To separately investigate the potential effects of shoulder on increasing interfacial bonded area and its mechanism, friction stir lap welding (FSLW) of 1.8 mm thick Al sheets without and with insert (copper foil or Al-12Si powders) was conducted using a special tool without pin, respectively. All the FSLW joints (without insert) fractured within top sheet but not along faying surface, suggesting that the shoulder plays an important role comparable or superior to pin in FSLW of thin sheets. Using several specially designed experimental techniques, the presence of forging and torsion actions of shoulder was demonstrated. The fracture surface of the joints with inserts indicates that interfacial wear occurs, which results in the oxide film disruption and vertically interfacial mixing over the area forged by shoulder with a larger diameter than a general pin, especially at the boundary region of weld. The boundary effect can be induced and enhanced by forging effect and torsion effect.