Effect of consolidation parameters and heat treatment on microstructures and mechanical properties of SiCp/2024 Al composites

1 Introduction

The development in SiC particle reinforced aluminum matrix (SiCp/A1) composites over the past 30 years has attracted the attention of material producers and end users because they have been successfully used as components in automotive, aerospace, opto-mechanical assemblies and thermal management [1]. The most significant aspect of SiCp/A1 composites was the increase in modulus compared with that of competitive aluminum alloys. At 30 vol% SiC reinforcement, the designer was able to match the CTE of the electrolysess nickel plating used on the reflective surface, the modulus of this composite was about 70% above that of unreinforced Al or Ti structural alloys, this increase in modulus was achieved with a material with a density one-third less than that of titanium.

Several challenges had to be overcome in order to intensify the engineering usage of SiCp/A1 composites. First, science of primary processing of SiCp/A1 composites needed to be understood more thoroughly. The understanding of the relationships between the processing parameters and microstructures was very important in order to improve the mechanical properties of the composites. The strength increased with increasing reinforcement content only as long as the composite was able to exhibit enough ductility to attain full strength [1, 2]. Particle reinforced aluminum matrix composites (PAMCs) are manufactured either by solid state [powder metallurgy (PM) processing] or liquid state (stir casting, infiltration and in situ) processes techniques at industrial level. Among others, the PM method was the most attractive for several reasons. Firstly, PM offers microstructural control of the phases [3] Secondly, the lower temperatures employed during the process accounted for the strict control of interphase kinetics [4]. As the content reached 30 vol% SiC, increase in strength tended to taper off. Ultimate tensile strengths of the composites were controlled by the type and temper of the matrix alloy, the parameters of preparation process and by reinforcement content. The aim of the aforementioned optimization was to ensure that the strength and stiffness contribution of the ceramic phase in the resultant composite material was significant.

Compared with the performance of the composite material, the studies were relatively few for the hot-pressing processes. Rahimian et al. [5] studied the effect of particle size, sintering temperature and sintering time on the properties of Al–Al₂ O₃ composites made by PM. The study showed that by increasing the sintering temperature, the density of the composite material was increased, in this case by 600°C sintering temperature and longer sintering time, the composite material compression strength decreased because of matrix alloy grain coarsening. Song and He [6] studied the effects of pressure and extrusion on the microstructures and mechanical properties of SiC reinforced pure aluminum composites, the study showed that both extrusion and increasing pressing pressure can substantially improve the strength and plasticity of the composites, The extrusion and increasing the pressure decreased the number of pores and increased the density and interfacial bonding strength. Hong and Chung [7] explored the volume fraction of the reinforcement, vacuum hot pressing temperature, vacuum hot pressing pressure, extrusion temperature, and extrusion ratio on the mechanical properties of SiC whisker reinforced 2124 Al composites. The effects of the sintering temperature and extrusion on the microstructures and mechanical properties of the composite were studied by Sun et al. [8].

Vacuum hot pressing sintering parameters have a greater impact on the performance of SiCp/A1 composites, but the system study of the hot pressing sintering temperature was small and the sintering parameters in the different reports exhibited large differences. There is no system research on hot pressing parameters as to how it affects interface structure and characteristics of microscopic mechanisms. Therefore, it is necessary to study the impact of hot pressing sintering parameters on microstructure and mechanical properties in the hope of getting optimum hot pressing parameters. In this paper, 30% SiC particulate reinforced 2024Al matrix composites (30% SiCp/2024Al) have been made using a traditional PM method. The effects of the vacuum pressing parameters and heat treatment conditions on the mechanical properties and microstructures of the composites have been studied.

2 Materials and methods

2.1 Raw materials

SiC particulate (SiCp) reinforced 2024Al-based MMCs were used in this article. The Al–4.5 Cu–1.2 Mg powders, with an average size of about 10 μm after sieving, were dry mixed thoroughly with 30 vol% of 40 μm sized SiC particles. Figure 1 shows the morphologies of these powders. The shape of the SiC particulate was irregular. No obvious defects or cracks were found.

Figure 1

SEM micrographs showing morphologies of as-received powders: (A) 2024Al, (B) SiC.

3 Results

3.1 Effect of consolidation temperature on microstructures and mechanical properties of SiCp/2024 Al composites

3.1.1 Mechanical properties of composites

The composites were fabricated with different vacuum hot pressing temperature from 520°C to 600°C under a pressure of 75 MPa. Table 1 shows the variation in tensile strength, hardness and relative density of 30%SiCp/2024Al composite in different vacuum hot pressing temperatures.

