Additive Friction Stir Deposition of Aerospace Al-Zn-Mg-Cu-Zr Alloys: Leveraging Processing and Metallurgical Science for Structural Repair

Additive Friction Stir Deposition is an emerging solid-state additive manufacturing process that leverages severe plastic deformation to deposit fully dense metallic parts. This is of particular interest for high-strength aluminum alloys in which the addition of copper to the alloy chemistry makes them susceptible to hot cracking. This plagues traditional 3D printing of metals which is based on melting and solidification. This work looks at a particular high-strength aluminum alloy AA7050, one of the most widely utilized alloys for complex aerostructures. One of the key traits allowing for its widespread use is its low quench sensitivity, which enables it to be formed into thick sections and still achieve adequate strength. This work studies the feasibility of printing AA7050 and achieving full strength in thin cross sections as well as the influence of the zirconium dispersoid particle on quench sensitivity when applied to thicker sections. It was found that AA7050 after AFSD has significantly more quench sensitivity than traditionally processed material and through STEM, it was determined that this was due to the Al3Zr dispersoid particles providing heterogeneous precipitation sites. It was demonstrated that removing Zr alleviates the quench sensitivity in the case of printing with a featureless tool; however, the breakup of large constituent particles with a protrusion tool increases the number of interfaces for heterogeneous nucleation that induces sensitivity. This work shows that the dynamic recrystallization necessary to deposit material is detrimental to the fundamental performance of the alloy, making it challenging for thick AA7050 to achieve peak strength. A separate study is shown in which AFSD was utilized to successfully repair analogous corroded fastener holes in AA7050 commonly observed in service. After repairing with AFSD, the AA7050 outperformed the baseline material in R=0.1 and R=-1 fatigue, even outperforming pristine material in the R=0.1 case. This was determined to be due to the breakup of Fe-rich constituent particles serving as fatigue crack initiation sites which effectively delays the crack initiation process.