Abnormal grain coarsening of laser surface remelting rapidly solidified TA15 titanium alloy

Rapidly solidified microstructure of a laser surface remelted TA15 titanium alloy as a function of laser scanning speed and solidification cooling rate was investigated. Abnormal coarsening of grains in the rapidly solidified remelted zone was readily observed for the laser glazed TA15 titanium alloy irrespective of the rapid solidification cooling rate. The grains in laser remelted zone of TA15 titanium alloy were abnormally coarsened and their sizes were up to 10 to 20 times larger than that of the grains or 32 to 62 times larger than that of the grains in the original material. The rapid solidification and grain growth of molten bath of laser surface remelting TA15 titanium alloy was a process of directly epitaxial growth of the grains of the unmelted substrate at the contact of the melt-pool bottom. The rapidly solidified grain size in the laser remelted zone was determined by the grain size of unmelted base metal at the bottom of molten bath. During laser heating and molten bath forming in the surface of the TA15 titanium alloy, grains of the unmelted base metal at the top of heat-affected zone nucleated, grew in an extremely short time owing to the extremely high temperature. Solid-state abnormal coarsening of grains on the unmelted base metal at the bottom of molten bath resulted in the abnormal coarsening of rapidly solidification structure in laser remelted zone of the TA15 titanium alloy.