Table 1

The variation in tensile strength, hardness and relative density of composite in different vacuum hot pressing temperature.

Hot pressing temperature (°C)	Tensile strength (MPa)	Hardness, HB	Relative density (%)
520	143	56.5	98.7
540	155	57.6	99.0
560	176	60.1	99.2
580	198	61.2	99.5
600	179	67.3	99.6

Mechanical testing from Table 1 showed that the mechanical properties of the specimens were sensitive to hot press temperatures. With an increase in hot press temperature from 520°C to 580°C, the tensile strength increased from 143 MPa to 198 MPa, indicating that increasing hot pressing temperature substantially improved the mechanical properties of the composite. The tensile strength increased with increased hot pressing temperature until 580°C at which point the tensile strength reached maximum value. When the hot pressing temperature continued to rise, the tensile strength began to decline. The results indicated that increasing the hot pressing temperature appropriately was the main factor involved in improving the mechanical properties of the composite, but excessive temperature would have had adverse results.

As seen in Table 1 the hardness and relative density increased with increase in the sintering temperature. We know that increasing the sintering temperature can decrease the number of pores and enhance the interfacial bonding strength between the SiC particles and the 2024Al matrix, thus inevitably improving the hardness and density of the composite. With an increase in the sintering temperature, the emergence of the liquid phase was conducive to the filling and binding of the powder particles gap, thereby producing high density. This agrees well with Shin’s reports [9].

3.1.2 Microstructures of the composite

Figure 2 shows SEM (BSE) micrograph of the composites under differing hot pressing temperatures. The gray area in the figure was the Al alloy matrix, the black particles was reinforcing SiC particles. It can be seen that under the hot pressing temperature of 580°C, the SiC particles were distributed uniformly in the Al matrix (Figure 2F). Sintering temperature had no effect on the particle size distribution. With the increase of temperature there did not appear to be a significantly depleted SiC region, this was different from Shin et al. observation [9]. At 520°C and 600°C, part of the SiC particles appear fragmented (Figure 2A, E). If the low hot pressing temperature was too low or too high, it was not conducive to the sample preparation.

Figure 2

Back-scattered electron SEM micrographs of 30%SiCp/2024Al composites made at hot pressing temperatures of (A) 520°C, (B) 540°C, (C) 560°C, (D) 580°C, (E) 600°C.

The XRD patterns and the microstructure images of the materials are shown in Figures 3 and 4, respectively. The XRD results showed that the composite consisted of three main phases, i.e., Al, SiC and Al₂ Cu (Figure 3). Some relatively large sized Al₂ Cu phase appeared as white particles in the microstructure (Figure 4). EDS spot analyses (Figure 4F) confirmed that the bright spots include Al and Cu element. When the hot pressing temperature at 600°C, the amount of Al₂ Cu phase was marginally reduced, as indicated by the SEM image in Figure 4E. Diffraction peaks become unclear with decreasing weight percentage [10]. From Figure 3D, just a very small Al₂ Cu peak was detected in the XRD analysis of composites hot pressed at the 600°C. When the composites hot pressed at 600°C, the liquid increased rapidly, in a vacuum state, the liquid easily exuded from the gap of the steel mold. Probably there was a tendency for Al₂ Cu dissolution with increased hot pressing temperature.

Figure 3

XRD spectra of the composites fabricated at hot pressing temperatures of (A) 540°C, (B) 560°C, (C) 580°C, and (D) 600°C.

Figure 4

Secondary electron SEM micrographs of 30%SiCp/2024Al composites made at hot pressing temperatures of (A) 520°C, (B) 540°C, (C) 560°C, (D) 580°C, (E) 600°C and (F) EDS of white particles from (E) image.

Pores were observed in all the specimens, as shown in Figure 4, the number of the pores in the matrix decreased with the increase in the hot pressing temperature. Increasing hot pressing temperature substantially improved the microstructures and decreased the pores of the composites. From Figure 4A, we know that the pores were clearly visible beside SiC particles, which indicated that interfacial bonding between the SiC particles and the matrix was relatively weak. But in Figure 4D, the pores were scarce in order to reveal the interfacial details of reinforcement and Al matrix, TEM was used to study the interfacial microstructure of the composites. Figure 5 shows the morphologies of the interface between the reinforcements and the Al matrix. As shown in the figure, the interface was very clean, smooth and straight, the interface between SiC and Al matrix was free from any interfacial reaction products. Also, it can be seen that good interfacial bonding exists between the matrix and SiCp, and the SiCp was capable of being well wetted by the matrix alloy.

