# 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
the bonding temperature to 510-530 \({}^{\circ}\)C while maintaining the same bonding time of 30 min enhanced the average joint strength to around 59.2-65.7 MPa, indicating that increasing the bonding temperature contributed considerably to the joint strength. Another factor that greatly affected the joint strength was the bonding time. At the bonding temperature of 500 \({}^{\circ}\)C, prolonging the bonding time to 60, 120, and 240 min enhanced the average joint shear strength to 61.5-70.3 MPa. However, at bonding temperatures of 510-530 \({}^{\circ}\)C, increasing the bonding time led to fluctuations in joint strength, in contrast to the increasing trend of bonding at 500 \({}^{\circ}\)C. The slight increase in average bonding shear strength at 510 \({}^{\circ}\)C is attributed to the higher bonding temperature due to the enhanced atomic diffusion. However, it seems that at bonding temperatures of 520-530 \({}^{\circ}\)C, the average joint shear strength could only reach around 60.0-66.7 MPa, despite the increased bonding time. The highest joint shear strengths were 89.15, 83.21, 79.66, and 80.33 MPa at bonding temperatures of 500, 510, 520, and 530 \({}^{\circ}\)C for 240 min, respectively. These results may be explained by considering the elevated bonding temperature, since it was close to the melting point of the base material. During the bonding process, grain growth is rather severe; thus, softening of the base material is likely to occur. The base materials applied in this study were identical, and no interlayer was implemented between the faying surfaces. Thus, mutual diffusion was the dominant mechanism and no IMC phase layer existed. This indicated that the results should be similar to those of the solid solution type of diffusion bonding [22], in which thermally activated mutual diffusion and softening of the base metal have a combined effect on the joint strength. The elevated bonding temperature enhanced the joint strength; however, it also led to softening of the base material and a resultant decrease in joint strength. To sum up, two influencing factors, namely, elevated bonding temperature and prolonged bonding time, both contributed to the fluctuation of the bonding strength of the diffusion bonded AA6061/AA6061 joints, yielding an ultimate bonding strength of 71.2 MPa. This strength is higher than that of NaOH surface-treated adhesive bonding (21.7 MPa) [25] and gas metal arc welded AA6061 lap-bonded joints (53.3 MPa) [34], but lower than that of Ag-28 Cu brazed joints (around 150-160 MPa) [14; 16]. This result indicates that the joint strength obtained by the diffusion bonding method is ideal and can be expected to provide  
Figure 5: 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 240 min. better joint reliability because the interface is free of defects and brittle IMC phase; in fact, most of the joint fractures reported for bonding with TLP and brazing were located in the IMC phase [17]. Furthermore, the surface morphologies of the shear-fractured samples diffusion bonded at 500-530 \({}^{\circ}\)C for 240 min are shown in Figure 7. The fractography of the sample bonded at 500 \({}^{\circ}\)C, shown in Figure 7a, featured a flat fracture surface with a mixture of voids and dimple-like fractures (black arrows) in some areas. The voids and the flat fracture surfaces indicated a fast failure region and thus a weak bond [18], but the dimple-like morphology indicated locations where local asperities yielded and strong metallic bonds were formed in the localized area [23]. Further increasing the bonding temperature revealed an increase in the number of dimpled areas distributed on the fracture surface (Figure 7b-d), and the voids were diminished.