Abstract
Aluminum alloys are of great interest to the aeronautics community due to their high strength-to-weight ratio and relatively low cost (compared to other aerospace alloys such as titanium). Specifically, 7xxx-series alloys are desirable due to their high tensile strength, moderate toughness, and low corrosion susceptibility in certain tempers. High deposition rate additive manufacturing (AM) methods represent a current technology that may play a role in future aircraft structures and would benefit from the high strength of 7xxx-series alloys. However, the ability to weld and additively manufacture structures from 7xxx-series alloys is limited by their susceptibility to hot cracking during fusion welding. This work investigates the AM compatibility and resulting properties of two common aerospace aluminum alloys, 2219 and 7075, deposited with the electron beam freeform fabrication (EBF3) process.
2219 is a weldable, Al-6.3Cu aerospace alloy used in rocket structures due to its moderate strength, excellent toughness, and good corrosion resistance. Linear walls and a brick-shaped deposit were fabricated, and the builds showed no evidence of cracking and minimal porosity, indicative of excellent AM compatibility. The microstructure and tensile properties of the deposits were studied in both the as-deposited (AD) and the artificially aged (T6) heat-treated (HT) conditions. The microstructure of 2219 in the AD condition included grains slightly elongated in the vertical direction, with an area-averaged grain diameter of 130 μm. The AD tensile properties were anisotropic, with yield strength (YS) and ultimate tensile strength (UTS) values ranging from 16.8 and 38.8 ksi in the longitudinal orientation to 21.5 and 40.7 ksi in the transverse orientation. All AD properties fell between typical values for 2219 wrought plate in the annealed (O) and naturally aged (T4) conditions. The HT tensile properties were isotropic, with an average YS of 43.4 ksi and an UTS of 63.0 ksi, and were consistent with typical 2219-T62 wrought product values (YS of 42.1 ksi and UTS of 60.2 ksi).
In comparison, 7075 is an Al-5.7Zn-2.4Mg-1.5Cu alloy with limited weldability and is used in aircraft structures due to its high strength. Linear wall deposits were fabricated with the EBF3 process to study the effects of beam parameters and baseplate conditions on the extent of hot cracking. In these deposits, both solidification and liquation cracks were observed, indicating that similar hot cracking mechanisms are in operation for both conventional welding and EBF3 of 7075. However, there were differences in the cracking behavior between the two processes due to the distinct geometry and thermal history of layer-by-layer AM. In the welding literature, solidification cracks typically occur parallel to the weld direction in the fusion zone, while liquation cracks occur in the partially melted zone of the base metal. In the 7075 EBF3 deposits, periodic solidification cracks appeared as large macrocracks transverse to the weld direction that propagated through the deposit layers. Between these solidification macrocracks, microstructural-scale liquation cracks developed below the topmost layer of the deposits.
As a first order effect, both the thickness and the initial temperature of the baseplate altered hot cracking density, with baseplate thickness affecting the periodicity and length of solidification cracks and temperature affecting the number and average length of liquation cracks. In addition, the composition of the fusion zone and its solidification characteristics contributed to cracking. During processing, Zn and Mg were vaporized from the molten pool due to their high vapor pressures, with losses ranging from 10% to 65% across the deposits. Solidification diagrams calculated from the Scheil solidification model were used to gain insight into both the hot cracking susceptibility (HCS) of the deposit compositions and the propensity for liquation cracking at the deposit-baseplate intersection.
A substantial reduction in cracking was observed when printing with a focused beam (keyhole) condition. The resulting deposit was free from solidification cracks in the steady state region and showed a 4x reduction in liquation crack density. The keyhole regions of the microstructure showed equiaxed grains on the order of 40 μm in diameter that disrupted solidification crack formation and lowered the liquation crack density. These improvements illustrate the impact of grain refinement in a material predicted to have a high HCS.
Analysis was also completed to explore future directions of alloy design for AM that would reduce the HCS of Al-Zn-Mg-Cu alloys. Results indicate that increasing the Cu content reduces cracking susceptibility by changing the solidification pathway, while maintaining a high concentration of Zn and Mg in the alloy retains strength. The HCS index of such Cu-rich 7xxx-series alloys was 1.5x higher than that of 2219 due to the Zn and Mg additions but 1.9x lower than that of 7075.
The results of this work indicate that achieving wrought 7075 properties with AM of 7xxx-series alloys is challenging due to Zn and Mg vaporization losses. These losses increase the susceptibility of 7075 to solidification and liquation cracking, inhibiting the use of 7xxx-series alloys in AM products. In addition, solute vaporization contributes to a loss of strength compared to wrought product, limiting the final application of the AM material. Future work involving alloy design in combination with in-situ grain refinement is pivotal to the successful deposition of high-strength aluminum alloys with EBF3.
