Origins of Embrittlement of an Al-Zn-Mg-Cu Alloy Post Additive Friction Stir Deposition

Additive Friction Stir Deposition (AFSD) is a solid state, bulk, metal additive manufacturing technology that seeks to replace certain castings and forgings wherever it is economically feasible among other applications. Critical to its deployment is an in depth understanding of how the solid state deposition process effects engineering alloys used in relevant applications. In this work, an aerospace aluminum alloy 7075 is evaluated both in the as deposited and heat treated condition via age hardening studies and tensile testing. It is found that an embrittlement phenomena occurs that is sensitive to processing parameters and quench rate during heat treatment. Through the use of SEM, TEM, and APT the embrittlement phenomena has been linked to excessive grain boundary precipitation caused by a combination of shear induced mixing and shear induced segregation which allow for the formation of phases at grain boundaries that are slow to dissolve, leaving the grain boundary in a non-equilibrium solute rich state. Critical to this process is the role of dispersoid particles, which are modified by shear processes which provide high energy spots for thermally stable precipitate nucleation. Removal of these dispersoid particles by an alloy modification had been shown to eliminate the embrittlement effect after depositing in a condition where embrittlement is expected for the unmodified 7075. Further work demonstrates the different relationships between processing conditions and the degree of embrittlement for three different tool types. Beyond the implications of the particular alloy studied, this work highlights the fundamental concepts involved when a manufacturing process operates at high strain rates and total strains which can be used for the design of alloys meant for AFSD.