Aluminum Alloy for Electron Beam Deposition

Application of additive manufacturing (AM) to aircraft structural components is limited by the lack of high-strength aluminum alloys optimized for AM processing. High strength aluminum alloys (such as 7050-T7451) are difficult to process by AM because of their solidification cracking susceptibility. In addition, these alloys contain volatile alloying elements such as Zn and Mg that can easily evaporate during the AM process and adversely affect mechanical properties. The performance of a proprietary Al-Cu-Zn-Mg alloy was evaluated for additive manufacturing by electron beam deposition. One objective was to determine whether tensile strengths comparable to wrought 7050-T7451 could be achieved through post-deposition heat treatment. Deposits of the Al-Cu-Zn-Mg alloy were fabricated using electron beam deposition and the resulting microstructure, composition, and level of porosity were correlated with deposition parameters. Tensile, fracture toughness, and corrosion properties were evaluated following heat treatment for two sets of deposition parameters. Generally, adjustments to deposition parameters yielded opposite trends in level of porosity and losses of high vapor pressure solute elements Zn and Mg. Increasing beam current and reducing travel speed reduced porosity but resulted in greater losses of Zn and Mg. Under focusing the beam reduced elemental losses but did not appreciably reduce porosity. Generally, porosity reduced ductility and fracture toughness and losses of Zn and Mg reduced tensile strength. In all deposition conditions, the Al-Cu-Zn-Mg alloy was susceptible to both intergranular and stress corrosion cracking. The strength of the Al-Cu-Zn-Mg alloy after post deposition heat treatment was lower than typical values for 7050-T7451, related primarily to the significant losses of Zn and Mg. Further enhancement of Zn and Mg content in feedstock wire of the proprietary alloy combined with adjustments to deposition conditions and post-deposition heat treatment may enable production of electron beam deposited material that has strength comparable to 7050-T7451 wrought products.