Microstructure and Mechanical Properties of Micro-SiCp and Nano-SiCp Reinforced AlSi10Mg Composites Using Spark Plasma Sintering

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Copyright: Zheng, Yamin
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Abstract
AlSi10Mg is a casting alloy commonly used in the automobile and additive manufacturing (AM) industry, such as the vehicle-mounted battery shell. As such its applications in additive manufacturing and casting have been widely studied in recent years. On the other hand, there are limited researches on the fabrication of AlSi10Mg using the powder metallurgy (PM) route. Spark plasma sintering (SPS) is an advanced sintering process for powder, which evaporates the surface of the powder by the high-voltage current to form connecting necks between powder particles. Objectives of this work are to evaluate the combined effect of high energy ball milling, SPS treatment and extrusion on the microstructure and mechanical properties of AlSi10Mg and its composites reinforced with micrometric or nanometric silicon carbide. Our results show that the grain size and the microstructure of the resulting AlSi10Mg samples were much refined as compared to those in casting AlSi10Mg alloy. Because of the lower processing temperature and short duration in SPS as compared to that in casting and liquid-phase sintering, the grain size of the alloy was 1-3μm, while for the casting alloy it was ~500μm. Together with extrusion after the SPS treatment, the morphology of eutectic Si AlSi10Mg samples also changed from eutectic Si to spheroidal Si. As for the alloy reinforced with nanometric SiC, it is worth noting that massive nanometric-SiC aggregated on the interface of Si in nanometric-SiC reinforced samples. The use of SPS for the sintering of AlSi10Mg tended to improve the mechanical properties of the monolithic samples. When compared to the hardness of cast AlSi10Mg (55HV), all as-SPSed and extruded samples processed greater hardness ≥62HV. For samples with 10vol% micrometric-SiC, they have the highest hardness: 99HV for as SPSed samples, 114HV for as-extruded samples, 125HV for as-solutionalised samples, and 127HV for T6 treated samples. An important observation was that the T6 treatment did not bring about significant improvement in the hardness. Thus, the main contributions to hardness came from the SPS, extrusion and reinforcement strengthening. Precipitation hardening did not have a pronounced effect on these already very high hardness samples; hence the hardness of these composites was independent on the aging time. Samples with nanometric SiC reinforcements, on the other hand, were found to have neglectable dispersion strengthening. For extruded samples, after adding 2vol% of nanometric SiC into the AlSi10Mg, the hardness increased from 63HV of the monolithic samples to 83HV of the samples reinforced with nanometric SiC. The very small impact on hardening by the nano-reinforcements was due to the aggregation of nanometric SiC in these samples. Furthermore, most nanometric SiC was found to be along the grain boundaries, further reducing the amount of nano-SiC for dispersion hardening. After long-time high-energy ball milling, the grain size of milled samples (2.0 µm) was not significantly decreased as compared to that of the unmilled samples (2.2 µm). Furthermore, ball-milling for 12 hours introduced more impurities such as Al95FeCr into the samples. Although Ar atmosphere was used during ball-milling, the major precipitation element Mg was inevitably oxidized during the ball-milling process, as shown in the XRD analysis. This caused the depletion of Mg for aging hardening in the milled monolithic samples. Consequently, as compared to the hardness of 79HV of the as-cast T6 AlSi10Mg, the hardness of milled monolithic samples was considerably less (64HV). The composites were much harder and possessed a higher tensile strength as compared to the monolithic ones. However, their ductility was expectedly low. For samples with 10% of micrometric SiC reinforcement, their elongation in the tensile test were only 0.1%. Fractographic studies also show the largely brittle fracture of the composites, while those of the monolithic AlSi10Mg samples exhibited micro void coalescence. In summary, this research provides valuable results on the effects of ball-milling, SPS and extrusion on AlSi10Mg. It shows that a long duration of ball-milling did not provide the desired effect of grain refinement or strengthening and in fact detrimental. to the age hardening of the monolithic samples. As for the SPS/extrusion treatment, it has been shown that these treatments were greatly beneficial to both monolithic AlSi10Mg and its composites.
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Author(s)
Zheng, Yamin
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Chan, Sammy
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Publication Year
2020
Resource Type
Thesis
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Masters Thesis
UNSW Faculty
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