# Effect of Post-Weld Heat Treatment on the Solid-State Diffusion Bonding of 6061 Aluminum Alloy
## 3 Results and Discussion
### Effect of PWHT on Diffusion Bonded AA6061 joints  
AA6061 is an age-hardenable alloy, and its mechanical properties can be improved by heat treatment. Conventionally, T6, the most widely applied temper grade, includes solution heat treatment followed by quenching and age hardening to improve the mechanical properties and weldability of AA6061 [28]. PWHT has also been utilized to improve the joint strengths of AA6061 after bonding to compensate for the thermal effects of various welding processes on the weld metal or bonding interface. Several studies have reported that T6-PWHT can also improve the joint properties of the weld metal in the TIGW process [4; 34] and the interfacial structure of ultrasonic additive manufacturing by changing  
Figure 6: Bonding strengths of AA6061/AA6061 joints diffusion bonded at 500–530 \({}^{\circ}\)C for 30–240 min [11; 25; 33]. the metallurgical structure [35]. However, the effect of PWHT on diffusion bonded AA6061 joints has not been reported yet. Thus, to evaluate whether PWHT could increase the joint strength, samples bonded at extreme conditions of 500 and 530 \({}^{\circ}\)C for 240 min were subjected to T4- and T6-PWHT, respectively. The interfacial microstructures after the treatment are shown in Figure 8, and the corresponding bonding strengths are presented in Figure 9. Since the diffusion bonding used temperatures are close to the AA6061 solid solution temperature, it should be difficult to form second phase particle at the bonding interface. T6 heat treatment was carried out after diffusion bonding, and the precipitation strengthening phase formed in the base metal and the interface, which increased the bonding strength. As shown in Figure 8b,d, the precipitates in the matrix of the base metal were very fine, indicating possible enhancement of the tensile strength of the base metal due to precipitation hardening [36]. However, the interface of the T4-tempered AA6061 joints exhibited some cracks at the edge of the joints and into the interior of the joint, as indicated by the black arrows in Figure 8a,c. In contrast, far fewer cracks developed in the samples subjected to T6-PWHT, and the interior of the joint was free of flaws (Figure 8b,d). Note that these cracks formed after T4-PWHT and not the original diffusion bonding process. Thus, the presence of the cracks was associated with the PWHT as this stage involved heating the AA6061 joint to 530 \({}^{\circ}\)C and holding it at that temperature for 1 h before subsequent quenching in room-temperature water (T4). The thermal shock of quenching could have led to sudden shrinkage of the two bonded AA6061 plates, and heavy mechanical loads generated by differential expansion could have acted at the periphery of the joint, which was the stress concentration area. The bonding strengths measured by SLS test shown in Figure 9 indicated that the presence of these cracks led to a significant decrease in joint strength. On the other hand, the T6-PWHT reduced the occurrence of interfacial cracking, providing a noticeable increase (about 40-50%) in joint strength. This increase is highly related to the T6 tempering treatment, for this PWHT  
Figure 7: Fractography of the fracture surfaces of AA6061 joints diffusion bonded for 240 min at temperatures of (**a**) 500 \({}^{\circ}\)C, (**b**) 510 \({}^{\circ}\)C, (**c**) 520 \({}^{\circ}\)C, and (**d**) 530 \({}^{\circ}\)C.  
led to elimination of the interfacial cracks through atomic interdiffusion during the aging process and further enhancement of the base materials due to precipitation hardening. Furthermore, the fractography after SLS tests of the AA6061 joints bonded at 500 and 530 \({}^{\circ}\)C and tempered with T4- and T6-PWHT are shown in Figure 10. The fracture surfaces of the T4-tempered AA6061 joints greatly resembled those of the samples diffusion bonded at 500 \({}^{\circ}\)C for 240 min. The fractography exhibited flat surfaces mainly composed of shear marks (white arrow) with some small dimples (black arrow).