UCLA

In today's society, the manufacturing of complex structures and assemblies is of vital importance in the design and engineering process. In this context, fusion welding has proven to be an invaluable method from large scale automotive assembly lines to high tech aerospace applications. With the growing call for energy-efficient solutions, the demand for high strength aluminum alloys in these applications is evident. Therefore, when incorporating these materials into profitable production processes, a material‘s weldability becomes crucial.

While high strength aluminum alloys such as AA7075 and AA2024 are considered unweldable due to their high susceptibility to hot cracking, the medium strength AA6061 is only weldable with limitations. The dissimilar filler metals used to mitigate hot crack susceptibility leave the weld metal with limited strength, ductility, corrosion resistance, and heat treatability.

Therefore, this study aims to eliminate the intrinsic susceptibility of introduced aluminum alloys using filler metal of similar chemical composition treated with refractory nanoparticles, i.e., nano-treating (NT). Filler metal containing a volume fraction of 0.8 v% of TiC (7075NT-C, 2024NT-C, and 6061NT-C) and a volume fraction of 0.8 v% TiB2 (7075NT-B, 2024NT-B, and 6061NT-B) nanoparticles were produced, matching the constituent element composition of their respective commercial aluminum alloys. In addition, the industrially applied filler metal ER5356 was nano-treated with 0.8 v% TiC (5356NT-C) and 0.8 v% TiB2 (5356NT-B), respectively. The welds performed with similar nano-treated filler metal showed a remarkable grain refinement and morphology control, whereas the use of TiC-containing filler metals was deemed more effective than their TiB2 -bearing counterparts.

The transversal tensile properties of the welded joints showed an increased weld metal hardness in as-welded condition and virtually wrought transversal tensile properties by applying a post-weld heat treatment to the specimens.

The Houldcroft hot crack susceptibility tests conducted showed a significant reduction in the weld metals’ susceptibility to hot cracking when comparing tests performed with nano-treated filler metal to tests performed with the respective base metal as filler metal. It was noted that the crack reduction achieved with NT-C filler metal slightly exceeded that of tests performed with NT-B.

An in-depth thermal analysis investigating the solidification behavior of nano-treated alloys revealed a three-fold mechanism caused by the refractory nanoparticles. It was shown that nanoparticles significantly alter an aluminum alloys solidification process from liquidus to solidus temperature and cause unprecedentet changes to the materials resultant microstructure. This new finding strongly contradicts observations made with the use of conventional grain refiners and mitigate an aluminum alloys susceptibility to hot cracking.