Effects of Cu Distribution on Solidification Microstructure in Transition Layer of Cu/Al Composite Ingot Prepared by Casting Aluminum Method

Copper distribution in the transition layer

Figure 2 shows the line scanning results of Cu and Al in the transition layer of the Cu/Al composite ingot prepared according to the above procedure with 60 s holding time. At the interface of the solid Cu and liquid Al, the Cu content is the highest and the Al content is the lowest, yet the Cu content decreases gradually and the Al content increases gradually away from the interface. Actual measurement results of the Cu content by the energy spectrum analysis signed with “+” in the solidification microstructure are shown in Figure 3. The Cu content is about 60 % (mass%, the following are the same) at the Cu/Al interface, then decreases to 5 % within 300 μm away from the interface, and then decreases to 0 % from 300 μm to 750 μm away from the interface. The Cu concentration gradient near the binding surface of Cu and Al reaches 1 %/μm.

Figure 2:

Line scanning results of Cu and Al in the transition layer.

Figure 3:

Actual measurement results and calculation results of the Cu content.

Solidification microstructure in the transition layer

The solidification microstructure of the transition layer is shown in Figure 4. Three kinds of microstructures are observed in the transition layer: the hypoeutectic microstructure close to the pure Al side, the eutectic microstructure in the middle of the transition layer and the hypereutectic microstructure close to the pure Cu side. A thin layer γ exists between the pure Cu and the transition layer. According to the literature [5], the β phases and γ phases shown in Figure 4 are CuAl₂ and Cu₉Al₄, respectively, so there are four microstructures in the transition layer of the Cu/Al composite ingot in total: α + eutectic (α + CuAl₂), eutectic (α + CuAl₂), CuAl₂ + eutectic (α + CuAl₂) and Cu₉Al₄.

Figure 4:

Metallograph of the solidification microstructure in the transition layer: (a) holding for 60 s; (b) holding for 110 s.

Relationship between the Cu distribution and solidification microstructures

The alloy in the transition layer is Al-Cu alloy. The hypoeutectic component is close to pure Al side and the hypereutectic component is close to pure Cu side. The pouring temperature and time from pouring to the forced cooling affect the Cu distribution. Because the liquidus temperature of the Al-Cu alloy in the transition layer is lower than the melting temperature of pure Al, the outer surface of the Cu plate will naturally cool from pouring to the forced cooling. The cooling rate is relatively low and the temperature gradient in the melt is lower. It can be confirmed from Figure 5 that the pure Al is first solidified during the solidification. The Cu atoms in the transition layer continue to diffuse from the pure Cu side to the transition layer after the pure Al solidifies, which causes the Cu concentration gradient to decrease gradually, as shown in Figure 5(a). At the same time, the eutectic composition of the Al-Cu alloy in the transition layer moves gradually to the pure Al side. The longer the time form pouring to the forced cooling, the closer to the pure Al side the eutectic composition and the thinner the α + eutectic (α + CuAl₂), as shown in Figure 5(c) and 5(d).

Figure 5:

Schematic diagram of the solidification process in the transition layer: (a) Concentration and temperature distribution; (b) Phase diagram of Al-Cu alloy; (c) Solidification process at 60 s after pouring; (d) Solidification process at 110 s after pouring.

The component changes in the transition layer do not affect the microstructure species, but affect the proportions of all kinds of microstructures. The larger the Cu concentration gradient in the transition layer, the larger the l_α/l_z value. It can be concluded that the lower the pouring temperature, the shorter the time from pouring to the forced cooling, the stronger the forced cooling intensity, thus the thinner the total thickness of the transition layer and the larger l_α/l_z value. The proportion of each microstructure to the total thickness in the transition layer is shown in Figure 6. The change of l_α/l_z value is in agreement with the above analysis. At the same pouring temperature, the values of l_(α+β)/l_z and l_β+(α+β)/l_z increase with prolonging time from pouring to the forced cooling, while the l_γ/l_z value barely changes.

