Analysis of the compaction behavior of Al–SiC nanocomposites using linear and non-linear compaction equations

Abstract

The compressibility behavior of Al–SiC nanocomposite powders was examined and the density-pressure data were analyzed by linear and non-linear compaction equations. SiC particles with an average size of 50 nm were mixed with gas-atomized aluminum powder (40 μm average size) at different volume fractions (up to 20 vol%) and compacted in a rigid die at various pressures. In order to highlight the effect of reinforcement particle size, the compressibility of micrometric SiC particles of two sizes (1 and 40 μm) was also examined. Analysis of the compressibility data indicated hindering effect of the hard ceramic particles on the plastic deformability of soft aluminum matrix, particularly at high volume fractions. More pronounced effect on the yield pressure was obtained for the nanometric particles compared with the micrometric ones. Nevertheless, better particles rearrangement was taken place when the ultrafine SiC particles were utilized. In light of the experimental and theoretical analysis, the densification mechanism of aluminum matrix composites and the effect of reinforcement particle size and volume fraction are discussed.

Introduction

SiC reinforced aluminum matrix composites offer several advantages due to their high specific-strength, stiffness, wear resistance and thermal conductivity accompanied with low thermal expansion coefficient [1], [2], [3]. Further improvement in mechanical properties is achieved by using nanometric SiC particles [4], [5], [6]. Nevertheless, nanoparticles are prone to agglomeration and clustering [7], [8], inducing difficulties in processing of metal matrix nanocomposite. Kang and Chan [6] showed that clustering of the nanoparticles degrades the mechanical properties especially when a high volume fraction of the reinforcement is utilized. Powder based techniques become the major processing approach as they offer advantages such as the ability of using high reinforcement volume fractions, homogeneous distribution of the particles throughout the metal matrix, and limited matrix-reinforcement reaction [9], [10], [11]. In spite of these advantages, the compressibility of composite powders is remarkably lower than that of the unreinforced matrix, which often results in insufficient strength to support secondary processing like sintering, machining or extrusion [12]. Hence, it is imperative to improve the compressibility of nanocomposite powders in order to achieve higher density during compaction and to decrease the deformation force required for full densification. So far, many studies have been carried out to investigate the compressibility of composite powders containing micrometric reinforcement particles. For instance, Lange et al. [13] studied the densification behavior of the mixed aluminum and steel powders under cold compaction; Gurson and McCabe [14] examined the yield function for mixed metal powders by using data from triaxial compression tests; Kim et al. [15] proposed a densification model for mixed copper and tungsten powders under cold isostatic pressing and die compaction; Tavakoli et al. [2] studied the compaction behavior of Al–SiC composite powders under monotonic and cyclic loading by using Heckel equation; Kim et al. [16] employed a hyperbolic cap model with the constraint factors proposed by Storåkers et al. [17] to investigate the densification behavior of Al alloy powders mixed with zirconia inclusions; Fogagnolo et al. [18] reported the effect of mechanical alloying on the compressibility of Al6061-AlN composite powder. In addition, there are a number of valuable analytical/numerical models, for example, Martin and Bouvard [19] and Skrinjar and Larsson [20], which enable prediction of the compaction behavior of composite powders developed by discrete-element models. Besides valuable knowledge gathered on the compressibility of composite powders, the densification behavior of nanocomposites has been investigated limitedly. Recently, the authors showed that addition of nanometric Al₂O₃ [21] and SiC [22] particles significantly alter the compaction behavior of aluminum powder. Following the previous work, the aim of the present study is to investigate the compressibility of Al–SiC nanocomposites by employing compaction equations. The analysis of the consolidation behavior by compaction equations is worthy as it gives insight on the densification mechanism. It is well known that the densification mechanism of metal powders under applied load can be considered as two-stage process including particle rearrangement (PR) at low pressures and plastic deformation (PD) at high pressures [23], [24]. These mechanisms may occur simultaneously, particularly at local (micro) scales. To analyze the compressibility data, modified Heckel [25], Panelli-Filho [26], and Ge [27] equations can be used:

$$\text{Heckel}:\mathit{Ln}\left(\frac{1}{1-D}\right)=K_{1}P+B_1$$

$$\text{Panelli-Filho}:\mathit{Ln}\left(\frac{1}{1-D}\right)=K_2\sqrt{P}+B_2$$

$$\text{Ge}:\mathit{Log}\left[\mathit{Ln}\left(\frac{1}{1-D}\right)\right]=K_3\mathit{Log}(P)+B_3$$

where D and P are the relative density (density of the powder compact relative to pore-free density) and applied pressure, respectively. K₁, K₂, and K₃ are termed “densification indexes” and related to the ability of a powder bed to densify by plastic deformation under an applied load. These linear compaction equations offer the advantages of studying the role of plastic deformation (PD) on the densification response of composite powders. In order to study the role of particle rearrangement (PR), the non-linear equation proposed by Cooper and Eaton [28] can be utilized:

$$\frac{{\epsilon }_0-\epsilon }{{\epsilon }_0}=a_R\exp \left(\frac{-k_R}{P}\right)+a_P\exp \left(\frac{-k_P}{P}\right)$$

where ε and ε₀ are the compact porosity and the bulk powder porosity, respectively. a_R and a_P are dimensional coefficients indicating the fraction of the theoretical compaction achieved at infinite pressure by PR and PD processes, and k_R and k_P correspond to the magnitude of the pressure at the start of PR and PD. Analysis of the compressibility data by this equation enables us to evaluate the contribution of each mechanism on the densification. We have examined the relationship between D and P experimentally and then the indexes and coefficients of the above equations were determined analytically by statistical analyses. The role of nanometric SiC particles on the densification of aluminum powder was then evaluated and compared with micrometric ceramic particles.