UCLA

Fusion welding is an important fabrication process used on a variety of engineered structures to create strong and permanent joints. Today, there is a growing call for structural lightweighting and sustainability, and the incorporation of light alloys like aluminum can reduce emissions, improve fuel economy, and contribute to closed-loop manufacturing. However, there is a glaring incompatibility between fusion welding and aluminum alloy joining. More specifically, marrying two dissimilar aluminum alloys has historically proven to be difficult, and current methods come with a penalty to either structural weight or the joint’s design freedom. First, the weldability of dissimilar aluminum alloys is low, especially when one or both are prone to hot cracking. This stems from issues involving the solidification of the alloy, which can result in the formation of defects such as cracks during solidification. Second, the potential for post-weld heat treatment is low, as the chemistry differences between the weld and the base metals vary. Third, dissimilar welds often suffer from residual stresses after solidification. Adverse effects like distortion can result from this, and conventional methods to alleviate residual stress become difficult to implement. Recently, nano-treating (NT) technology has been utilized to address these problems and improve the fusion welding of dissimilar aluminum alloys. Nano-treating is the addition of a low concentration of refractory nanoparticles into metals and alloys to elicit changes in processability, microstructure, and properties. Previous research into nano-treating strongly suggests a high potential for the technology to address the presently posed challenges. In this dissertation, weldability of dissimilar aluminum alloys was first investigated by fabricating joints using high-strength grades of wrought alloys and nano-treated ER5183 filler with approximately 1 vol.% TiC nanoparticles. Specific alloy combinations were AA2024, AA6061, or AA7075 (similar welds) along with AA2024 to AA5083 or AA2024 to AA7075 (dissimilar welds), with comparisons drawn to welds made with commercial ER5356 filler when possible. Joints created using commercial filler displayed large dendritic microstructures, continuous secondary phase networks, and a tendency for defect formation while joints with nano-treated fillers displayed refined, equiaxial, non-dendritic grains, dispersed secondary phases, and a notable elimination of harmful defects. High joint quality was quantified using microhardness and tensile testing. This study showed a successful extension of nano-treating technology to dissimilar welding of high-strength aluminum alloys. The second study focused on the natural aging behavior of dissimilar high-strength aluminum alloy joints made between AA6061 and AA7075 (with nano-treated AA6061 or AA7075 fillers) or AA2024 and AA7075 (with nano-treated AA2024 or AA7075 fillers). Short-term data over eight to ten weeks consisted of weekly microhardness measurements, while long-term data was collected after over a year of natural aging. It was noted that all dissimilar joints showed increases of over 30% in microhardness over the short term. In particular, joints created with 7075-NT filler increased by over 40% in weld metal microhardness in the short term, and moreover, continued to increase over the long term. Tensile tests confirmed improvements in strength after aging. Computational phase diagrams and microstructural studies suggest differences in the behavior of specific strengthening precipitates owing to the variance in compositions. The study shows the promise of nano-treating in improving dissimilar weld joint strength without the need for heat treatment. Additional investigation on the welding of AA2024 with nano-treated ER2319 filler confirmed improvements in microhardness during natural aging, but tensile testing showed improvements only as welded compared to commercial ER2319, necessitating deeper analysis of this system. The third study evaluated the reduction of residual stress using nano-treating technology. Experiments related to angular distortion, outward thermal flux from the weld metal, in-situ strain, and microstructure were conducted to understand the effect of nano-treating on residual stress development in fusion welds of high-strength alloys AA2024, AA6061, and AA7075. Macroscopic observations noted significant reductions in average deflection and axial strain, while the inclusion of nanoparticles altered the release of heat during weld solidification. A theoretical mechanism based on the experimental results and established nano-treating effects is proposed to elucidate the nanoparticle-enabled control of residual stress by reducing constraints on solidification-induced shrinkage. In summary, this dissertation extends the use of nano-treating technology to dissimilar welding of high-strength wrought aluminum alloys to expand the capabilities of fusion welding and build upon previously established research cementing the fabricability of high-strength aluminum alloys with nano-treating technology. Furthermore, it provides guidance on dissimilar joint design in terms of potential improvements to strength while circumventing limitations regarding heat treatment. Finally, this work proposes a new way of combatting a long-standing problem in welding residual stress using nano-treating to improve the process of fusion welding aluminum alloys.