Figure 6:

Proportion of the thickness of every kind of structure to the total thickness of transition layer l_x – the thickness of every kind of structure; l_γ/l_z –10 times of the actual value.

Theoretical calculation of Cu distribution

The Cu atoms are dissolved into the molten Al at speed of v_d to form the Al-Cu alloy. The dissolved Cu atoms first accumulate at the interface between the solid Cu and the liquid Al, and then enter the liquid Al by Cu atoms diffusion and liquid Al flow. The distribution state of the Cu atoms is shown in Figure 7. Ignoring the flow, and the dissolved Cu atoms enter the molten Al only by diffusion, the following equation represents this process:

where D_L is the diffusion coefficient of Cu atoms in the molten Al, C is the Cu concentration, x is the distance away from the pure Cu, and t is the time.

Figure 7:

Schematic diagram of the concentration distribution of Cu in the molten aluminum after the Cu dissolution v_d -the dissolution rate of Cu; x- the distance.

In Eq. (1), when t = 0，CCu=0. The Cu concentration CCu∗ at the interface between the solid Cu and the liquid Al cannot be precisely calculated, so it can be only estimated. We investigated the dissolution process of solid Cu in the molten Al in a previous study, and found that the AlCu₂, Cu₅Al, CuAl₂ and Cu₂Al₃ could be formed on the binding surface after the liquid Al contacted the solid Cu, so these compounds constitute the transition layer between solid Cu and liquid Al. These compounds or atomic groups were immediately separated by molten Al to form high-Cu-content liquid. The weight percentage of Cu in this part liquid can be estimated based on the atom percentage of the compounds and their contribution to break apart the solid Cu.

If the Cu₅Al is formed, the CCu∗value will equal to 92.17 %. If the Cu₂Al is formed, the CCu∗value will be 82.49 %. If the Cu₂Al₃ is formed, the CCu∗value will be 61.09 %. If the CuAl₂ is formed, the CCu∗value will be 54.08 %. These compounds all contribute to break apart the solid Cu. Because the dissolution rate of solid Cu in the molten Al is relatively fast, it can be assumed that the Cu₅Al plays a major role in the process, but the proportion responsible for breaking apart the solid Cu is very difficult to determine. The average concentration of the Cu in the compounds or atomic groups is 72.59 %, and the solidification microstructure close to the pure Cu mainly includes CuAl₂, so the CCu∗ value is set to 65 % for calculating.

The boundary conditions in Eq. (1) are as follows：

Results of a previous study [20] show that the dissolution rate of solid Cu has an average value of 3.6 μm/s when the contact time of Cu and Al is 60 s. The diffusion coefficient D_L of Cu in molten Al is 1~3 × 10⁻⁵cm²/s.

The calculation results of the Cu distribution are shown in Figure 8. The dissolved Cu atoms first accumulate at the interface of solid Cu and liquid Al, so the Cu concentration is higher close to the interface between solid Cu and liquid Al. Cu atoms diffusion and liquid Al flow result in the gradual decrease of the Cu concentration with increasing the distance to the interface. The calculated distribution trend of Cu in the molten Al is consistent with the actual measurement results (Figure 2), and the distribution according to qualitative analysis (Figure 5).

Figure 8:

Calculation results of Cu concentration distribution in the transition layer.

The calculation results of the Cu concentration shown in Figure 3 when the diffusion coefficient D_L equals to 1 × 10⁻⁵cm²/s or 3 × 10⁻⁵cm²/s are closer to the actual measurement results when the diffusion coefficient D_L is 1 × 10⁻⁵cm²/s, though the diffusion coefficient D_L is generally 3 × 10⁻⁵cm²/s [18]. It is analyzed that the diffusion direction of the Cu in the transition is upwards, when the method shown in Figure 1 is adopted to prepare the Cu/Al composite, so the diffusion rate cuts down owing to the gravity affect. More accurate calculation results can be achieved if diffusion coefficient of the Cu in the molten Al is appropriate.