Effect of Composition and Travel Speed on Solidification Mode in Iron-Nickel Alloys

This study investigated the effect of composition and travel speed on solidification mode in binary Fe-Ni alloys. Small Fe-Ni ingots were cast with compositions ranging from 10.6 wt. % Fe to 99.3% Fe. Cellular mode solidification growth mode was observed below 94.4 wt.% Fe, while solid-state phase transformations obscured the solidification microstructure above 96.8 wt.% Fe. Laser welds were then completed on each ingot with two different travel speeds (50 and 100 mm/s). All fusion zone microstructures exhibited cellular morphology up to compositions of 96.8 wt.% Fe. Differences in cross-section weld geometry were observed as a function of composition. At travel speeds of 50 mm/s, intergranular cracking was observed with increasing nickel content. Cracking was not observed in the 100 mm/s welds. The difference in cracking was attributed to a change in the weld geometry from a keyhole weld at 50 mm/s to conduction mode welds at 100 mm/s.