Figure 5

TEM micrographs of SiC-Al interface in 30%SiCp/2024Al composites fabricated at hot pressing temperatures of 580°C.

3.1.3 Fracture surfaces of the composite after tensile testing

Figure 6 shows the SEM fracture surfaces of the composite at different hot pressing temperature after tensile testing. It can be seen that the fracture surfaces of all the specimens had similar characteristics, including both ductile and brittle fracture features. All the fracture surfaces consisted of numerous dimples in the matrix and fragmentation and decohesion of the SiC particles from the matrix.

Figure 6

The Secondary electron SEM micrographs of fracture surfaces of the composite hot pressed at (A) 520°C, (B) 580°C and (C) 600°C.

The dimples could have been a result of the void nucleation and subsequent coalescence by strong shear deformation and fracture process on the shear plane, whereas the fracture and decohesion of the SiC particles could have been explained by work-hardening and the fragmentation of the ceramic phase caused by high stress concentration. The main difference of the fracture surfaces was that increasing the hot pressing temperature increased the ductile feature.

It can be seen from Figure 6A that no obvious plastic deformation could be observed along the fracture surfaces when the sintering temperature was 520°C. On the other hand, spherical aluminum powders were clearly observed, indicating the poor cohesion of the powders and thus the degraded mechanical properties. This agrees well with the microstructures observation in Figure 4 and the density testing and the hardness testing in Table 1 under the sintering temperature of 520°C. It can also be seen from Figure 6B that a mixed fracture mode including typical ductile dimples of the matrix and a small amount of decohesion of the SiC particles from the matrix can be observed. Many of the small dimples exist along the fracture surfaces generated by the plastic deformation of the matrix. When the hot pressing temperature reached 600°C, the internal cracks increased in the SiC particles affecting the mechanical properties of the material. Some large voids with of similar size to SiC particles could also be observed, generated by decohesion of the SiC particles from the matrix.

3.2 Effect of hot pressing pressure on mechanical properties of SiCp/2024 Al composites

Figure 7 shows the variations in tensile strength and relative density of composites with varying vacuum hot pressing pressure under a hot pressing temperature of 580°C after sintering for 3 h. It can be seen that the tensile strength and relative density of the composite hot pressed at 45 MPa was much lower than those of composites with higher pressures of 75 MPa and 90 MPa. As the liquid phase of the matrix alloy was squeezed into clusters of particles during hot pressing, it was believed that extrusion could have decreased the number of pores and improve the interfacial bonding strength between the SiC particles and matrix. In addition, high pressure were broken up the oxide layer at the surface of the aluminum powders and enhance cohesion between the powders. Consequently, the enhanced densification of the composite increased the tensile strength. The tensile strength increased with increasing vacuum hot pressing pressure from 45 to 75 MPa because of the dominant effect of the enhanced density. However, the high pressure also increased the damage to the SiC particles, and thus cannot effectively improve the strength when the pressure was >75 MPa.

Figure 7

The variations in tensile strength and relative density of composites with varying vacuum hot pressing pressure.

3.3 Effect of hot pressing time on microstructure and mechanical properties of SiCp/2024 Al composites

Figure 8 shows the variations in the effect of hot pressing time on tensile strength and hardness of composites after sintering at 580°C. The tensile strength and hardness increased with the sintering time prolonged from 2 h to 3 h, but increased the sintering time from 3 h to 4 h, lead to a reduction in tensile strength and hardness.

Figure 8

The variations in the effect of hot pressing time on tensile strength and hardness of composites.

The dependence of diffusion over time may be explained by Equation (1) [5].

(1)r=2.4Dt (1)

Where r is radial distance, D is the diffusion coefficient and t is the sintering time.

It can be seen that the atomic displacement was proportional to the square root of time. But when the hot pressing time was too long, this was responsible for the atomic diffusion leading to grain coarsening. Figure 9 shows the effect of sintering time on the microstructure. The grain at the sinter time of 4 h was larger than that at 3 h. This may be explained by the fact that at the sintering temperature of 580°C and sintering time of 4 h grain growth has occurred which according to Hall-Petch theory has led to lower strength and hardness.

