New insight into microstructure and mechanical properties of heterostructures 7075 Al joint prepared via single mode laser welding

Abstract

The application of lightweight and high-strength Al alloys are crucial to reduce energy consumption and emissions in today's world. The lightweight and high strength 7 series aluminum alloy is widely used in aerospace, but its poor welding performance has been a bottleneck restricting the further lightweight of structural parts. In this work, a lightweight and high-strength Al joint with a tensile strength of ∼400 MPa was successfully prepared by single mode laser beam welding using non-weldable 7075 aluminum alloy as the experimental mode. The superfine laser beam reduces the volume of molten pool and increases the solidification rate of molten pool, which significantly inhibits the growth behavior of each microstructure, thus refining the microstructure. The geometrically necessary dislocations (GND) density of each microstructure of 7075 joint after tensile test was: 1.32 × 1014 m−1 for EZ, 1.21 × 1014 m−1 for CLZ, HAZ is 1.17 × 1014 m−1 for HAZ. The synergistic effect of heterogeneous microstructure and work hardening enhanced the tensile strength of 7075 joint. Furthermore, TEM, FIB and EBSD were used to characterize the fracture behavior of the 7075 joint in detail. TEM images showed that the discordant interface between α-Al grain boundary and SiO2 particle was the potential source of crack initiation. The microstructure refinement design and technical selection strategy of the work provide a new insight for laser welding of lightweight and high strength 2xxx/7xxx aluminum alloy.