Multilayer friction stir plug welding: A novel solid-state method to repair cracks and voids in thick aluminum plates

In the present paper, a novel solid-state welding method is introduced and investigated which can be used as a repair method for thick aluminum plates. The process is named "multilayer friction stir plug welding". This process was proposed to overcome issues regarding friction hydro-pillar processing of aluminum alloys including a high oxidation rate and extreme softening at elevated temperatures. In this process, a hole with a special cross-section is drilled to remove the crack or void. Then, the hole is filled using several plugs with the same composition of the parent material. These plugs are welded to the parent material and each other using a set of non-consumable rotary tools. Heat input is the most important parameter which should be controlled during the process. Therefore, tool rotational speed and dwell time were investigated as the main process parameters that dominate the heat input during the process. Tensile tests and optical microscopy were used to investigate the mechanical properties and microstructure of the welds. The fractographic analysis was also carried out to investigate the failure mode. The results indicate the fabrication of sound welds. Higher tool rotational speed and lower dwell time lead to a stronger joint. The highest achieved strength was 124 MPa with rotational speed of 3000 rpm and dwell time of 5 s. The presence of dimples and micro-voids with some evidence of shear failure in fractographic analysis indicates the ductile failure. (C) 2020 CIRP.