Figure 9

The effect of sintering time on the microstructure of composite (A) 3 h (B) 4 h.

3.4 Effect of heat treatment on mechanical properties of SiCp/2024 Al composites

Figure 10 illustrates the tensile properties and elongation of the composite after the solution treatment at a solution temperature of 495°C for 2 h and aged at 190°C for 10 h. It can be seen from Figure 10 that the tensile strength and the elongation of the composite increased initially with the sintering temperature up to 580°C, after which they decrease.

Figure 10

The tensile strength and elongation as a function of the T6 heat treatment for the composite under different sintering temperatures.

Figure 11 shows the SEM micrographs of the composite samples assembled at 580°C in solution treatment conditions. It can be seen that most of the second phase particles disappeared, a few coarse particles could still be observed in the composite (Figure 11A). After the solution treatment, only GP zones could be formed at room temperature, and they were not discernible under SEM. So the remaining coarse particles in the solutionized composites should be undissovled second-phases. These phases mainly contained Al, Cu and Mn elements by the EDS analyses, as shown in Figure 11(B). It was reported that the manganese-containing phases, e.g., mostly Al₂₀ Mn₃ Cu₂, were formed at the interface of the liquid and solid phases [11]. The manganese of the 2024 aluminum matrix forms intermetallic particulates, which were then readily coarsened owing to the high diffusivity of manganese in the semisolid state and the small solubility of manganese in aluminum. These particles were present even after the subsequent heat treatment process, and most of them found at the previous solid liquid boundary, indicating that the Al₂₀ Mn₃ Cu₂ phase was more thermally stable than the Al₂ Cu phase.

Figure 11

The SEM micrographs of the composite samples made at 580°C in solution treatment conditions (A) SEM (BSE) images, showing few precipitates present in the matrix and at interfaces; (B) EDS spectrum of a precipitate shown in Figure 11A.

Figure 12 shows TEM micrographs of the precipitate morphology in the composite. A number of large spherical precipitates were also visible on the grain boundary and the matrix of the composite, as shown in Figure 12. The energy dispersive X-ray analysis (EDAX) in TEM revealed that these large precipitates mainly contained Al and Cu elements, as shown in Figure 13(A). In addition, the index of the selected area electron diffraction pattern was in accord with the crystal structure of tetragonal system Al₂ Cu, as shown in Figure 13(B). From the above mentioned, it can be confirmed that these precipitates were Al₂ Cu.

Figure 12

TEM micrographs of the precipitate morphology in the composite.

Figure 13

Spectrum obtained via EDAX in TEM and SADP of the spherical precipitates at the matrix of the composite (A) EDAX, (B) SADP.

Figure 14 shows the HB hardness curves of the composites aged at different times at 190°C. It can be seen that the hardness increased with increasing sintering temperature no matter what the aging time was. Most importantly, the hardness increased initially with the aging time up to peak value, after which the hardness began to stay constant with the aging time. The higher the sintering temperature, the faster the time taken to achieve hardness peak.

Figure 14

The HB hardness as a function of the aging time for the composite under different sintering temperatures.

It is generally accepted that the precipitation sequence in Al-Cu alloys can be expressed as: G.P. zone →θ″ phase→θ′phase→θ phase when the aging temperature was below 190°C [12]. So the increase of the hardness of the composite at under-aged stage was attributed to the formation of the G.P. zone and θ″ phase and the peak-aged hardness was attributed to the formation of the θ′ phase. The hardness began to stay constant with the formation of the stable θ (Al₂ Cu) phase.

This phenomenon can be explained by the precipitation process of the Al-Cu matrix during aging. During aging, new precipitates nucleate and grew from the supersaturated matrix [13]. The increase in the strength at the under-aged stage were due to the increase in the volume fraction of the precipitates [14]. A high number of volume-fractioned precipitates effectively inhibited the movement of the dislocations, generated more geometrically necessary dislocations and reached the critical dislocation density for fracture earlier during deformation, and thus increased the strength. Once the excess solute atoms were totally exhausted, the growth stage of the precipitates was terminated. At that time, the strength of the composite reaches the maximum. Then the volume fraction of the precipitates remained constant.

