# Effect of Post-Weld Heat Treatment on the Solid-State Diffusion Bonding of 6061 Aluminum Alloy
## 3 Results and Discussion  
### Interfacial Microstructure and Mechanical Properties of the Fabricated AA 6061 Joints  
Diffusion bonding is a process that involves atomic interdiffusion at the faying surface, and the weldability is highly dependent on the process temperature and time [22]. In the case of AA 6061 joints, since this process is categorized as bonding between similar metals, the interface is expected to be free of brittle IMCs. The interfacial microstructures of the fabricated AA 6061 joints after diffusion bonding at 500-530 \({}^{\circ}\)C for various bonding times are presented in Figures 2-5, and the relation between bonding temperature and average bonding shear strength of AA 6061 joints for corresponding bonding times is shown in Figure 6.  
As shown in Figure 2, the matrix of the AA 6061 aluminum alloy had light-colored intermetallic precipitations dispersed within it. The IMCs were rich in iron (10.13-11.87 at.%) and silicone (6.84-7.75 at.%) and also contained manganese and chromium, so they were identified as (Fe, Mn, Cr)\({}_{3}\)SiAl\({}_{12}\) phase [32]. Figure 2 shows the joint microstructure corresponding to the bonding temperature range of 500-530 \({}^{\circ}\)C under a constant bonding time of 30 min. The original bonding interface for each sample condition, if clearly visible, is identified by white arrows. For crack-free interfaces, the approximate joint interface is indicated by dotted lines. Under a static bonding pressure of 5 MPa and a holding time of 30 min, the oxide film of aluminum was torn by deformation and fractured in the localized area, which provided direct metal-to-metal contact to allow atomic diffusion. As shown in Figure 2a-c, although some interfacial voids still existed at the interface of the AA6061/AA6061 joints in the samples bonded at 500-520 \({}^{\circ}\)C, no IMC layer formed, and the bonded areas were more than 80%. This implies that the bonding time of 30 min and temperature of 500-520 \({}^{\circ}\)C allowed the second stage of diffusion bonding [22]. Further increasing the bonding temperature to 530 \({}^{\circ}\)C with sufficient heat, as shown in Figure 2d, obviously promoted the atomic interdiffusion, as few interfacial voids or cracks were observed at the interface. The results indicated that elevating the bonding temperature increased the bonded area. In addition, prolonging the bonding time to 60, 120, and 240 min exhibited similar results; given sufficient time, diffusion led to void shrinkage and crack reduction. As shown in Figures 3-5, the joint interfaces were difficult to distinguish because of the similarity in composition of the base metal and the high-quality metallurgical bonding. Figure 2: Microstructures of AA6061/AA6061 bonding interfaces after diffusion bonding at (**a**) 500 \({}^{\circ}\)C, (**b**) 510 \({}^{\circ}\)C, (**c**) 520 \({}^{\circ}\)C, and (**d**) 530 \({}^{\circ}\)C for 30 min. The joint shear strengths measured by the SLS test are shown in Figure 6. Compared with brazing using Al-Si-Cu filler [11] or adhesive bonding [25; 33], diffusion bonding of 6061 aluminum alloy has higher bonding shear strength. The joint strength with diffusion bonding was strongly dependent on two factors. One was the increasing bonding temperature, which promoted the atomic interdiffusion. With a short bonding time of 30 min and bonding temperature of 500 \({}^{\circ}\)C, the average joint strength could only reach around 42.7 MPa, possibly due to the cracks and voids at the interface. Further elevating  
Figure 4: Microstructures of AA6061 / AA6061 bonding interfaces after diffusion bonding at (**a**) 500 \({}^{\circ}\)C, (**b**) 510 \({}^{\circ}\)C, (**c**) 520 \({}^{\circ}\)C, and (**d**) 530 \({}^{\circ}\)C for 120 min. Figure 3: Microstructures of AA6061 / AA6061 bonding interfaces after diffusion bonding at (**a**) 500 \({}^{\circ}\)C, (**b**) 510 \({}^{\circ}\)C, (**c**) 520 \({}^{\circ}\)C, and (**d**) 530 \({}^{\circ}\)C for 60 min.