# Effect of Post-Weld Heat Treatment on the Solid-State Diffusion Bonding of 6061 Aluminum Alloy
## 2 Experimental  
The base material used in this study was rolled 6061 aluminum alloy plate with T6 heat treatment. The plate was cut into samples measuring 50 mm \(\times\) 10 mm \(\times\) 3 mm for diffusion bonding. The chemical composition of AA 6061 is provided in Table 1. Prior to bonding, the sample surfaces were ground flat with 1200# to 2000# silicon carbide papers and immersed in an acetone bath in an ultrasonic cleaner to eliminate surface contaminants. The polished specimens were stacked in a lap configuration with an overlap distance of approximately 5 mm. Increasing the temperature increases the diffusion rate. According to the study of [PERSON] et al. [31], the yield strength of AA6061 is less than 10 MPa when the temperature is above 550 \({}^{\circ}\)C. It is necessary to have a high temperature for diffusion and without macroscopic plastic deformation of AA6061 during the diffusion bonding process. Thus, the temperatures used for diffusion bonding are between 500 and 530 \({}^{\circ}\)C. To form the metallurgical bonds, the stacked AA6061 specimens were diffusion bonded under a static pressure of 5 MPa and vacuum of 10\({}^{-2}\) torr in a vacuum hot press furnace (Zhenhua Electric Heating Industry Co., Ltd., Guangdong, China). The specimens were heated at 40 \({}^{\circ}\)C/min to final temperature ranges of 500-530 \({}^{\circ}\)C and held for various time periods for identification of suitable process parameters. A schematic diagram of the dimensions of the bonded specimens is provided in Figure 1. After the bonding process, the lap-bonded samples were subjected to shear loads on the two ends of the specimen, known as a single-lap shear (SLS) test, to measure the joint strength with a tensile testing machine (Shimadzu AG-10 TE.) at room temperature. Three measurements of joint shear strength were performed for each diffusion bonding condition and the average was calculated. In addition, the specimens bonded at 500 and 530 \({}^{\circ}\)C were post-weld heat treated with the T4 and T6 temper grades to enhance the joint strength. Note that the T4 and T6 tempering treatments were based on the dimensions of the sample (AA6061 sample plate) [28], in which the heat treatment parameters were as follows: first, solution heat treatment at 530 \({}^{\circ}\)C for 1 h, then removal from the furnace for water quenching at room temperature (T4), and finally artificial aging at 160 \({}^{\circ}\)C for 18 h (T6). The AA6061/AA6061 joints were cross-sectioned and the interfacial microstructures were observed by a field-emission scanning electron microscope scanning electron microscopy (FE-SEM, JSM-7800F) coupled to an energy-dispersive X-ray spectroscopy (EDX). The composition of the interface was analyzed with EDX. Moreover, the fractography of the fractured surfaces after shear tests was also observed by SEM and the fracture mechanisms were analyzed.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Al** & **Mg** & **Si** & **Cu** & **Cr** & **Mn** \\ \hline Bal. & 1.10 & 0.61 & 0.25 & 0.12 & 0.01 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical composition of AA 6061 aluminum alloy (wt.%).  
Figure 1: Schematic diagram of the geometry and dimensions of the diffusion bonded specimens subjected to SLS tests.