4 Discussion

For SiC particles reinforced aluminum metal matrix composites, the SiC particle was the main strengthening factor. During deformation, the external applied stress was be transferred from the Al matrix to SiC particles, and thus the yield strength increased as did the tensile strength [15, 16]. Another strengthening mechanism was when the composites were heat treated, the dislocation density of the Al matrix increased due to the difference in the thermal expansion coefficient between the SiC particles and the Al matrix [17, 18]. The interaction of these increased dislocations could also increase the strength of the composites.

In general, the increase in the strength of the materials by the introduction of the SiC particles was accompanied by the decrease in the plasticity, as SiC particles were also the microcrack initiators during deformation. In a ceramic phase, SiC is brittle and has high strength. During deformation, two types of the microcracks will be initiated by the SiC particles. First, if the interfacial bonding between the SiC particles and matrix was strong, the SiC particles would fracture to nucleate microcracks when the local strain and dislocation density reached critical values by high stress concentration. Second, if the interfacial bonding between the SiC particles and matrix was weak, decohesion between the SiC particles and matrix occurred causing nucleate microcracks before the SiC particles fractured. Thus, a strong interface improved both the strength and plasticity of the composites as weak interfaces will nucleate microcracks at rather low external applied stress.

As seen in Figure 2 specimens made under different hot pressing temperatures showed a similar characteristic in microstructure with the composites, in which the pore was clearly observed and the increasing hot pressing temperature substantially decreased the volume fraction and density of the pores. With the increasing sintering temperature, the liquid content increases. At around 580°C, the liquid volume fraction reached 5%, a continuous network of a liquid phase spread around the SiC particles; the emergence of liquid phase was conducive to the filling and binding of the powder particles gap, which may be an indication of enhanced wetting conditions (Figure 2), thereby producing a high density. We know that decreasing the number of pores and increasing the interfacial bonding strength and density will inevitably increase the strength and plasticity of the composite.

Relevantly, in a recent study Kevorkijan [19] found that the presence of a small amount of liquid was beneficial in achieving full densification in vacuum hot pressing. As the vacuum hot pressing temperature increased, the 2024 Al matrix became more deformable and this resulted in less damage to particulate during vacuum hot pressing. When the hot pressing operation was at 600°C, this temperature exceeded the melting point of Al₂ Cu (590°C), Al₂ Cu phase will decreased. It must be noted that the Al₂ Cu precipitates, which were residues from a liquid phase acted as failure initiation sites. Figures 3D, 4E also show that most of the relatively large precipitates of Al₂ Cu had been taken into solution, leaving only a small amount of reduced size particles in the structure. Meanwhile, matrix extrusion from the mold increased significantly due to an increase in the liquid phase. Thus, the decrease of the elongation of the composite sintered at 600°C may have been due to partial melting of intermetallic compounds Al₂ Cu with low melting point along the grain boundaries during the sintering process.

The increase in tensile strength of 30%SiCp/2024 composite with increasing vacuum hot pressing temperatures up to 580°C is in considered due to be to increases both in the relative density and the interfacial bonding strength and there was more Al₂ Cu phase. When the composites were hot pressed at 600°C, the liquid aluminum extrusion and Al₂ Cu dissolution caused the reduction of phase Al₂ Cu. 580°C was the optimum vacuum hot pressing sinter temperature.

5 Conclusions

This paper studied consolidation parameters and heat treatment on the microstructures and mechanical properties of powder metallurgy made 30%SiCp/2024Al particulate metal matrix composites. Several conclusions have been drawn:

1. Increasing the hot pressing temperature can significantly improve the density and mechanical properties of the composites by accelerating the element diffusion, decreasing the number of pores and improving the interfacial bonding.

2. The composite made at 580°C exhibited the optimum strength. The fracture mechanism was the matrix ductile fracture and the SiC particle fracture. A higher relative density, a stronger interfacial bonding strength and the more Al₂ Cu phase existed in composites were the main reason for the high strength.

3. Many SiC particulates were fractured in the composite consolidated at a low temperature, the presence of a small amount of liquid was beneficial for avoiding SiC particulate damage in vacuum hot pressing, whereas micro voids formed by the interfacial debonding were found in the composite consolidated at a high temperature. Excessive sinter temperature easy to produce micro-cracks, and the liquid aluminum extrusion caused the reduction of Al₂ Cu phase.

4. Solution treatment plus aging of the composites resulted in a significant improvement (25%) in the strength and hardness. The higher the sintering temperature, the faster the time in achieving hardness peak.