RESEARCH PAPER Effects of supporting plate hole and welding force on weld formation and mechanical property of friction plug joints for AA2219-T87 friction stir welds Bo Du 1 & Xinqi Yang 1 & Kaixuan Liu 1 & Zhuanping Sun 1,2 & Dongpo Wang 1 Received: 5 August 2017 /Accepted: 9 March 2019 # International Institute of Welding 2019 Abstract Friction plug welding (FPW) experiments were performed on 8-mm-thick 2219-T87 FSW welds to investigate the weld forma- tion, microstructure, hardness distribution, mechanical property, and fracture behaviors of FPW joint. The main findings are as follows: geometric size of supporting plate hole and welding force exert great effect on weld formation and mechanical property. Recrystallized zone (RZ) with varied width is observed on the bonding interface between the plug and base metal due to huge friction heat and deformation. Softening is found near the bonding interface due to the disappearance of cold working and transformation of constituent particles. The ultimate tensile strength (UTS) and elongation of FPW joint can reach 336.3MPa and 8%, respectively. The initial fracture of tensile specimen is prone to locate at the lower part of RZ. The tensile fracture morphology of FPW joint is characterized by dimples. Keywords Friction plug welding . AA2219-T87 aluminum alloy . FSW weld . Supporting plate hole . Microstructure . Mechanical property 1 Introduction The aluminum alloy 2219-T87 has become the preferred structural material in liquid rocket propellant storage tank structure due to its low density, high strength, and admirable cryogenic property compared with other aluminum alloys [1, 2]. Friction stir welding (FSW) and bobbin tool friction stir welding (BTFSW) have been widely applied in manufactur- ing longitudinal and circumferential welds of storage tank for its great advantages such as energy efficient, environment friendly, and versatile [3, 4]. However, a keyhole defect (it is a through hole for BTFSW) will be left at the termination of weld, which should be perfectly repaired. Friction plug welding (FPW) is a solid-state welding tech- nique exhibiting dominant advantages such as high joint strength, low residual stress, and very small distortion over tungsten inert gas welding (TIG) methods and has become the most promising technique in repairing terminal keyhole or through hole, as well as other defects formed in a TIG or FSW weld. As Hartley revealed, the ultimate tensile strength (UTS) of FPW joint increased by 20% compared with tradi- tional TIG repair method [5]. Since the invention of FPW technique, Lockheed Martin, Marshall Flight Center and TWI initiate the investigation on FPW owing to its dominant advantages. To obtain high- strength FPW welds, the design of plug and plate hole was optimized [6, 7]. In addition, the Btop hat^ and Bheat sink^ were designed on the plug to increase the friction heat and pressure and eliminate weak-bonding defects [8, 9]. Metz and Barkey investigated the microstructure and fatigue behav- ior of 2195 Al-Li FPW joints that contain FSW welds. In their study, a 30–122-μm recrystallized layer and a 35% reduction of hardness were observed at the weld interface. The fatigue behavior decreased compared with FSW joint, which was de- termined by a complex effect of base metal, friction stir weld, Recommended for publication by Commission III - Resistance Welding, Solid State Welding, and Allied Joining Process * Xinqi Yang xqyang@tju.edu.cn 1 Tianjin Key Laboratory of Advanced Joining Technology, School of Materials Science and Engineering, Tianjin University, Tianjin 300350, People’s Republic of China 2 Tianjin Long March Rocket Manufacture Co., Ltd., Tianjin 300451, People’s Republic of China Welding in the World https://doi.org/10.1007/s40194-019-00731-2