Spark Plasma Sintering of Nanostructured Aluminum: Influence of Tooling Material on Microstructure
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- Liu, D., Xiong, Y., Li, Y. et al. Metall and Mat Trans A (2013) 44: 1908. doi:10.1007/s11661-012-1533-6
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The influence of tooling material, i.e., graphite and WC-Co, on the microstructure of a spark plasma sintering (SPS) consolidated, nanostructured aluminum alloy is studied in this paper. The results show that tooling selection influences microstructure evolution, independent of process parameters. The influence of tooling on microstructure is rationalized on the basis of the following factors: heating rate, electrical current density, localized heating, and imposed pressure. A theoretic framework, based on the physical properties of graphite and WC-Co, is formulated to explain the observed behavior.
Spark plasma sintering (SPS) is a recently developed powder consolidation technology that is capable of being applied to metals, alloys, ceramics, and composites.[1–3] In SPS, a pulsed high-amperage, direct current is used concurrently with a superimposed uniaxial pressure to consolidate powders. In comparison with well-established consolidation methods such as hot pressing, where the samples are heated externally, the thermal energy during SPS is generated in situ, i.e., by the mold elements, as well as by the powder being sintered in case of conductive powder. This technique possesses the advantage of sintering samples under conditions of high heating rate [up to 1000 K/minutes (1000 °C/minutes)] and high pressures (up to 1000 MPa), which also depend on the mold material and sintering temperature used. Consequently, materials can be consolidated at relatively low temperatures and under shorter time intervals, thereby minimizing changes to the starting microstructures, relative to conditions generally present during conventional consolidation methods.[1,3,4]
There is a wide body of literature showing that SPS process conditions significantly influence the microstructure of the consolidated material and thereby mechanical response.[5–7] Not surprisingly, numerous investigators have sought to optimize process parameters via studies aimed at understanding densification mechanisms. In related work, Zadra et al. found that temperature and pressure work together to enhance the neck growth between the powder particles and the application of the sintering pressure at high temperature is beneficial to the improvement of mechanical property of the consolidated aluminum samples in comparison with that at room temperature. Santanach et al. investigated the influence of SPS consolidation parameters on α-Al2O3 powder (0.14 μm) and proposed that sintering can be divided into two stages: densification without grain growth at lower temperatures and grain coarsening with further densification at higher temperatures. The results of Munir et al. showed that an electrical current can accelerate the solid state reaction between Mo and Si, but the growth rate of the product phases, which are MoSi2 and Mo5Si3, are independent of the current direction and the pulse patterns. Kubota et al. reported on the SPS consolidation behavior of nanostructured pure aluminum powder using an applied pressure of 49 MPa at 873 K (600 °C) for 1 h. Interestingly, following SPS consolidation, the nanostructured material consisted of a mixture of nanostructured grains approximately 300 nm with interdispersed coarse grains 2 to 5 μm in size. In a study that combined numerical analysis with experiments, Grasso et al. reported that the application of a low sintering pressure (i.e., 5 to 20 MPa) led to a high temperature with an accompanying high thermal gradient that was thought to be responsible for the observed non-uniform microstructure when using a constant electrical current to consolidate WC powder by SPS. In contrast, a low temperature and hence low thermal gradient led to the formation of a uniform microstructure in experiments involving a high sintering pressure (i.e., 60 to 80 MPa) for the same current conditions corresponding to low sintering pressures. In recent work, Liu et al. reported on the influence of processing conditions, e.g., the pressure loading mode, starting microstructure (i.e., atomized vs cryomilled powders), sintering pressure, sintering temperature, and powder particle size on the consolidation response and associated mechanical properties of compacts consolidated from nanostructured Al 5083 powder. In another study, Xiong et al. developed a numerical framework using COMSOL Multiphysics modeling and simulation software to provide insight into the mechanisms responsible for densification of the nanostructured Al powder.
An inspection of the above-described experimental and numerical studies, however, shows that in almost all cases, the SPS experiments were conducted using mold-plunger tooling assemblies fabricated out of graphite; graphite is an attractive material given its stability, high-temperature resistance, and machinability characteristics. Graphite tooling assemblies, however, are limited by their compressive strength, which is on the order of 200 MPa at room temperature. Hence, in the case of SPS experiments that require high pressures (e.g., >200 MPa), tungsten carbide (WC) is the preferred material for SPS tooling. WC/Co has a high compressive strength at room and elevated temperatures, and hence may be readily used as SPS tooling for applications that require high pressure and high temperature. For instance, the compressive strength of WC-6 wt pct Co is 5.4 GPa at room temperature. Additionally, the WC-10 wt pct Co shows a compressive strength as high as 2.0 GPa and 1.0 GPa at temperatures of 973 K and 1273 K (700 °C and 1000 °C), respectively. Interestingly, information on the influence of SPS tooling material on the resultant microstructure of consolidated materials is essentially non-existent. Accordingly, the present work was motivated by the following questions. First, what is the influence of tooling materials (e.g., graphite and WC/Co) on the microstructure of nanostructured Al during SPS? Second, can the thermal fields resulting from the different tooling materials be correlated to any observed microstructure changes in the case of nanostructured Al? The selection of nanostructured Al for the present study was motivated by recent interest in consolidation of bulk nanostructured metals from powders.[19–22]
The nanostructured powder used in the present study was prepared as follows. First, inert gas atomized Al 5083 (Al-4.5Mg-0.57Mn-0.25Fe in wt pct) powder was milled in a slurry of liquid nitrogen (LN) for 8 h in a modified Svegvari attritor using stainless steel balls. The ball-to-powder ratio was 32:1 and 0.2 pct stearic acid (CH3(CH2)16COOH) was added prior to milling to prevent agglomeration during milling. At the completion of the milling experiments, the powder/LN slurry was collected in a stainless container and then transferred to a nitrogen glove box to minimize atmospheric contamination. Detailed information on cryomilling can be found in the literature.[23,24] In the present study, the size of the cryomilled powder particles used for the SPS experiments was in the range of 10 to 25 μm, as established by sieving. Moreover, sieving was performed under a controlled atmosphere (i.e., inside a glove box) to minimize atmospheric contamination.
