High deposition rate additive manufacturing such as gas metal arc-based wire arc additive manufacturing (WAAM) is ideally suited to produce large and highly complex shapes. The inherent requirement for low density materials in ground transportation and aerospace applications necessitates high strength aluminum alloys. However, high strength Al alloys in the 2XXX, 6XXX, and 7XXX alloys are considered “unweldable” with arc welding methods. In this study, secondary phase particles were added to high strength aluminum feedstocks that act as grain refiners upon solidification and prevent hot cracking. Modeling and experiments were combined to understand the microstructure and mechanical properties affected by the grain refinement.
A robotic Fronius gas metal arc (GMAW) system was used to deposit both standard 6061 and inoculated 6061 aluminum additive builds. The builds were sectioned, and solidification structure was characterized using optical microscopy and electron backscattered diffraction (EBSD). For the heat treatment study, additive builds were sectioned and put through stress relief, solution and age (T6), and direct age heat treatments. Mechanical properties were evaluated with microhardness testing. The microstructure in the heat-treated condition was characterized using scanning electron microscopy (SEM).
Results and Discussion:
Solidification cracking along large, columnar grains was evident in the standard 6061 build, while the inoculated build was crack free with a fine, equiaxed grain structure. Analytical solidification models were developed to understand the difference in microstructure development between the standard and inoculated 6061 builds. The models predict that grain refinement should occur in the inoculated build, but the morphology observed in the experiment are not yet predicted by the models. Possible reasons for discrepancies between model and experiment are discussed. The significant grain refinement observed in the inoculated 6061 build leads to increases in hardness in the as-built condition as well as after heat treatment.
Hot cracking has been eliminated in 6061 aluminum WAAM through inoculation additions into the feedstock that act as grain refiners. Solidification morphology is compared to analytical model predictions which show reasonable agreement, and improvements to the modeling are being pursued. Significant increases in hardness are measured for all conditions in the inoculated 6061 build due to grain refinement. The results of this study open new doors for large scale additive manufacturing of high strength aluminum alloys for a wide range of applications.
Keywords: WAAM, high strength aluminum, solidification cracking, inoculation, solidification, dendritic growth