Figure 2 shows schematic diagram of dissolution of Al in Sn melt. Table 3 shows the diameter data of Al in Sn melt with different temperature. According to the phase diagram of Al-Sn, it can be seen that no intermetallic compound is formed between Al-Sn, which means that there is no reaction diffusion at the Al/Sn solid-liquid interface during the dissolution process, but only interdiffusion behavior (McAlister and Kahan, 1983).

The dissolution flux of Al wire in Sn melt can be expressed in J, J = − m A = − ρ A l d V d t / A (1) m is the dissolving mass rate of Al wire, A the diffusion interface area, ρ _Al the density of Al wire, V the volume of Al and t the time. The volume change dV of aluminum can be expressed as: d V = 2 π r l d r (2) r is the radius of aluminum and l is the calculated length of aluminum. The dissolution flux of Al wire in Sn melt can be expressed as follows: J = − ρ A l d r / d t = − ρ A l v (3)

Combined with the data in Table 1, the dissolution flux of Al calculated at 450, 500, and 550 °C are 0.324, 0.504, and 0.905 (g/m²·s), respectively. The dissolution behavior of Al in Sn melt is based on the thermal activation, so the dissolution flux can be described by Arrhenius equation: J = J 0 ⁡ exp − Q / R T (4)

J ₀ is dissolution-diffusion flux constant. Take the logarithm of both sides of Eq 4, we have I n J = I n J 0 + − Q / R • 1 T (5)

Using the dissolution flux data under different Sn melt temperature conditions, the InJ-1/T curve is ploted, as shown in Figure 3. After linear fitting, it can be seen that l n J = 0.345 − 6082 / T and the Q = 50.5657 kJ/mol

According to Eqs 3, 4, the relationship between the dissolution rate of Al in Sn melt and the temperature can be obtained as: v = J 0 ρ A l exp − Q R T (6)

By substituting the density of Al and the calculated J ₀ and Q values, the dissolution rate of Al in Sn melt can be expressed as: v = 0.000523 ⁡ exp − 6082 T (7)

Figure 4 shows the solid-liquid interface of the Al-Sn alloy after holding at 500 °C for different times. From the above test results, it can be seen that the Al wire gradually dissolves in the Sn liquid as the holding time increased. The Sn atoms dissolved in the Al surface layer under the action of interdiffusion. Since the melting point of embedded particles is strongly dependent on the nature of the particle/matrix interface (Allen et al., 1980), the melting will occurs when its melting point lower than the ambient temperature (Sn melt temperature). The liquid Sn/solid Al interface gradually moved to the side of the Al wire, and the diameter of the Al gradually decreased, which means that the Al wire gradually dissolved in the Sn melt. Once the Al wire starts to dissolve, the alloying reaction becomes self-propagating, as it is exothermic in nature, eliminating the need for additional heat supply (Baras and Politano, 2018). Figure 4B shows that the Sn liquid infiltrates along the surface defects of the Al wire to form a dissolution channel during this process. If the dissolution channel continues to extend and multiple dissolution channels cross each other, it will cause small Al particles in the surface of the Al wire to fall off.

Figure 5 shows the SEM and EDS images of Al and Sn at the solid-liquid interface at 500°C for 30 min. Interestingly, the Al/Sn interface is not a planar structure, but a dendritic structure with a certain orientation. From Figure 4B; Figure 5, small pieces of Al peel off from the solid-liquid interface during the dissolution process of the Al-Sn held at 500 °C for 30 min. After exfoliation, more small aluminum particles quickly moved to the surface of Sn melt and formed a large number of unmelted Al particles due to their density was much lower than that of Sn melt. So only partially exfoliated small pieces of Al were observed in the images. Since the Sn melt on the surface is direct contact with the air, the falling Al partiles were easily oxidized and aggregated to form unmelted Al particle clusters, resulting in insufficient Al-Sn alloying.

Concerning the microstructure and morphology of the diffusion and dissolution behavior of Al/Sn, the dissolution mechanism of solid Al in Sn melt can be inferred as shown in Figure 6. During the initial stage of dissolution, the concentration gradient causes the Sn melt to selectively dissolve the Al present at grain boundaries or other defects, resulting in the formation of small pits on the surface of the Al. At this time, the diffusion behavior of Sn to Al dominates the Al/Sn interdiffusion. Then the Sn melt penetrates these small pits and continuously promotes the dissolution of Al to the interior of the solid Al. Due to the randomness of Al grain boundaries and internal defects, these dissolution channels gradually bend to form the aforementioned dendritic structure. A large number of unmelted Al are released after the dissolution channels are connected. Since the density of the Al particle is much lower than that of the Sn melt, the Al particles will float to the upper surface of the Sn melt to aggregate under the action of buoyancy, resulting in insufficient alloying during the preparation of the Al-Sn alloy.

In order to solve the problem of insufficient alloying, the dissolution behavior of Al in Sn melt under the action of electromagnetic fields was studied. The applied traveling wave magnetic field excitation current parameters are 100A, 20 Hz. Figure 7 shows the SEM micrographs of Al/Sn interface with traveling wave magnetic field at 500°C. Compared with the Al/Sn interface without electromagnetic stirring (Figure 5), the interface under electromagnetic stirring is relatively flat, and the dendritic structure grooves basically disappear. At the same time and temperature, the Al hardly dissolves in Sn melt in Figure 4, while the dissolution of Al in Sn melt has been basically completed in Figure 7D, which indicates that electromagnetic stirring can promote the dissolution of Al in Sn melt. The electromagnetic force induces vigorous convection, which enhances solute exchange at the interface, narrows the diffusion zone, and inhibits the formation of bulk structure. Under the action of the traveling wave magnetic field, the Sn melt forms a symmetrical eddy current. The movement of the flow field formed by the traveling wave magnetic field can accelerate the heat transfer in the melt, thereby weakening the temperature gradient. The model diagram of Al/Sn dissolution under the action of electromagnetic field is shown in Figure 8.

The Al wire is easily oxidized and a dense Al₂O₃ oxide film is formed on the surface (Zhao and Li, 2019). The oxide film hinders the diffusion of Al atoms and reduces the Al dissolution rate in the absence of a magnetic field. The implementation of an electromagnetic field can create a continuous scouring and impacting effect on the surface of the Al wire, which can cause the oxide film to break, crack or fall off. The fresh surface of the Al wire is more likely to inter-diffuse with Sn, which further accelerates the dissolution of the Al wire.

The concentration gradient of solute elements at the solid-liquid interface of the melt plays a significant role as a driving force for the dissolution process. In the absence of an external electromagnetic field, the inter-diffusion zone on the surface of the Al tends to be wide, and the concentration gradient is relatively small. As a result, the dissolution and diffusion rate is low because the small diffusion driving force limits the rate at which solute elements can dissolve and diffuse. When an external electromagnetic field is applied, the dissolved Al element at the solid-liquid interface is rapidly removed through forced convection. This leads to a reduction in the width of the inter-diffusion zone and an increase in the concentration gradient. Hence, as the diffusion driving force increases, the dissolution and diffusion rate also increase.

Through the above analysis, it can also be found that the high-speed flow of Sn melt is affected by the external electromagnetic field, which benefits the homogenization of the alloy melt. The uniform alloy structure can be obtained by the subsequent rapid solidification process. That is to say, the external electromagnetic field can improve sufficient alloying in the Al-Sn alloy.