The effect of individual and combined zirconium and manganese additions have been compared for an AA2198 6 mm thick sheet in T351 temper regarding their influence primarily on recrystallisation resistance and secondly on fracture toughness and overageing resistance. A complete characterisation of the dispersoid distributions was carried out for a deeper understanding of the effects of the Al3Zr and Al20Cu2Mn3 particles, involving studying their formation from the as-cast and homogenised stage.The most important finding in this work was the lower recrystallisation resistance in the alloy containing 0.1 wt%Zr + 0.3 wt%Mn compared to that containing only 0.1 wt%Zr. This result was rather unexpected, if one considers the opposite microsegregation patterns of Zr and Mn during casting, which leads to dispersoids occupying the majority of the grains’ volume and minimising dispersoid-free zones that could be potential sites for nucleation of recrystallisation. The other two alloys with dispersoid additions 0.05 wt%Zr + 0.3 wt%Mn and 0.4 wt%Mn, were partially and fully recrystallised respectively in the rolled T351 condition.Equally important in this work, was the observation that the opposite microsegregation trend of Zr and Mn sufficed to restrict grain growth in unrecrystallised areas. The 0.1Zr-0.3Mn alloy exhibited the lowest grain size of all alloys, both in the T351 temper and after annealing at 535oC for up to 144 hours. The reason for this was the combined action of Al20Cu2Mn3 dispersoids and Mn solute in the regions where the Zr concentration was low (i.e. near the grain boundaries), which offered additional pinning pressure to those areas compared to the 0.1Zr alloy.The lower recrystallisation resistance of the 0.1Zr-0.3Mn alloy was explained on the grounds of two main factors. The first was the lower subgrain size and hence stored energy within bands of Al20Cu2Mn3 dispersoids, which increased the driving force for recrystallisation in these regions. The second was the interaction between Zr and Mn that led to a decrease in the Al3Zr number density and pinning pressure. Since Zr was the dominant dispersoid family in terms of inhibiting recrystallisation, inevitably this alloy became more prone to recrystallisation. The Al3Zr pinning pressure was found to be much lower especially within bands of Al20Cu2Mn3 dispersoids. The detrimental effect of the Mn addition on the Al3Zr distribution was proven not to result from the dissolution of Zr within Mn-containing phases, and several other phases, at the grain interior and also in grain boundaries. The observed effect could not be precisely explained at this stage.Concerning mechanical properties, the 0.1Zr alloy exhibited the best combination of properties in the Kahn tear tests for fracture toughness. Further, it had a higher overageing resistance compared to the 0.1Zr-0.3Mn alloy.As an overall conclusion from this work, considering all the studied properties here that are essential for damage tolerant applications, the addition of 0.1 wt%Zr to the AA2198 6 mm thick sheet was found to be superior to that of the combined addition of 0.1 wt%Zr + 0.3 wt%Mn.

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. / Doctor of Philosophy / Additive Friction Stir Deposition (AFSD) is an emerging additive manufacturing technique that utilizes severe plastic deformation instead of melting to 3D print metals. This work focuses on one of the most prominent aluminum alloys used in aerostructures (AA7050) and its performance after printing. It was found that printing AA7050 in thick sections has further challenges and that modifying the alloy chemistry can alleviate losses in strength. The understanding of AA7050 and AFSD was utilized for a specific application, the repair of corroded fastener holes on the coupon level. It was found that repairing the simulated corroded hole improved the fatigue performance of the coupon indicating a successful means for repairing components.

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. / Doctor of Philosophy / Additive Friction Stir Deposition (AFSD) is a new 3D printing process for metals where deformation is used to deposit material in an additive fashion. This work involves understanding and solving an embrittlement issue that occurs during heat treating after deposition for a particular aluminum alloy (7075). In this work, the origins of the embrittlement phenomena are uncovered which have to do with the degree and severity of deformation. Several solutions including alloy development and process control are successfully demonstrated.