Effects of melt temperature on the magnetic treated refinement of eutectic and primary phases in Al-Fe binary alloy melt by measuring thermopower
Introduction
Aluminum alloys have been widely applied in aerospace and automobile industries. Fe presents as one of the most common impurities in aluminum alloys. Because the Al-Fe intermetallics are in the form of needles, flakes and lath shapes, which dissever substrate and deteriorate the mechanical properties [1], [2], [3], [4], [5]. Meanwhile, some researchers found that the refined Al-Fe intermetallics can increase the strength and high temperature resistance of the aluminum alloys [6], [7], [8], [9]. Therefore, it is important to develop some methods to reduce the size and control the morphology and distribution of Al-Fe intermetallics. Magnetic field treatment is an effective way to refine the eutectic and primary phases.
Researchers have found the beneficial influences of magnetic field on the solidified microstructures. The magnetic field can decrease eutectic and dendrite arm spacing and refine cells/dendrites [10], [11], [12], as well as achieve an excellent transition and distribution of phases [13], [14]. The alternating magnetic field parameters impact the primary phases during the solidification process [15]. Furthermore, the efficiency of magnetic field is affected by the conditions of magnetic field (intensity, frequency and gradients of magnetic field) [14], [16], [17], [18], treatment temperature, treatment duration [19], cooling rate [20], and alloy composition [21], [22]. Among the above factors, the treatment temperature range mainly determines the efficiency. Because initial nucleation and following growth stages involve some mechanisms controlled by melt temperature.
Widely used investigations of metal melt are post-solidified examination methods, called the metallographic observation, due to the difficulty of metal melt observation at elevated temperature. In this work, online thermopower measurement provides the real-time and continuous check for the melt, as well as a new insight into the phase evolution treated by magnetic field. Electrical parameters depended on temperature have been theoretically and experimentally investigated by many researchers [23], [24]. Meanwhile, the electrical parameters of liquid metal have gradually drawn much attention to reveal the melt structure variation, even in the external fields [25], [26], [27], [28]. Liu et al [25] have investigated the ultrasonic irradiation refines and homogenizes the short ordered structures in the liquid alloys, based on the resistivity drop. Li et al [26], [28] have separately indicated the decreased Seebeck voltage as the growth speed increased and the refinement of primary dendrite in directional solidification by magnetic field. However, as one of the important electrical parameters, Seebeck voltage is detected during the directional solidification rather than the one under the magnetic field. It is noted that the relationships between electrical parameters and solidified microstructures help us understanding what happened to the melt under the magnetic field.
This work presents both the thermopower and the solidified microstructures of Al-Fe binary alloy melt treated by the alternating magnetic field over different temperature ranges, which derives from the nucleation and growth of eutectic and primary phases. The results reveal the mechanisms of magnetic field on refining eutectic and primary phases during the selected solidification stages.
Section snippets
Alloys preparation and treatment
As-cast billet was hypereutectic Al-3.66 wt% Fe binary alloy prepared by commercial pure aluminum and iron. The liquidus and eutectic temperature of Al-3.66 wt% Fe alloy was calculated to be 1001 K and 928 K suing the JMatePro software. Billet was cut into small blocks (5 × 10 × 100 mm3). Then some small blocks were placed into a corundum crucible and re-melted by resistance furnace to make sure the same composition for each experiment. Alloy was heated up to 1033 K and isothermal held for
Thermopower
Fig. 2(a) gives the thermopower during the direct cooling process. The curves are divided into three stages. Initial thermopower shows stable value during the isothermal holding in the absence of magnetic field (denoted I). Thermopower increases in the time range of 300–1900 s, which corresponds to the cooling ranging from 1033 K to 933 K. Thermopower increases near linearly with the decreasing melt temperature. Thermopower changes from three different contributions, namely electron, phonon and
Conclusion
Effects of alternating magnetic field on the Al-Fe alloy during cooling process are quantitatively investigated by measuring thermopower and observing the solidification microstructures. Refinement efficiencies are discussed by applying the magnetic field over the different temperature ranges, which correspond to the nucleation and growth of eutectic and primary phases.
The results indicate that thermopower increases near linearly with melt temperature during the direct cooling process.
CRediT authorship contribution statement
Qing Lan: Conceptualization, Methodology, Writing - original draft, Writing - review & editing. Qichi Le: Supervision, Funding acquisition, Project administration. Ruizhen Guo: Data curation, Validation. Jianfeng Zhang: Investigation, Methodology, Formal analysis.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Funding: This work is supported by the National Key Research and Development Program of China [2017YFB0305504]; the National Natural Science Foundation of China [51974082].
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