The microstructure of the cryomilled Al 5083 powder particles consists of two types of grains, i.e., equiaxed grains (> 90 vol. pct) and elongated grains (<10 vol pct), please refer to. In Reference 14 and the present study, an elongated grain is defined as a grain with an aspect ratio that exceeds 2. Because the content of the elongated grains is relatively small, they are not included in the grain size distribution histogram. Figures 3(a) and (b) show the microstructure and grain size distribution in the cryomilled Al 5083 powder, respectively. From Figure 3(b), it can be seen that most of the grains fall in the nanoscale regime (i.e., 10 to 100 nm), whereas some with a diameter of 100 to 130 nm are occasionally observed.
Processing Conditions and Density of the SPS-Consolidated Samples
673 K (400 °C) × 3 min @ 100 MPa
WC-5wt pct Co
Four factors are proposed to contribute to the observed differences in microstructure of the materials studied herein, i.e., the heating rate of the sample, the current density passing through the sample, the localized heating, and the localized pressure at the contacts between the particles. These are discussed in detail below.
4.1 Comparison of the Heating Rate of the Samples
It should be noted that this value represents an upper bound of the heating rate ratio in the case of HRR > 1, for the following reasons. First, there is heat transfer between layers II/I, layers II/III and sample/mold interfaces. In fact, thermal diffusion between the interfaces accelerates the heating of the mold and decreases the heating rate of the sample. Second, the contact resistance at the sample/plunger interface will likely be higher than the ideal contact conditions assumed in the present model, causing a reduction in the flow of electrical current and heating rate of the sample. Despite these simplifications, the proposed model provides a useful framework to analyze and compare the thermal characteristics of the mold and the sample during SPS.
The density of the compact changes proportional to the displacement of the SPS ram head during SPS.
The electrical resistivity of Al 5083 at 673 K (400 °C) is threefold that at RT (in reference to aluminum) and the electrical resistivity varies proportional to the temperature increment. The other parameters, i.e., the specific heat, density and electrical resistivity of graphite and WC-Co, specific heat, density of Al 5083, used in this paper are their room temperature values (see Appendix, Table AI).
The relative density of the compact was 64 pct prior to sintering and 99 pct after the pressure was loaded, considering that the relative density of the random packing powder was 64 pct and the sample had a relative density of 99.1 pct after SPS consolidation.
4.2 Comparison of the Amperage Passing Through the Samples
In order to investigate the effect of the temperature on the electrical current distribution between the sample and mold, the Is/I was calculated under the assumption that the consolidation occurs at room temperature and the change in sample density is the same as that associated with RT—673 K (400 °C) sintering, as illustrated in Figure 7(a). The results are shown in Figure 7(b). From Figures 7(a) and (b), it can be seen that the dependence of the electrical resistivity of Al alloy on temperature shows a marked influence on the electrical current distribution in the case of loose powder and a relatively small influence in the case of a dense compact when using GMPTA. For instance, at time = 116 seconds [temperature = 613 K (340 °C) for RT—673 K (400 °C) sintering and prior to the application of pressure], the Is/I for the RT—673 K (400 °C) sintering (Figure 7(a)) is 58 pct, whereas the sample accounts for 79 pct for RT consolidation (Figure 7(b)). When time = 200 seconds [the temperature is 673 K (400 °C) for RT—673 K (400 °C) sintering and after the application of the targeted pressure], the percentage of the electrical current shared by the sample is 93 pct for the RT—673 K (400 °C) sintering and 98 pct for room temperature consolidation. However, when using WCMPTA, the temperature rise results in a significant reduction of the value of Is/I (Figure 7(a)) relative to room temperature consolidation (Figure 7(b)). For instance, when time = 112 seconds [the temperature is 613 K (340 °C) for RT—673 K (400 °C) sintering and prior to the application of pressure], the Is/I is 2.1 pct for the RT—673 K (400 °C) sintering (Figure 7(a)) and 5.5 pct for RT consolidation (Figure 7(b)). At time = 200 seconds [the temperature is as high as 673 K (400 °C) for RT—673 K (400 °C) sintering and after the application of the pressure], the sample accounts for 19 pct of the total electrical current for RT—673 K (400 °C) sintering and 41 pct for RT consolidation.
4.3 Localized Heating
4.4 Localized Pressure
Al 5083 nanostructured powders were consolidated via SPS using graphite and WC-Co tooling. The resultant differences in microstructure were discussed using a theoretic framework based on the physical properties of graphite and WC-Co and revealed two important findings. First, the powders experienced different heating rates and thus different thermal profiles under two circumstances, although the molds experienced almost identical thermal profiles during SPS. Second, differences in sample electrical current density, local temperature coupled with local pressure influenced microstructural evolution during SPS.
This paper is based upon work supported by the US Army TACOM-ARDEC under contract No. W05QKN-09-C-118 and the Office of Naval Research with grant No. N00014-07-1-0745. Part of D. Liu’s work is also supported by the Young Scientist Foundation of Shandong Province, China (No. BS2009CL043), and the innovation foundation of Shandong University (2012TS032).