Abstract: Friction pull plug welding is a kind of solid phase joining technique, which has obvious advantages, such as high strength of welding joint, small residual stress and distortion. Friction pull plug welding has a potential application for aerospace field. The influence of axial force on the forming and joint properties of 2A70 aluminum alloy friction pull plug welding is studied. The welding defects, microstructure and fracture morphology are analyzed. The results show that good weld forming can be obtained with axial force in the range of 20-30 kN. When the axial force is 20 kN, the lack of bonding can be observed on the bonding interface. When the axial force is increased to 22 kN or above, the lack of bonding is completely eliminated. And the axial force is increased to 28-30 kN, the tensile strength of the welded joint can reach 376 MPa and the joint coefficient is 83.6%. When the axial force is low (20 kN), the bonding interface has weak bonding defects. The microscopic characteristic is micropores along the interface. The weak bonding defects are important reason for the tensile strength and elongation decline. In the welding joint, the hardness of the heat affected zone on the side of the plug is the lowest, and the analysis show that the morphology and size of the grain don't change significantly, while the θ' phase dissolves and the θ phase coarsens, leading to a decrease of local strength. The fracture morphology shows that the fracture has toughness characteristics under the optimized parameters. The research can provide a good reference for the analysis of the welding process and mechanism of aluminum alloy friction pull plug welding.

Key words: 2A70 aluminum alloy, friction pull plug welding, axial force, microstructure, mechanical properties