Title: Evaluating the Effects of CTE Mismatch on the Formability and As-Formed Mechanical Properties of Al-Mg/Al2O3 Nanocomposites
Abstract: Aluminum (Al) alloys are widely used throughout the aerospace industry for their high strength-to-weight ratio and cost effectiveness. However, common aerospace alloys such as Al 2024-T3 and 7050-T7 are precipitation hardened and require multistep heat treatments to achieve service properties. NASA’s integrally stiffened cylinder (ISC) flow forming process is poised to reduce the weight, manufacturing time, and cost of stiffened aerospace structures such as rocket cryogenic fuel tanks and aircraft fuselages. However, commercial implementation is hindered by low formability in heat treated alloys, while distortions and additional expenses are incurred by heat treatment after forming. Thus, work hardenable alloys are of interest to eliminate the need for a post-forming heat treatment and further increase the manufacturing rate benefits of the ISC process. Moreover, the addition of ceramic nanoparticles to form work hardenable Al matrix nanocomposites (AMNCs) is expected to further increase the as-formed strength, but a significant amount of research remains to understand the tradeoffs of strength with formability, since the mechanisms that govern the nanocomposites’ strength in the formed condition (e.g. Orowan strengthening and coefficient of thermal expansion (CTE) mismatch strengthening) will impede dislocation motion and slip during ISC deformation.
The proposed research explores the effects of the CTE mismatch strengthening mechanism on formability and as-formed strength in Al-Mg/Al2O3 nanocomposites. Vacuum hot pressed and extruded powder metallurgy nanocomposite alloys with an Al-4.5Mg matrix and Al2O3 reinforcement particles in 1% and 3% volume fractions will be investigated during the course of the work. It is hypothesized that the formability of AMNCs can be increased by reducing the equilibrium size of the geometrically necessary dislocation (GND) punched zone through a specially designed anneal. Formability testing and flow forming trials will be conducted on nanocomposites of differing CTE mismatch levels to assess this theory.
The research will be carried out in three interconnected tasks. Task 1 will investigate the development of annealing treatments to optimize the CTE mismatch levels to achieve a high-mismatch and a low-mismatch condition for each composite. Nanoscale characterization of the punched zones will be performed with transmission electron microcopy (TEM), in addition to characterization of the nanoparticle-matrix interfaces and the interactions of the GNDs with statistically stored dislocations at different strain levels. Average matrix dislocation densities will be measured through x-ray diffraction (XRD) peak broadening analysis and compared with CTE mismatch models. Task 2 will utilize coupon-scale compressive and tensile formability testing and forming limit diagram construction to screen the four AMNC conditions (two volume fractions with two CTE mismatch levels each) for flow forming candidacy. The measured yield strengths will be compared with AMNC models using the dislocation densities measured in Task 1 and with other AMNCs in the literature to understand how thermal processing affects prediction of the equilibrium punched zone size. Task 3 will investigate room temperature stepwise flow forming trials on 100 mm diameter cylinders using NASA LaRC’s new vertical flow forming machine (WF VUD-600). These trials will be used to validate the forming limit diagram predictions by detecting the critical thickness reduction for the nanocomposites as a function of volume fraction and CTE mismatch level. Smooth-walled cylinders of the maximum allowable thickness reduction for each composite will then be flow formed and samples extracted for tensile testing to measure the as-formed mechanical properties. The results from the flow forming trials will be compared to the property targets to discuss the outlook for flow formed AMNC structures and to inform future composite designs.
Dr. Sean Agnew, MSE, Chair
Dr. James Fitz-Gerald, MSE, Advisor
Dr. Tao Sun, MSE
Dr. Christopher Goyne, MAE
Dr. William Harrigan (Gamma Alloys, Inc.)
Dr. Karen Taminger (NASA Langley Research Center)
Meeting ID: 917 8958 3380
All interested persons are invited to attend.