Aluminum-based Material Fabrication by Friction Stir Processing: Microstructural Evolution and Mechanical Properties

Friction stir processing (FSP) is an energy efficient solid-state material processing technique for microstructure modification of commercial high-strength Al alloys. Many variant techniques were developed in recent years that enabled light-weight and high-strength structure fabrication. Identifying relationship among process conditions, microstructures, and mechanical properties is of critical importance to facilitate the practical implementation of these new techniques. The research in the dissertation focusses on developing two main techniques of the FSP: a) friction stir back extrusion (FSBE) of 6063 aluminum alloy for tube making and b) FSP of 7075 aluminum alloy from powder feedstock. FSBE fabricated Al 6063 alloy tubes with refined grains in the inner walls and less orientation asymmetry, which is beneficial to the thin-walled light-wight structure building. The local concentrated precipitation and the grain-size gradient throughout the wall thickness illustrated the non-uniform straining applied by the current FSBE design. Such non-uniformity can be utilized for a selective microstructural modification in the extrusion process. FSP from premixed powder realized complete in-situ Al₃Ni formation in Al 7075-Al₃Ni composite. Nano-scale Ni additive in the start material produced fine-sized Al₃Ni, which generated more effective hardness enhancement than the micron-scale Al₃Ni reinforcement by the Orowan strengthening effect. The Ni addition also introduced precipitation kinetics change in Al 7075 alloy including Cu depletion from Al matrix and promoted T- Al₂Mg₃Zn₃ precipitation. The current FSP set up on powder enabled fabrication of large-scale consolidated composite with homogenous microstructure. The FSP on Al 7075 powder produced fully consolidated, highly recrystallized grain structure comparable to the solid pieces after FSP. The microhardness was mainly governed by the resultant morphology and content of the strengthening intermetallics. However, the excessive Fe-rich intermetallics (Al₇Cu₂Fe and Al₂₃Fe₄Cu) were introduced possibly by the tool’s wearing. The Fe-rich inclusion as well as the other possible residue features in powder triggered further grain refinement and reduction of main strengthening η'/η phases in the post heat treatment. The hardness reduction of the material from powder in the post heat treatment indicated the necessity of modifying material preparation and the post-process for the perfection of this FSP techniques.