física de Metales, Caracterización de Materiales and Materiales of Centro Atómico Bariloche are acknowledged for the mechanical assistance, SEM and microhardness measurements, respectively.
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