Influence of Grain Refinement and Microstructure on Fatigue Behavior for Solid-State Additively Manufactured Al-Zn-Mg-Cu
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TRADITIONALLY, Al-Zn-Mg-Cu alloys have had extensive use within the aerospace industry for flight-critical components due to their high strength-toweight ratio. However, these components can often be complex and expensive to fabricate, whereas additive manufacturing (AM) affords a solution to this problem, by providing rapid and cost-effective fabrication of high strength-to-weight materials for complex structural components.[1] Despite increased interest in AM, certain materials have proven to be difficult to process using fusion-based AM technologies. One such material, aluminum alloy (AA) 7075, is a precipitate-strengthened D.Z. AVERY, B.J. PHILLIPS, C.J.T. MASON, M. PALERMO, M.B. WILLIAMS, C. CLEEK, P.G. ALLISON, and J.B. JORDON are with the Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL, 35487. Contact e-mail: [email protected] O.L. RODRIGUEZ is with the Materials and Processes Laboratory, NASA Marshall Space Flight Center; Huntsville, AL, 35812. Manuscript submitted December 13, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
material that has good machinability and formability but suffers from hot cracking. The AA7075, in the T6 temper, derives its strength from the formation of two primary strengthening phases, g¢ (MgZn2) and g (MgZn2), that are formed from Guinier–Preston (GP) zones.[2–6] The g¢ is a fine dispersed semi-coherent metastable phase that is considered the main phase responsible for the strength of the AA7075.[7] The g¢ is a hexagonal precipitate with lattice parameters a = 0.496 nm and c = 1.402 nm, which is primarily found within the grain interior with average diameter of 3 to 10 nm.[3,7–10] The g phase is an incoherent equilibrium phase that forms from g¢ particles at elevated temperatures.[7] The g phase is primarily formed within grain boundaries and has a hexagonal lattice, a = 0.521 nm and c = 0.860 nm, with average particulate diameter > 50 nm.[3,7–10] Additionally, while g¢ and g phases are found in the grain and grain boundary, therein lies a precipitate-free zone between grain matrix and boundary with an average width of 250 A˚. This soft precipitate-free zone favors intergranular fracture due to the lack of strengthening phase.[11] Furthermore, additional secondary phases will
precipitate out of solution such as S-phase (Al2CuMg), and the T-phase E-phase (Al18Mg3Cr2), (Al32(Mg,Zn)49), which can be found throughout the microstructure specifically at grain boundaries.[11–13] These particulates are used for grain and subgrain pinning, which increases the strength of the AA7075.[13] In addition to the strengthening precipitates, two main types of constituents particles are found in wrought AA7075: iron-rich (Al6(Fe,Mn), Al3Fe, Al(Fe,Mn,Si), Al23Fe4Cu, and Al7Cu2Fe) and silicon-based intermetallics (Mg2Si).[14,15] These brittle coarse constituent particles range in size from 1 to 50 lm.[14] During mechanical loading, cracking of the constituent particles occurs, which can lead to void coalescence and void growth or cracking of the aluminum matrix under cycl
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