Effects of High-Density Pulse Currents on the Solidification Structures of Cu-SiCp / AZ 91 D Composites

1 e Coordinative Innovation Center of Taiyuan Heavy Machinery Equipment, Taiyuan University of Science and Technology, Taiyuan 030024, China Shanxi Provincial Key Laboratory of Metallurgical Equipment Design and Technology, Taiyuan University of Science and Technology, Taiyuan 030024, China School of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, China School of Materials Science and Engineering, North Minzu University, Yinchuan 750000, China


Introduction
Magnesium matrix composites are widely used in aerospace, construction, marine, and mineral processing industries due to their low density, good mechanical properties, and good corrosion resistance [1][2][3].However, the performance of traditional magnesium alloys can no longer meet the needs of social development, and people are committed to the preparation of high-performance magnesium alloys.At present, the commonly used preparation methods of magnesium matrix composites include powder metallurgy, stirring casting, in situ synthesis, and melt infiltration methods [4][5][6].After treatment, the reinforced particles are evenly distributed, and the properties of alloys are improved.However, these preparation methods require strict pretreatment procedures, high manufacturing costs, and complex operation.As a new technology, the treatment of alloy melt by using high-density pulsed current has attracted the attention of researchers, and some research results have been achieved.Especially under the action of high-density pulsed current, it can effectively inhibit the segregation of the second phase and refine the solidification structure.At the same time, this technology has the characteristics of in situ synthesis technology for some composite materials [7][8][9].
SiC particles are often used as reinforcing phase for magnesium matrix composites.However, the surface activity of micron and nano-SiC particles is easy to agglomerate, and the wettability of SiC particles is poor [10].e wettability can generally be improved by increasing the surface energy of the reinforcing phase to lower the surface tension of the melt.Preheat treatment of reinforcing particles, addition of appropriate elements to melt, surface coating, and ultrasonic dispersion can be adopted.erefore, the modification of the SiC surface is beneficial to promote the wettability of interface and the uniform distribution of reinforcing particles in the process of liquid casting.At present, the volume fraction of SiC reinforcing phase added by the full liquid stirring casting method is 5 vol.%∼10 vol.% [11].In this paper, a new type of full liquid treatment device under the condition of electric pulse is used.SiC particles coated with copper film are added into the AZ91D matrix as reinforcing phase to improve the wettability of the matrix and reinforcing phase.
e Cu-SiC p / AZ91D composites prepared under different conditions are studied.e microstructures of AZ91D composites and the strengthening mechanism are discussed, which provides a new design concept and method for future research.

Experimental
Cu-SiC p /AZ91D composites contain 10 vol.%Cu-SiC p , in which a layer of 0.095 μm copper film is deposited on the surface of SiC p .e mass of Cu is 17.3% of that of Cu-SiC p .
e sample size of AZ91D magnesium alloy is 16 mm × 16 mm × 30 mm. e chemical composition of the AZ91D alloy is shown in Table 1 [12].e magnesium alloy was used as the matrix in the experiment, and 10 μm SiC particles were chosen as the reinforcer to prepare Cu-SiC p /AZ91D composites.
Figure 1 shows the schematic diagram of the electric pulse melting device used in this study.e preparation process is described as follows.Firstly, magnesium alloy samples were placed in the boron nitride crucible, and then, SiC p was placed on the surface of magnesium alloys.e pressure in the vacuum box was pumped to 2 × 10 −4 Pa, and then, argon gas was filled to the pressure of 50 kPa.en turning on the high-frequency induction power supply, the samples were heated to 700 °C in the vacuum box, and all the samples were melted in the crucible.en, the metal melt was heated for 10 min.Finally, the electric pulse was applied to the metal melt for 5 min.
e electric pulse treatment process of the metal melt was provided as follows.First, set the pulse width 10 μs, the frequency is 30 Hz, and start the power, then read the required current peak on the oscilloscope by adjusting the voltage, and finally the heating device was closed to start the cooling process in the furnace.When the body was completely solidified, the pulse power supply was turned off.
e structure and composition of the samples were analyzed by SS-550 Shimadzu scanning electron microscope, Phoenix EDAX-2000 energy dispersive spectrometer, and X-ray diffractometer (6000×).

XRD Analysis of Solidification Structures. XRD analysis
was carried out with the prepared magnesium matrix composites (Figure 2).As shown in Figure 2, the Cu-SiC p / AZ91D magnesium matrix composites prepared under conventional conditions mainly consist of three phases: α-Mg, β-Mg 17 Al 12 , and Mg 2 Si. e composites prepared under pulse currents mainly consist of two phases: α-Mg and Mg 2 Si, and the diffraction peaks of Mg 2 Si are enhanced.

