3.2. Microstructure of Base Material and the Bonding Zone
a shows the microstructure of the as-polished Al/Mg2
Si MMC. The microstructure consists of primary Mg2
Si reinforcement particles, eutectic α-Al/Mg2
Si, and the α-Al phase, as labeled in Figure 2
a. Figure 2
b shows the bonding interface of sample Cu-2. The interface line was not straight across the bonding line probably due to the following reasons: (1) The as-received base material had many porosities due to the casting process. Therefore, if these porosities were located in the bonding interface, the formed liquid at the bonding temperature would flow into the porosities and change the shape of the interface; and (2) using Cu powder as an interlayer, due to the inherent interstitial space between the powder particles, could have the same effect as porosities in the matrix and hinder the liquid to spread homogenously at the bonding interface.
shows the microstructure of the Cu-0.5 sample. It can be observed that three zones exist in the bond region:
Athermally Solidified Zone (ASZ), which solidified by decreasing temperature from bonding temperature to room temperature.
Isothermally Solidified Zone (ISZ), which solidified at the constant temperature (bonding temperature) and segregation of reinforcement particles was obvious in this zone.
Base Material (BM), which did not have an effect on this zone with increasing temperature.
Formation of three different zones in TLP bonding of Al/Mg2
Si MMC could be explained according to the phase diagram of Al-Cu, as shown in Figure 4
. In the first stage, the copper atoms from the interlayer diffuse to the base metal (aluminum) and aluminum atoms diffuse from the base metal into the copper interlayer. By continuing the diffusion of copper to the aluminum, the copper composition of the contact region between the base metal and interlayer might reach more than CαL
composition and these regions could start to be melted and form a liquid in the interface. Then, the copper interlayer and the top layer of the base metal, which were already in contact with the interlayer, could completely dissolve and therefore, the liquid in the bonding interface could reach a chemical composition between CLα
. Gradually, this interlayer became wider (second stage); dissolving the reinforcement particles that have been trapped in the Al matrix.
The concentration of copper atoms in the interface is higher than the top layer of the base metal, which was in contact with the liquid. The surface of the base metal (Al), which was in contact with the liquid, had a chemical composition of CαL and the liquid had a chemical composition of CLα. From this moment onwards, the isothermal solidification could start by nucleation of the solid embryos on the base metal surface (third stage) and these nuclei have a chemical composition equal to CαL. The solid/liquid interface tries to maintain a thermodynamical balance between CLα of the liquid and CαL of the solid, but since the establishment of such a balance needs solid state diffusion, the speed of moving solid/liquid interface is very slow and this step is time-consuming (fourth stage).
As shown in the Cu-0.5 sample micrograph (Figure 3
), the athermally solidified zone had formed, implying that the isothermal solidification stage (third stage) had not been completed. In TLP bonding, if the sufficient time for the isothermal solidification was provided, the melt completely solidifies isothermally and the final structure would be solid-phase with CαL
composition. However, if the bonding time was not sufficient, as was the case in our study, while the melt is at the bonding temperature, it continues to isothermally solidify and when the holding time ends, the bonding temperature drops and the melt solidifies conventionally and forms an athermally solidified zone.
shows the SEM micrograph of sample Cu-2. According to the color contrast in Figure 5
, three different phases can be detected in the bonding line: Darkest phase, dark phase, and bright phase. According to the EDS analyses, the darkest phase, with an irregular shape morphology (pointed out in Figure 5
), had 64 at.% of Mg and 35 at.% of Si, representing Mg2
Si, the reinforcement particles. The dark phase, which is spread on the entire sample, had 98 at.% Al, representing α-Al. Lastly, the bright phase had 64 at.% Al and 34 at.% Cu, representing CuAl2
, which is the most possible phase, according to the phase diagram of Al-Cu [20
].The XRD analyses from the fracture surface of the bonded surface, as shown in Figure 6
, also confirmed the result of the EDS analyses.