Effect of Mg-rich filler metal on weld zone properties in pulsed laser-welded ultra-fine grain AA6061: A Taguchi optimization study

This study investigates the effects of filler metal with high magnesium (Mg) content at different heat inputs on microstructural evolution, strengthening mechanisms, and reduction of welding defects in pulsed laser welding (PLW) of ultra-fine-grained (UFG) AA6061 sheets. The dominant mechanism in UFG-welded AA6061 specimens, produced by accumulative roll bonding (ARB), was attributed to the grain boundary strengthening (GBS) effect due to grain size reduction and an increase in dislocation density. High heat input and remelting during PLW with AA5356 filler destroy the UFG structure, causing grain growth in the heat-affected zone (HAZ) and weld zone (WZ). It is shown that weld No. 7, with a heat input of 112 J/mm, and the use of high Mg filler metal contributed to the improvement of WZ strength due to increased fluidity, uniform distribution of this alloying element, and precipitation of Mg₂Si strengthening phase in the WZ. After welding under optimal conditions using a filler metal along with high Mg content, the strengthening mechanisms changed from the GBS effect and increased dislocation density to solid solution strengthening and precipitation of Mg₂Si as the strengthening phase. Scanning electron microscopy images show that laser welding using AA5356 filler metal eliminates the delamination effect and local necking, which are the leading causes of AA6061-UFGed failure. The failure in weld No. 7 indicates ductile fracture due to the heterogeneous distribution of dimples.