SEM Analysis of Solidification Structures
. In order to further analyze the tissue changes, the samples were photographed at a high solution for elemental scanning.e scanning results are listed in Table 2. e XRD spectrum of Figure 2 and Table 2 indicated that the material was composed of α-Mg phase (Spectrogram 1 and Spectrogram 4).
e black skeletal dendrite is α-Mg 17 Al 12 phase (Spectrogram 2).e new phase Mg 2 Si is formed on the dendrite of β-Mg 17 Al 12 (Spectrogram 3).e gray-white structure near Mg 2 Si is composed of α-Mg and Al 4 C 3 as well as less attached Cu (Spectrogram 5).
Figure 3 shows high-resolution SEM photographs of AZ91D magnesium alloy, Cu-SiC p /AZ91D magnesium matrix composite, and Cu-SiC p /AZ91D composite in pulsed electric fields.
e AZ91D magnesium alloy is mainly composed of gray-black α-Mg and dark-black skeletal eutectic β-Mg 17 Al 12 (Figure 3(a)).e phase of β-Mg 17 Al 12 grows along the grain boundary.Figures 3(b) and 3(c) show that the hard strengthening phase Mg 2 Si grows along the β-phase after adding Cu-SiC p , and the Mg 2 Si phase replaces the β-Mg 17 Al 12 phase after applying pulsed electric fields.
is is consistent with the bright white structure shown in Figures 4(b) and 4(c), the formation of Mg 2 Si diffraction peaks in Figure 2, and the disappearance of Mg 17 Al 12 diffraction peaks.e results show that Mg 2 Si precipitates as a heterogeneous nucleation point at the grain boundary.SiC and Mg 17 Al 12 peaks were not observed in the XRD spectra (Figure 2).It was inferred that the formation of Mg 2 Si was ascribed to the reaction of SiC p and Al [6]: SiC p in the copper-SiC p was depleted, and Cu was distributed among the dendrites.Formed Al 4 C 3 was dispersed in the vicinity of Mg 2 Si [2].Al 4 C 3 was mixed with the matrix α-Mg, thus changing the color from gray to graywhite (red circle in Figure 3(c)), which was consistent with Spectrogram 5 in Table 2. e color change in Figure 3(b) was not obvious because SiC p was not mixed evenly, and the content of Al 4 C 3 was too small.As a heterogeneous nucleation point, Al 4 C 3 increased the nucleation rate and refined grains [13,14].
Copper-SiC p used in this study could effectively change the wettability between particles and melt, but segregation of copper-SiC p still occurred, and a large number of copper-SiC p had not yet reacted with Al in the melt.Dendrites in the structure were mainly composed of β-Mg 17 Al 12 and Mg 2 Si phases.Al 4 C 3 had fewer heterogeneous nucleation points and poor homogeneity, and the grain refinement effect was Note: reproduced from Zhang et al. [12], under the Creative Commons Attribution License/public domain.
2 Advances in Materials Science and Engineering not observed (Figure 3(b)).When high-density pulse currents were applied, the Lorentz force was produced by the metal melt under the action of electric elds [15], and the rst cyclotron force of the Lorentz force formed strong convection together with the melt, thus resulting in the decreased temperature gradient, widened two-phase zone in the melt.erefore, the segregation of Cu-SiC p was e ectively restrained, and Cu-SiC p was allowed to join the melt and maintained in the uniformly mixing state.Due to the wetting e ect and the increase in the contact area, Cu-SiC p reacted with Mg 17 Al 12 su ciently, thus resulting in phase replacement of β-Mg 17 Al 12 phase by the vermicular Mg 2 Si (Figure 3(c)).In addition, the formation of a large number of heterogeneous nucleation points Al 4 C 3 increased the nucleation rate, promoted heterogeneous nucleation, obtained uniform structures, and further re ned the grains [16] (Figure 4(c)).
In the magnesium alloy of AZ91D, α-Mg formed the matrix, and the β-Mg 17 Al 12 phase was distributed along the crystal boundary with the large crystal grain.In the Cu-SiC p / AZ91D composite material obtained under conventional conditions, the tissue was mainly composed of three phases: β-Mg 17 Al 12 , Mg 2 Si, and a small amount of Al 4 C 3 phase.Under pulsed electric elds, the composite material showed the uniform structure, and the vermicular Mg 2 Si phase replaced the β-Mg 17 Al 12 phase.In the vicinity of the Mg 2 Si phase, α-Mg was mixed with Al 4 C 3 to form the gray-white zone.When a high-density pulse current is applied, the Lorentz force is generated in the melt under the electric eld [16], and the melt is strongly convected by the Lorentz force, resulting in a decrease in the temperature gradient inside the melt and a widening of the two-phase region.In this way, the nucleation rate of Cu-SiC p /AZ91D increased, and crystal grains were re ned.After adding Cu-SiC p particles, the Al 4 C 3 phase was not detected because the following hydrolysis reaction occurred in the sampling preparation process [6]: (3)

