Towards elucidating the process-structure-performance relationships of lightweight structural alloys
Additive manufacturing processes have become a leading technology for research innovation. Additive manufacturing offers the capacity to fabricate complex, near net shape components and the possibility to repair existing components. The vast majority of additive processes are fusion-based, relying on melting and solidification, which can lead to poor mechanical performance due to intense thermal gradients leading to solidification cracking and columnar dendritic grain growth. Additive Friction Stir-Deposition (AFS-D) is a novel technique that implements solid-state severe deformation to create depositions additively. As such, the AFS-D process offers potential to fabricate fully dense components with wrought-like mechanical performance and microstructure. Likewise, AFS-D avoids the intense thermal gradients of fusion welding that leads to solidification cracking in lightweight materials in aerospace applications, such as aluminum alloys. In this research, the process-structure-property relationship is quantified by means of microstructure characterization and mechanical evaluation of AFS-D AA6061. To understand the process-structure-property relationships of AFS-D as-deposited AA6061, test specimens in two orthogonal directions, longitudinal and build, were subjected to quasi-static monotonic tension and strain-controlled fatigue testing. Microstructural evaluation revealed the refinement of constituent particles in AFS-D AA6061, in addition to dynamic recrystallization and grain refinement. Mechanical results indicated homogeneous strength between the two directions investigated at a similar strength to wrought AA6061-O, and fatigue performance similar to the wrought in the longitudinal direction. Microstructural examination of a standard heat treatment for the T6 temper of AA6061 on AFS-D AA6061 was conducted. This led to mechanical performance superior in strength to the control wrought AA6061-T651 in monotonic tension tests, and similar fatigue performance to the as-deposited AFS-D AA6061 and wrought AA6061. Fractography revealed an evolution in the deformation behavior for post deposition heat treated AFS-D AA6061 compared to the as-deposited specimens. Lastly, the mean strain effects of heat treated AFS-D AA6061 and a lightweight rolled aerospace aluminum alloy, AA2099-T83, are quantified and captured in this work. The tensile stable cycle mean stress proved detrimental to the fatigue performance of aluminum alloys. A modified strain-based Morrow model is proposed in this work that successfully captures the effect of tensile mean strain loading conditions on these aluminum alloys.