The microstructural evolution and phase transformations have been investigated during the partial remelting of a bulk alloy prepared by the cold pressing of A356, pure Ti and pure Al powders. A dropping experiment was used to investigate the reaction kinetics of Ti powders and the Al matrix simultaneously. The results show that a semi-solid microstructure with ne and spheroidal primary α-Al particles suspended in the liquid phase could be obtained after the bulk alloy was heated to 595 C for 30 min. The microstructural evolution process was divided into four stages, involving the transformation of the powders into primary particles, the formation of a liquid phase and the increase in its amount, which results in the formation of a continuous liquid layer, the rapid coarsening of the primary particles and the increase in the liquid phase amount, and the nal coarsening of the primary particles. Chemical reactions between the Ti powders and the Al element in the matrix occurred simultaneously. Next, core-shell structured, reinforced particles composing both an intermetallic shell and a soft Ti metal core formed in situ. The compact shell subsequently ruptured and peeled off when its thickness increased to a given value for a given size of Ti powder particles. Finally, the Ti powders were consumed completely because of the formation and the subsequent peeling of the shell. Results of the dropping simulation experiment show that the reaction product layer grows in a linear kinetic manner characterized by an activation energy of 374 kJ/mol.
Abstract:The mechanical behaviors of the thixoforged in situ Mg 2 Si p /AM60B composite at elevated temperatures were evaluated. The results indicated that the thixoforged composite exhibits higher UTS (ultimate tensile strength) than that of the thixoforged AM60B at the cost of elongation. As the testing temperature rises from 25 to 300 • C, the UTS of both these two materials decreases while their elongations increases. The enhanced dislocation motion ability, the softened eutectic β phase at 120 • C, the activated non-basal slipping and the dynamic recovery and recrystallization mechanisms at 150 • C are responsible for the change in tensile properties with testing temperatures. The fracture mode transforms from the ductile into the brittle as the initial strain rate increases from 0.01 to 0.2 s −1 at 200 • C.
The effects of mold temperatures on the microstructures and mechanical properties of thixoforged Grp (graphite particles)/AZ91D composites have been investigated, followed by partial remelting and thixoforging technology. The results indicate that the best semi-solid microstructure could be obtained after being partially remelted at 600 °C and held for 60 min. Correspondingly, under a mold temperature of 300 °C, the best tensile properties were obtained by thixoforging. The UTS (ultimate tensile strength) and elongation of the thixoforged Grp/AZ91D were up to 304.1 MPa and 13.9%, respectively, which increased 11.3% and 43.1% in comparison with the thixoforged AZ91D, respectively. The variation of the tensile properties responded to the influences of mold temperatures on the amount of eutectic phase, the distribution of Grp, and the grain size of α-Mg. Meanwhile, HRTEM (High Resolution Transmission Electron Microscope) showed good bonding between Grp and AZ91D, and many edge dislocations were found in the inverse FFT (Fast Fourier Transform) image. And the result showed that the increase in tensile properties is attributed to the synergistic effect of load transfer, dislocation strengthening, and Orowan looping mechanisms from the Grp strengthening the matrix.
Abstract:The effects of reheating temperature and time on microstructure and tensile properties of thixoformed in situ Si p /ZA27 composites have been investigated. Simultaneously, the effect of the Si particles on the tensile properties has also been studied. The results indicate that the two parameters have similar effects on the composition, fraction and segregation degree of the liquid phase and the size and shape of the Si particles, and thus the solidification behaviour, the resulting microstructure, the fracture regime and the tensile properties. But in contrast, the effects of the reheating temperature are larger than those of the reheating time; the effects of the changes in the Si particle size and shape are smaller than those of the changes in the other factors. The optimum tensile properties, the ultimate tensile strength of 363 MPa and the elongation of 0.97%, can be obtained after being thixoformed under reheating for 70 min at 465°C. The Si particles exhibit two behaviours during tensile testing; namely fragmentation and interfacial de-bonding. This indicates that the load transfer mechanism is the main strengthening mechanism of the Si particles.Keywords: Si p /ZA27 composites; thixoforming; microstructure; tensile properties; fracture.Reference to this paper should be made as follows: Wang, B., Chen, T.J. and Zhang, S.Q. (2017) Biographical notes: B. Wang is a graduate student and studied on the effect of process parameters on microstructure and tensile properties of thixoformed in situ Si p /ZA27 composites at Lanzhou University of Technology (LUT).T.J. Chen is a Professor at LUT. His research focuses on fabrication and forming of nonferrous alloys and their composites, such as traditional casting and semisolid forming of Al, Zn, Mg-based alloys and their composites. His research area also covers corrosion behaviours and surface modification of Mg alloys.Effects of reheating temperature and time on microstructure 127 S.Q. Zhang is a doctoral student and studied on the effect of process parameters on microstructure and tensile properties of thixoformed in situ Mg 2 Si/AM60B composites at LUT.
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