Re nement Mechanism of Solidi cation Structures.
Figure 3 shows the metallographic pictures of AZ91D magnesium alloy, Cu-SiC p /AZ91D magnesium matrix  (marked by the blue ellipse), and the grains near the bright white structure are refined Cu-SiC p that can provide potential nucleation points for the melt, thus increasing the number of fine grains in the composites.e copper-SiC p particles have the pinning effect on grain boundaries and inhibit the grain growth [17].As shown in Figure 4(c), after pulsed electric fields and adding Cu-SiC p , all the gray-white reticulated structures become the finer bright white structures with dense dendrites and fine grains.
According to the theory of electromagnetic field dynamics, under the action of pulse currents, the particles and the whole melt are affected by the changing electromagnetic force.is vibration will produce the following effects on the aggregation of particles in the melt.Firstly, the vibration of the electromagnetic force can break coagulated Cu-SiC p into smaller particles, as shown in Figure 5(a).Secondly, under the action of pulse currents, the SiC particles of different sizes are also affected by inertial force, which results in the relative motion between them.e effect of relative motion also weakens the coagulation effect of Cu-SiC p , as shown in Figure 5(b).Furthermore, under the action of pulsed electric fields, the undercooling of the alloy melt increases, thus leading to the increase in the viscosity of the alloy melt and weakening the coagulation effect of Cu-SiC p particles [10,18].
In order to study the effect of pulse currents on the nucleation rate of alloy melt, the nucleation rate of classical nucleation theory [15] is expressed as follows: where h is the Planck constant; n is the number of atoms per unit volume; K is the Boltzmann constant; T is the absolute temperature; SL is the surface free energy; T m is the melting point; ΔT � (ΔT m − T) is the undercooling of the alloy melt; L m is the latent heat of melting; ΔG A is the liquid atom nucleation barrier.When the axisymmetric current j → � j(r)e z → passes through a cylindrical conductive melt, the magnetic field B → � B(r)e 0 → is formed.Pulse currents usually affect the undercooling of the alloy melt through the generated Joule heat and electromagnetic force.In addition, when pulse currents are applied during the melt solidification process, more solute atoms are stimulated to break the energy barrier and enter the matrix due to the effect of instantaneous discharge.At the same time, pulse currents enhance the vibration of atoms deviating from the equilibrium position, reduce the energy barrier, and change the nucleation barrier.erefore, in the original nuclear rate equation (1), ΔG A � (ΔG 0 + ΔG E ), where G 0 is the thermodynamic barrier for the nucleation without applying an external field and G E is the thermodynamic barrier for the nucleation after applying an external field.en, we get Among them, K 1 is a parameter related to materials; J is the pulsed current density; σ 0 is the conductivity of disordered dielectrics; σ n is the conductivity of nuclei; V is the volume of nuclei; and K is the Boltzmann constant.For the crystalline melt, if σ n > σ 0 , then ξ > 0. erefore, it can be concluded that the pulse currents reduce the nucleation barrier in the alloy melt.e effect of pulse currents increases the nucleation rate in the alloy melt, and the increase in the nucleation rate leads to the grain refinement in the alloy melt.When EPT is applied after heat preservation, the pulse currents contact directly with the melt and the nucleation growth stops, thus forming an equiaxed region, which can effectively improve the nucleation rate of liquid metal and semisolid metal and trigger the heterogeneous nucleation mechanism [19].Fine structures were obtained during the rapid solidification because the increase in the undercooling promoted the nucleation rate.e mechanism of dendrite breakage induced by Loren magnetic force under electric pulse treatment allowed the grain refinement [16,20].

Conclusion
Cu-SiC p /AZ91D composites prepared without high-density pulse currents mainly consisted of three phases: α-Mg, β-Mg 17 Al 12 , and Mg 2 Si.By applying high-density pulse currents, the structures of Cu-SiC p /AZ91D composites were transformed into the phases of α-Mg and Mg 2 Si.
Cu-SiC p /AZ91D composites were prepared by different testing methods.
e results showed that the Cu-SiC p / AZ91D composites under high-density pulse currents had uniform structures, and the grains were significantly refined.
e nucleation barrier was reduced, and the nucleation rate Advances in Materials Science and Engineering was effectively increased by applying high-density pulse currents.erefore, the fine structure was obtained.e microstructures of Cu-SiC p /AZ91D composites were transformed into α-Mg, Al 4 C 3 and Mg 2 Si phases under the action of high-density pulse currents.Al 4 C 3 and Mg 2 Si phases as heterogeneous nucleation points increased the nucleation rate of the composites.e Al 4 C 3 phase was not detected in the obtained tissues due to the hydrolysis reaction.

Figure 5 :
Figure 5: Particle condensation process under the influence of electromagnetic force.

Table 2 :
AZ91D and Cu-SiC p /AZ91D magnesium matrix composite element scanning atomic content table.