An in-situ study of the interface microstructure of solid-state additive deposition of AA7075 on AZ31B substrate

Abstract The influence of post processing on the interface microstructure, nano and tensile properties of solid-state deposition of AA7075 aluminum alloy on AZ31B cast Mg alloy is studied in detail. Transmission electron microscopy (TEM) microstructural analysis identified a 200–300 nm layer of Mg17Al12 intermetallic with a columnar grain at the interface of the samples annealed at 200 °C. The in-situ TEM observation during annealing showed that the evolution of grains and growth of the new phases were controlled by the diffusion of voids and the interfacial reaction between Al/Mg. X-ray diffraction (XRD) analysis shows that Mg2Al3 at the Al side and Mg17Al12 at the Mg side were formed at the interface of the sample annealed at and above 300 °C. The thickness of the intermetallic increased significantly with increasing the annealing temperature. The nano-indentation revealed that the intermetallic layers achieved considerably better nano-mechanical properties, including elastic modulus and hardness, than the AA7075 coating and AZ31B substrate. With increasing the annealing temperature from 200 °C to 400 °C, the fracture strength of the coating reached a maximum value of ~130 MPa, then decreased to ~100 MPa. The interfacial microstructure analysis shows debonding during the tensile tests with debonding strength increasing with the annealing temperature. Dimples were identified on the coating fracture surfaces of the sample annealed at 300 °C. It is believed that the formation of intermetallic due to the diffusion enhanced the mechanical properties.