2A12-T4 aluminum alloy was friction stir spot welded, and the microstructural characteristics and mechanical properties of the joints were investigated. A softened microstructural region existed in the joint, and it consisted of stir zone (SZ), thermal mechanically affected zone (TMAZ), and heat affected zone (HAZ). The minimum hardness was located in TMAZ, and the average hardness value in SZ can be improved by appropriately increasing welding heat input. The area of complete bonding region at the interface increased with increasing welding heat input because more interface metals were mixed. In a certain range of FSSW parameters, the tensile shear failure load of the joint increased with increasing rotation speed, but it decreased with increasing plunge rate or decreasing shoulder plunging depth. Two kinds of failure modes, that is, shear fracture mode and tensile-shear mixed fracture mode, can be observed in the tensile shear tests, and the joint that failed in the tensile-shear mixed fracture mode possessed a high carrying capability.

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In order to study the effect of the frequency and the intensity of high-frequency alternating-current (AC) electric fields on lean combustion, the effects of high-frequency (5–30 kHz; 5 kV) and high-voltage (0–5 kV; 15 kHz) AC electric fields on lean (excess air ratio 1.2/1.4/1.6) premixed CH4/N2/O2 flames are evaluated experimentally in the paper in detail. Results show that the mean flame propagation speed increases with the electric field frequency at first. However, the transition frequency, after which the mean flame propagation speed reaches to the maximum and then tends to be stabilized, is shown to be around 25 kHz. The combustion peak pressure increases minimally at various frequencies, but the timing of it decreases apparently; additionally, the effect of frequency on the combustion pressure change is very close to that on the mean flame propagation speed. The initial duration and main duration also have a similar tendency with the increase of the frequency. Besides, with the increase of the applied voltage, the mean flame propagation speed increases nearly exponentially. Meanwhile, the combustion peak pressure increases, and the timing of it decreases. Similarly, the effect of applied voltage on the combustion pressure change is also close to that on the mean flame propagation speed, and the initial duration and main duration also declines with the increase of the applied voltage. The result shows that the combustion of lean mixtures can be enhanced effectively by high-frequency (5–30 kHz) and high-voltage (1–5 kV) AC electric fields, which has a positive meaning in the further study of the electric-field-assisted combustion theory.

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