Due to one of the highest strength-to-weight ratios among aluminum alloys, Al7075 is a popular alloy in the automotive and aerospace industries. However, the widespread application of this precipitation-hardened alloy is significantly hindered by limitations in manufacturing processes, such as casting and fusion welding, mainly due to solidification cracks. Several investigations have been conducted to mitigate this issue through welding parameter optimization, adjusting the fusion zone composition, or adopting a solid-state welding process instead. Recently, several studies reported crack-free joints after fusion welding using nanoparticles, but the governing mechanisms are not yet clearly understood. Therefore, the first stage of this research investigates the micro-mechanisms behind solidification crack elimination in fusion welding of Al7075 alloy using 1 vol.% TiC-nanoparticle enhanced Al7075 filler metal. By incorporating the TiC nanoparticles into the welding process, the study proposes two critical mechanisms in solidification crack elimination: (i) fusion zone grain morphology and size alternation due to the presence of TiC-nanoparticles in the early stage of solidification, acting as nucleation sites; and (ii) deleterious continuous eutectic precipitates replacement with favorable discontinuous ones.
In the second stage of this research, a modified physical model is presented, illuminating the role of fusion grain morphology alternation in reducing solidification crack susceptibility. The modified proposed model showed that altering the fusion zone grain morphology from dendritic to equiaxed (i) reduces strain accumulation on the coherent solid network in the mushy zone by reducing the coherency temperature, and (ii) prevents the growth of potentially formed voids between grains by maintaining the total pressure at void/liquid interfaces in the positive range.
In the third and last stage, using the comprehensive understanding of solidification crack elimination mechanisms, the study introduces a practical approach to solidification crack elimination in fusion welding of Al7075, involving TiC-nanoparticle incorporation using a fabricated paste. This approach eliminated the solidification cracks in Al7075 TIG welding in a cost-effective way, by simplifying the fabrication process.
These findings collectively underscore the potential of TiC nanoparticles as a strategic tool to enhance solidification crack resistance and enlighten the mechanism behind solidification crack elimination, which can contribute to further promoting and broadening the use of Al7075 in aerospace and automotive applications