Friction stir lap welding of AA2024-T4 with drastically different thickness

Friction stir lap welding is a promising technology to obtain high-strength joints of Al alloys in aviation and aerospace industries. Despite extensive studies performed on thin-wall structures, few works were conducted to join the plates with drastically different thickness, such as the joining of aircraft skins and panels. In this paper, 2.5-mm and 25-mm-thick AA2024-T4 plates were joined with a circumferential notches shape pin. The influence of pin length on joint features and mechanical properties was investigated. The pin improved the material flow behavior and stirred the materials violently, which were beneficial to reducing the interface migration and facilitating the joining of interfaces. The hook defects were eliminated. The failure load firstly increased with the pin length from 2.6 mm to 2.7 mm and then decreased when the pin length increased from 2.7 mm to 3.1 mm. The failure load reached 7.97 kN due to sufficient material intermixing with strong mechanical joining and large bearing area with metallurgical bonding. The selection of pin length should consider the deformation of thick plates, which acted as backing plates with lower elasticity modulus than typical backing plates made of steel.