**1. Introduction**

Aluminium alloys have been widely used in the aviation, aerospace, shipping, and transportation industries because of their advantages of high specific strength, good fatigue performance, and strong corrosion resistance [1–3]. However, the welding of aluminium alloys presents great challenges. Traditional welding methods are not suitable for aluminium alloys, particularly dissimilar aluminium alloys, since they are prone to generate defects such as porosity and crack during solidification [4,5]. Friction stir welding (FSW)—which is a cost-effective and environmentally friendly solid-state method developed by the Welding Institute in 1991 [6]—is a promising welding method that avoids the above-mentioned issues. According to the microstructures and thermal effect, FSW joints can be typically divided into four zones: nugget zone (NZ), thermo-mechanical affected zone (TMAZ), heat affected zone (HAZ), and base material (BM).

Several researchers have investigated the friction stir welding of aluminium alloys and found that FSW could significantly improve the strength of welded joints compared with traditional welding methods [7–13]. However, due to the large amount of heat generated by friction and plastic deformation, the welded joint shows an obvious thermal softening effect, leading to the strength of the HAZ being lower than the BM [14,15]. Taking this into account, a forced cooling medium was applied during the welding process to reduce the thermal softening and improve the strength of the joint. Zhang et al. [16] pointed out that the tensile strength and corrosion resistance of FSW AA2014 joints were enhanced via water cooling. Sharma et al. [17] studied the effects of air, water, and liquid nitrogen cooling on AA7039 FSW, and suggested that water cooling was more helpful in improving the mechanical properties of the joints. Benavides et al. [18] investigated the mechanical properties of FSW AA2024 joints cooled by liquid nitrogen, and revealed that (1) the grain size in the NZ reduced, (2) the softening area of the joint narrowed, and (3) the hardness of the TMAZ and HAZ increased. Heirani et al. [19] focused on the influence of underwater FSW on the microstructure and mechanical properties of AA5083. They found that the HAZ of the joint disappeared, and the strength and hardness were improved. Other studies also indicated that a forced cooling medium is beneficial for improving the mechanical performances of FSW aluminium alloy joints [20–23].

The 5A06 is a corrosion-resistant non-heat treatable aluminium alloy (Al-Mg alloy), while 6061 is a high-strength heat-treatable aluminium alloy (Al-Mg-Si alloy) [24]. In the present work, a forced air cooling system that is cheap and easy to use was added to the commercial friction stir welding machine. The main purpose was to understand the effect of forced air cooling on the microstructure and mechanical properties of FSW AA5A06-AA6061 joints. The microstructures, tensile properties, hardness distribution, and fracture features of the joints welded in different conditions were systematically investigated and discussed.
