# Effect of Post-Weld Heat Treatment on the Solid-State Diffusion Bonding of 6061 Aluminum Alloy
## 1 Introduction  
Aluminum alloys have been widely used in engineering structures in the automotive and aerospace industries due to their unique physical properties, such as low density, high specific strength, good formability, and excellent corrosion resistance. To allow the utilization of aluminum alloys in these machinery manufacturing industries, the joining method is a major concern in the provision of complex structures for a wide variety of applications. Traditionally, welding technologies such as laser beam welding (LBW) [1], tungsten inert gas welding (TIGW) [2; 3; 4], and friction stir welding (FSW) [5; 6] have been adopted for a wide range of AI alloy joint structures. However, the application of welding to aluminum alloy is not favored because the process involves melting the base material, which causes residual stress from the shrinkage of solidifying material and the heat affected zone (HAZ), which eventually degrades the joint strength. Brazing is considered as an alternative to welding because of the lower brazing temperature [7; 8; 9]. The higher tolerance of the process parameter window in brazing comes with the advantage of cost-effectiveness for mass production. However, common filler metals, such as BAlSi-4 and Al-12 Si alloys, exhibit high brazing temperatures (above 566 \({}^{\circ}\)C) that are close to or above the melting points of some aluminum alloys [10; 11], leading to the risk of joint degradation. Some low-temperature filler metals and the transient liquid phase (TLP) bonding technique have beenadopted to address this issue [12; 13], and bonding materials such as Cu, Ag, and Mg, which are melting point depressants that form eutectics or intermetallic compounds (IMCs) with Al alloy, have been developed to avoid the risk of melting the base material. For example, silver and copper-based filler metal [14] and Cu or Ag foils as interlayers [12; 13; 15; 16] that form Al-Cu or Al-Ag IMCs at the interface have been utilized to provide sound interfaces. However, research showed that the failure and breakage of the joint usually initiated in the IMC phase and led to deterioration of the joint strength due to embrittlement [17; 14]. Based on the difficulties encountered, solid-state diffusion bonding has become an attractive technique due to the ease and the good quality that can be achieved with similar and dissimilar materials [18; 19; 20]. The major difference between diffusion bonding and other conventional joining methods is that it allows lower bonding temperatures. Two nominal surfaces of the base material are directly bonded under external pressure for specific bonding times. This approach avoids heat effects and the formation of IMCs while maintaining satisfactory bond strength without significant deformation [21; 22]. Diffusion bonding of aluminum alloys is thus greatly influenced by bonding conditions such as temperature, external pressure, suppression, holding time, and surface roughness [22; 23]. [PERSON] [24] performed 1 h diffusion bonding of 2014 aluminum alloy with copper interlayer at 475 \({}^{\circ}\)C, under a pressure of 4 MPa. The best bonding tensile strength was 188.36 MPa. However, a 1.9 \(\upmu\)m thick Al\({}_{2}\)Cu intermetallic compound was formed on the interface. The formation of brittle intermetallic at the interface may cause the toughness of the joint to decrease. The main difficulty in the diffusion bonding of aluminum alloys is their readily oxidized nature. The adherent oxidation layer on the surface continues to be a challenge for diffusion bonding of aluminum alloys because it prevents close contact of the base material and acts as a diffusion barrier, impeding atomic interdiffusion and resulting in poor metallurgical bonds [23].