Superior strength of laser-arc hybrid additive manufactured Al-Zn-Mg-Cu alloy enabled by a tunable microstructure

Achieving high strength in the precipitation-strengthened Al-Zn-Mg-Cu alloys produced via additive manufacturing (AM) remains challenging. This work focused on an innovatively developed pulsed laser-arc hybrid process for Al-Zn-Mg-Cu alloy, and further heat treatment for improving the properties was proposed. The as-deposited sample displayed an alternating microstructure featuring coarse columnar grains in the arc zone (AZ) and equiaxed grains in the laser zone (LZ). By simulating the thermal history, it was determined that the formation mechanism of the microstructure was mostly due to the sharp temperature gradient and solidification rate during the pulsed laser duration, as well as the accelerated fluid flow. The stable Zn- and Mg-enriched phases were dispersed within the grains in the LZ, whereas aggregated as the network in the AZ. The two-stage solid solution and aging treatments promoted the generation of numerous disk-like η′ nanoprecipitates and minor needle-like GP-II zone. Moreover, investigations revealed that heat treatment not only contributed to simultaneously increasing the strength and elongation, but also significantly decreasing the anisotropy. This results in a favorable strength (ultimate tensile strength of 602.3 ± 7.6 MPa), which was superior to that of conventional wrought 7075 aluminum alloy. Yield strength modeling analysis demonstrated that the enhanced properties were chiefly attributable to the synergistic effect of Orowan strengthening by the η′ precipitates and dislocation shearing given by the coherent GP-II zone. Our approach of laser-arc hybrid will be applicable to other alloy systems where AM is difficult to perform, and it will have a wide range of applications in the aerospace industry.