Influence of Grain Refinement and Microstructure on Fatigue Behavior for Solid-State Additively Manufactured Al-Zn-Mg-Cu Alloy

A solid-state severe deformation-based additive manufacturing process, additive friction stir-deposition (AFS-D), offers an innovative solution to achieve wrought-like mechanical performance from metals that are susceptible to solidification cracking. In this work, the microstructural evolution and fatigue mechanisms of an Al-Zn-Mg-Cu alloy (AA7075) manufactured via a rapid solid-state deposition process are quantified for the first time. The AFS-D process deposits feedstock via frictional heat and severe plastic deformation, while avoiding the deleterious effects of solid–liquid phase transformation. A fully dense AA7075 deposit was manufactured without the need for additional alloying elements. The microstructural characterization of the as-deposited AA7075 employed optical, scanning electron microscope, and electron backscatter diffraction. The as-deposited AA7075 exhibited a refinement of the constituent particles and grains within the microstructure. Additionally, to quantify the fatigue behavior of the as-deposited AA7075, strain-life experiments were conducted, where a reduction in fatigue resistance was observed compared to the heat-treated feedstock, due to coarsening of strengthening precipitates η′ and η (MgZn2). Post-mortem analysis of the as-deposited AFS-D AA7075 revealed a change in the fatigue nucleation and growth mechanisms compared to the control feedstock. Lastly, a microstructure-sensitive fatigue life model was utilized to elucidate process-structure–property fatigue mechanism relations of the as-deposited and feedstock AA7075 materials.